The December 2011 debris flows in the
Pohara-Ligar Bay area, Golden Bay: causes,
distribution, future risks and mitigation options
M. J. Page
G. J. Stevens
R. M. Langridge
K. E. Jones
GNS Science Consultancy Report 2012/305
December 2012
The December 2011 debris flows in the PoharaLigar Bay area, Golden Bay: causes, distribution,
future risks and mitigation options
M. J. Page
G. J. Stevens
R. M. Langridge
K. E Jones
GNS Science Consultancy Report 2012/305
December 2012
Society has an ever-increasing need to manage landscapes. To do this effectively requires
improved understanding of the way landscapes behave, and the controls on that behaviour.
This is certainly the case where hazard mitigation involves the management of the generation,
transport and deposition of sediment.
DISCLAIMER
This report has been prepared by the Institute of Geological and
Nuclear Sciences Limited (GNS Science) exclusively for and under
contract to Tasman District Council. Unless otherwise agreed in
writing by GNS Science, GNS Science accepts no responsibility for
any use of, or reliance on any contents of this Report by any person
other than Tasman District Council and shall not be liable to any
person other than Tasman District Council, on any ground, for any
loss, damage or expense arising from such use or reliance.
The data presented in this Report are available to GNS Science for
other use from December 2012.
BIBLIOGRAPHIC REFERENCE
Page, M. J.; Langridge, R. M.; Stevens, G.; Jones, K. E. 2012. The
December 2011 debris flows in the Pohara-Ligar Bay area, Golden
Bay: causes, distribution, future risks and mitigation options,
GNS Science Consultancy Report 2012/305. 91 p.
Project number 430W1490
Confidential 2012
CONTENTS
EXECUTIVE SUMMARY..................................................................................................... VII
1.0
2.0
INTRODUCTION ........................................................................................................1
1.1
Purpose of the report ..................................................................................................... 2
1.2
The brief ........................................................................................................................ 2
1.2.1
Project Objectives ..............................................................................................2
1.2.2
Scope of Services/Project Design ......................................................................2
1.2.3
Deliverables ........................................................................................................3
1.2.4
Contributions from TDC......................................................................................3
1.3
Terminology ................................................................................................................... 5
1.4
Previously studied debris flow events in New Zealand ................................................. 6
1.4.1
Tapawera, Motueka ............................................................................................6
1.4.2
Matata .................................................................................................................7
1.4.3
Te Aroha .............................................................................................................8
1.4.4
Stony Creek, Westland .......................................................................................8
CONDITIONS CONTRIBUTING TO THE 2011 POHARA-LIGAR BAY DEBRIS
FLOWS .......................................................................................................................9
2.1
Geology ......................................................................................................................... 9
2.1.1
Fan deposits (Q2a) (Quaternary) .......................................................................9
2.1.2
Limestone (On) (Oligocene), siltstone (Mb) (Miocene) and sandstone
(Eb) (Eocene) .....................................................................................................9
2.1.3
Separation Point Granite (Ksg) and Granodiorite (Ksd) (Cretaceous) ...............9
2.1.4
Limestone and calcareous mudstone (Oma) (Cambrian-Devonian)..................9
2.1.5
Pikikiruna Schist (Omp) (Late Cambrian-Devonian) ..........................................9
2.1.6
Faults and earthquakes ....................................................................................10
2.2
Geomorphology ........................................................................................................... 12
2.3
Land use and settlement history ................................................................................. 14
2.4
December 2011 storm ................................................................................................. 16
3.0
NATURE OF THE DEBRIS FLOWS AND RESULTING DAMAGE .......................... 21
4.0
ANALYSIS OF STORM DAMAGE ........................................................................... 43
4.1
4.2
Data and method of analysis ....................................................................................... 43
4.1.1
Field data ..........................................................................................................43
4.1.2
Aerial photography ...........................................................................................43
4.1.3
LiDAR ...............................................................................................................43
Results ......................................................................................................................... 43
4.2.1
Rock type and vegetation cover distribution on a catchment basis .................43
4.2.2
Landslide bare ground distribution on a catchment basis ................................43
4.2.3
Landslide bare ground distribution in relation to vegetation cover and
rock type ...........................................................................................................44
4.2.4
Catchment parameters – the Melton Ratio ......................................................44
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5.0
6.0
7.0
8.0
EVIDENCE FOR PREVIOUS DEBRIS FLOWS ........................................................ 49
5.1
