L A N D
M A N A G E M E N T
H A N D B O O K
61
Managing Forested Watersheds for
Hydrogeomorphic Risks on Fans
2009
The Best Place on Earth
Ministry of Forests and Range
Forest Science Program
Managing Forested Watersheds for
Hydrogeomorphic Risks on Fans
D.J. Wilford, M.E. Sakals, W.W. Grainger,
T.H. Millard, and T.R. Giles
The Best Place on Earth
Ministry of Forests and Range
Forest Science Program
The use of trade, �rm, or corporation names in this publication is for the information and
convenience of the reader. Such use does not constitute an official endorsement or approval
by the Government of British Columbia of any product or service to the exclusion of any
others that may also be suitable. Contents of this report are presented as information only.
Funding assistance does not imply endorsement of any statements or information contained herein by the Government of British Columbia. Uniform Resource Locators (URLs),
addresses, and contact information contained in this document are current at the time of
printing unless otherwise noted.
Library and Archives Canada Cataloguing in Publication Data
Managing forested watersheds for hydrogeomorphic risks on fans / D.J.
Wilford ... [et al.].
Includes bibliographical references.
ISBN 978-0-7726-6119-7
1. Mass-wasting--British Columbia--Forecasting. 2. Landslide hazard analysis--British
Columbia. 3. Forests and forestry--Environmental aspects --British Columbia. 4. Forest
management--British Columbia--Planning. 5. Forest hydrology--British Columbia.
6. Alluvial fans--British Columbia. 7. Colluvium--British Columbia. I. Wilford, D. J.
(David J.), 1950- II. British Columbia. Ministry of Forests and Range III. British Columbia.
Forest Science Program
SD387.E58M36 2009
634.961
C2009-909966-7
Citation
Wilford, D.J., M.E. Sakals, W.W. Grainger, T.H. Millard, and T.R. Giles. 2009. Managing
forested watersheds for hydrogeomorphic risks on fans. B.C. Min. For. Range, For. Sci. Prog.,
Victoria, B.C. Land Manag. Handb. 61.
www.for.gov.bc.ca/hfd/pubs/Docs/Lmh/Lmh61.htm
Prepared by
D.J. Wilford
Ministry of Forests and Range
Smithers, BC
T.H. Millard
Ministry of Forests and Range
Nanaimo, BC
M.E. Sakals
Ministry of Forests and Range
Smithers, BC
T.R. Giles
Ministry of Forests and Range
Kamloops, BC
W.W. Granger
Grainger and Associates Consulting Ltd.
Salmon Arm, BC
Prepared for
B.C. Ministry of Forests and Range
Research Branch
Victoria, BC
© 2009 Province of British Columbia
Copies of this report can be obtained from:
Crown Publications, Queen’s Printer
PO Box 9452 Stn Prov Govt
563 Superior Street, 2nd Flr
Victoria, BC V8W 9V7
1 800 663-6105
www.crownpub.bc.ca
For more information on Forest Science Program publications, visit:
www.for.gov.bc.ca/scripts/hfd/pubs/hfdcatalog/index.asp
ABSTRACT
Fans are linked to their watersheds by hydrogeomorphic processes—�oods, debris �oods, and debris
�ows. These processes move water, sediment, and
debris from the hillslopes of a watershed through
channels to the fan. Fans in British Columbia are
often the site of residential developments, and transportation and utility corridors, as well as high-value
habitat for �sh and high-productivity growing sites
for forests. Collectively, these features are termed
“elements-at-risk” because they may be vulnerable
to watershed-generated hydrogeomorphic processes
that issue onto the fan. These processes may be natural or result from land use activities, and can cause
the partial or total loss of some or all of the elements
on the fan.
In British Columbia, forest harvesting and road
building is associated with increased hydrogeomorphic hazards. The downstream effects of these forestry activities in source areas may be far-reaching
and extend beyond the scope of conventional siteoriented planning. A �ve-step approach is presented
to assist land managers undertake risk analyses and
assessments that place their proposed developments
within the watershed-fan system. The �ve steps are:
1) identify fans and delineate watersheds; 2) identify
elements-at-risk on fans; 3) investigate fan processes;
4) investigate watershed processes; and 5) analyze
risks and develop plans. This scheme is applicable to
forested watersheds throughout British Columbia.
ACKNOWLEDGEMENTS
The need for this handbook was raised by the Forestry Committee of the Council of Forest Industries
and represents 3 years of collaborative research
involving many people throughout British Columbia. The concept for the �ve-step approach was
created by Bill Grainger and we are indebted to him
for his foresight. This handbook has gone through
many revisions over the past 3 years, incorporating
suggestions from workshop participants; this ver-
sion has had the bene�t of very thorough reviews by
Todd Redding, Robin Pike, Ian Smith, Steve Webb,
Rita Winkler, and David Maloney. This handbook
has bene�ted from a meticulous editorial review by
Steve Smith. We are indebted to the Forest Investment Account–Forest Science Program, BC Timber
Sales, and the B.C. Ministry of Forests and Range for
�nancial support.
iii
CONTENTS
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iii
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
iii
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
The Five-step Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
Step 1 Identify Fans And Delineate Watersheds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 1.1 The Fan-watershed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 1.2 Fan Identi�cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 1.3 Watershed Delineation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2
4
7
Step 2 Identify Elements-at-risk on Fans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 2.1 Human Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 2.2 Anthropogenic Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 2.3 Natural Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
8
8
10
Step 3 Investigate Fan Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 3.1 Hydrogeomorphic Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 3.2 Event Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 3.3 Event Magnitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
10
12
13
Step 4 Investigate Watershed Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 4.1 Watershed-fan Process Linkages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 4.2 Office Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 4.3 Field Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 4.4 Synthesis of Watershed Processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
14
16
18
19
Step 5 Analyze Risks and Develop Plans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 5.1 Understanding Risk Analysis and Risk Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 5.2 Consequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 5.3 Hazard. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 5.4 Risk Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 5.5 Assessing Risk and Making Management Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Step 5.6 Document, Monitor, Evaluate, and Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
20
20
21
23
23
25
Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
57
appendices
1 Wathl Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2 Eagle Summit Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
3 Shale Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
4 Hummingbird Creek case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
46
iv
Tables
1 Characteristics of hydrogeomorphic process deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
2 Forest management focus for different hydrogeomorphic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
3 Predictive models for dominant hydrogeomorphic processes using the relative
relief number and watershed length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
4 Example of long-term probabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
5 Qualitative frequency de�nitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
6 Example of qualitative hazard analysis matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
7 Qualitative risk analysis matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
A1.1 A matrix combining hazard and consequence to determine risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A1.2 Consequences, hazards, and risks of all the identi�ed hazards and elements-at-risk, except
the coarse sediment loading hazard and the consequence of potential impacts to residences
and human safety, which are dealt with separately . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A1.3 Coarse bedload sediment risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A2.1 Qualitative frequency de�nitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A2.2 A qualitative hazard matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A2.3 A qualitative risk matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A4.1 Relative relief numbers for the Swansea Point fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A4.2 Forest management focus for different hydrogeomorphic processes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A4.3 A matrix combining hazard and consequence to determine risk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
31
31
32
39
40
40
51
55
56
Figures
1 An aerial oblique of a fan—the clearcut harvested portion of the landscape . . . . . . . . . . . . . . . . . . . . . . . .
3
2 An aerial photograph with notation indicating fan areas in red and potential �ow pathways in blue . . . .
3
3 A landform map with alluvial and colluvial fans highlighted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
4 An example of an actively growing fan resulting from high sediment transport from the watershed
to the fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5 An example of a dissected fan resulting from abundant water but limited sediment transport
6
from the watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
6 A topographically de�ned watershed above a fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
7 A water intake structure at the apex of a fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
8 This building was damaged by a debris �ow caused by a drainage diversion related to a forest road . . . .
9
9 This drainage structure is not designed to accommodate the water, sediment, and debris from
naturally occurring hydrogeomorphic events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
10 A debris �ow initiated as a result of hydrophobic soils in the Kuskanook watershed,
damaging homes and the highway on the fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
A1.1
A1.2
A2.1
A2.2
A2.3
The Wathl Creek watershed and fan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
A view of the Wathl Creek fan and Kitamaat Village looking east into the watershed. . . . . . . . . . . . . .
28
Eagle Summit Creek watershed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
Gully and fan reaches of Eagle Summit Creek, looking south . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
A view of the drainage structure on the Trans-Canada Highway at Eagle Summit Creek . . . . . . . . . .
36
v
A2.4
A2.5
A3.1
A3.2
A3.3
A3.4
A4.1
A4.2
A4.3
A view of the upper steep reach in Upper Eagle Summit Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
37
A view looking upstream in the lower reach of Eagle Summit Creek . . . . . . . . . . . . . . . . . . . . . . . . . . . .
38
Location map for Block SKI 101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
42
Shale Creek watershed and fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
Channel network on Shale Creek fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
Stream 2 �owing in an older and larger channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
Map of Hummingbird and Mara Creek watersheds, and the Swansea Point Fan . . . . . . . . . . . . . . . . . .
47
Distribution of new debris �ow sediment on the Swansea Point fan, mapped on July 14, 1997 . . . . . .
48
Looking downstream from the con�ned valley reach of Hummingbird Creek to the apex
of the Swansea Point fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
A4.4 The “Hinkelstein” at the apex of the Swansea Point fan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A4.5 Aerial photograph BC2616: 76 of lower Hummingbird Creek, 1959, showing a recent landslide
to the north on similar steep, northwest-aspect terrain, a possible relic debris slide colonized
by a younger forest stand, and the location of the 1997 debris avalanche . . . . . . . . . . . . . . . . . . . . . . . .
A4.6 Pre- and post-development runoff-contributing areas to the culvert located above the
debris avalanche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A4.7 Aerial photograph of lower Hummingbird Creek watershed with the debris avalanche
below the road in right centre . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
vi
49
51
53
53
INTRODUCTION
British Columbia’s mountainous landscape contains numerous forested watersheds connected to
fans that have human development or high natural
resource values. Fans are the cone-shaped deposits
of sediment formed where stream channels leave
the con�nes of mountain valleys (Bull 1977). Fans
are desirable sites for development because of their
gentle gradients and workable materials. Unfortunately, fans are often dynamic landforms. They
are run-out areas for hydrogeomorphic events (i.e.,
debris �ows, debris �oods, and �oods) originating
in the watersheds. Fans commonly have uncon�ned
stream channels, resulting in the broadcasting of
water and sediment, and channel avulsions. Risks
from hydrogeomorphic events in�uencing a fan
must be carefully considered when natural changes
occur in a watershed (e.g., wild�re or mountain pine
beetle) or prior to watershed development (e.g., forest
harvesting or other developments). This is because of
the potential to change water and sediment regimes
that can lead to changes in the timing, magnitude,
and frequency of hydrogeomorphic events affecting fans. Even without land use changes in watersheds, natural hydrogeomorphic events on fans have
resulted in the loss of life and high �nancial costs
throughout British Columbia (Septer and Schwab
1995) and world-wide (Sidle et al. 1985). It is therefore
important that resource development in watersheds
above fans be planned and undertaken with an understanding of the fan-watershed system and a critical consideration of the risks to downstream features
on fans (Jakob et al. 2000).
This manuscript describes �ve steps used to
recognize and analyze risk in fan-watershed systems.
The information presented builds upon a body of
knowledge regarding fan-watershed systems from
British Columbia and beyond. Notable sources
include a multi-year investigation of forested fans
across British Columbia (Wilford 2003; Wilford et
al. 2002, 2003, 2004a, 2004b, 2005a, 2005b, 2006),
Land Management Handbook 56: Landslide Risk
Case Studies in Forest Development Planning and
Operations (Wise et al. 2004), and Debris Flow
Hazards and Related Phenomena (Jakob and Hungr
2005a). This handbook provides direction to those
who, during the course of their work, encounter
fans and need to analyze associated risks (e.g., forest
practitioners and planners, engineers and geoscientists, regional planners). This manuscript attempts to
provide an understanding of watershed-fan processes and the planning and assessment of watershed
activities to help better manage potential environmental, social, and economic risks. It also helps to
identify when it is prudent to engage hydrogeomorphic specialists (i.e., hydrologists, terrain specialists,
and others) in the process.
THE FIVE-STEP METHOD
Forest-land management in British Columbia has a
largely site-level approach to planning. While many
strong programs exist to address the importance of
site-level management issues, some situations call
for a wider perspective. For example, landslides and
peak �ows in a watershed can lead to serious hydrogeomorphic impacts on a fan located many kilometres away, destabilizing the fan surface, affecting
infrastructure, and altering aquatic habitat. These
effects are a result of connections in the fan-watershed system, and risk analyses on fans should match
the scale of these interactions between the fan and
the upstream watershed.
The Gully Assessment Procedure (British Columbia Ministry of Forests and British Columbia
Ministry of Environment 2001) presents an approach
to assessing fan-watershed systems, but it is applicable only to coastal areas of British Columbia.
The Watershed Assessment Procedure Guidebooks
(BCMOF and BCMOE 1999a) indirectly address risks
on fans at a watershed scale. While the Mapping and
Assessing Terrain Stability Guidebook (BCMOF and
BCMOE 1999b) states that off-site downslope/downstream elements-at-risk and possible consequences
should be considered, assessments have been generally conducted at the site level and often stop short
of a risk assessment by focussing primarily on the
hazards (e.g., initiation zones) (Schwab and Geertsema 2008). In 2006, the Province of British Columbia
formalized landslide risk assessments for residential
1
developments (Association of Professional Engineers
and Geoscientists of British Columbia 2006), which
should result in better management of new activities
on fans, given existing hydrogeomorphic risks. However, these formal guidelines do not address incremental risks to existing elements (e.g., homes, roads,
drainage structures) on fans due to natural processes
or land use developments in the watershed.
This handbook provides the �rst comprehensive,
provincial-level framework for assessing risk on fans
from upstream watershed activities. The following
�ve steps for analyzing risk in fan-watershed systems
are used as a framework for this handbook:
Step 1: Identify Fans and Delineate Watersheds
• Identify the physical inter-connections that exist
between areas of potential activities (watersheds)
and areas of potential impacts (fans).
Step 2: Identify Elements-at-risk on Fans
• Recognize and inventory values on the fan that
may be affected by the hydrogeomorphic processSTEP 1
IDENTIFY FANS AND DELINEATE WATERSHEDS
Step 1.1 The Fan-watershed System
A fan is a cone-shaped deposit of sediment formed
where a stream channel leaves the con�nes of a
mountain valley (Bull 1977) (Figure 1). It is an expression of its watershed; the fan is created by and
represents a summary of the hydrologic and geomorphic processes in the watershed. The watershed,
or catchment, is the source area for water, sediment,
and woody debris, and the stream channels are the
transport zone. The local climate provides the water
and solar energy; geology and biota provide the
transportable material and control the watershed
morphology. Water and organic and inorganic materials are moved from hillslopes, into stream channels through various pathways. Materials eventually
move out of the watershed and onto the fan. This
strong linkage between the watershed, channel,
2
es in the watershed (i.e., either natural processes
or those related to proposed management actions).
Step 3: Investigate Fan Processes
• Identify the nature of hazardous hydrogeomorphic processes (type, frequency, and disturbance
extent).
Step 4: Investigate Watershed Processes
• Identify watershed features controlling hydrogeomorphic processes (e.g., watershed hydrology,
geomorphology, and the role of vegetation cover).
• Identify the potential for incremental hazards associated with management activities.
Step 5: Analyze Risks and Develop Plans
• Develop planning options for the watershed and
assess the associated risks.
• Document the process and establish a plan to
monitor, evaluate, and report.
and fan requires a watershed-level perspective.
Watersheds can be of various scales. For example,
a small cutblock may comprise most of the runoff-contributing area of a culvert along a mainline
logging road. The same cutblock may be only a very
small portion of the watershed contributing water,
sediment, and debris to a fan containing a highway
bridge-crossing located several kilometres from the
block. Medium-sized watersheds (on the order of
�fty to hundreds of square kilometres) may include
numerous tributary junction fans—each with its
own watershed. Water and sediment leaving the
tributary junction fans subsequently in�uence mainstem channel conditions and transport regimes. In
other situations, mid-slope fans may cause the dominant �ow pathway to change from one downstream
watershed to another as the stream naturally moves
about on the fan surface (Figure 2).
Figure 1 An aerial oblique of a fan—the clearcut harvested portion of the landscape.
This is a “classic” fan, radiating as a cone-shaped landform where the stream
leaves the confines of the hillslope.
Figure 2 An aerial photograph with notation indicating fan areas in red and potential
flow pathways in blue. Depending on the dominant flow pathway on the
mid-slope fan, water and sediment can be delivered to any of the valleybottom fans.
3
Step 1.2 Fan Identification
A “classic” fan has a cone shape; however, the development of a fan is affected by the space into which
it builds and the hydrogeomorphic processes that
occur there. Many fans are bounded on one or both
sides by hillslopes, often resulting in an asymmetric
shape to the fan. Some fans build into a broad valley
that is not affected by any other hydrogeomorphic
process while some fans build into a con�ned valley
with a valley-bottom river. The valley-bottom river
may remove sediment from the toe of the fan, thus
limiting the size of the fan and potentially the spatial
extent of sediment and debris deposits on the surface
of the fan. Some fans build into lakes or the ocean
(fan deltas), affecting both the spatial extent of the
fan and the internal structure of the fan.
Identifying fans that may be affected by management activities in and beyond the planning area
is a critical component in developing appropriate
management strategies and prescriptions. A given
planning unit may encompass no fans but may
belong in the contributing areas of many separate
fan-watershed systems. Conversely, a planning area
may include little of the contributing areas, but
many fans. Also, multiple scales of fan-watershed
systems may be present, so it is important to identify
fans, including their apexes and their boundaries.
Various methods are available, and any method that
identi�es the fans, particularly the fan apexes, is appropriate.
A common method is to use stereoscopic aerial
photographs. A range of aerial photograph scales
(e.g., 1:30 000 and 1:60 000) allows different vertical
exaggerations and different levels of visible detail to
be reviewed. A degree of aerial photographic interpretation skill is required, but can be developed
through practice and by interpreting areas where
one is familiar with ground conditions.
On topographic maps the typical “fan-shaped”
morphology is often apparent where stream con�nement ends at valley-bottom �oodplains or lakes. Using stereo aerial photographs and topographic maps
together will usually result in the most accurate fan
delineation. Using either or both of these methods,
the size of fans that can be identi�ed will vary depending upon the scale of available photographs or
topographic maps.