Reported possible debris flows ................................................................................... 49
5.2
Rainfall records ............................................................................................................ 50
5.3
Geomorphic and stratigraphic evidence ...................................................................... 50
5.3.1
Fans and fan morphology.................................................................................50
5.3.2
Channel morphology ........................................................................................51
5.3.3
Paleo-debris flow deposits (C
14
dating) ............................................................51
LIKELY RESPONSE TO FUTURE RAINFALL EVENTS ......................................... 61
6.1
In the immediate term (next few years) ....................................................................... 61
6.2
In the short term (up to ten years) ............................................................................... 61
6.3
In the long term (up to 1,000 years) ............................................................................ 61
6.4
Climate-change forecast ............................................................................................. 62
AREAS AT RISK FROM DEBRIS FLOWS AND DEBRIS FLOODS ........................ 63
7.1
Catchments likely to generate debris flows and debris floods .................................... 63
7.2
Susceptible fan areas .................................................................................................. 66
MITIGATION OPTIONS ............................................................................................67
8.1
Land use and land management ................................................................................. 67
8.2
Engineering works ....................................................................................................... 70
8.2.1
Nyhane Drive fan ..............................................................................................70
8.2.2
Pohara Valley fan .............................................................................................71
8.3
Early warning systems ................................................................................................ 74
8.4
Avoidance .................................................................................................................... 74
9.0
CONCLUSIONS .......................................................................................................75
10.0
RECOMMENDATIONS.............................................................................................77
11.0
ACKNOWLEDGEMENTS .........................................................................................79
12.0
REFERENCES .........................................................................................................79
13.0
GLOSSARY ..............................................................................................................82
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FIGURES
Figure 1.1
Extent of study area and location of main debris-flow sites at Pohara Valley and Ligar
Bay (Nyhane Drive). Aerial photography, taken in January 2012, shows extent of
December 2011 landslides and debris flows/floods...................................................................... 4
Figure 2.1
Geological Map of the study area, showing the distribution of rock units/main lithological
groupings and major faults. From QMap 1:250 000 Geological Map 9 Nelson (Rattenbury
et al. 1998). Outline of study catchments and catchment numbers are shown (see
sections 2.2 and 3.0). ................................................................................................................. 11
Figure 2.2
Location of catchments and fans. ............................................................................................... 13
Figure 2.3
Matenga Rd area in 1911 showing steep hills in pasture with some areas of scrub. Note
old road to Wainui zigzagging up the hills to the ridge on the sky line (Courtesy of Alan
Swafford). ................................................................................................................................... 14
Figure 2.4
Vegetation cover map of study area in 2011. ............................................................................. 15
Figure 2.5
Isohyet map of 48 hour rainfall for 13-15 December 2011 rain storm......................................... 17
Figure 2.6
Histogram of largest storm rainfalls in past 50 years. ................................................................. 17
Figure 2.7
Moving rainfall totals (at 15 minute intervals) for given time periods (1hr, 2hr, 6hr, 12hr,
24hr, 48hr) plotted against 1 hour actual rainfall, for the December 2011 storm event at
the Kotinga rain gauge site. ........................................................................................................ 19
Figure 2.8
Moving rainfall return periods (at 15 minute intervals) for given time periods (1hr, 2hr,
6hr, 12hr, 24hr, 48hr) plotted against 1 hour actual rainfall, for the December 2011 storm
event at the Kotinga rain gauge site. .......................................................................................... 19
Figure 3.1
Channel in upper reaches of Catchment 4.0. Note the channel is steep, with a small
width to depth ratio, and has been has been scoured to bedrock. The channel has a
roughly U-shaped form typical of repeated debris flows (TDC photo). ....................................... 22
Figure 3.2
Woody debris deposited along the valley floor just upstream of the fan in Catchment 4.0
(TDC photo)................................................................................................................................ 22
Figure 3.3
Incision of previously in-filled valley floor of Catchment 4.0 (upstream of Figure 3.2) (TDC
photo). ........................................................................................................................................ 23
Figure 3.4
Sediment and woody debris deposited by a debris flood onto the fan associated with
Catchment 4.0. Abel Tasman Drive is in lower left foreground (Courtesy of Tim Cuff)............... 23
Figure 3.5
Landslide in pine forest in Catchment 8.0 (TDC photo). ............................................................. 24
Figure 3.6
Channel in upper reach of Catchment 8.0 (TDC photo). ............................................................ 25
Figure 3.7
Landslides on lower hillslopes logged in late 2010/early 2011 in Catchment 9.1 (TDC
photo). ........................................................................................................................................ 25
Figure 3.8
Looking down fan of Catchment 8.0. Sediment deposited by debris flood (TDC photo)............. 26
Figure 3.9
Looking up fan of Catchment 8.0 from Abel Tasman Drive. Distal end of debris-flood
deposit (TDC photo). .................................................................................................................. 26
Figure 3.10
Sediment deposited by a debris flood onto the lower part of the fan associated with Ligar
Bay Villas catchment (7.3). Note sediment from this and other nearby catchments has
entered the estuary behind Tata Beach (Courtesy of Tim Cuff). ................................................ 27
Figure 3.11
Large landslide in regenerating forest in Catchment 10.1 (Nyhane Drive). Landslide
occurred across Old Wainui Hill Rd (see Figure 2.3) (TDC photo). ............................................ 28
Figure 3.12
Main channel of Catchment 10.1, ~270 m downstream of Figure 3.11. Note large boulder
and channel eroded to bedrock (TDC photo). ............................................................................ 28
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Figure 3.13a
Main channel of Catchment 10.1, ~320 m downstream of Figure 3.12. Arrow points to
deposit shown in Figure 3.13b (TDC photo). .............................................................................. 29
Figure 3.13b
Enlargement of possible debris-flow deposit shown Figure 3.13a (TDC photo). ........................ 29
Figure 3.14
Woody debris deposited at the head of the Nyhane Drive fan (TDC photo). .............................. 30
Figure 3.15
Nyhane Drive subdivision showing the path of the debris flood down the fan to the coast.
Note a branch of the debris flood has spread across the fan to link up with a debris flood
from the adjacent catchment (Courtesy of Braeden Lobb). ........................................................ 30
Figure 3.16
Upper part (apex) of Nyhane Drive fan (see Figure 3.15) showing houses impacted by a
debris flow. Note large amount of woody debris (Courtesy of Tim Cuff). .................................... 31
Figure 3.17
The two uppermost houses on the Nyhane Drive fan (see Figure 3.16) showing partial
burial by sediment deposited by the debris flow. These houses were subsequently
condemned (Courtesy of Tim Cuff). ........................................................................................... 31
Figure 3.18
Debris-flow deposit surrounding house shown in Figure 3.17. Note shed has been
moved by debris flow (TDC photo). ............................................................................................ 32
Figure 3.19
Close up of house at right of Figure 3.17, showing impact of woody debris (TDC photo). ......... 32
Figure 3.20
One of many landslides in Matenga Rd West catchment which supplied sediment and
debris to the stream (TDC photo). .............................................................................................. 33
Figure 3.21
Debris-flow deposit across Nyhane Drive West. Note small diameter culvert under road.
Below this point the debris flow entered a narrow gorge before flowing onto Matenga Rd
(TDC photo)................................................................................................................................ 34
Figure 3.22
Landslides on the steep hillslopes in Catchment 14.2 (Winter Creek) (Courtesy of
Braeden Lobb)............................................................................................................................ 35
Figure 3.23
Section of Winter Creek scoured by a debris flow as it passed downstream. Falconer Rd
at bottom left (Courtesy of Tim Cuff). ......................................................................................... 35
Figure 3.24
Debris flow deposit surrounding property at 64 Pohara Valley Rd (TDC photo). ........................ 36
Figure 3.25
Accumulation of woody debris at 64 Pohara Valley Rd (TDC photo). ........................................ 36
Figure 3.26
Water Treatment Plant partially buried by debris flood sediment (TDC photo). .......................... 37
Figure 3.27
Debris flood sediment at the junction of Pohara Valley Rd and Abel Tasman Drive
(Courtesy of Tim Cuff). ............................................................................................................... 37
Figure 3.28
The large landslide that supplied the majority of the sediment that generated the debris
flow in the Ellis Creek catchment (Courtesy of Paul Woperies). ................................................. 39
Figure 3.29
Debris flow and debris flood sediment deposited on the Ellis Creek fan (Courtesy of
Paul Woperies). .......................................................................................................................... 40
Figure 3.30
Debris flow deposit along Ellis Creek near the head of the fan (TDC photo). ............................. 41
Figure 3.31
Severely damaged house on Ellis Creek fan (TDC photo). ........................................................ 41
Figure 3.32
Sediment-laden flood waters (possible debris flood) travelling down Ellis Creek on 15
December (Courtesy of Braeden Lobb). ..................................................................................... 42
Figure 3.33
Ellis Creek on 16 December (cf. Figure 3.32). Numerous cobbles and boulders have
been deposited along the banks (TDC photo). ........................................................................... 42
Figure 4.1
Scatterplot of percentage hillslope bare ground and catchment area for the 29 study
catchments. Catchments are classified according to debris flow/debris flood occurrence
in the December 2011 storm event. ........................................................................................... 44
Figure 4.2
Scatterplot of Melton Ratio and catchment length for the 29 study catchments.