Soil, sur�cial geology, and landform mapping has
4
been completed for many areas of British Columbia at 1:50 000 scale and generally identi�es larger
fans in mapped areas (e.g., Runka 1972). Due to the
relatively small map scale, many smaller fans will be
amalgamated into larger polygons and thus may not
be represented. More recently, landform mapping
has been done in association with terrain stability
mapping, which is usually completed at 1:20 000
scale and therefore is much better at identifying
smaller fans (Figure 3). Much of this work is also
available in digital format (available at: www.em.gov.
bc.ca/mining/geolsurv/terrain&soils/frbcguid.htm).
However, in some areas, low-gradient terrain units
are combined into large polygons (e.g., colluvial and
morainal blankets with fans) and the location of individual fans may not be indicated. When using any
mapped information, it is important to understand
what the purpose of the work was, what the methods
were, and at what scale the original mapping was
completed.
Attempts have been made to automatically identify fans using geographic information systems (GIS);
success was limited due to challenges in identifying
the distal (outer) boundaries of fans. The boundaries
frequently transition into �oodplains with very little
change in gradient.
Ground-truthing is validating remotely sensed
information through �eld checks. It is recommended
to ground-truth potential problem areas or to randomly validate mapping. Once in the �eld, fans can
be difficult to identify because: they can be large,
making it difficult to conceive of the entire landform
without seeing it; distal portions may grade into wetland areas, making �eldwork difficult or impossible;
and fan boundaries may grade into other low-gradient surfaces such as �oodplains, making fan boundary delineation difficult. Not withstanding these
challenges, �eldwork is essential to complete Steps 2
and 3 of a fan-watershed assessment.
There is considerable discussion in the literature
regarding the temporal origins of fans. In some
situations, �eld evidence (e.g., elevated fan surfaces)
supports the position that fan origin is linked to lateglacial sedimentation episodes unrelated to modern
conditions (Ryder 1971a, 1971b; Ritter et al. 1993).
During these periods, considerable volumes of sediment were deposited as fans. As watersheds revegetated, sediment supply declined and streams from
the watersheds eroded into the fan surface, leaving
Figure 3 A landform map with alluvial (Ff in yellow) and colluvial (Cf in orange) fans
highlighted. Of note: one fan was missed by the mapper but identified during
fieldwork—it was mapped as a complex of morainal blanket and morainal veneer
(Mbv) and is located just below the centre of the map and highlighted in orange
(a portion of polygon 100).
elevated fan surfaces and lower surfaces that are
in�uenced by contemporary hydrogeomorphic processes. Inactive elevated or “paraglacial” fan surfaces
can be found on many fans in British Columbia, but
other types of fans are also found. Watersheds with
abundant sediment transported to their fans may
have fans that are actively growing (Figure 4) (Beaty
1970). Watersheds that have abundant water but
limited sediment transport may have dissected fans
(Figure 5) (Hunt and Mabey 1966). Some fans are
considered to be in a steady-state, dynamic equilibrium with sediment supply and water (Denny 1965,
1967; Hooke 1968). The �eldwork described in Steps 2
and 3 will identify the nature of speci�c fans.
5
Figure 4 An example of an actively growing fan resulting from high sediment transport from the watershed
to the fan.
Figure 5 An example of a dissected fan (A) resulting from abundant water but limited sediment transport
from the watershed. The stream channel (B) is incised into the fan surface 6–10 m.
6
Step 1.3 Watershed Delineation
It is necessary to identify the watershed upstream of
each fan in the planning area, and the fans outside
of the planning area if activities are planned in the
watersheds of these fans. A watershed, catchment, or
drainage basin is de�ned as the area where the water
arriving at the surface will drain to a point of interest (Strahler and Strahler 1987). Properly delineating a watershed is one of the most important steps
toward understanding the fan-watershed system.
Watershed boundaries can be determined though
a variety of methods, ranging from a pencil and a
topographic map to advanced GIS.
For individual or a small number of watersheds,
the best method is manually drawing watershed
boundaries on appropriately scaled paper or digital
topographic maps, as described below and illustrated
in Figure 6.
1. Determine the point of interest. In the case of fan
studies, the point of interest is usually the apex, or
top, of the fan.
2. Draw the boundary moving upslope from the
point of interest, crossing contour lines perpendicularly. An exception is commonly required in
the immediate vicinity of the apex where the scale
of the map is generally insufficient to properly
characterize the surface topography.
3. Continue drawing until a closed polygon has been
created de�ning the catchment area above the fan.
Figure 6 A topographically defined watershed above a fan.
7
In this way, topographic watershed boundaries
are readily de�ned and reasonably accurate and thus
are the standard for hydrology, geomorphology, and
engineering studies. There may be errors, such as in
areas of low relief where surface water �ow pathways may not follow the mapped surface expression.
Where groundwater �ow paths do not conform to
topographically de�ned watershed boundaries, errors can also be introduced.
STEP 2
IDENTIFY ELEMENTS-AT-RISK ON FANS
A key part of risk analysis is to identify what is at
risk on fans, and these are referred to as “elementsat-risk”—human safety, public and private property
(including building, structure, land, resources,
recreational site, and cultural heritage features),
transportation system/corridor, utility and utility
corridor, domestic water supply, �sh habitat, wildlife
(non-�sh) habitat and migration, visual resource,
and timber (BCMOF and BCMOE 2002).
Once all the involved watersheds and fans to be
included in the analysis have been identi�ed, the
elements-at-risk on the fans need to be inventoried.
Both natural and human-made features should be
included. This step involves office work (aerial photographs and maps) and �eldwork. Include all elements that are on the fan surface as de�ned in Step 1
and as re�ned in the �eld during this step.
The information collected for each element should
include: description, location, value (qualitative or
quantitative), a description of fan features in the
vicinity of the element (e.g., immediately beside an
old channel), and a description of the exposure (e.g.,
summer residences only, or office building occupied
10 hrs/day). Identifying elements-at-risk early in the
risk analysis is important so that the intensity of the
remainder of the investigation can be adjusted accordingly. If the elements-at-risk are high value (e.g.,
human safety), more effort should be applied than
if the elements-at-risk are of lower value (e.g., forest
stand values).
Step 2.1 Human Safety
Potential injury or loss of human life is generally
considered the highest-value element in risk analyses
(Wise et al. 2004). Therefore, the presence of humans
on the identi�ed fans creates a strong potential for
8
Manual delineation can also be completed with
GIS. The advantage of using GIS is that not only can
the watershed boundary be easily altered but watershed area and other relevant watershed metrics can
be readily and accurately attained. A planimeter or
digitizer can be used to determine watershed area if
the topographic information is in paper format.
legal liability. A positive note is that humans are
mobile features that: 1) may not always be present in
that location, and thus have limited temporal exposure to fan hazards; and 2) can vacate the area
if given enough notice.
Step 2.2 Anthropogenic Features
While humans can be mobile, their improvements
frequently are not, and any damage may result in
litigation. Thus anthropogenic features must be
included as elements-at-risk. This refers to any
human-made features on the identi�ed fan(s).
Elements-at-risk on the higher surfaces should be
described and their speci�c exposure to hydrogeomorphic events should be evaluated. Assessment
of human-made features requires an awareness not
only of direct costs (e.g., repairing the railway following a debris �ow) but also the indirect costs (e.g.,
loss of business revenues due to delays). Indirect
costs can be very signi�cant, as in the case of transcontinental rail lines, where one day of traffic closure
may cost $10 million (Jacob and Hungr 2005b).
Common anthropogenic features are water intakes
(Figure 7), roads (including bridges), railways,
houses (Figure 8), institutional buildings, power
transmission lines, and buried �bre optic lines.
Some features such as drainage structures on the
fan may not just be elements-at-risk, but—if poorly
designed to accommodate the water, sediment, and
debris from naturally occurring hydrogeomorphic
events—may increase the likelihood of negative
impacts on the fan (Figure 9). If there is a history
of elements affected by �oodwater or debris from
hydrogeomorphic events, subsequent activities in
the watershed may be blamed for any future damage.
Thus, documenting the fan history prior to proposed
Figure 7 A water intake structure at the apex of a fan.
Figure 8 This building was damaged by a debris flow caused by a drainage diversion related to a
forest road.
9
Figure 9 This drainage structure is not designed to accommodate the water, sediment, and debris from
naturally occurring hydrogeomorphic events. This is an additional risk factor for forest management in the watershed.
or existing watershed activities is critical, not only to
identify the hydrogeomorphic issues that need to be
addressed, but to provide a defence should impacts
occur that are unrelated to those watershed activities.
Step 2.3 Natural Features
Fish habitat and forest sites are common natural
features on forested fans. Special habitats or rare feaSTEP 3
INVESTIGATE FAN PROCESSES
The fan surface is investigated from the office and in
the �eld to identify the nature of hydrogeomorphic
processes—their type, frequency, and disturbance
extent. This step can be undertaken while investigating the fan for elements-at-risk. Further information
on the topics covered in this section can be found in
Wilford et al. (2005b) and Jakob and Hungr (2005a)
(particularly Chapters 2, 8, and 17).
10
tures may also exist and should be researched. With
any feature, an understanding of why it exists on the
fan is important to correctly managing upslope areas
to preserve the conditions. A common example is
where high-quality spawning gravels and �sh habitat
are present in the stream channel on the fan. To preserve the natural conditions, management within the
watershed should not lead to degradation of water
quality, changes in bedload movement, or signi�cant
changes to peak stream�ows.
Step 3.1 Hydrogeomorphic Processes
The three principal hydrogeomorphic processes
in�uencing fans—debris �ows, debris �oods, and
�oods—leave characteristic deposits on fan surfaces
(Table 1). Evidence of all three processes can be
found on some fans. For example, it is common for
debris �ow fans to also experience debris �oods and
Table 1 Characteristics of hydrogeomorphic process deposits (after VanDine 1985; Smith 1986; Pierson and Costa 1987;
Wells and Harvey 1987; Costa 1988)
Characteristics
Flood
Debris �ood
Debris �ow
Mode of deposition
Grain-by-grain, dominated
by traction processes
(bouncing or rolling along
the streambed)
Rapid grain-by-grain
aggradation from both
suspension and traction
En masse
Strati�cation
Massive or horizontal
strati�cation (with cross
strati�cation)
None or horizontal
strati�cation
None
Grading
Variable: as a result of
sequential processes rather
than a single process
Frequently normal-graded
(coarse on bottom, �ne
on top)
None; reverse; reverse to
normal
Sediment characteristics
and texture
Clast-supported with an
open framework or
distinctly �ner grained
matrix of in�ltrated sand;
rounded clasts; wide range
of particle sizes; sorting
from front to tail; bmax
<10 cm to >20 cm
Clast-supported, with
predominantly coarse sand,
moderate to poorly sorted,
bmax typically <80 cm but
may be larger
Matrix-supported; rarely
clast-supported; very poor to
extremely poor sorting;
extreme range of particle
sizes; bmax 60–230 cm, may
contain megaclasts up to
400 cm
Orientation of clast
long-axisa;
imbricationb
Always perpendicular to
�ow; usually well
imbricated
Large cobbles to boulders are
usually perpendicular to �ow;
pebbles to small cobbles are
usually parallel to �ow; weak
imbrication and collapse
packing
Variable, based on location
within �ow; parallel to �ow
is most prominent; weak to
no imbrication
Landforms and deposits
Bars, fans, sheets, and
splays; channels have large
width-to-depth ratio
Similar to water �ood but
deeper deposits
Marginal levees, terminal
lobes, trapezoidal to
U-shaped channel
a A clast (e.g., pebble, gravel, cobble) has three axes: “a” is the longest, “b” is the second longest, and “c” is the shortest. The “b” axis
determines the sieve size that the clast will pass through. The term “bmax” refers to the “b” axis of the largest clast that can be found
in a deposit.
b Imbrication is the shingling or overlapping of clasts with the upper edge of each clast inclined downstream, similar to a deck of
tilting cards. Clasts are moved into this position by stream�ow.
�oods, and while �oods and debris �oods can occur
more frequently than debris �ows, debris �ows can
be far more damaging. Identifying hydrogeomorphic processes is a key element of the fan-watershed
assessment because each process is the result of
watershed factors that need to be addressed when
planning activities in the watersheds.
Debris �ows are homogeneous (single phase)
sediment–water mixtures similar to wet concrete,
with sediment concentrations between 70 and 90%
by weight (or 47–77% by volume), and result in the
formation of marginal levees and terminal lobes
(Costa 1984, 1988; VanDine 1985; Smith 1986; Pierson
and Costa 1987; Wells and Harvey 1987; Hungr et al.
2001). Debris �ows in British Columbia commonly
have peak discharges 2–50 times larger than the
200-year �ood (Jakob and Jordan 2001). Debris �ow
levees are steep-sided features that are often in parallel pairs, marking the path of the debris �ow. Levees
are formed as a debris �ow traverses a fan—the core
continues downslope while the slower-moving edges
of the �ow come to rest as deposits of large-diameter
sediment. Levee height is proportional to event �ow
depth, and is typically 1–2 m high.
11
Debris �oods, also referred to as hyperconcentrated �ows, occur when a greater volume of streambed materials is mobilized than in a �ood. As with
�oods, the mixing of sediment is not complete and
there is a rapid increase in solids concentration
toward the bed. Debris �oods have sediment concentrations between 40 and 70% by weight (or 20–47%
by volume); sediment deposits are bars, fans, sheets,
and splays (Costa 1988; Hungr et al. 2001). While
debris �oods move more and larger sediment than
�oods, peak discharges are at most 2–3 times that
of major �oods (e.g., 100-year �ood) (Hungr 2005).
Debris �oods commonly have a maximum particle
size of the same order as the peak depth of major
�oods—for example, up to several tens of centimetres (< 1.0 m) in the case of typical small mountain
streams (Hungr et al. 2001).
Floods have sediment concentrations between
1 and 40% by weight (or less than 20% by volume);
sediment deposits are bars, fans, sheets, and splays
(Wells and Harvey 1987; Costa 1988; Hungr et al.
2001). Characteristically the bars, sheets, and splay
deposits from �ood events are not as extensive as
debris �ood deposits, are better sorted, and have
a sediment size that is not as large. For example,
a �ood in a 2 m wide stream channel may deposit
sediment several metres each side of the channel,
while a debris �ood may deposit sediment 10 metres
each side of the same-sized channel.
Fans can be formed from one or a combination
of these three processes, any of which may no longer
be actively forming the fan. For example, the major
portions of many fans in British Columbia were
formed as the glacial ice was melting approximately
10 000 years ago, and there was abundant meltwater
and exposed, unconsolidated sediment. These are
referred to as “paraglacial fans” (Ryder 1971a, 1971b),
and sediment deposits on the fan surface may not
re�ect contemporary processes. For example, if a
fan does not have recent evidence of debris �ow
activity, but debris �ow levees are present on a fan
surface with 500-year-old trees and well developed
soil horizons, it is likely that the deposits re�ect
processes that are not currently active in the fan’s
watershed. However, caution must be used when
deciding whether evidence is no longer re�ective of
contemporary hydrogeomorphic processes. Jakob
and Jordan (2001) identify return periods of up to
500 years for some debris �ows, and, historically, the
12
return period of events considered too dangerous
for residential construction in British Columbia was
1 in 475 years or less (British Columbia Ministry of
Transportation and Highways 1996).
Step 3.2 Event Frequency
Investigations of hydrogeomorphic activity on fans
clearly indicate that events are neither rare nor
extreme (Innes 1985; Jakob and Jordan 2001). To
illustrate this point, evidence of recent hydrogeomorphic activity was investigated with a random
sample of 51 fans in the Bulkley Timber Supply Area,
an area with a mix of plateaus and mountainous
terrain. There had been little or no human activity in the watersheds of the sample fans. While this
area would not be characterized as overly unstable,
hydrogeomorphic activity was observed on 41 fans,
indicating that 82% of the fans had disturbance
occurring on at least a portion of the fan surface in
the last 50 years. A similar study was undertaken
on the southern British Columbia coast, and recent
hydrogeomorphic activity was found on 49 of 55 fans
(89%) (Millard et al. 2006). On the other hand, in the
drier southern interior of the province there are watershed-fan systems that have recently experienced
hydrogeomorphic activity on their fans outside of
stream channels, after several hundred years of inactivity (Giles et al. 2005). These longer return period
systems, with events often related to widespread
forest health issues or severe forest �res, can result in
underestimation of the hydrogeomorphic hazards.
Different methods can be used to determine the
frequency of events. In populated areas it is common
to �nd records of events in local archives (Septer and
Schwab 1995). Dendroecology—using scars, dating
cohorts of trees, and analyzing tree ring widths for
abrupt, long-lasting growth changes—is a useful
method of establishing a year, and in some cases a
season, of events (Strunk 1997). However, if recent
events have disturbed more of the fan surface, it is
common to lose some or all of the “tree record” of
older events. This method requires a degree of preparation of increment cores from trees—sanding and
microscopic identi�cation of growth rings to identify
growth changes. For a more detailed description of
dendroecology techniques refer to Dendroecology—A
Guide for Using Trees to Date Geomorphic and Hydrologic Events (Wilford et al. 2005a).
Dendroecology can be as simple as determining
a possible date for a hydrogeomorphic-event deposit
on a fan by determining the stand age on top of the
deposit. In �re-dominated ecosystems, the lack of
evidence of a stand-establishing �re on a fan surface (e.g., burnt stumps and logs and ash in the soil)
suggests that the stand was established following a
hydrogeomorphic event, and the stand age will correspond closely to the age of the event (Grainger and
Wilford 2004). Where there is evidence of standestablishing �re on a fan surface, it is difficult to
determine how many stand rotations may have
occurred on the deposit, and the deposit may be
hundreds (if not thousands) of years old.
Trees can be partially buried by sediment in the
hydrogeomorphic riparian zone, and will not die if
the roots have sufficient oxygen and water. Characteristically these trees do not have a basal or “butt”
�are, but the �are can re-establish over a period of
time. While lack of butt �are and recent deposits of
sediment are indications of hydrogeomorphic activity, it is possible for trees to recover after a period
of time (as little as 25 years) and re-establish basal
�are. Dendroecology is a useful means of establishing whether trees have in fact been buried and have
recovered a “normal” form (e.g., there will be an
abrupt reduction in radial growth rings immediately
following the event, and continuing reduced growth
for a period of 10 or more years, then a gradual release as the tree establishes a new root system
just below the soil surface).
On �ood fans, individual events may be less important than the overall �ood and sediment regime.