Catchments are classified according to debris flow/debris flood occurrence in the
December 2011 storm event. ..................................................................................................... 47
Figure 5.1
a) Sedimentary characteristics of debris flows. b) Degree of sorting and imbrication of
clasts. ......................................................................................................................................... 51
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Figure 5.2
a) Paleo-debris flow deposits exposed in the stream in Matenga Rd West catchment. b)
Note the unstratified, poorly sorted nature of the deposits, and the matrix-supported
angular clasts (GNS Science photos). ........................................................................................ 52
Figure 5.3
Exposure in true right bank of Nyhane Drive stream, ~130 m upstream of the fan,
showing 2011 debris-flow deposit overlying an older debris-flow deposit with a
radiocarbon age of 133 ± 17 yr BP (GNS Science photo). ......................................................... 54
Figure 5.4
Exposure in true left bank of Nyhane Drive stream, ~130 m upstream of the fan, showing
~2.5 m of debris-flow deposit, below the present surface, beneath which is ~5 cm of
cemented sands overlying another debris-flow deposit which has been dated at 1319 ±
22 yr BP (GNS Science photo). .................................................................................................. 55
Figure 5.5
Close up of lower debris-flow deposit shown in Figure 5.4. Sample for radiocarbon dating
was taken from yellowish brown soil between clasts (GNS Science photo). .............................. 56
Figure 5.6
True left bank of Nyhane Drive stream, ~230 m from the apex of the fan, showing buried
soil overlain by ~ 2.5 m of gravelly sand grading upwards into brown sands. The buried
soil has a carbon date of 4563 ± 21 yr BP (GNS Science photo). .............................................. 57
Figure 5.7
Location of tank pit and trench on Nyhane Drive fan. ................................................................. 58
Figure 5.8
Tank pit in Nyhane Drive fan. A debris-flow deposit (as yet undated) occurs at the base
of the exposure (TDC photo). ..................................................................................................... 59
Figure 5.9
House at 41 Nyhane Drive, a) boulders excavated from pit for water tank, b) the boulders
used as landscape features (Courtesy of Hagen Jurke). ............................................................ 59
Figure 5.10
a) Steps and b) Weir perched above present stream level in Matenga Rd West
catchment (GNS Science photos). ............................................................................................. 60
Figure 5.11
a) Debris-flow deposit exposed in the gorge of Matenga Rd West catchment, and
radiocarbon dated at 7906 ± 25 yr BP. b) Close up of boxed area in Figure a) (GNS
Science photos).......................................................................................................................... 60
Figure 8.1
Small debris dam, largely consisting of woody debris, blocking a channel in Catchment
8.0 (TDC photo).......................................................................................................................... 69
Figure 8.2
Debris retention structure in Canada, which retains coarse debris (boulders and logs),
while allowing through-flow of finer sediment and flood water. ................................................... 71
Figure 8.3
Debris flow retention net in Japan. ............................................................................................. 72
Figure 8.4
Mitigation works on a steep debris flow fan in Italy. The structure confines the passage of
debris flow and debris flood material across the apex of the fan (high hazard zone). ................ 72
Figure 8.5
Possible sites for debris retention structures in Winter Creek (Catchment 14.2) and Haile
Lane Creek (Catchment 14.4), above the Pohara Valley settlement. ......................................... 73
TABLES
Table 2.1
Maximum 24, 12, 6 and 3 hour rainfalls at Kotinga Bridge rain gauge during the 13-15
December 2011 rain storm. ........................................................................................................ 16
Table 2.2
Rainfall totals in the Takaka area for the 13-15 December 2011 rain storm............................... 16
Table 4.1
Melton Ratios (R), catchment lengths (CL) and occurrence of debris flows and debris
floods for the study catchments. ................................................................................................. 46
Table 5.1
Conventional radiocarbon ages and calibrated calendar year ranges for samples taken
from debris flow deposits, buried soils and tree stumps. ............................................................ 52
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APPENDICES
APPENDIX 1: CATCHMENT ROCK TYPES (FOR CATCHMENT AREA UPSTREAM
OF FAN APEX).........................................................................................................84
APPENDIX 2: CATCHMENT VEGETATION COVER (FOR CATCHMENT AREA
UPSTREAM OF FAN APEX). ...................................................................................86
APPENDIX 3: CATCHMENT BARE GROUND FOR DECEMBER 2011 STORM (FOR
CATCHMENT AREA UPSTREAM OF FAN APEX).................................................. 88
APPENDIX 4: LANDSLIDE BARE GROUND DISTRIBUTION IN RELATION TO
VEGETATION COVER .............................................................................................90
APPENDIX 5: LANDSLIDE BARE GROUND DISTRIBUTION IN RELATION TO
ROCK TYPE .............................................................................................................91
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EXECUTIVE SUMMARY
Between the 13 and 15 December 2011 the Tasman District experienced an extreme rain
storm which caused severe flooding, landsliding and a number of debris flows which affected
homes and properties, and resulted in the declaration of a civil defence emergency. In the
Pohara-Ligar Bay area in eastern Golden Bay, properties in Nyhane Drive and Pohara Valley
were severely affected by debris flows and debris floods. The houses in both these areas are
built on fans which have formed over thousands of years, and will continue to do so through
the deposition of sediment eroded from their upstream catchments. Debris flows and debris
floods are two of the processes that deliver this sediment, and their recurrence intervals are
important for natural hazard risk management. An earlier debris flow event may have
occurred in the historic record, but this has not been verified. Rainfall records since 1932
show that the rainfall that fell in the December 2011 storm was much greater than in any
previously recorded event. The estimated return period for the 24-hour rainfall that generated
the debris flows is ~200 years, which is similar to the interval between two dated debris flow
deposits in the Nyhane Drive catchment. However, this recurrence interval may reduce,
based on NIWA-modelled changes in precipitation for this century (using an IPCC scenario
of a 2°C increase in temperature), which suggest that present 100-year average recurrence
interval 24-hour rainfall totals are projected to increase by around 60-80 mm, and to occur
about twice as often (i.e. become 50-yr events).
A debris flow is a very rapid to extremely rapid flow of water-saturated sediment and debris
that travels down a steep channel. Material is contributed from landslides on the surrounding
hillslopes and from sediment and vegetation scoured from the channel as the debris flow
travels downstream. It is capable of carrying large boulders and logs and can be destructive
when it reaches a fan or other depositional surface. A debris flood is a very rapid surging flow
of water, heavily charged with debris, in a steep channel. A debris flood almost always
occurs as a continuation downstream of a debris flow, but can occur in the absence of a
debris flow. Debris flows were responsible for the worst damage to houses and other
infrastructure in Nyhane Drive and Pohara Valley. This mainly occurred where the streams
flowed onto the upper parts of the fans. Further down the fans, as the larger debris was
deposited and velocities reduced, debris floods continued often to the coast and many
houses and properties were affected by the deposition of finer sediment.
The number of debris flows that occurred during the December 2011 storm in Golden Bay,
and also during a storm in the Tapawera area in 2010 has highlighted a hazard that until now
was either not recognised, or regarded as occurring very rarely in the Tasman District. These
areas are largely underlain by deeply weathered, and highly erodible Separation Point
Granite, a lithology which is reasonably common in the Tasman District. The majority of
these debris flows deposited their sediment onto fan surfaces which are favoured locations
for housing and other development, and it can be expected that there will be increased
pressure for such development in the future.
Of the twenty-nine catchments identified in the Pohara-Ligar Bay area, only some generated
debris flows and debris floods. There are threshold values for catchment length and
catchment ruggedness (measured using catchment relief and catchment area) which, when
combined identify those catchments capable of generating debris flows and those capable of
debris floods. These thresholds are likely specific to catchments dominated by Separation
Point Granite, and require refinement with analysis of further catchments. Such analysis may
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provide a high level or district-wide assessment, however, site investigations are always
necessary where development is proposed.
For the Nyhane Drive and Pohara Valley communities, mitigation options largely involve
engineering works, land use and land management. At Nyhane Drive there are still areas of
the fan that are not built on, and which could be modified to contain debris flow and debris
flood sediment. The existing channel which delivers sediment to the fan could also be
enlarged and possibly lined, and moved slightly eastwards away from existing houses to flow
onto areas of the adjacent fan which have not been built on. The Pohara Valley settlement
occupies the entire fan which is confined between rows of hills, severely limiting mitigation
options on the fan. The stream channels could be enlarged and lined, and culverts
significantly enlarged. Consideration could be given to building debris-retention structures in
the upper reaches of the two largest catchments that drain onto the fan.