The spatial extent and complexity of the channel
network on the fan can be used to assess factors such
as sediment supply or the type of vegetation on the
fan (Millard et al., in press)
The fan surfaces identi�ed in Step 1 may include
elevated surfaces that have not been in�uenced by
contemporary hydrogeomorphic processes. On forested fans this can be evident in lower site productivity on the elevated surfaces of some fans (Wilford
et al. 2006); however, site productivity may not vary
in some climatic regions of British Columbia. In
those situations, differences in stand age and soil
pro�le development may indicate a lack of contemporary hydrogeomorphic disturbance.
The output from this part of the fan investigation
should be the identi�cation of how often a particular type of event has occurred in the past. In some
situations the best one can do is determine the time
since the last event—because evidence of previous
events has been removed by subsequent events. In
these situations, the time estimate is an estimate of
the event return period (T), or the number of years
between event occurrences.
Step 3.3 Event Magnitude
The disturbance extent of hydrogeomorphic events is
an indication of the power and volume of water and/
or material that issues from a watershed. The disturbance extent of hydrogeomorphic events on forested
fans can be determined from aerial photographs and
�eldwork. Evidence on aerial photographs can range
from strips of bare sediment to strips of century-old
trees (cohorts). However, some evidence may not
be apparent on aerial photographs due to the narrow swath cut through the forest, and some events
may not be powerful enough to clear forests—sediment is spread on the fan surface beneath trees, and
�eldwork is necessary to identify these disturbances.
A zone of potential disturbance outside the stream
channel on a fan is de�ned as the hydrogeomorphic
riparian zone. This zone can range from a few metres
wide to hundreds of metres wide. For a more detailed discussion of identifying hazardous zones on
fans refer to Forest Management on Fans—Hydrogeomorphic Hazards and General Prescriptions (Wilford
et al. 2005b).
Incised stream channels on fans may give the
impression that hydrogeomorphic events will not
in�uence the fan surface. However, depending upon
the con�guration of the channel and banks, debris
�ows can deposit up to 6 m of material in a single
event, resulting in signi�cant disturbance beyond
an incised channel (Osterkamp and Hupp 1987).
Evidence of over-bank disturbance from hydrogeomorphic events and channel conditions should be
explored prior to assuming that elements are not at
risk. Channels with a high likelihood of debris jamming may require an extra degree of caution when
developing forest management recommendations.
The natural disturbance extent of hydrogeomorphic events can also be modi�ed by anthropogenic
features and practices (e.g., riparian harvesting).
Some features such as dikes tend to convey the
disturbance downstream to lower positions on the
fan; however, if features are constructed of local
streambed materials without armouring, they are
13
more likely to be eroded and fail. On fans, both
inadequate drainage structures and roads that climb
in elevation toward stream channels can increase the
extent of disturbance (Wilford et al. 2003). Removal
of the hydrogeomorphic riparian zone through
forest harvesting, or through agricultural or residential clearing, increases the potential for greater
disturbance on the fan surface (Wilford et al. 2003).
The fan investigation should document the potential hydrogeomorphic implications of any existing
anthropogenic in�uence.
STEP 4
INVESTIGATE WATERSHED PROCESSES
The fan investigations provide an understanding of
past hydrogeomorphic processes that formed the
fan, and thus identify the processes produced by the
watershed. Investigating the watershed above the fan
is necessary to understand watershed characteristics, how they are linked to the processes observed
on the fan, and how that linkage creates the existing hydrogeomorphic hazard. Only then one can
determine the degree to which planned development
activities in the watershed could in�uence those
processes and potentially alter the existing hazard,
creating additional or incremental hydrogeomorphic
hazards.
A series of avenues are pursued when investigating a watershed—including both office work and
�eldwork. The watershed investigation is similar to
that undertaken using the Forest Practices Code Watershed Assessment Procedures (BCMOF and BCMOE
1999a) with a focus on geomorphic processes issuing
from the watershed, particularly debris �ows and
debris �oods.
Step 4.1 Watershed-fan Process Linkages
This section discusses how watershed-initiated
hydrogeomorphic processes are connected to the
fan and which watershed processes could lead to
increased or incremental hydrogeomorphic hazards.
Subsequently, this identi�es an appropriate management focus (presented in Table 2 and in italics in the
descriptions of the hydrogeomorphic processes).
Flood
There are two basic types of �oods: hydrologic and
dam-release. Rainfall and/or snowmelt generate
14
The outcome from this part of the investigation
should be an estimate of the probable magnitude,
runout zone, power, and potential damaging nature
of the event to pair with the frequency of occurrence
determined in Step 3.2. It is not uncommon that
more than one kind of event has occurred on the fan,
or on different parts of the fan, and that these separate types have different frequencies, and different
implications for management in the watershed (see
the Eagle Summit Creek case study in Appendix 2).
hydrologic �oods, which are the most common type
of �ood in British Columbia. Forests can play an
important role in hydrologic �oods, particularly in
snowmelt-dominated watersheds where forest harvesting or natural deforestation (e.g., mountain pine
beetle or wild�re) can alter snow accumulation and
melt rate processes (Winkler et al. 2005), potentially
resulting in increased peak �ows. Roads, trails, and
soil compaction (e.g., landings and other compacted
areas) can also in�uence �ood generation (Harr et
al. 1979; Wemple and Jones 2003). Equivalent clearcut
area (ECA) and percent of the watershed in roads and
compacted areas are two important watershed indicators where �ood impacts to fans are possible (BCMOF
and BCMOE 1999a). However, attention must be paid
to mass wasting because land use–related landslides
can increase sediment delivery to �ood fans, resulting in channel avulsions and fan destabilization (see
Shale Creek Case Study presented in Appendix 3).
Dam-release �oods are caused by a rapid release
of large volumes of stored water such as moraine,
snow avalanche, or debris �ow dam ruptures, glacial
lake outbursts, beaver dam failures, and downstream
dilution of debris �ows and debris �oods. Dam-release �oods are relatively rare, but they do occur in
British Columbia. There is a record of glacial outburst �oods occurring in northern British Columbia
(e.g., Alsek River, Tulsequah Lake, Salmon, and
Bear Rivers near Stewart, and the Kitimat River)
(Septer and Schwab1995); however, with the melting of alpine glaciers, the occurrence could increase
(Geertsema and Clague 2005). Landslides that block
stream channels and subsequently release rapidly
have been documented throughout British Columbia
(Jakob and Jordan 2001). Beaver dams may fail as the
Table 2 Forest management focus for different hydrogeomorphic processes. Key issues are bolded.
Process
Initiation
Management focus
Flood
Runoff
ECA – harvest, �re, forest health
Road density
Fire – reduced soil in�ltration
Landslides – increasing sediment
load to channels on the fan
(in�lling channels)
Dam rupture
Dam rupture – beavers, road
drainage structures
Landslide dams – harvest and
roads (coast), gentle-over-steep
(interior)
Runoff
ECA – harvest, �re, forest health
Road density
Fire – reduced soil in�ltration
Landslide
Harvest and roads on unstable
terrain and gentle-over-steep in
the interior
Dam rupture
Dam rupture – beavers, road drainage
structures
Landslide dams – harvest and roads
(coast), gentle-over-steep (interior)
Landslide
Harvest and roads on unstable
terrain and gentle-over-steep in
the interior
Runoff—in-channel
initiation
ECA – harvest, �re, forest health
Road density
Fire – reduced soil in�ltration
Debris �ood
Debris �ow
result of a variety of processes, including high-intensity precipitation, rapid snowmelt, animals burrowing through the dam, human destruction of portions
of dams, lack of maintenance by beaver, and collapse
of upstream dams (Hillman 1998; Butler and Malanson 2005). A dam-release �ood can exceed a design
stream�ow �ood (e.g., 100-year return period) by
a factor of up to 100 (Jakob and Jordan 2001). The
potential for dam-release �oods should be explored,
particularly where landslide-prone terrain has previously, or could potentially, produce a landslide dam.
In addition to local knowledge regarding past damrelease �oods, forest cover on a fan may contain
dendroecological evidence of past events—cohorts
of trees and damage to older trees (Wilford et al.
2005a).
Debris �ood
There is a continuum from water �oods through
debris �ows, with debris �oods being an intermediary process. Debris �oods should be viewed as an
extraordinary form of �ood in which an extremely
large volume of sediment with a wide range in grain
size is moved (essentially the whole streambed) and
deposited in a short period of time (Smith 1986).
Where debris �ows have been observed closely, some
evolve into debris �oods as they proceed downstream due to addition of water from tributaries
(Pierson and Scott 1985). Thus the factors that lead to
both debris �ows and �oods should be explored in debris �ood watersheds—increase in sediment delivery
to streams and the potential to increase peak stream�ows, and sediment and debris loading in stream
channels and gullies.
15
Debris �ow
Valley-con�ned debris �ows in humid temperate
forests characteristically have three forms of initiation (Takahashi 1981). The most common in British
Columbia is by impulse loading of channels, where
an open-slope failure (debris slide or debris avalanche) that enters a steep stream channel destabilizes sediment in the channel and proceeds down
channel by the force of gravity as a debris �ow (Sidle
et al. 1985; Jordan 1994; Millard 1999). This highlights
the need to identify landslide-prone terrain that is
tributary to steep gullies and stream channels and
to manage the hazards appropriately. The two other
forms of initiation occur during a major runoff event
with the mobilization, or bulking, of sediment and
debris stored in the stream channel, or the rupture
of a debris jam. Particularly in snow-dominated watersheds, in-channel mobilization of sediment can be
affected by changes in peak �ow runoff due to changes
to the forest canopy—as a result of wild�res, insect
infestations, or forest management activities including road density.
All three forms of initiation may occur following
extreme climatic inputs, which are assumed to occur
randomly. While climatic events are essential (Septer
and Schwab 1995), other conditions must be present
for either an open-slope failure or mobilization of
channel sediments (Miles and Kellerhals 1981; Dagg
1987). For example, if there is little or no sediment
and debris load in a channel, an in-channel debris
�ow is less likely to occur. A recent debris �ow may
have removed sediment and debris from a channel,
and the time to the next signi�cant event generally
depends on how rapidly the sediment load in the
channel is recharged. The exception is where very
large open-slope landslides enter stream channels
and have enough material to develop into signi�cant debris �ows regardless of the rate of sediment
entrainment along the pathway. The rate of sediment delivery to channels varies widely between
watersheds, depending on geology and vegetative
cover. Some channels experience rapid sediment
recharge and debris �ows can occur more than once
a year, while others require centuries or millennia
after an event to recharge channel sediment enough
to experience another debris �ow. A key factor in
determining debris �ow potential is to examine the
sediment and debris loading in stream channels and
gullies that is available for entrainment by an event,
and to understand the hillslope-channel conditions in
a watershed.
16
Step 4.2 Office Investigations
The �rst phase of office investigations is to compile
information related to watershed processes, to gain
an overview of the watershed, and to focus subsequent �eldwork.
Morphometrics
In mountainous or “graded” watersheds, topographic maps or GIS analyses can be used to generate
several basic topographic measurements that can be
used to provide a �rst approximation of the hydrogeomorphic processes. The relative relief number
is watershed relief divided by the square root of watershed area (Melton 1965). The relative relief number
has been used in various locations to differentiate
debris �ow, debris �ood, and �ood watersheds (Table
3). There are some cautions when using this approach. First, it is only an approximation and should
not be used to guide management decisions in the
watershed unless it is supported by �eld observations
and the detailed fan investigation undertaken in Step
3. Second, research shows regional variation within
British Columbia, and so this approach should be
used cautiously, particularly in areas not previously
studied. Third, the method is designed for “graded”
mountain watersheds and does not apply to watersheds in plateau terrain. For example, the Yard Creek
watershed near Sicamous is 100 km2 with a large
low-gradient upland plateau. The relative relief number predicts that the dominant hydrogeomorphic
process will be �ooding. However, there are extensive debris �ow deposits at the fan apex, originating
from a small steep-gradient tributary just above the
fan, which cannot be identi�ed except through aerial
photograph interpretation and �eldwork. While the
large fan is formed predominantly by �oods, forest
management planners must be aware that debris
�ows can originate from a particular part of the
watershed and that these hazards must be managed
accordingly.
Sediment production and movement: surface
erosion, landslides, and stream channels
The general location and nature of sediment sources
in a watershed can be determined from aerial photographs, topographic maps, and interpretative maps.
Speci�c details on sediment production are explored
during �eldwork (e.g., sediment textures, connectivity to stream channels, causes of landslide initiation).
Table 3 Predictive models for dominant hydrogeomorphic processes using the relative relief
number (RRN) and watershed length (the distance from the fan apex to the most
distant point on the watershed boundary)
Hydrogeomorphic
process
Class limits
Source
Flood
RRN < 0.30
Jackson et al. 1987
Wilford et al. 2004b
Debris �ood and debris �ow
Debris �ood
Debris �ood
Debris �ow
Debris �ow
Debris �ow
RRN > 0.30
Millard et al. 2006
Jackson et al. 1987
RRN > 0.30 and Length > 2.7 km
RRN 0.30–0.60
RRN > 0.60 and Length < 2.7 km
RRN > 0.52
RRN > 0.60
Aerial photographs provide a perspective on both
where landslides have occurred and the connectivity
of the landslide-prone terrain to stream channels.
Comparison of recent and historic aerial photographs is useful, particularly in situations where
forest growth obscures the evidence of landslides.
Topographic maps are used to identify speci�c
steep slopes and gentle-over-steep situations, and to
provide indications of connectivity of hillslopes to
stream channels. GIS analysis can be used to identify
steep slopes that could be prone to landslide activity;
however, this approach is strengthened when other
factors such as slope water movement and landslide
history are included in the modelling analysis (Pack
et al. 1998).
Sur�cial geology and terrain maps provide
information on the nature of sur�cial materials in
the watershed. This is particularly useful in areas
with lacustrine deposits that have a high potential to
degrade water quality—either in the natural setting
or as a result of land use activities. Terrain stability
maps identify landslide hazards, but are generally
restricted to either operating areas or the timber
harvesting land base (i.e., the maps do not identify
slope stability issues in alpine areas, which in some
cases can be signi�cant, as described by Geertsema
et al. 2006).
Forest �re severity maps of burned areas are valuable in identifying areas with reduced soil in�ltration—water repellency, removal of surface cover, and
soil sealing (Larsen et al., in press). These soil conditions have resulted in signi�cant increases in rainfall
Wilford et al. 2004b
Millard et al. 2006
Wilford et al. 2004b
Bovis and Jakob 1999
Millard et al. 2006
runoff (Scott and Van Wyk 1990; Moody and
Martin 2001) and debris �ows (Jordan et al. 2004)
(Figure 10). The implications of hydrogeomorphic
impacts to fans can be signi�cant.
Naturally unstable areas in the lower portion of a
watershed have been found to more strongly in�uence the nature of hydrogeomorphic processes (e.g.,
more sediment can be delivered to fans during a
hydrogeomorphic event) than unstable areas that are
located further from the fan in the headwaters of the
watershed (Wilford et al. 2005b).
Aerial photographs are used to identify stream
reaches. The Channel Assessment Procedure Guidebook (BCMOF and BCMOE 1996a) is a useful reference. If the channels are wide enough it is possible
to identify the general nature of the bed materials
and channel form (e.g., Hogan 2008 notes 30 m wide
channels with 1:50 000 aerial photographs down to
10 m wide channels with 1:5 000 aerial photographs).
Aerial photographs are also used to identify gullied
terrain and determine the degree of connectivity
between the gullies and stream channels. The ability
of stream channels to transport sediment is a key
factor in determining the downstream consequences
of sediment that enters a stream channel (Hogan and
Wilford 1989).
Flood generation: forest cover and road density
Forest cover maps are used to identify the extent and
location of natural and anthropogenic disturbance,
including wild�res, forest health issues, and past forest harvesting. These maps are also used to identify
17
Figure 10 A debris flow initiated as a result of hydrophobic soils in the Kuskanook watershed,
damaging homes and the highway on the fan. (Source: D. Boyer, B.C. Ministry of
Environment.)
potential changes to forest cover, such as the spread
of mountain pine beetle. Speci�c issues relating to
forest health should be explored, as the implications
to forest cover can be signi�cant in both immature
and mature stands (Woods et al. 2005). The Vegetation Resource Inventory (VRI) is the source of
information on forest cover; however, data are generally not entered on the heights of regeneration until
the openings have been declared free-to-grow—a
period of up to 20 years. Data on the extent of forest disturbances and the subsequent regeneration
are used to calculate the equivalent clearcut area
(ECA)—a measure of hydrologic disturbance to the
forest cover (BCMOF and BCMOE 1999a). The location
of hydrologically recovering stands is important in
snowmelt-dominated watersheds due to the more
rapid snowmelt in openings and immature stands
(Winkler and Roach 2005). Snowmelt from higherelevation disturbed areas can synchronize with
lower-elevation melt, increasing peak stream�ows.
Conversely, accelerated low-elevation snowmelt
could desynchronize runoff, tending to decrease
peak stream�ows.
Road density can be determined using GIS. Additional areas should be included in the calculation
of compacted areas in the watershed (e.g., landings
18
and skid trails). Research has identi�ed that as little
as 5% of a watershed in roads and compacted areas
can in�uence peak �ows (Harr et al. 1979; King and
Tennyson 1984). The value determined in the office
may be an overestimate that can be re�ned during
�eldwork. For example, deactivated roads and speci�c road segments may not be in�uencing hillslope
hydrology and peak �ow generation (Wemple and
Jones 2003).
Step 4.3 Field Investigations
Fieldwork in the watershed is undertaken to explore
key factors identi�ed in the office work, particularly
to verify data and collect information that cannot be
generated in the office. Fieldwork is undertaken at
both the watershed scale and the proposed development scale. The watershed scale explores the key
processes identi�ed in the previous �eld and office
work. A �eld review of the area under plan (proposed roads, harvesting, and other developments)
should focus on areas identi�ed during the office
work as potentially problematic—surface erosion
and landslide-prone terrain in particular. This will
verify whether the situations are problematic and
lead to the identi�cation of management options.
Sediment production and movement: stream
channels, gullies, landslides, drainage structures,
and surface erosion
An overview of the channel network is undertaken
to describe key features in the reaches identi�ed from aerial photographs. Descriptions should
include bed and bank materials, riparian condition,
and an assessment of sediment and debris loading. A
useful aid is the Channel Assessment Procedure Field
Guidebook (BCMOF and BCMOE 1996b). The riparian zone can be explored during this �eldwork for
evidence of hydrogeomorphic activity (e.g., dendroecology).