A significant amount of large woody debris was deposited on the fans, some of which added
significantly to the structural damage of houses. Logging of the exotic forests in the steep
catchment areas requires careful management to reduce the amount of slash. The optimal
mix of vegetation cover to minimise the volume of sediment and woody debris carried by
debris flows and debris floods requires further research.
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1.0
INTRODUCTION
Between the 13 and 15 December 2011 the Tasman District experienced an extreme rain
storm which caused severe flooding, landsliding and a number of debris flows which affected
homes and properties, and resulted in the declaration of a civil defence emergency. In the
Pohara-Ligar Bay area in eastern Golden Bay, landslides, floods and debris flows inundated
a number of homes and properties, and closed roads including the road to Wainui Bay.
Eleven properties were “red stickered” as unsuitable for occupation and requiring subsequent
evacuation. Of these, four were the result of debris-flow impact, and one the result of a
debris flood (the other six were due to other types of landslides). There was no formal
warning of debris flows, and residents self-evacuated during the event. Many other
properties were affected by debris floods, but without structural damage to buildings. The
Pohara water supply was affected (both the intake and treatment plant), as was the Port
Tarakohe supply, by silting up of the dam. Damage was done to storm water services and
some watercourses changed course.
Over 48 hours, 674 mm of rain was recorded at the Council’s Kotinga rain gauge near
Takaka, with 454 mm in 24 hours. This 48 hour total represents a third of Takaka’s normal
annual rainfall, (average December rainfall is 207 mm), and is more than twice that of any
other 2-day rainfall on record. TDC has estimated that while the rainfall event had an
estimated return period of around 600 years, the debris flows were triggered after about 450
mm in 24 hours, which has an estimated return period of about 200 years. The event was
also unusual in that rainfalls were highest near the coast. Normally the largest totals are seen
at higher altitudes, usually around twice as much as observed at low altitude. In this case, in
the Takaka area the totals at altitude were only 40% of those seen in Takaka itself. There
were two reasons for this; unusually warm and moist air in the lower levels of the
atmosphere, and a relatively low wind speed. The ground was already wet, with 210 mm
having fallen during the previous 10 days.
Landsliding was particularly severe in the hills east of Takaka, where associated debris flows
damaged or destroyed a number of houses on fans in the Pohara Valley and at Nyhane
Drive in the Ligar Bay area. Several residents (Nyhane Drive) report that their property and
houses were hit by debris-charged flood waters (debris flow and or debris flood), early on the
evening of 14 December. Therefore, these debris flows were initiated about 24 hours into the
storm, after about 450mm of rainfall. The debris contained boulders and large logs, which
significantly increased the damage done to dwellings. The logs were derived from the mature
pine forests, and to a lesser extent the native/indigenous vegetation, that cover a large
proportion of the hills above the fans. This has again focused attention on the offsite impacts
of both forest management practices, and forestry land use in general, especially on the
Separation Point Granite lithology that occurs in the district.
While the debris-flow hazard, hitherto was unrecognised, the District Council and community
are now faced with decisions about how to reduce the risk and consequences of future
debris flows for those people already living on these fans. Decisions will also be needed
about whether, or under what conditions, development occurs on a number of other fans,
which as yet have not been extensively built on.
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1.1
PURPOSE OF THE REPORT
The objective of the report is to present an analysis of the December 2011 debris flow event
in the Pohara-Ligar Bay area, and to assess the likely future risks. The purpose is to provide
Tasman District Council and the people of Golden Bay with background information to
enable informed decisions on debris flow hazard-mitigation options, and future land use
around Golden Bay.
1.2
THE BRIEF
This report has been prepared for and funded by Tasman District Council. It has been
prepared by Mike Page, Rob Langridge and Katie Jones from the Institute of Geological &
Nuclear Sciences (GNS Science), in collaboration with Glenn Stevens, Resource Scientist
from Tasman District Council. Mike Page is a Geomorphologist with extensive experience of
erosion processes and sediment transfer, from hillslope to catchment scale, encompassing
both contemporary and geologic timescales. Mike also has wide experience in landslide
hazard mapping and zonation. Rob Langridge is an earthquake geologist with experience in
stratigraphic interpretation and dating of deposits. Katie Jones is a remote sensing and GIS
specialist. The report was reviewed by Mauri McSaveney, who has studied a number of
debris flow events and has contributed to an international state-of-the-art text on debris flow
hazards and their mitigation, and Grant Dellow who is an engineering geologist with
experience in landslide hazard and risk assessment.
1.2.1
Project Objectives
•
Determine the catchment processes that contributed to the initiation of the debris flows.