Gullies identi�ed during the office work should be
examined to identify sediment and debris loading,
evidence of debris �ow activity, and connectivity to
the stream system. While the Gully Assessment Procedure (BCMOF and BCMOE 2001) was developed for
coastal British Columbia gully systems, the approach
is worthy of consideration for assessing gullies in
other areas of the province.
An overview of the road system, including nonstatus roads, is needed to address linkages to all
three hydrogeomorphic processes. The age of the
road system could provide useful information, as
construction standards changed signi�cantly with
the introduction of the Forest Practices Code in 1995
(Horel 2006). It is necessary to identify past and
potential landslide risks from road systems, and the
potential linkage to stream channels.
Drainage structures should be examined across
stream channels or gullies where hydrogeomorphic
hazards have been identi�ed. The issue to assess is
whether the structures will pass or block potential
events, and determine the consequences.
If an element-at-risk in a watershed is domestic water production, �eldwork should explore for
evidence of surface erosion issues. Selective sampling
for surface erosion may be advisable (e.g., Carson et
al. 2007). A challenge to be aware of is that evidence
of past surface erosion can be masked by revegetation, road reconstruction, or grading and surface
maintenance. The Wathl Creek Case Study (Appendix 1) also explored other issues associated with
drinking water—contamination from wood leachate,
hazardous materials, and wildlife.
Flood generation: forest cover and roads
The status of regeneration should be examined, particularly in watersheds where peak �ows have been
identi�ed as a factor for hydrogeomorphic processes.
It is likely that the VRI information is not current,
and this can have signi�cant implications for the
calculation of equivalent clearcut area.
Forest health issues identi�ed in the office should
be examined in the �eld from two perspectives. It is
important to integrate the current hydrogeomorphic
implications of forest health issues as well as the
potential future issues. This is particularly relevant
in watersheds that have a high potential impact to
the forest cover (e.g., mountain pine beetle). In watersheds subject to signi�cant forest health issues, it
is likely that there will be hydrogeomorphic consequences, regardless of human activities.
Areas that have had intense forest �res in the past
5 years should be examined to determine the degree
of reduced soil in�ltration and explored for evidence
of erosion or overland �ow. The issue of hydrophobic
soils has only recently become apparent in British
Columbia, but the hydrogeomorphic consequences
have been signi�cant (Jordan et al. 2004).
A �eld examination of the road system identi�es
the degree to which roads are in�uencing hillslope
hydrology and peak �ow generation. Emphasis in the
�eld should be on identifying the degree of deactivation of non-active roads (i.e., determine if drainage
structures are still in place, and whether roads have
compacted surfaces that are in�uencing water movement) and the quality of road cross-drainage (i.e.,
identifying if ditchlines are collecting subsurface
�ows and delivering the water to streams rather than
back onto the forest �oor). The office determination
of percentage of the watershed in roads and compacted areas should be re�ned with this �eldwork.
Step 4.4 Synthesis of Watershed Processes
Compiling the �eld and office information will give
an overview of the current and projected hydrogeomorphic situation in a watershed that could affect
the watershed-fan system and potentially increase
the watershed hazard.
The next step is to develop or review plans in light
of the potential for proposed activities to in�uence
those watershed processes, and assess the potential
risks from those activities to elements on the fan.
This would include updating ECA calculations,
reviewing harvest locations for the potential to
synchronize or desynchronize snowmelt runoff,
identifying road and harvesting location relative to
19
landslide-prone terrain, projecting road density, and
exploring any other activities that could increase
identi�ed hazards in the watershed. If forest health
or other natural issues are in�uencing the forest
STEP 5
ANALYZE RISKS AND DEVELOP PLANS
In Step 5, all the previously gathered information is
integrated in an analysis of existing and potential
incremental risks in the watershed-fan system. This
understanding is then used to guide management
decisions concerning natural impacts to the forest
cover (�re, forest health) or proposed (harvesting,
road) activities in the watershed. Risk de�nitions
and procedures in this section are consistent with
Land Management Handbook 56, Landslide Risk
Case Studies in Forest Development Planning and
Operation (Wise et al. 2004), which can be referenced for further discussion of these topics.
Step 5.1 Understanding Risk Analysis
and Risk Assessment
Risk is a measure of the probability of a speci�c
event occurring, and the consequence, or adverse
effects, of that event on speci�c elements, such as
human health, property, or the environment. Simply
stated, risk is the product of hazard and consequence.
Risk analysis includes determining hazards and
consequences. Hazards are the potentially damaging
events and the hazard analysis includes information
such as the process, magnitude (including spatial
extent), and likelihood of occurrence. Consequence
refers to the likelihood of damage or losses to an
element-at-risk in the event of a speci�c hazard;
analysis of the consequence includes the spatial
and temporal exposure and the vulnerability of the
elements-at-risk. Consequence can also include the
value of losses or damage to elements. The concepts
of risk, hazard, and consequence are useful tools for
land managers to employ, but completing hydrogeomorphic risk analyses is the realm of quali�ed
professionals.
Risk analysis includes using some process to combine the hazards and consequences to estimate the
risk to elements. In the forest management context
20
cover in a watershed, it is important to develop a
forecast of background changes that can be anticipated regardless of any new planning initiatives.
this means understanding both the existing risks
and the incremental risks as a result of proposed or
imposed forest management activities in a watershed.
Risk assessment includes evaluating the results
of the risk analysis and determining whether that
risk is acceptable. Generally this entails comparing the risk analysis results with public, corporate,
and/or legislative standards of acceptable risk. Risk
assessment can include hydrogeomorphic hazard
specialists and other experts, but the �nal decision
regarding risk acceptability is generally the responsibility of land managers and regulatory decision
makers. Under the British Columbia Forest and
Range Practices Act (2006), the determination of
acceptable risk, and the decision to proceed with any
forest management activities on the basis of that risk
assessment, is the responsibility of forest licensees
and BC Timber Sales.
If the risk assessment determines that the estimated risks are unacceptable or intolerable, two options
are available: cancel further plans, or identify risk
control and mitigation measures to reduce the hazards and/or reduce the consequences. Identi�cation
of measures involves hydrogeomorphic hazard specialists, who can determine the effectiveness of risk
control and mitigation measures. With this information, land managers can revisit the risk assessment
in light of the control and mitigation measures and
their associated costs.
The �ve-step procedure presented in this handbook addresses risk analysis and risk control measures. For a further discussion of risk assessment and
acceptable risk, the reader is referred to Cave (1992),
Fell (1994), Wise et al. (2004), and APEGBC (2006).
Step 5.2 Consequence
In risk analysis it is common to analyze hazards
prior to analyzing consequences. However, within
the forestry context it is common for managers to
want to know what the potential consequences are
�rst, because the level of consequence frequently
identi�es the level of effort needed in all stages of
the risk analysis. This approach also ensures that the
consequence, which can be far away from the area
managed, is not overlooked or paid less attention
than it deserves. The development area is generally
investigated for hazards, but the consequence area, if
left until a later planning stage, can mean that work
in the development area is either wasted or misdirected. Thus, in keeping with the approach taken in
the forest sector, consequences are discussed prior to
hazards.
Consequence considers the effects of the speci�c
hazardous hydrogeomorphic event on the elementsat-risk on the fan (probability between 0 and 1, or
qualitatively low, moderate, high). This can include
an analysis of the likelihood the element will be at the
site at the time the event occurs, which is sometimes
called the temporal exposure. For �xed elements,
such as houses, highways, or forests, this probability
is always equal to 1 and does not need to be analyzed
further. For highway and railway traffic, or human
occupation of residences or institutions, there is generally a temporal exposure probability of < 1.
Vulnerability of a given element will vary depending on the severity of the hydrogeomorphic
event and the sensitivity of the element. Damage can
be minor, partial, or total, and this can be expressed
quantitatively (between 0 and 1) or qualitatively
(low, moderate, high). This determination is generally a joint effort involving hydrogeomorphic hazard
specialists and other appropriate specialists such as
�sheries biologists, foresters, and highway or railway
transportation experts, depending on the elements
potentially at risk.
Element exposure and vulnerability can be combined to further re�ne the consequence, as described
in more detail by Wise et al. (2004). The information
gathered in Steps 2 and 3 of this process should allow
such further re�nements, if necessary.
Risk analysis can include estimating the worth of
individual elements to produce a monetary consequence value. This can be expressed quantitatively
as a numerical dollar value, or qualitatively as a low,
moderate, or high dollar value. A qualitative approach may be more appropriate for some elements,
such as species at risk. Human lives should be assigned the highest possible value in any consequence
rating scheme.
For the risk analysis process described in this
handbook, the consequence value should be described qualitatively (low, moderate, high). Other
more quantitative procedures are possible, as described by Wise et al. (2004).
Step 5.3 Hazard
In this analysis, hazard is de�ned as the probability
or likelihood of occurrence of a hydrogeomorphic
event of a speci�c process and magnitude, and with
a speci�c power and spatial disturbance extent. It
can be expressed as a quantitative probability with
a value between 0 and 1, or qualitatively a value between very low and very high.
The investigations described in Steps 3 and 4
should determine the type, magnitude (size, power),
and runout or impact area of an expected event, and
the frequency (how often the event is likely to occur
over time). This describes the existing hydrogeomorphic hazard, which is the result of natural climatic,
geologic, vegetation, and hydrologic watershed
conditions, as well as any existing human land-use
changes in the watershed. Its likelihood of occurrence can be expressed as the return period T, or the
frequency (annual probability) Pa, which is simply
1/T.
Section 5 describes the watershed processes contributing to the various hydrogeomorphic hazards.
An understanding of the existing hazards is the �rst
step in analyzing the potential hazards that could
result from proposed or naturally imposed activities
in the watershed, as also described in Section 5 and
summarized in Table 2.
Following the procedures described there, one
should arrive at a description of the developmentrelated, or incremental hydrogeomorphic hazard,
which should include event type, magnitude, and
frequency, as a result of a particular forest activity.
It is important to note, however, that the probability
of occurrence of forest development-related hazards
will be different than the existing natural hazard,
because forest development effects usually have a
relatively limited time frame compared to natural
hazards. For example, the hydrological effects of
harvesting, �res, or insect-related mortality decrease
over several decades as the stand re-grows (BCMOF
and BCMOE 1999a); roads can have a speci�ed use
period after which they are deactivated, and potential hazards decrease relative to the level and efficacy
of the deactivation; and hazards associated with
21
hydrophobic soils in severe burns generally decrease
signi�cantly after several years. A recent study of
landslides on northern Vancouver Island found that
the greatest frequency of landslides from harvested
cutblocks occurred in the �rst 5 years following harvesting (Horel 2006).
The long-term probability of a hydrogeomorphic
event (Px) resulting from a particular forest development can be calculated if the frequency, or annual
probability of occurrence (Pa) and the duration of
effect of the forest development (x) are known, from:
Px = 1-(1-(Pa))x
Equation 1
For example, if previous experience suggests that
a road in a particular landscape position is expected
to result in one landslide every 50 years, and that
landslide could initiate a debris �ow in the channel
it affects, the forest road–related debris �ow hazard
has a return period (T) of 50 years and an annual
frequency (Pa) of 1/50= 0.02. If the road will be deactivated to remove the landslide hazard after 20 years
(x), the long-term probability of the forest road–
related debris �ow (Px ) is:
P20 = 1-(1-(0.02))20 = 0.33 Equation 2
Therefore, there is a probability of 0.33 that a
debris �ow will be initiated during the 20-year life of
the road.
Table 4 shows the long-term probabilities
calculated using Equation 1, for a range of annual
probabilities of events and a range of life spans of the
watershed activities contributing to those events.
Table 5 summarizes the ranges of quantitative
probabilities from Table 4, described by each qualitative hazard term, and de�nes those qualitative terms.
Continuing the previous example, there is a probability of 0.33, or a high likelihood, that a particular
20-year road would initiate a debris �ow of a particular magnitude and runout extent within that
20-year period. This is the incremental (watershed
activity–related) hazard at that site.
The �nal step in the hazard analysis is to assign a
qualitative value to the event magnitude, based on its
power, which is a measure of its likelihood of causing
damage, and its extent, which is a measure of how
large an area and what risk elements it could affect.
Table 6 is an example of a hazard analysis matrix.
Assuming the debris �ow in our example would
reach risk elements on the fan, one could conclude
that, since debris �ows are generally high-power
destructive events and their extent includes risk
elements, the event magnitude would be considered
high. Therefore, with a high frequency and a high
magnitude, the debris �ow hazard in this case would
be very high.
Table 4 Example of long-term probabilities (adapted from Wise et al. 2004, Table A4.3)
Pa =
annual
prob
1
2
5
1/1
1/2
1/5
1/10
1/20
1/50
1/100
1/200
1/250
1/500
1/1000
1/2000
1/2500
1/5000
1
0.50
0.20
0.10
0.05
0.02
0.01
0.01
0
0
0
0
0
0
1
0.75
0.36
0.19
0.10
0.04
0.02
0.01
0.01
0
0
0
0
0
1
0.97
0.67
0.41
0.23
0.10
0.05
0.02
0.02
0.01
0
0
0
0
22
Px, long-term probability of occurrence: Px = 1-(1-Pa)x
x = life of watershed activity (years)
10
20
25
50
100
200 250
500
1
1
0.89
0.65
0.40
0.18
0.10
0.05
0.04
0.02
0.01
0
0
0
VL
1
1
0.99
0.88
0.64
0.33
0.18
0.10
0.08
0.04
0.02
0.01
0.01
0
1
1
1
0.88
0.72
0.40
0.22
0.12
0.10
0.05
0.02
0.01
0.01
0
1
1
1
0.93
0.92
0.64
0.39
0.22
0.18
0.10
0.05
0.02
0.02
0.01
1
1
1
0.99
0.99
0.87
0.63
0.39
0.33
0.18
0.10
0.05
0.04
0.02
L
1
1
1
1
1
0.98
0.87
0.63
0.55
0.33
0.18
0.10
0.08
0.04
1
1
1
1
1
0.99
0.92
0.71
0.63
0.39
0.22
0.12
0.10
0.05
1
1
1
1
1
1
0.99
0.92
0.87
0.63
0.39
0.22
0.18
0.10
M
1000
1
1
1
1
1
1
1
0.99
0.98
0.86
0.63
0.39
0.33
0.18
2000 2500
1
1
1
1
1
1
1
1
1
0.98
0.86
0.63
0.55
0.33
1
1
1
1
1
1
1
VH
1
1
0.99
0.92
0.71
0.63
0.39
H
Table 5 Qualitative frequency definitions (adapted from BCMOF and BCMOE 2002, Table A10.2)
Quantitative Probability
Qualitative Hazard
Qualitative Description
Px>0.64
Very high
Event is imminent after the
watershed activity (i.e., any phase
of forestry operations)
0.18<Px ≤0.64
High
Event is probable during the
lifetime of the watershed activity
0.04<Px ≤0.18
Moderate
Event is possible, but not likely
during the lifetime of the watershed
activity
0.02<Px ≤0.04
Low
Remote likelihood of event during
the lifetime of the watershed activity
Px≤0.02
Very low
Very remote likelihood of event
during the lifetime of the watershed
activity
Table 6 Example of qualitative hazard analysis matrix
Table 7 Qualitative risk analysis matrix (adapted from Wise
(adapted from Wise et al. 2004)
Frequency
High
Very high
High
Moderate
Low
Very low
Very high
Very high
High
Moderate
Low
Magnitude
Moderate
Very high
High
Moderate
Low
Very low
et al. 2004)
Low
Hazard
High
High
Moderate
Low
Very low
Very low
Very high
High
Moderate
Low
Very low
Very high
Very high
High
Moderate
Low
Step 5.4 Risk Analysis
The �nal step in the risk analysis procedure is to
combine the hazard and consequence to arrive at an
estimate of the hydrogeomorphic risks from speci�c
watershed activities to speci�c elements on the fan.
Table 7 is an example of a qualitative risk matrix;
other matrices are possible. In this way, the qualitative consequence and risk values determined in the
previous sections are combined to yield a qualitative
risk value.
Depending on whether or not consequence exposure, vulnerability, or worth have been taken into
account, the result could be an analysis of partial
risk, speci�c risk, or speci�c value of risk (Wise et al.
2004). In any case, this last step completes the input
from technical specialists—the risk analysis.
Consequence
Moderate
Very high
High
Moderate
Low
Very low
Low
High
Moderate
Low
Very low
Very low
Step 5.5 Assessing Risk and Making
Management Decisions
With the results of the risk analysis and technical
support where necessary, it is the role of land managers to complete the risk assessment and determine if
the estimated risks are acceptable or tolerable. Three
options are usually available:
1. the risks are considered to be unacceptable and
the project is cancelled,
2. the risks are considered to be acceptable with the
planned forest development or if forest management strategies are applied to reduce the potential
incremental hazards, or
3. the risks are considered to be acceptable if the
consequences can be reduced by designing, constructing, and maintaining protective structures
on the fan such as dikes or berms (VanDine 1996).
23
In some cases where the risks are considered to be
acceptable, options 2 and 3 are both implemented.
The hydrogeomorphic hazards in a speci�c watershed are identi�ed in Steps 3 and 4, and summarized
in Table 2. This knowledge leads directly to management strategies to reduce the potential incremental
hazards. In debris �ow and debris �ood watersheds
it is essential to avoid initiating landslides, particularly those that could directly affect the channel.
A key step in this direction is to complete terrain
stability mapping and develop plans that recognize and adequately manage unstable terrain. Field
investigation may indicate that avoidance of unstable
terrain is a preferred management option, but it may
also determine that reasonable options are available.
In coastal British Columbia, approximately one-half
of all landslides occur in clearcuts and are not roadrelated (Jakob 2000); proposed harvesting must be
assessed and managed accordingly.
It is widely acknowledged that forest roads are
a major factor in landslide initiation, due both to
poorly constructed roads and to their in�uence on
hillslope hydrology. In a coastal logging–landslide
study, no difference in soil disturbance was found
between helicopter and conventional yarding, but
there was a higher incidence of open-slope landslides
on the conventionally yarded areas—due primarily to the subtle but signi�cant in�uence of roads
on hillslope hydrology (Roberts et al. 2004). In the
interior of British Columbia, most signi�cant landslides associated with forestry operations are related
to drainage diversion by forest roads and trails
located on gentle to moderately sloping terrain, some
distance upslope from steeper, more landslide-prone
terrain (gentle-over-steep) (Grainger 2002). Speci�c
terrain stability �eld assessment techniques and
strategies to manage these potential hazards have
been described by Grainger (2002).