•
Determine if there is any evidence of past debris flows and determine their frequency of
occurrence.
•
Assess the factors that led to property damage and/or situations where damage was
avoided from the debris flows.
•
Assess the likely response to future rainfall storms.
•
Identify the rainfall, geological and topographical conditions that could result in debris
flows in the wider vicinity and hence identify areas where there is significant risk from
debris flows.
•
Recommend possible mitigation options to minimise the risk from debris flows for these
locations in the future.
1.2.2
Scope of Services/Project Design
1.
Detailed aerial photo interpretation and geomorphic mapping of the debris flows and
associated flooding to delineate source areas from debris flow and flood deposits in the
Ligar Bay and Pohara Valley Rd area. Inspection for geomorphic evidence of past
debris flows;
2.
A fieldtrip to inspect the debris-flow deposits to infer the processes involved, inspect
catchment conditions, and to collect samples of datable material to use to determine
the past frequency of these events;
3.
Analysis of the samples to provide dates for past debris flow events.
4.
Analysis and review of catchment conditions that led to the initiation of the debris flows,
including:
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a.
Catchment geology and geomorphology and their effects on headwater slope
stability;
b.
Land cover and effect of vegetation on stability and erosion potential;
c.
Storm rainfall
d.
Sediment transport processes active during the storm;
e.
Possible damming of debris in streams;
f.
Channel erosion.
5.
GIS analysis to identify catchments in the area between Tata Beach and Ellis Creek
(Clifton) with a similar assemblage of catchment conditions as the affected area, and
where a similar elevated debris flow hazard might exist (see Figure 1.1 for extent);
6.
Assessment of the factors that led to property damage and/or situations where damage
was avoided from the debris flows;
7.
Assessment of the likely response of the affected catchments in the study area to
future heavy rainfalls for the following time scales:
8.
a.
The immediate term, in relation to sedimentation in the catchments and recently
deposited material in the channels, and its vulnerability to movement downstream
in rain and associated runoff events expected at least several times a year;
b.
The short term (up to ten years), i.e. significant but relatively common events;
c.
The long term (up to 1,000 years), i.e. low probability extreme events.
Recommendation of options to manage the risk and mitigate the damage caused by
debris flows in the future.
1.2.3
•
•
1.2.4
Deliverables
A report presenting:
˗
an analysis of the geological and runoff processes during the extremely intense
rain of the December 2011 storm in the Ligar Bay and Pohara areas of Golden Bay
˗
recommendations to manage future risks and the consequences of debris flows
in the study area.
GIS layers and maps of the following:
˗
the extent of debris flows and associated flooding
˗
detailed geomorphic mapping of the debris flows, differentiating source and
deposition zones
Contributions from TDC
1.
Orthorectified aerial photo coverage of the areas of interest (Golden Bay) in both
electronic and hard-copy form
2.
Photos (and their locations) of the debris flows and associated damage taken
immediately following the event.
3.
Pre- and post- event LiDAR of the affected areas.
4.
Rainfall and hydrological data
Delivery: 30 November 2012
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Figure 1.1
Extent of study area and location of main debris-flow sites at Pohara Valley and Ligar Bay (Nyhane
Drive). Aerial photography, taken in January 2012, shows extent of December 2011 landslides and debris
flows/floods.
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1.3
TERMINOLOGY
Debris flow is a term used by natural hazard specialists to describe a particular type of
landslide. However, the general public, especially those who have experienced debris flows,
often refer to them as “floods” or “flash floods” because of the speed at which the sedimentladen water travels, both within a stream channel and also once it overflows onto a fan
surface. However, the behaviour and composition of a debris flow is very different from that
of a conventional flood. Debris flows only occur under specific catchment and rainfall
conditions, and involve a number of different processes. They are generally restricted to
small, steep catchments where slope sediment and channel sediment are plentiful. For the
same amount of rain, a debris flow has a much higher discharge than a flood, it contains
more and often larger rock debris, and it moves faster. As a consequence debris flows are
far more dangerous.