Mitigation opportunities for landslides may exist
in some watersheds. Addressing current or potential
erosion from non-status roads may be an appropriate strategy. Alternative yarding systems should be
explored, although unconventional systems such as
helicopters may not result in a reduction in landslides, particularly in gullied terrain (Roberts et al.
2004).
Maintaining an appropriate ECA is important
in �ood and debris �ood watersheds, and also in
watersheds where there is a potential for in-channel
debris �ow initiation. There is no speci�c ECA that
24
guarantees that no incremental hazard will occur in
all situations. However, a series of factors should be
considered when establishing a conservative ECA, including watershed-speci�c processes and characteristics, past impacts to elements on the fan, the nature
of the elements-at-risk, and location of past and proposed forest harvesting or natural disturbance to the
forest cover. Watershed-speci�c mitigative opportunities may exist through desynchronizing snowmelt
runoff; for example, by locating forest harvesting on
mid- to low-elevation or southern-aspect slopes
There is ongoing research on cumulative forest
harvesting and ECA effects on stream�ows. Differences have been highlighted between watersheds
dominated by rainfall, rain-on-snow, and spring
snowmelt. Huggard (2006) describes the different hydrological effects over time of dead standing
timber and salvage harvesting at the stand level,
and the Forest Practices Board (2007) explored the
issue at the watershed level. The effect of speci�c
watershed characteristics on the harvesting-related
stream�ow impacts were examined by Whitaker et
al. (2002) and Schnorbus and Alila (2004). Land
managers should involve experts with an appropriate
knowledge of watershed processes, linkages between
cumulative impacts and stream�ows, and local
conditions.
Road density can be a peak-�ow hazard factor
where inadequate cross drains allow long sections
of road to direct intercepted �ows into stream
channels. Strategies to limit road density include
total-chance watershed-level planning to avoid
constructing unnecessary roads, to deactivate roads,
and to provide frequent cross drainage to limit the
potential for ditch lines becoming a part of the
stream channel network (Wemple et al. 1996). A
mitigative option would be to assess and deactivate
non-status roads in the watershed.
Non-licensee stakeholders often have great reservations regarding forest harvesting in watersheds.
Often they will refer to the protective services of
those forests to various ecological and watershed
functions. It is poorly understood that forests must
be managed to maintain high levels of protective
services because forests are not static entities (Sakals
et al. 2006). Forest licensees may gain the support of
watershed stakeholders if they commit to a longterm management program to enhance the protective services of forests.
Both forest harvesting and lack of forest management (e.g., not undertaking post-wild�re rehabilitation as described by Curran et al. 2006) within a
watershed can have a high likelihood of affecting the
downstream fan. With an awareness of the total risk
situation and appropriate accommodations for reducing those risks, forest harvesting and forest management will be able to continue in many watersheds
with minimal, and potentially positive, effects.
Step 5.6 Document, Monitor, Evaluate,
and Report
A key element in watershed management is to evaluate the effectiveness of plans and practices. The objective in applying the �ve-step approach is to limit
the hydrogeomorphic in�uence on a fan as a result
of forest management in the associated watershed.
Documenting information collected and decisions
made will allow for retrospective analysis as time
unfolds. Documentation is critical because it could
be a long time before climatic events occur that
trigger hydrogeomorphic responses from the watershed. When such events occur it is essential that
monitoring be undertaken to determine whether
responses on the fan were “beyond the natural range
of variability.” Information collected on the fan in
Step 3 will prove valuable for this assessment. The
collection of additional watershed information may
be required to update Step 4. Results of the evaluation should feed into an update of Step 5, and should
be applied where appropriate to future fan-watershed
risk analyses. Reporting results to interested parties
(e.g., the public or government agencies) is critical
to maintain support for the forest development, and
may be necessary in situations where hazards are
identi�ed.
CASE STUDIES
Four case studies from across British Columbia are
presented in the Appendices to illustrate the application of the �ve-step approach for analyzing risk in
fan-watershed systems in different geographic settings and with different management issues. Wathl
Creek watershed is a community watershed with the
village of Kitamaat located on the fan. The watershed
has had no forest development to date, and the case
study demonstrates the application of the �ve-step
approach in providing guidance to forest development planning. Since there was not a development
plan to analyze, some aspects of the risk analysis could not be completed. Eagle Summit Creek
watershed has had past forest development, and the
Trans-Canada Highway crosses the lower fan. The
watershed has produced debris �ows and debris
�oods in the past, and the highway culverts are not
able to pass all the material—impacts have occurred
to the highway. The �ve-step approach was applied to
provide forest development direction that will limit
incremental hydrogeomorphic hazards. In the Shale
Creek case study, the �ve-step approach was used
in analyzing impacts from historical logging on the
fan and in the watershed. This information was then
used to provide guidance for harvesting secondgrowth stands on the fan. The Hummingbird Creek
case study uses the �rst four steps of the �ve-step
approach to describe the circumstances leading to a
debris �ow and the resulting impacts to infrastructure on the fan. The �ve-step method is then used
to provide guidance—based on what could have
been determined prior to the debris �ow, along with
knowledge that we now have regarding debris �ows
in that terrain setting.
25
SUMMARY
The �ve-step approach for analyzing risk in fanwatershed systems is designed to move forest management from a site-level focus to a watershed-level
perspective. This broader perspective allows for the
recognition of the linkages between fans and their
watersheds, the hydrogeomorphic processes in�uencing the fans, and an assessment of incremental
risks associated with watershed development plans
or natural factors in�uencing the forest cover.
Changes to the forest cover in a watershed, either
through forest harvesting or natural factors, can
have signi�cant effects on downstream fans. For this
reason, when management decisions are to be made,
we encourage the use of the fan-watershed system as
a conceptual framework to be applied across a range
of spatial scales.
The �ve steps for analyzing risk in fan-watershed
systems are:
Step 1: Identify Fans and Delineate Watersheds
• Characterize the physical inter-connections that
exist between areas of potential activities (watersheds) and areas of potential impacts (fans).
26
Step 2: Identify Elements-at-risk on Fans
• Recognize and inventory values on the fan that
may be affected by the hydrogeomorphic processes in the watershed (e.g., either natural processes or those related to proposed management
actions).
Step 3: Investigate Fan Processes
• Identify the nature of hazardous hydrogeomorphic processes (e.g., type, frequency, and disturbance extent).
Step 4: Investigate Watershed Processes
• Identify watershed features controlling hydrogeomorphic processes (e.g., includes watershed hydrology, geomorphology and the role of
vegetation cover).
• Identify the potential for incremental hazards
associated with management activities.
Step 5: Analyze Risks and Develop Plans
• Develop planning options for the watershed and
assess the associated risks.
• Document the process and establish a plan to
monitor, evaluate, and report.
APPENDIX 1 Wathl Creek case study
Wathl Creek is located near the head of Kitimat Arm
of Douglas Channel, and is a designated community
watershed (Figure A1.1). The watershed-fan system
was the subject of a hydrogeomorphic assessment
(Grainger and Wilford 2008), undertaken prior to
any signi�cant forest development planning for the
watershed (one block had been harvested via a low
watershed divide). This case study demonstrates the
application of the �ve-step approach to guide forest
development planning in an undeveloped watershed.
Step 1 Identify fans and delineate
watersheds
The boundaries of Wathl Creek watershed and fan
were identi�ed manually using topographic maps
and aerial photographs, and mapped at a scale of
1:20 000. A series of small fans is present throughout the watershed—primarily in the mid and upper
reaches. Individual tributary fan watersheds were
not delineated, but the fans were identi�ed on a landform map (Denny Maynard and Associates 2001).
Figure A1.1 The Wathl Creek watershed and fan.
27
Step 2 Identify elements-at-risk on fans
Natural features – Salmon habitat is present on the
fan; a waterfall in the lower reach of the watershed is
an impassable barrier.
Anthropogenic features – Kitamaat Village is located
on the Wathl Creek Fan (Figure A1.2). Structures
include a school, administrative buildings, and many
houses. Two groundwater wells on the fan provide
domestic water to the village. Low �ows in the creek
and groundwater have been an issue during the
summer, and in July 2003 one well temporarily ran
dry. A bridge crosses Wathl Creek, providing access
to the west side of the fan where most of the village
is located. A long dike constructed of primarily local
channel material is present along the full channel
length on the fan, providing protection for most of
the village.
Human safety – Approximately 800 people live in
Kitamaat Village on the Wathl Creek Fan.
Step 3 Investigate fan processes
Hydrogeomorphic process – Much of the fan surface
has been modi�ed by human activity (forest removal, construction of roads and dike). A dike has
been constructed along the western stream channel
bank on the fan in response to signi�cant �ooding
between 1964 and the late 1970s. The dike has cut off
several old channels that �owed across the western
portion of the fan. The eastern portion of the fan is
elevated, well above any river �ood level. Given the
evidence on the fan, we concluded that the dominant
hydrogeomorphic process in�uencing the fan was
�ood. The highest instantaneous peak �ows are generated by rain-on-snow or rain events in September,
October, and November. The largest monthly stream
discharges occur in June and July during the snowmelt freshet.
Event frequency – The largest storms for this area
probably occurred in 1891 and 1917 and it is likely
that the area has not experienced storms of this magnitude since (Schwab 2000). Local residents reported
large �oods and bridge washouts on the fan in 1974,
1978, and 1988, and these events were con�rmed by
Septer and Schwab (1995). The impacts on the Wathl
Creek fan could have been related to inadequate construction of the dike and bridge. Some of the largest
�oods in regional stream discharge records occurred
in 1991 and 1992. It may be that these �ows were not
as great in Wathl Creek, or if they were as large, they
were less memorable because the new bridge was
Figure A1.2 A view of the Wathl Creek fan and Kitamaat Village looking east into the
watershed.
28
designed to handle large storm �ow and no bridge
washout occurred.
Event magnitude – Peak �ows are currently being
con�ned by the dike, although it is possible for the
dike to be over-topped—surveys show that the
200-year �ood level is higher than the existing
berm near the fan apex.
Step 4 Investigate watershed processes
Office investigations
Morphometrics – The watershed has an area of 127
km2 and relief of 1.6 km, and is graded (i.e., mountainous). The relative relief number is 0.14, suggesting that Wathl Creek is a �ood watershed (Wilford
et al. 2004b).
Sediment production and movement – Landform,
terrain stability, and sediment transport maps were
available for the watershed. These maps and aerial
photographs were used to identify nine stream
reaches and divide the watershed into four sections
based on different levels of sediment production
(e.g., frequency of gullies, presence of stability class
IV and V, and general slope angles).
Flood generation – Aside from one small block, the
watershed has natural forest cover and no roads. The
potentially operable forest land is below 900 m and
is approximately 30% of the watershed.
Field investigations
The watershed was inspected by helicopter with several stops to explore �oodplains, channels, and wood
quality in the forest.
Sediment production and movement – Connectivity
between unstable terrain and stream channels was
inspected in the �eld. Most gullies had a direct connection to the stream channel. Zones of the watershed with unstable slopes have a very limited valley
�at. Streambed, streambank, and sediment storage
characteristics were described for each of the nine
reaches. Eight reaches were identi�ed as non-alluvial
transport zones (primarily bedrock), and only one
reach had an alluvial channel.
Flood generation – The logged area had very low
relief and represented only a small portion of the
overall watershed. A signi�cant portion of the watershed is steep, with shallow soils, and limited forest
cover. No forest health issues were identi�ed.
Synthesis of watershed processes – Office work and
�eldwork identi�ed signi�cant areas with slope
stability concerns, and this terrain is generally directly connected to the stream channel. The terrain
stability maps accurately describe unstable slopes in
the watershed. The stream channel in the watershed
has very limited storage capability and efficiently
conducts materials through to the fan.
Step 5 Analyze risks and develop plans
Consequence – elements of concern
Water quality – The groundwater wells are located
less than 20 m from Wathl Creek and are 18–20 m
deep. The nature of the subsurface materials is not
known. A conservative position is to assume that
the aquifer has coarse sediment and that there is a
direct connection to the stream channel. Suspended
sediment would most likely be trapped; however, any
dissolved chemical or biological contaminants may
or may not be removed. Currently, the stream rarely
runs turbid, and since the wells have been installed
there has not been an issue with suspended sediment
in the water supply.
The clean water in Wathl Creek has provided a
good environment for �sh and other organisms.
Fish species present include chum, coho, pink, and
sockeye salmon.
Water supply – The oral record suggests that there
have been periods where groundwater levels are so
low that supply is compromised.
Residences, infrastructure, and human safety –
Most of Kitamaat Village is located on the southwest
side of Wathl Creek, on the lower-elevation portion
of the fan. The village is protected by the dike;
however, the dike may not be adequate to protect the
village from a 200-year �ood. If the dike is breached,
�ow in the channel will be reduced, resulting in the
deposition of bedload, channel in-�lling, and
subsequently more �oodwater issuing forth across
the fan surface.
Hazards
Potential forest development – It was suggested that
it would be prudent to use the Community Watershed
Guidebook (BCMOF and BCMOE 1996c) as a minimum set of standards to be followed for any forest
operations within the watershed. It was noted that
the hydrologic and geomorphic impact of any forest
development, regardless of the level of planning, is
dependent on how operations are carried out in the
watershed. It was suggested that the licensee, Haisla
29
Forestry Limited, which is owned by the residents
of Kitamaat Village, should keep the community
informed of any forestry activities in the watershed.
Water quality – Several water quality issues were
raised by band elders and councillors:
Biological contamination by wild animals We
are not aware of any cases where forest development in a coastal setting has resulted in contamination of previous pristine waters supplies as a
result of wildlife feces contamination. Based on
this, it is our opinion that there is a very low likelihood that forest development would result in an
introduction of contamination of water supplies by
wild animals. However, there are some uncertainties and we recommend monitoring wild
animal populations in the watershed, particularly
in the north and south Plateau areas.
Contamination by wood leachate We are not
aware of any cases where forest harvesting has
caused noticeable contamination in downstream
domestic water supplies by wood leachate. There
are cases of contamination from wood sorts and
log dumps where there is a large amount of wood
concentrated in a small area. These activities
should not be undertaken in the watershed. We
conclude that there is a very low likelihood that
forest development would cause chemical contamination by wood leachate.
Contamination by hazardous material spills
The record of this type of contamination in forestry operations suggest that the likelihood of a
signi�cant spill is low. Steps can be taken to considerably reduce this hazard. Following appropriate procedures, there is a very low likelihood that
forestry operations will result in contamination of
Wathl Creek by a hazardous materials spill.
Sediment loading Given the granitic bedrock
in the watershed, the sur�cial materials have a
relatively low amount of �ne-grained soils. The
landform maps identify the location of several
�ned-grained glacio-lacustrine sediment deposits, and these could become a �ne-grained sediment source if not managed properly. Overall,
there is a low likelihood of �ne-grained sediment
impacts to surface water quality and �sh habitat.
There is a very low likelihood of �ne-grained sediment impacts to groundwater-sourced domestic
water supplies.
30
The additional or incremental coarse bedload
sediment hazard due to forestry activities is a
function of two things: the likelihood of erosion
or landslides, and the likelihood of a signi�cant
amount of the mobilized sediment reaching
Wathl Creek. A signi�cant amount of sediment
means that it will make a noticeable difference in
sediment loads, compared to the relatively high
natural background sediment levels already in
Wathl Creek. Given the terrain stability mapping, we are able to conclude that: there is a
very high incremental sediment hazard from both
forest harvesting and road construction in the
Upper Mountain Valley area and in the gullies of
the larger tributary streams in the Mid Mountain
Valley and South Plateau areas. Given the limited
amount of unstable terrain and lack of connectivity to the stream channel in the lower channel reaches, there is a low incremental sediment
hazard from harvesting and road construction
on the open slopes of the Mid Mountain Valley, in
the North Plateau area, and in the South Plateau
area.
Water quantity
Peak �ows – Given that the operable forest land
base occupies only 30% of the watershed, and that
harvesting within this area will be constrained due
to terrain, riparian protection, old-growth management, and other harvesting restrictions, the actual
area harvested will be less than 30% of the watershed. We conclude that the incremental peak �ow
hazard is very low.
Low �ows – Research has shown that, except in cases
where there has been signi�cant soil compaction
or there is signi�cant fog drip, harvesting generally
increases or has no measurable effect on low �ows.
Given the limited operational area in the Wathl
Creek watershed there is a very low likelihood of
development-related impacts to low �ows.
Except for the coarse sediment loading hazard in
some portions of the watershed, all other identi�ed
incremental forestry-related hazards were judged to
be low to very low.
Risk analysis and recommendations
Risk is the product of hazard (or likelihood an event
occurring and affecting a value) and the consequence (or the exposure, vulnerability, and worth
of the element-at-risk; for example, the high-quality
domestic water supply). While we have suggested
consequence values in order to carry out the following risk analyses, it is worth repeating that the �nal
determination of the consequence values and the
subsequent risk analysis are the responsibility of the
licensee, regulatory authorities, and stakeholders, as
is the determination of acceptable risk and the decision to proceed with any development based on that
analysis. To manage all potential hazards and associated risks, two general recommendations are made:
– The Community Watershed Guidebook (BCMOF
and BCMOE 1996c) should be used as a minimum
set of standards.
– Water users should be kept informed of any
forestry activities planned or happening in the
watershed, and of how potential hazards and
risks are being managed.
For the risk analysis we use Table A1.1, and the
consequences, hazards, and risks for a series of the
elements-at-risk are presented in Table A1.2.
The coarse bedload sediment risk from proposed
development in various portions of the watershed to
residences, other buildings, and human safety on the
fan is presented in Table A1.3.
Table A1.1 A matrix combining hazard and consequence to
Table A1.2 Consequences, hazards, and risks of all the
identified hazards and elements-at-risk, except
the coarse sediment loading hazard and the consequence of potential impacts to residences and
human safety, which are dealt with separately
Issue
Incremental
hazard from
forestry –
Consequence likelihood
Risk
Domestic
water – suspended
sediment
Low to
Moderate
Very low
Very low
Domestic
water – industrial
chemicals
High
Very low
Low
Domestic
water – biological
High
Very low
Low
Domestic
water – wood
leachates
High
Very low
Low
Fish – suspended
sediment
Moderate
Low
Low
Fish – low �ows
High
Very low
Low
Peak �ows
High
Very low
Low
Low �ows
High
Very low
Low
determine risk
Hydrologic
hazard
High
Very high
High
Moderate
Low
Very low
Very high
Very high
High
Moderate
Low
Consequence
Moderate
Very high
High
Moderate
Low
Very low
Low
High
Moderate
Low
Very low
Very low
31
Table A1.3 Coarse bedload sediment risk
Consequence – impacts to
buildings and human safety
Forest development
sediment hazard
Coarse sediment
risk
Upper Mountain Valley
High
Very high
Very high
Large tributary gullies –
Mid Mountain Valley,
South Plateau
High
Very high
Very high
Mid Mountain Valley –
except large tributaries
High
Low
Moderate
South Plateau – except
large tributaries
High
Very low
Low
North Plateau – all
High
Very low
Low
Area
Development planning and mitigative strategies
Recommendations include:
•
•
•
•
32
No harvesting or road building in the unstable
Upper Mountain Valley portion of the watershed.
No harvesting in the large tributary gullies of the
mid-watershed.
Road building with caution through the large
tributary gullies. Carry out detailed on-site
assessments to determine where there may be
acceptable gully crossing sites, use experienced
road layout personnel, thorough geotechnical assessment and engineering, and cautious
construction and maintenance practices. All this
increases costs and there will be locations where
gully crossings with an acceptable level of risk
will not be feasible.
On Mid Mountain Valley open slopes, with
good forestry practices through all planning and
implementation phases (including total chance
planning based on thorough geotechnical and
hydrological assessments), much of the area can
be accessed and harvested with an acceptable
level of risk to elements-at-risk on the fan.
A series of forest management planning scenarios
may be developed using these recommendations
along with other considerations (e.g., timber quality and equipment availability). These scenarios,
along with mitigative strategies, should be evaluated
against the risk analysis. In the case of Wathl Creek,
it is likely that the residents of Kitamaat Village will
have input to any �nal forest management plans.
We noted in the report that there is a potential
risk to residences, infrastructure, and human safety
in Kitamaat Village because of apparent problems
with the dike along the southwest side of Wathl
Creek. These problems exist whether any forest
development takes place in the watershed or not. It
was recommended that the Band Council undertake
further investigation of the level of safety the dike
is providing the village, with respect to the current 200-year design �ood level, and with respect to
expected future storm magnitudes and sea-level rise
resulting from ongoing global climate change.
APPENDIX 2 Eagle Summit Creek Case Study
Eagle Summit Creek is located in the Monashee
Mountains near the headwaters of the South Thompson River, upstream of Shuswap Lake and just east
of the Thompson River–Columbia River drainage
divide, about 30 km west of Revelstoke, B.C. The
watershed-fan system was the subject of a hydrogeomorphic assessment (Grainger 2007). The report is
summarized using the �ve-step approach.
This case study demonstrates that there can be
different processes in a watershed producing different hydrogeomorphic events with different occurrence intervals and magnitudes. Knowledge of the
different watershed processes and the different ways
those processes can be affected by various forest
management activities allows land managers to focus
their decisions where they will be most effective in
managing risks.
north side of the Eagle River, adjacent to the toe of
the Eagle Summit Creek fan.
Human safety – Hydrogeomorphic impacts to the
Trans-Canada Highway could compromise human
safety and life. A small debris slide from the lower,
steep, Eagle River valley walls caused a fatality in the
spring of 2002 when a machine operator was swept
into Clanwilliam Lake and drowned, while clearing
debris from a previous natural landslide. In December 1967 a debris �ow in Camp Creek, on the north
side of the Eagle River Valley, and approximately 15
km west of Eagle Summit Creek destroyed the Camp
Creek Bridge over the TCH, resulting in two fatalities
to passengers in a car that entered the debris �ow
path due to loss of the bridge.
Step 1 Identify fans and delineate
watersheds
Debris �ow – There are well developed levees on
the steep concave fan adjacent to the channel, with
boulders up to 2.0 m in diameter. Fan gradients are
50% near the apex, 30% through the mid-fan, and
decrease to 10% near the toe, downslope of the TCH.
The fan surface is irregular, with abandoned channels and large boulders and blocks underlying the
vegetated surface.
There is a mature western redcedar, Douglas-�r,
and western hemlock stand with trees from 0.45
to 0.55 m diameter, estimated to be 90 years old,
growing on the fan surface, including the boulder
levee. Except for a couple of trees growing within
a few metres of the channel, no scarring was noted
on mature trees, even those immediately adjacent to
the channel at the fan apex. There are also burned
stumps throughout the fan, thought to be from the
stand-establishing �re for the current stand. If the
stand had been established by a hydrogeomorphic
event any evidence of the previous stand would have
been obliterated. This indicates that there has been
almost no hydrogeomorphic disturbance outside
the channel in at least 250 years, and possibly for
much longer. It was concluded that there have been
large debris �ows in Eagle Summit Creek that have
extended down the fan to the present position of
the TCH, but that none have occurred in at least 250
years.
The boundaries of Eagle Summit Creek watershed
and fan were identi�ed manually using topographic
maps and aerial photographs and �eld reviews,
and mapped at a scale of 1:20 000. The fan is relatively small, about 240 m wide and 170 m long, but
it blocks the narrow Eagle River valley-bottom,
forming Clanwilliam Lake just upstream of the fan
(Figure A 2.1). The watershed has some steep alpine
mountains at its upper extent and a broad, gently to
moderately sloping plateau forming the mid watershed. The stream is in a steep, 1600 m long gully
dissecting the plateau (Figure A 2.2).
Step 2 Identify elements-at-risk on fans
Natural features – Timber on the fan. There are no
recognized �sh values in Eagle Summit Creek or
Clanwilliam Lake.
Anthropogenic features – An approximately 200-m
section of the Trans-Canada Highway (TCH) crosses
the lower fan. The highway crossing of Eagle Summit Creek consists of two 1.0 m high oval culverts
(Figure A 2.3). Hydrogeomorphic impacts could
disrupt automobile and truck traffic on the main
highway connection between Alberta and southern
British Columbia. The CPR mainline runs along the
Step 3 Investigate fan processes
33
Figure A2.1 Eagle Summit Creek watershed (2001 airphoto). Proposed cutblocks are shaded.
34
Figure A2.2 Gully and fan reaches of Eagle Summit Creek, looking south.
35
Figure A2.3 A view of the drainage structure on the Trans-Canada Highway at Eagle Summit
Creek.
Debris �ood – In the Eagle Summit Creek channel on the fan, recently moved sediment is mainly
cobbles and small boulders < 0.75 m diameter. Some
larger blocks have been introduced to the channel
from high failing cliffs immediately above it.
A small group (cohort) of young western redcedar trees occupies a small gravel sediment wedge
adjacent to the channel. Dendroecology analysis
(Wilford et al. 2005a) of the trees indicated that they
were established in the early 1950s and show impact
scars dating from 1967 and 1982. The fact that these
young trees were not destroyed indicates that it was
not a high-powered debris �ow that affected them.
Three hydrogeomorphic events are reported to have
affected the TCH in the last 35 years: in 1967, in the
early 1980s, and one at an unspeci�ed date (Thurber
1987), which corresponds well with the dendroecology �ndings. The early 1950s cohort establishment
suggests that at least three and possibly four events
have occurred in the channel in the last 55 years.
These events appear to be largely contained in the
channel and extend to the TCH. It is noted that the
capacity of the existing TCH crossing is very likely
too small to pass a debris �ood. At least some of
36
the recent debris �oods have blocked the culverts
and deposited sediment on the highway, disrupting traffic. No injury or other damage was reported
(Thurber 1987)
As discussed in Step 3.1, debris �oods have peak
discharges that are, at most, two to three times that
of major �oods (Hungr 2005), and have maximum
particle size of the same order as the peak depth of
major �oods—up to several tens of centimetres
(< 1.0 m) in the case of typical small mountain
streams (Hungr et al. 2001), which describes the
�ow magnitude and sediment size observed in Eagle
Summit Creek on the fan. It was concluded that
there is an existing debris �ood hazard that extends
to at least the TCH and has a return period (T) of
approximately T = 14 years (i.e., a 55-year period of
record with four debris �ood events).
Step 4 Investigate watershed processes
Office investigations
Morphometrics – The watershed has an area of
6.4 km2 and relief of 1.68 km. The relative relief number is 0.66, suggesting that Eagle Summit Creek is a
debris �ow watershed (Jackson et al. 1987; Wilford et
al. 2004b).
However, relative relief number is designed for
“graded” or concave mountainous watersheds, and
there is a wide, moderately to gently sloping plateau
in the middle of the watershed. We have observed
that morphometrics designed for graded watersheds
often do not give the correct results for watersheds
with signi�cant upland plateaus.
Sediment production and movement – Terrain
stability mapping and air photos show that Class IV
(potentially unstable) and Class V (unstable) terrain is ubiquitous along the steep Eagle River valley
walls, with debris slides, rock falls, and debris �ows.
On the upland plateau, terrain was largely Class I to
Class III (stable). Little sediment was being delivered to the channel on the plateau, or transported
through it to the steep reach above the fan.
Flood generation – Stream�ow records show that
peak �ows occur during the spring freshet snowmelt in late May and most commonly in early June.
Detailed analysis of damaging �oods in Eagle
Summit Creek and debris �ow in a smaller tributary
stream in the region in 1967 and 1968 show that rain
on melting snow caused those large runoff events
(Thurber 1987).
There had been extensive harvesting on the plateau area of the watershed throughout the 1980s. The
existing equivalent clearcut area (ECA) at the time of
the assessment was approximately 21%.
Field investigations
Areas proposed for harvest and road building were
investigated on the ground, as was Eagle Summit
Creek channel, from the plateau down to its con�uence with the Eagle River. The channel and surrounding areas were also observed from the air in
a helicopter �y-over.
Sediment production and movement – Most sediment production and delivery to the channel occurred in the steeper stream reaches along the south
Eagle River valley wall. This section was subdivided
into two reaches based on sediment regime characteristics (Figure A 2.2).
The upper steep reach is about 1.0 km long with
35–55% channel gradients. Gully sideslopes range
from moderate (30–50% gradient) to very steep
(< 90% gradient) with bedrock slab failures and talus
slopes of large blocks. There were also lesser till and
colluvium slopes, with some soil slumping observed
above the channel. The channel is generally 6–8 m
wide, with 2–3 m deep substrate of large blocks and
boulders from 1.0 to 4.0 m diameter (Figure A 2.4).
The lower reach has similar channel gradients,
with mixed gully sideslopes of bedrock slab failures
and thick till and colluvium with numerous failing
Figure A2.4 A view of the upper steep reach in Upper Eagle Summit Creek.
37
gullies delivering sediment directly to the Eagle
Summit Creek mainstem from small tributary debris
�ows. The channel has a mixed substrate of blocks
and boulders, cobbles and gravel, with some sections
scoured to bedrock. Immediately upstream of the
fan there is a 50% gradient reach with mainly gravels, cobbles, and small blocks (< 1.0 m diameter) and
abundant woody debris, partially derived from pre1970s harvesting on steep gully sidewalls. In spite of
the reported recent events there was a high sediment
load in the channel immediately above the fan, and
also in other sections of the channel, particularly
near the con�uence of tributary debris �ows that are
actively delivering sediment to the main Eagle Summit Creek channel (Figure A 2.5).
Figure A2.5 A view looking upstream in the lower reach of
Eagle Summit Creek. The small boulders and
fine sediment originate from sidewall debris
flows. This material is draped over the large
block substrate of Eagle Summit Creek.
38
Synthesis of watershed processes – It was concluded
that the upper steep reach could be the source of a
debris �ow if a sizeable landslide should affect the
channel. In-channel debris �ow or �ood initiation
of the 1.0–4.0 m diameter sediment by high stream
�ows is considered unlikely. From the channel loading observed, it was estimated that 15–25 m3 of blocky
sediment could be mobilized in a debris �ow from
each metre-long section of channel. Depending on
where along the channel a debris �ow originated,
from 25 000 to 40 000 m3 of blocky sediment could
be mobilized, which would reach the fan and deposit
anywhere along the 200 m of the TCH crossing
the fan. This is an order of magnitude greater than
events observed in the last 55 years. Debris �ows
have not been observed in Eagle Summit Creek for at
least 250 years. Thus there is an existing debris �ow
hazard with a very low frequency and a very large
magnitude (power and extent). Landslide risk is the
watershed process that needs to be focussed on when
considering forest activities that could have an impact on the upper steep reach of Eagle Summit Creek
on the Eagle River valley wall.
It was also concluded that the frequent debris
�oods observed in the last 55 years all originate in
the steep lower reach, where there is more abundant
�ner-grained sediment (gravel to small boulders and
blocks) in the channel. There appears to be a high
rate of sediment recharge to the lower steep reach. It
was concluded that, in the lower steep reach, debris
�oods can be initiated by small tributary debris �ow
damming of the channel or by in-channel initiation
in the mainstem by high stream �ows. Because of the
sediment loading and the low return period between
events of approximately 14 years, the lower steep
reach is near its debris �ood failure threshold most of
the time. Therefore, there is an existing debris �ood
hazard with a high frequency and a much smaller
magnitude (power and extent within the channel),
which does however affect the TCH. It is considered
very unlikely that development on the plateau could
initiate landslides that could affect the lower steep
reach above the fan. Therefore, the watershed process
that needs to be addressed when considering forest
activities is an increase in peak �ows from harvesting on the plateau.
Step 5 Analyze risks and develop plans
The debris �ow return period in Eagle Summit Creek
is at least 250 years, and possibly more. Therefore, the
frequency, or annual debris �ow probability, is estimated to be at least 1/250 or 0.004. Debris �oods in
Eagle Summit Creek have a return period of approximately 14 years, a frequency of 1/14 or 0.07. These are
the existing natural hazards at the site.
Incremental landslide and debris �ow hazard
and risk
Proposed developments included some harvesting at
the edge of the upland plateau, with some cable harvesting proposed on the upper slopes of the steeper
Eagle River valley walls. Also, an old road along the
edge of the plateau would be upgraded to access this
block. Extensive harvesting on the plateau located
up to 2.0 km from the steep Eagle Creek valley walls
could increase the existing watershed ECA by about
50%, to nearly 30% of the watershed.
The harvesting and road were a concern, particularly from possible disturbance and diversion
of poorly incised ephemeral streams in the steeper
cable harvest area, and from the potential for the
road to intercept and divert hillslope drainage onto
steeper downslope areas.
The road was proposed to be long term (20 years).
Given this time frame and the sensitivity of the
gentle-over-steep terrain in this area to drainage
concentration by roads, it is appropriate from a risk
management perspective to include a margin of error
with respect to road drainage. It is possible that during the life of the road, a drainage diversion could
occur that would elevate the likelihood of a landslide
reaching Eagle Summit Creek. An assumption was
made that the landslide frequency would increase
from the historical value of 0.004 to 0.05 (1 in 20
years). From Table A 2.1 this would be considered a
high incremental frequency.
The power and destructive force of a 25 000–
40 000 m3 debris �ow comprised in large part of
large boulders and blocks would be very high, and
this very high-powered event would extend to the
TCH anywhere along the 200 m where it is on the
Eagle Summit Creek fan. This was considered a large
to very large magnitude event. From Table A 2.2, with
a high incremental frequency and a high to very high
magnitude, there is a very high incremental debris
�ow hazard.
This large, powerful debris �ow would likely bury
some of the TCH, disrupting traffic along one of the
three main car and truck routes between British
Columbia and the rest of Canada. The Eagle Summit
Table A2.1 Qualitative frequency definitions (adapted from
B.C. Ministry of Forests 2002, Table A10.2)
Quantitative
long-term
frequency
(annual
probability)
Qualitative
frequency
Qualitative
description
> 0.64
Very high
Event is imminent after
the watershed activity
> 0.18, < 0.65
High
Event is probable during
the lifetime of the watershed activity
> 0.04, < 0.19
Moderate
Event is possible, but not
likely during the lifetime
of the watershed activity
> 0.01, < 0.05
Low
Remote likelihood of
event during the lifetime
of the watershed activity
< 0.02
Very low
Very remote likelihood of
event during the lifetime
of the watershed activity
39
Table A2.2 A qualitative hazard matrix (adapted from Wise
•
et al. 2004)
Frequency
High
Magnitude
Moderate
Low
Very high
High
Moderate
Low
Very low
Very high
Very high
High
Moderate
Low
Very high
High
Moderate
Low
Very low
High
Moderate
Low
Very low
Very low
Creek crossing would likely be seriously damaged
and would require rebuilding or replacement. If a
vehicle and its occupants were hit by the debris �ow
it would probably result in a fatality or fatalities. If
a vehicle ran into the deposit, injury and possibly a
fatality could result.
The managing foresters considered the possibility of more detailed consequence analysis, including
exposure and vulnerability of traffic and human
safety, but decided that it would not add value to
the risk analysis. Their decision was that any serious
forestry-related impact to the TCH was a high to very
high consequence.
From Table A 2.3 it was concluded that with a high
to very high incremental debris �ow hazard, and
high to very high consequences on the fan, there was
a very high incremental risk from the harvesting
and road upgrading being considered near the Eagle
Summit Creek gully.
To reduce the debris �ow hazard and thus the
risk, risk control measures were adopted as part of
the planning process. Regarding the road above the
steep Eagle Summit Creek gully walls, and the proposed cable harvesting on upper gully wall slopes,
a Terrain Stability Assessment (TSA) recommended
the following:
Table A2.3 A qualitative risk matrix (adapted from Wise et
al. 2004)
Hazard
High
Very high
High
Moderate
Low
Very low
Very high
Very high
High
Moderate
Low
40
Consequence
Moderate
Very high
High
Moderate
Low
Very low
Low
High
Moderate
Low
Very low
Very low
•
•
•
•
•
On > 60% gradient gully wall slopes, the outer
2 m of the existing road prism �ll should not be
disturbed. Any widening of the road should take
place by removing cutslope material, which was
located on the moderate to gently sloping plateau.
Excavated material should not be placed on the
outer 2 m of the existing prism, but used to raise
the grade or end-hauled.
A drainage plan was prepared after a detailed
assessment of hillslope drainage was undertaken
by a geotechnical professional during a spring
freshet. Drainage control structures (culverts and
ditch blocks) were sized, located, �agged, and
painted in the �eld, and their location mapped
and documented.
Deactivate the road by removing all culverts and
installing cross ditches at all culvert locations
after harvesting and replanting.
The geotechnical professional should conduct
�eld reviews during road construction and
following deactivation, or at any time during
operations if any unexpected sub-surface �ows
were encountered that would require any change
to the drainage plan.
The lower block area with easily disturbed, poorly incised ephemeral streams should be deleted
from the harvesting plan.
Any bladed skid and backspar trails in the
remaining block should be deactivated by full
pullback and slope recontouring immediately
following yarding and prior to the next season’s
freshet.
It was concluded that these measures would
reduce the incremental landslide and debris �ow
hazard to low.
Incremental peak �ow and debris �ood hazard
and risk
The watershed and fan analyses concluded that
existing peak �ows are likely close to the natural
threshold stream �ow values required to initiate
debris �ooding in the steep lower Eagle Summit
Creek reach—a reach with abundant gravel to small
boulder sediment where debris �oods could be initiated. Fieldwork and reports of events indicate that
this threshold is exceeded on average every 14 years.
Therefore, any noticeable peak �ow increase due to
upslope forest development could push the system
over that threshold and initiate debris �oods more
frequently.
Proposed clearcut harvesting on the gentlegradient plateau extended up to 2 km upslope from
the steep Eagle Summit Creek gully on the Eagle
River valley walls. Recent research has shown that
in the snowmelt-dominated watersheds of southern
interior British Columbia the hydrological effects
of clearcut harvesting can lead to increases in all
sizes of peak �ows (Schnorbus and Alila 2004).
Therefore, there was the potential for proposed
clearcut harvesting to increase the existing debris
�ood frequency of 0.07 to some unknown higher
frequency. This increased frequency would exist for
at least 20–30 years, when hydrological green-up
would signi�cantly decrease the clearcut debris �ood
hazard. Although the speci�c effect that upslope
harvesting could have on peak �ows was not known
to any degree of certainty, it was concluded by the
risk analysis team that a noticeable increase in peak
�ows could entail at least a moderate, and possibly a
high, incremental debris �ood frequency.
Based on past occurrences, a debris �ood would
be largely contained in the channel, at least until
it reached the TCH crossing, which it would affect.
This was considered a moderate magnitude event.
Therefore, from Table A 2.2, with a potential high
incremental �ood frequency and moderate magnitude, there was potentially a moderate, and possibly
a high, debris �ood hazard.
A debris �ood would affect the highway crossing,
most likely by blocking the culverts and overtopping
the highway and disrupting traffic. The event would
probably not cause a fatality directly, but could cause
a vehicle pile-up that could result in damage to
vehicles, and possibly human injury. Again, the land
managers decided that any impact to the TCH that
would disrupt traffic and potentially result in human
injury was a high consequence. Therefore, from
Table A 2.3 there was a potential high to very high
debris �ood risk, which was considered unacceptable
by the land managers.
Because of the potential for harvesting-related
elevated peak �ows, the Terrain Stability Analysis
recommended that a forest hydrology professional
review the proposed harvesting further for potential
increased peak �ow effects. That review concluded
that the proposed clearcut harvesting above the H60
line (the line above which contains 60% of watershed
area, see Figure A 2.1) could lead to stream�ow effects
that could increase the frequency of �ood and/or debris �ood events on the fan. The hydrologists report
recommended deleting the proposed harvest area
above the H60 line until either:
•
•
the B.C. Ministry of Transportation and Highways are noti�ed that previous events have shown
that the existing culverts at the TCH crossing are
inadequate to pass the frequent debris �oods that
occur in Eagle Summit Creek, and that will occur
at some interval in the future whether additional
forest harvesting occurs in the watershed or not,
and a suitably designed span is installed over
Eagle Summit Creek, or
hydrologic green-up of existing harvested areas
has achieved a stand height and canopy closure
such that upper watershed ECA is reduced to an
acceptable level.
Harvesting of proposed areas below the H60
elevation was not considered an increased peak �ow
hazard. In fact, lower-elevation harvesting could
advance snowmelt in those areas early in the spring
freshet, desynchronizing melt from the lower-elevation area from peak �ow–generating snowmelt in the
upper watershed area. This could result in sustained
periods of moderately high, but not extreme, �ows.
41
APPENDIX 3 Shale Creek Case Study
British Columbia Timber Sales (BCTS) is developing harvesting plans for a cutblock located on the
alluvial fan of Shale Creek, Moresby Island, Haida
Gwaii Forest District (Cutblock SKI 101, Figure
A3.1). The fan was previously harvested in 1951, and
harvesting within the watershed of Shale Creek commenced shortly after the fan was logged. Evaluation
of the watershed and fan processes, and the effects of
the initial harvesting of the Shale Creek fan and its
watershed, are important for planning appropriate
cutblock layout and fan management.
The evaluation used aerial photographs from
1964, 1977, and 1994, as well as recent satellite imagery (circa 2007) to examine watershed and fan
conditions. Maps provided additional information.
Fieldwork on the fan examined the channel network
and other fan features.
Figure A3.1 Location map for Block SKI 101.
42
Step 1 Identify fans and delineate
watersheds
The Shale Creek watershed-fan system is located on
the Skidegate Plateau area of Moresby Island. Figure
A3.2 shows the watershed boundary of Shale Creek
and the fan boundary. The watershed is approximately 10.6 km2 in area, with an elevation range
of 80–620 m (Figure A3.2). Shale Creek fan builds
into a 1 km wide valley that trends west–east, which
includes Skidegate Lake. To the immediate west of
the fan, the valley is con�ned by hillslopes projecting into the valley from the northwest and southeast,
and the valley narrows to about 0.5 km in width. The
fan is limited in its ability to expand to the southwest
by the hillslope con�nement, but extensive fan expansion to the east is possible. The fan toe transitions
Figure A3.2 Shale Creek watershed and fan. Mapped stream channels shown on the fan are not correct, as is often the case with
fan channel networks.
into a �oodplain that extends east to Skidegate Lake,
a distance of about 1.5 km (Figure A3.1). The fan toe
is partially de�ned by wetter soils and a change in
timber type.
Step 2 Identify elements-at-risk on fans
Located in a relatively remote area, there is no human habitation on the fan. A mainline logging road
and power line cross the fan. Streams on the fan and
their high-value �sheries are the main element-atrisk.
Step 3 Investigate fan processes
The fan is low gradient (< 2%) and shows evidence of
site-level, high-power, water �ood events (Wilford
et al. 2005). Two channels currently exist on the fan
in the central and western portions (Figure A3.3).
Stream 1 is the main channel that carries water �ows
and sediment from the watershed and ranges from 10
to 25 m wide, with the greater widths near the apex
of the fan. Upstream of about 0 + 700 m on Stream
1 there are abandoned or slightly active channels
that avulse from, and return to, Stream 1. Stream 2
is smaller, with channel widths typically less than
5 m. This stream is inset into an older channel that
is 15–20 m wide, similar in width to the presently
active Stream 1 (Figure A3.4). Upstream of 0 + 349
m on Stream 2 there is a broad area of recent gravel
deposits (less than about 50 years in age) that have
buried the well de�ned older channel edges visible
downstream of 0 + 349 m. This broad gravel area
extends upstream to the left bank of Stream 1 near
43
Figure A3.3 Channel network on Shale Creek fan. Preliminary proposed harvesting areas are shown in pink.
Figure A3.4 Stream 2 flowing in an older and larger channel. Old channel banks are outlined in yellow.
Groundwater re-emergence results in limited flow variability, little sediment transport, and
previously active gravel bars becoming vegetated.
44
0 + 724 m (Stream 1 measurement). The blockage of
Stream 2 has isolated it from Shale Creek �ows; only
a tributary source and groundwater re-emergence
results in channel �ow.
Step 4 Investigate watershed processes
The relative relief number (Melton 1965) for Shale
Creek watershed is 0.17, indicating that debris �ows
or debris �oods are not likely delivered to the fan
(Millard et al. 2006). Although most of the watershed has low likelihood of landslides, there are some
slopes that are subject to landslides. In particular,
the channel reach immediately upstream of the fan
apex is narrowly incised into steep unstable slopes,
where most landslides deposit directly into Shale
Creek. The channel gradient of Shale Creek immediately upstream of the fan for a distance of almost 1
km is 2–4%, indicating that landslides that initiate in
this area and deposit sediment into the creek will not
continue to the fan as debris �ows. Field investigation of the fan did not �nd any evidence for debris
�oods or debris �ows, which con�rms that Shale
Creek is a �ood-only fan.
History of forest management and geomorphic
response
The fan area was harvested in 1951. The 1964 air photos show Stream 2 as the active channel that carries
water and sediment from the watershed. The 1964
air photos also show that much of the lower portion
of Shale Creek watershed was harvested by 1964,
including the steeply incised slopes in the channel
reach directly upstream from the fan apex. By 1964,
at least 14 logging-related landslides had occurred on
these valley slopes immediately upstream of the fan,
with additional slide areas visible in the 1977 photos.
Most of these landslides deposited sediment directly
into Shale Creek. Although individual landslide
events were not transported directly to the fan as
debris �ows or debris �oods, the channel would have
transported much of the landslide sediment to the
fan through subsequent �ood events. It is likely that
the sediment from these landslides resulted in the
extensive gravel deposits near 0 + 724 m on Stream 1.
Although Stream 2 appears to be the only active
channel in the 1964 photos, by 1977 it appears to have
been blocked.
Stream 1 parallels the Moresby Mainline road for
approximately 200 m (Figure A3.3), which is built
above the surface of the fan and acts as a dike. No
channels are evident on the downstream side of
the mainline road along the section where Stream 1
parallels the road. The alignment of Stream 1 with
the road, and the lack of channels below the road in
the area of the stream alignment, indicates that the
road was present prior to the existence of Stream 1.
This evidence, in combination with the large gravel
deposits blocking the entrance of Stream 2 and the
presently small channel size of Stream 2 inset into
an older, larger channel, all support the air photo
evidence that Stream 2 was the only active channel on the fan prior to watershed logging, and that
the blockage of Stream 2 resulted in the creation of
Stream 1 below 0 + 730 m.
With the exceptions of Stream 1, Stream 2, and
partially active channels near the apex of the fan,
only a few very faint remnants of channels were
found on the Shale Creek fan. All other surfaces
appeared old, with no �uvial activity for at least centuries, and possibly millennia. The lack of channels
across the surface of the fan indicates high channel
stability. Increased sediment deposition on the fan,
such as that which occurred from numerous landslides after the 1950s and 1960s logging of the slopes
upstream of the fan, is likely the main cause of channel instability.
Step 5 Analyze risks and develop plans
The primary element-at-risk on the fan is �sheries
habitat. The �sh habitat is likely best protected by
avoiding avulsions, which appear to be rare on this
fan, and by avoiding introducing sediment into the
channels. Harvesting of almost all trees on the fan
in 1951 did not appear to have caused the avulsion on
the fan, although streamside logging likely resulted
in damage to stream banks. Logging-related landslides within the watershed likely led to an avulsion.
The history of the watershed-fan system indicates
that to avoid management-caused avulsions on the
fan, watershed management is at least as important
as fan management.
BCTS has agreed to follow the Haida Gwaii
Strategic Land Use Agreement for Block SKI 101.
This agreement has the objective of retaining active
�uvial units by maintaining forests within 1.5 tree
lengths of the outer edge of the active �uvial unit.
For fans, the active �uvial unit is de�ned as the hydrogeomorphic riparian zone.
45
The hydrogeomorphic riparian zone on Shale
Creek fan extends from the left (or northeast) channel bank of Stream 1 to the right (or southwest)
channel bank of Stream 2. Upstream of about
0 + 700 m on Stream 1, the active �uvial unit includes some abandoned or slightly active channels
to the northeast of Stream 1. This zone encompasses
both the currently active channel (Stream 1) and
Stream 2, which has become isolated from sediment and water �ows of Shale Creek. Since this fan
appears to exist primarily as a single-thread channel
network and not a complex of channels across the
surface of the fan, this likely represents an oversized
active �uvial unit. All of the SKI 101 block areas are
at least 2 tree lengths from the active �uvial unit.
This exceeds the stated objective in the Haida Gwaii
Strategic Land Use Agreement and should provide
protection to bank stability.
The avulsion of the fan channel sometime
between 1964 and 1977 indicates that the channel
APPENDIX 4
Conclusion
The forest management history of the Shale Creek
watershed-fan system demonstrates that effective
fan management must include consideration of
watershed management. Planning for the SKI 101
block incorporated fan and watershed assessments
to understand how the channel network on the fan
responds to fan and watershed disturbances.
HUMMINGBIRD CREEK CASE STUDY
Hummingbird Creek is located in the Hunters Range
on the east side of Mara Lake near Sicamous in the
southern interior of British Columbia (Figure A 4.1).
Mara Creek joins Hummingbird Creek just below
the apex of the fan, and the joint fan is locally known
as Swansea Point. On July 11, 1997, a debris avalanche
initiated in the watershed, entered Hummingbird
Creek, and initiated a debris �ow. The debris �ow
travelled down the channel to the apex of Swansea
Point fan where sediment deposition began. The
volume of the debris �ow was estimated at 92 000 m3,
which resulted in the destruction of two houses and
damage to several other houses, as well as to Highway 97A and local roads.
This case study uses the �rst four steps of the
�ve-step method to describe the Swansea Point fan
and Hummingbird watershed, the elements-at-risk,
and the hydrogeomorphic processes that occur,
including the 1997 debris �ow event. A retrospective
application of step �ve describes how the use of the
method might have prevented the 1997 debris �ow,
highlighting the fact that relatively subtle changes in
a watershed can have very signi�cant implications
on the fan. Knowledge of the potential hydrogeomorphic processes that can occur in a watershed
allows land managers to focus their decisions where
46
network on Shale Creek fan is sensitive to watershed
processes. The avulsion was likely caused by loggingrelated landslides that led to extensive sediment deposits on the fan, blocking the then-active channel.
These events demonstrate the sensitivity of the Shale
Creek fan to forestry practices within the watershed.
Future harvesting within the Shale Creek watershed
should consider the potential for fan destabilization
as a result of landslides that could lead to increased
sediment deposition on the fan.
they will be most effective in managing risks for a
downstream fan.
Step 1 Identify fans and delineate
watersheds
The Swansea Point fan is built out into the waters of
Mara Lake. The watershed boundaries of Hummingbird and Mara Creeks and the Swansea Point fan are
illustrated in Figure A 4.1. Hummingbird watershed
is approximately 16.4 km2 in area. The lower half of
Hummingbird Creek follows a well de�ned, southwest-trending faultline and then turns abruptly west
as it reaches the fan. The fan extends out into Mara
Lake; the sub-aerial portion of the fan is approximately 0.6 km2. The distance from the fan apex to the
lakeshore is approximately 1.3 km. The fan gradient
ranges from approximately 18% at the apex to 5%
along the lakeshore.
Step 2 Identify elements-at-risk on fans
Human safety – Approximately 250 people live
permanently in the Swansea Point area (Statistics
Canada 2007), and during the summer more come
to stay at resorts on the fan.
Figure A4.1 Map of Hummingbird and Mara Creek watersheds, and the Swansea Point
Fan. The dashed line follows the 1500-m contour, which is the break between the gentle upland plateau and the steeper escarpment.
Anthropogenic features – Non-indigenous settlement began on Swansea Point fan in the early 1900s
and there are presently approximately 277 private
dwellings, of which 122 are occupied year-round
(Statistics Canada 2007). A large condominium
complex developed in 2007 was reviewed by the
Columbia-Shuswap Regional District but the concerns of residents on the fan were primarily the sewage disposal system, upgrading of the water system,
increased boat use on Mara Lake, highway access,
population density, beach access, and noise—very
little attention was paid to the hydrogeomorphic
hazards.
Highway 97A, connecting Sicamous to the Okanagan Valley, climbs the fan towards the apex and
there are numerous local roads on the fan, a poten-
tially problematic situation (Wilford et al. 2005). In
1997, Hummingbird Creek passed under the highway
through a 3-m elliptical multi-plate culvert. A trash
rack was in place to remove boulders immediately
upstream of the culvert. During the debris �ow event
in 1997, the trash rack rapidly blocked with debris
and diverted most material from the channel. The
culvert was relatively undamaged during the event
and remains in use today. In the aftermath of the
debris �ow, the upper channel was widened and a
levee was constructed and armoured with the largest
boulders available in the vicinity. The culvert outlet
and downstream banks were armoured extensively
to promote channel stability.
Swansea Road, the main access road on Swansea
Point running west from the highway down to Mara
47
Lake, was heavily damaged by the �ner-grained
fraction of the debris �ow and required rebuilding.
Natural features – As a result of the passage of the
1997 debris �ow and subsequent mitigation works,
there is little or no complexity to the fan channel
(i.e., the channel is quite straight with predominantly bare sand, gravel, and boulders along the margins;
there are no vegetated banks and very little wood
or organic material). Mara Lake is home to a wide
variety of �sh, including several types of salmon and
trout. Fish are not present in the Hummingbird watershed: the steady gradient of the lower channel precludes �sh passage, and upper portions have no lakes
or wetlands for �sh habitat. Rainbow trout are noted
in the Mara Creek watershed, likely in the lakes on
the upper plateau. Fish passage into Mara Creek is
obstructed at the con�uence with Hummingbird
Creek by a very steep section of channel.
Step 3 Investigate fan processes
Hydrogeomorphic processes – From �eld observations, dendroecology, anecdotal reports, and review of various sets of aerial photographs, there is
abundant evidence that debris �ow and debris �ood
events have occurred on the fan in the past, with
several lines of evidence pointing to a large event in
1935 (Jakob et al. 2000). Human activities on the fan
prior to 1997 have probably altered morphological
evidence of previous events, such as boulder lobes
and levees. However, there are exposures of debris
�ow deposits overlying �uvial sediments, indicating
a previous history of debris �ows on the fan (Interagency Report 1997).
The 1997 debris �ow – The Swansea Point fan can be
divided into upper and lower sections by Highway
97A (Figure A 4.2): the upper fan section is approximately 600 m long and the lower section is approximately 700 m long.
The upper fan is con�ned by bedrock ridges, has
moderate gradients between 10 and 18%, and has
coarse sedimentary deposits. The active hydrogeomorphic riparian zone (Wilford et al. 2005) remains
relatively con�ned, about 50 m wide, for the �rst
400 m below the apex, then widens to over 100 m
wide at the highway crossing. The typical particle
size near the fan apex is up to 1 m (b-axis). One
exceptionally large boulder (the “Hinkelstein,” 9 m
tall, 8 m wide, and 6 m long) can be seen in Figures
A 4.3 and A 4.4.
The lower fan has gentler gradients and �ner
sedimentary deposits. Most of the coarse material
(particle size > 0.4 m) was deposited on the upper
fan, but �ner debris �ow material and water crossed
both north and south of the highway culvert. To the
south, the debris �ow carved a new channel across
the road and then returned most of the �ow back
into the lower channel. To the north, much of the
Figure A4.2 Distribution of new debris flow sediment on the Swansea Point fan, mapped
on July 14, 1997 (from Inter-agency Report 1997). The shaded area incorrectly follows Mara and not Hummingbird Creek at the lower right.
48
Figure A4.3 Looking downstream from the confined valley reach of Hummingbird Creek
to the apex of the Swansea Point fan. Note presence of bedrock on channel
margins, large boulders in the channel, and the “Hinkelstein” boulder in the
centre.
Figure A4.4 The “Hinkelstein” at the apex of the Swansea Point
fan. This huge boulder was transported during the
1997 debris flow.
49
�ow was carried straight down Swansea Road or utilized other old channel segments on the fan surface,
before reaching Mara Lake in several locations.
Above the apex of the fan, the creek �ows in an
incised, steep-sided, bedrock-cored channel 8–10 m
wide, with a gradient between 18 and 21% (Figure
A 4.3). The average gradient between where the debris
avalanche entered the creek and the fan apex is 27%.
Event frequency – Two methods are used to reconstruct debris �ow occurrences on the Swansea Point
fan: a collection of anecdotal information and an
aerial photograph review.
Anecdotal information – One account, presumed
from the 1930s, indicates that the fan was covered by
a layer of boulders 1.2 m in diameter, with the largest
boulder having dimensions of 2.4 by 3 m (Jakob et
al. 2000). The highway bridge across Hummingbird
Creek was destroyed in this event.
Aerial photograph review – Aerial photographs
show that the main channel has migrated in recent
history. At the beginning of the 20th century, the
main channel was located on Railway Belt surveys
as heading more westerly across the fan (Fuller
2001). It remained in this position until after 1928
(as seen on aerial photograph A360: 28). In the 1928
aerial photograph there are two road alignments, the
newer of the two being roughly the present alignment of Highway 97A. On the 1951 aerial photographs (BC1292: 32 and 33) the creek �ows west until
it passes under Highway 97A, then turns southwest
and �ows into Mara Lake. This is likely the result of
a debris �ood or debris �ow event reaching the fan
and blocking the channel, and water being diverted
onto the old road alignment. This remains the present channel location.
Synthesis of fan processes
Compilation of all evidence suggests that there was
a relatively large event that occurred between 1928
and 1951, most likely in 1935. The classi�cation of
the event as either a debris �ow or debris �ood is
unclear, but it apparently had sufficient material to
block the existing channel and cause diversion onto
an old roadbed, which has subsequently become the
present-day channel. While there is some evidence of
more frequent smaller events, there have likely been
two large potentially damaging events in the last 75
years.
50
Step 4 Investigate watershed processes
Office investigations
While none of the aerial photographs dating back
to 1928 shows evidence of recent landslides on the
steep valley sidewalls above the lower channel, there
is evidence of older vegetated linear features and
possibly a large relic landslide. There is also a recent
large debris slide on similar terrain just north of
Hummingbird Creek (Figure A 4.5).
Morphometrics – The relative relief number (RRN)
(Melton 1965) has been found useful for determining
the hydrogeomorphic event that will emanate from
a watershed (Wilford et al. 2004b). It has been found
that for RRN < 0.3 the dominant hydrogeomorphic
process is �ooding; for 0.3–0.6 and > 0.6 with watershed length > 2.7 km the process is debris �ood; for
> 0.6 with watershed length < 2.7 km debris �ows
can occur (Wilford et al. 2004b). However, the RRN
is designed for “graded watersheds”—watersheds
that have steep terrain in the headwaters and gentle
terrain in the lower reaches. Caution must be used
when applying the RRN to reversely graded watersheds and in de�nition of the area of a watershed.
There are two distinct watersheds feeding onto
the Swansea Point fan, Hummingbird and Mara.
The individual watersheds indicate that Hummingbird (RRN = 0.44) and Mara (RRN = 0.39) are debris
�ood watersheds, but the combined area suggests it
is more likely a �ood watershed (RRN = 0.29) (Table
A 4.1).
Hummingbird and Mara watersheds are clearly
not graded; the headwaters have gentle slopes and
the lower portions are much steeper—a situation
that is referred to as “gentle-over-steep.” To compensate for this, subdivision of the watershed to
isolate the steeper sections was completed (Figure
A 4.1). For Hummingbird Creek, subdivision of the
watershed into the steeper section below the 1500 m
contour gives an RRN of 0.51. This ratio indicates that
the steeper portion of the watershed, which is more
susceptible to landslides, is likely to be characterized
by debris �ood events moving through the channels
and reaching the fan. Given that the 1997 event was
a debris �ow, a watershed RRN between 0.29 and 0.51
indicates �ood or debris �ood, which underestimates
the actual hydrogeomorphic hazard, and also suggests that the two watersheds should be treated as
separate for the RRN analysis.
Figure A4.5 Aerial photograph BC2616: 76 of lower Hummingbird Creek, 1959, showing a recent landslide
to the north on similar steep, northwest-aspect terrain, a possible relic debris slide colonized by
a younger forest stand, and the location of the 1997 debris avalanche.
Table A4.1 Relative relief numbers for the Swansea Point fan
Area km2
Elevation Range
(km)
RRN
Hummingbird
16.4
1.79
0.44
Mara
21.2
1.79
0.39
Combined
37.4
1.79
0.29
1.10
0.51
Hummingbird
< 1500m
4.61
Sediment production and movement – Terrain
stability mapping of the Hunters Range completed
after the Hummingbird debris �ow shows that Class
IV (potentially unstable) and Class V (unstable) ter-
rain is common along the steeper face above Mara
Lake. On the upland plateau, the terrain was generally Class I to Class III (stable) with shallow bedrock
covered by a thin veneer of sediment. Channels are
moderately incised into the plateau surface, and sediment production and delivery to streams is limited
to the stream escarpment slopes. The steeper slopes
on the Mara Lake face show more signs of sediment
delivery to the channels with thin linear strips visible
in the tree cover on aerial photographs (Figures A 4.5,
A 4.6, and A 4.7).
Several other creeks along the Hunters Range
above Mara Lake, including Mara, Rodgers, and
Johnson Creeks, have widened channels and exposed
sediments in the 1951 aerial photographs (BC1292:
31–35 and BC1285: 34–37). There is no evidence on the
51
aerial photographs of logging within the upper watersheds, and recent landslide scars are not apparent,
suggesting that in-channel �oods or debris �oods
might have been the cause. Sparse or young timber
types across the top of the Hunters Range hint at the
occurrence of widespread wild�re.
Flood Generation – Snow packs of 2–3 m thickness
are typical in this area. Limited stream�ow records
on smaller watersheds in the southern interior offer
only partial information on timing and volumes of
�ows from the Hunters Range. Chase Creek watershed (approximately 40 km west of Mara Lake) is the
closest gauged watershed that drains directly from
an upland area. Chase Creek experiences peak �ows
generated from snowmelt between mid-April to
mid-June, but maximum elevations are 500 m lower.
Coldstream Creek watershed (about 50 km south of
Mara Lake) is the next closest station and has peak
�ows generated between April and mid-June, but
maximum elevations are 600 m lower. From these
records, peak �ows in Hummingbird and Mara
Creeks are estimated to occur between mid-April
through late June.
Field investigations
On July 11th, 1997, a debris avalanche initiated on an
open slope directly below a culvert on the Skyline
Forest Service Road (Inter-agency Report 1997). A
skid trail in a 1994 cutblock diverted water, which
increased the drainage area leading to the culvert by
approximately a factor of 3 (Figure A 4.6). This interception and concentration of hillslope runoff to the
culvert is considered a “major contributing factor in
initiating the debris avalanche” (Inter-agency Report
1997).
Snowpacks across the North Okanagan-Shuswap
were 25% above normal in 1997, and followed record
levels of precipitation in the area over the preceding
9 months (October 1996 through June 1997), resulting in antecedent soil moisture conditions that were
substantially higher than normal. Heavy precipitation recorded at local climate stations between July
5 and 12, (60- to 85-year return period) contributed
more water to the already wet soils, and late on July
11 the debris avalanche was triggered.
The slope gradient below the culvert was up to
70% and had a cover of sur�cial sediment < 1 m thick
over glacially smoothed, benchy, granitic-gneiss
bedrock. The debris avalanche widened gradually
downslope over a total slope distance of 560 m, from
5 m at the headscarp to 110 m where it entered
52
Hummingbird Creek (Figures A 4.6 and A 4.7). The
debris avalanche then rapidly transformed into a
debris �ow, which travelled 2.4 km down Hummingbird Creek onto the fan (Figure A 4.7). An estimated
25 000 m3 of wood and sediment entered the creek
from the debris avalanche. A further 67 000 m3 of
debris, or 28 m3 of debris per linear m of channel,
was estimated to be scoured from the creek channel.
The total volume of the debris �ow was calculated at
over 92 000 m3 (Jakob et al. 2000). Deposition was
estimated as 49 000 m3 on the upper fan and 36 000 m3
on the lower fan, and 7 000 m3 reached Mara Lake.
Following the debris �ow, the channel has a reduced
amount of stored sediment, and a landslide entering
the channel now would have less sediment to
entrain.
Synthesis of watershed processes
The upper portion of Hummingbird Creek watershed is unlikely to generate debris �ows or debris
�oods. The relatively low channel gradients are less
able transport signi�cant sediment loads to the lower
half of the watershed. Floods may be generated from
snowmelt in the upper watershed, but the timing of
known events in late June and early July suggests
that snowmelt has less of an effect than rainfall.
Lower sections of the watershed have higher probabilities of generating events because there is direct
connectivity of steep, potentially unstable slopes
adjacent to the channel, and the channel gradient
is relatively steep (Millard 1999). The lower channel
has little or no �oodplain to arrest landslide debris,
and consequently landslides have the potential to
transform into a debris �ood or debris �ow. The 1997
debris �ow occurred as the result of intense longduration rainfall on top of unusually high antecedent moisture conditions and thus there was an
increased amount of stream�ow to propagate the
landslide sediments through the creek to the Swansea Point fan.
Step 5 Analyze risks and develop plans
It is useful to review the 1997 debris �ow from the
perspective of the �ve-step method for managing
risks on fans from upstream watershed activities.
In particular, to address the question “could the
method have in�uenced how forest development in
the watershed took place and possibly reduced the
likelihood that a debris �ow would have occurred?”
The following analysis uses the risk analysis pro-
Figure A4.6 Pre- and post-development runoff-contributing areas to the culvert located above the debris
avalanche. (Source: R. Winkler, B.C. Ministry of Forests and Range and D. Anderson, B.C.
Ministry of Environment.)
Figure A4.7 Aerial photograph of lower Hummingbird Creek watershed with the debris avalanche below
the road in right centre (15BCC04021: 202, taken in 2004). The debris avalanche developed
into a debris flow that proceeded down the channel to the Swansea Point fan.
53
cess to review what was known, or could have been
known, prior to the logging and road building in
the watershed that preceded the 1997 debris �ow. It
should be noted that, while hindsight is useful, this
analysis is not meant as a criticism of licensee or
Ministry of Forests practices in Hummingbird Creek
watershed prior to the 1997 debris �ow, which in our
understanding were to the standards of the day (Forest Practices Board 2001).
Summary
Step 1 Identify fans and delineate watersheds
This would bring to the attention of forest planners the connection between the upper plateau
areas of the watershed where forest activities
were planned, and the fan several kilometres
away from those activities.
Step 2 Identify elements-at-risk on fans
The presence of widespread risk elements (high
density of residences, major provincial highway
and other infrastructure) that could potentially
suffer losses would be identi�ed. Damage to residences, infrastructure, and the highway would be
considered a high consequence. Potential human
injury or loss of life by residents or highway users
would be considered a high to very high consequence, depending on the risk matrix employed.
These high consequences would require that the
remainder of the investigation and risk analysis
be carried out very thoroughly.
Step 3 Investigate fan processes
This would reveal the physical presence of debris
�ow deposits on the fan. With the potential for
high consequences, a thorough review of past
events, by various methods, would be appropriate. A review of historical aerial photographs
would show that a major channel avulsion had
occurred—further evidence of a signi�cant hydrogeomorphic event. A review of published or
oral history in the area, as was done subsequent
to the 1997 event, would con�rm the historic
occurrence of at least one large damaging event
over half a century ago. From this it could be
concluded that Hummingbird Creek could produce a large damaging event, and that it had been
some time since one occurred.
Step 4 Investigate watershed processes
This would include a traverse along the channel
above the fan to assess sediment and woody debris loads available to be mobilized. High levels
54
of entrainable material would have been found
prior to the 1997 event. This would have informed
investigators of the potential for a large event.
The texture of the material could give an indication of whether mobilization of the channel bed
would require a landslide impact or if in-channel
initiation could occur from peak �ows.
Investigation of fan and watershed processes
would lead to the conclusion that a large, potentially damaging hydrogeomorphic event had occurred on the fan circa 1935, and that a debris �ow
had also occurred some time in the past. It was not
clear whether the 1935 event was a debris �ow or
debris �ood. Therefore the linkages to be considered
between watershed processes and forest development
would include the production and delivery of both
sediment and runoff (Table A 4.2).
The potential for landslides that could initiate
debris �ows or debris �oods exists along the lower
stream reach. Given the state of knowledge prior
to 1997 (Hungr et al. 1984; Van Dine 1985, 1996), it
would have been considered possible, but unlikely,
that a landslide affecting the relatively low-gradient (average 27%) channel would have initiated a
debris �ow. Subsequent to the Hummingbird event,
and several other events in southern interior British Columbia, there is an increased awareness of the
potential of large debris �ows in relatively low-gradient channels.
There remains the possibility that a large landslide
could dam the relatively narrow V-shaped channel,
causing stream�ow to back up until the dam was
breached, initiating a large debris �ood. Management of incremental landslide hazards on the lower
steep slopes would be identi�ed as a concern.
Landslide hazards could be from operations on
the lower steep slopes, or from operations on the
moderate to gently sloping plateau some distance
above those slopes. Prior to the Hummingbird
event, management of road and trail drainage on
“gentle-over-steep” plateau terrain was not widely
recognized as a serious hazard. Subsequent to the
1997 debris �ow, and partially because of it, management of forest development in gentle-over-steep
terrain has been addressed through research and the
development of best practices (Grainger 2002; Jordan
2002).
Investigation of channel-sediment texture in the
steeper reach could give some indication of whether
Table a4.2 Forest management focus for different hydrogeomorphic processes. Key issues are
bolded.
Process
Initiation
Management focus
Flood
Runoff
ECA – harvest, �re, forest health
Road density
Fire – reduced soil in�ltration
Landslides – increasing sediment
load to channels on the fan
(in�lling channels)
Dam rupture
Dam rupture – beavers, road
drainage structures
Landslide dams – harvest and
roads (coast), gentle-over-steep
(interior)
Runoff
ECA – harvest, �re, forest health
Road density
Fire – reduced soil in�ltration
Landslide
Harvest and roads on unstable
terrain and gentle-over-steep in
the interior
Dam rupture
Dam rupture – beavers, road drainage
structures
Landslide dams – harvest and roads
(coast), gentle-over-steep (interior)
Landslide
Harvest and roads on unstable
terrain and gentle-over-steep in
the interior
Runoff—in-channel
initiation
ECA – harvest, �re, forest health
Road density
Fire – reduced soil in�ltration
Debris �ood
Debris �ow
in-channel debris �ow or debris �ood initiation
due to peak �ows could occur, and whether ECA
and other peak �ow watershed linkages would need
management attention.
Step 5 Analyze risks and develop plans
If one were to complete this step today for a similar set of conditions as existed at Hummingbird
Creek prior to 1997, and with the knowledge of
forest development and watershed processes we
now have, the available evidence would indicate
that there was a moderate to high likelihood of a
landslide affecting the channel and resulting in
a hydrogeomorphic event that would reach the
fan. Given the uncertainties in the analysis, this
would be considered a moderate to high hazard.
With the high to very high consequences existing
on the fan, obtained from standard risk matrices
(Table A 4.3), there would be a high to very high
incremental risk due to proposed forest development on the upland plateau, and further risk
management would be appropriate in any subsequent plans. From a forestry perspective, the
most feasible course of action would be to reduce
the incremental development-related hazards.
With the hindsight we now have regarding management of gentle-over-steep terrain and resulting
landslide hazards (Grainger 2002), it would be recognized as appropriate to fully deactivate the roads
and trails within the cutblock immediately following harvesting to re-establish or maintain natural
55
Table A4.3 A matrix combining hazard and consequence to
determine risk
Hazard
High
Very high
High
Moderate
Low
Very low
Very high
Very high
High
Moderate
Low
Consequence
Moderate
Very high
High
Moderate
Low
Very low
Low
High
Moderate
Low
Very low
Very low
drainage paths. This deactivation would likely have
prevented enlargement of the runoff-contributing
area to the culvert above the slide. Other actions to
reduce hillslope runoff due to harvesting include:
•
•
•
56
Road construction methods should minimize
subsurface �ow interception in high cut slopes,
such as decreasing the road width or using a
wider �ll to reduce cut slope height.
Drainage redirection should be avoided by
designing roads with no ditches or cross-drain
culverts. Road cross-drainage can be maintained
by out-sloping the road surface or by road construction with engineered continuous sub-grade
drainage.
In high-risk situations, it is professionally
responsible for the geoscientist or engineer
undertaking terrain assessment or road engineering design to prescribe site visits during and
after road building and harvesting, to assess site
conditions during operations and recommend
changes to prescriptions or designs if necessary,
and to ensure that operations are or were carried
out in conformance with recommendations or
designs (APEGBC Quality Management Bylaw
14(b)(4)).
If a steep area downslope of a potential development is judged to have a moderate or higher natural
landslide potential, the prudent decision may be that
the development should not occur within a speci�ed distance of the marginally stable slope. This was
probably not the case at Hummingbird Creek prior
to 1997 (Inter-agency Report 1997). Although we can
never know for sure what the outcome of various
management options would have been, it is likely
that if some or all of the above risk-mitigation steps
had been taken, harvesting and road building on the
upland plateau above Hummingbird Creek could
have occurred without causing a major debris �ow.
Gentle-over-steep terrain similar to the Hummingbird case study is not unique. This case study
presents a sobering example of what can happen
when apparently subtle changes in a watershed result
in signi�cant impacts to the downstream fan. Application of the �ve-step method may not always prevent such impacts, but it does provide a framework
to rigorously explore hydrogeomorphic hazards,
consequences, and risks.
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