A debris flow usually commences in a short, steep stream when a landslide or a number of
landslides are initiated during a period of intense (or prolonged) rainfall. Failure slopes are
generally between 25°-40°. These initiating slides are often only a few tens of cubic metres in
volume each, but may coalesce to form larger debris avalanches. The debris enters a steep,
confined channel, and as it moves rapidly downstream its volume increases due to scouring
of sediment stored in the bed and along the banks of the stream. A debris flow can also be
initiated by the blockage of a channel, often by a landslide or woody debris, and build-up of
water followed by a sudden breach or burst of the dam. A number of smaller debris flows in
tributary channels may coalesce to form a larger debris flow in the main channel. A debris
flow commonly moves in distinct surges or slugs of debris, separated by watery inter-surge
flows. A debris flow event may consist of one surge or many tens of surges. Surging arises
for a number of reasons: some result from flow instability caused by longitudinal sorting of
the debris flow material. Such surges are characterised by boulder fronts (boulders and other
large (woody) debris). The main body of the surge is a finer mass of liquefied debris, and the
tail is a dilute, turbulent flow of sediment-charged water, similar to a debris flood (see below).
The debris flow typically deposits this fast moving sediment onto a colluvial/debris fan, which
has developed at the base of the steep catchment where the slope reduces and the stream
becomes unconfined. It is through the accumulation of sediment from floods and debris flows
over thousands of years that these fans have formed. The coarsest debris is deposited near
the head of the fan, so that towards the distal end of the fan, sediment concentration, particle
size and flow velocity have reduced, and the process is then referred to as a debris flood.
Debris floods are not as damaging as debris flows as they do not carry such large debris.
Because of the often massive deposition at the head of the fan, successive pulses of debris
flows are readily diverted by the deposits of earlier pulses, making the direction of flow of a
debris flow on a fan very unpredictable.
The technical definitions of the various terms used in this report are taken from Hungr (2005):
•
A debris avalanche is a very rapid to extremely rapid (5-~20 m/s, 15-60 km/hr), shallow
slide or flow of partially or fully water-saturated debris on a steep slope, which is not
confined within an established channel.
•
A debris flow is a very rapid to extremely rapid (5-10 m/s, 15-30 km/hr) flow of watersaturated, non-plastic (granular) debris in a steep channel. Speeds are often faster
than a fit human can run. The sediment has a consistency of wet concrete, with
sediment concentrations often in excess of 60% by volume (80% by weight) compared
to flood waters, where sediment concentrations are generally <4% by volume (10% by
weight).
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•
A debris flood is a very rapid (up to ~5 m/s), surging flow of water, heavily charged with
debris, in a steep channel. A debris flood almost always occurs as a continuation
downstream of a debris flow, but can occur in the absence of a debris flow.
The term debris flow is also used to describe the entire phenomenon from initiating landslide
on a steep slope, the rapid flow along a confined channel, and the deposition on a debris fan
(Hungr 2005). There is almost always a debris flood as a continuation downstream of a
debris flow, and it is usual to extend the term debris flow to include the associated debris
flood when referring to the entire event. But it should be noted that a debris flood can occur
in the absence of a debris flow. When a debris flood occurs without an associated debris
flow, the distinction between the two is usually easiest made on the basis of peak discharge
during an event. Peak discharge during a debris flood is limited to at most 2-3 times that of a
major flood as it results in relatively low flow depths. On the other hand, debris flows produce
extremely large peak discharges by eroding and incorporating sediment from the stream’s
bed and banks as well as the stream’s water as it surges down the channel at a faster speed
than the flooded stream can flow. Peak discharges from a debris flow can be as much as 50
times as large as those of a major flood. Their destructive potential therefore, is much greater
than that of a flood.
The colluvial/debris fans onto which the debris is deposited are sites favoured for
development because they are gently sloping, above flood range of major rivers, well drained
and afford good views. However, the capacity for transporting large boulders and large
quantities of woody debris, together with the velocity at which they travel, make debris flows
highly destructive, to both buildings and people. Yet debris flow risks on fans are often under
estimated because debris flows tend to occur infrequently in any given catchment (due to
need for extreme rainfall and an abundant sediment source), and so may not feature in local
knowledge. Following the removal of significant amounts of slope and channel sediment
during a debris flow event, it may take 10s to 100s of years for this sediment to be replaced.
1.4
PREVIOUSLY STUDIED DEBRIS FLOW EVENTS IN NEW ZEALAND
As mentioned above, debris flows do not occur that frequently, and are not well recognised in
New Zealand. However, several debris flow events have been documented and studied in
the last 30-40 years.
1.4.1
Tapawera, Motueka
A debris flow event occurred in the Tapawera area of the Tasman District on 16 May 2010.
As in the December 2011 Pohara-Ligar Bay event, a localised cell of very high intensity rain
caused extensive landsliding and associated debris flows in an area of c. 8000 ha centred on
the lower Wangapeka and Baton Rivers and extending downstream as far as Woodstock.
The following summary of the event is taken from Basher (2010).
Antecedent soil moisture was high prior to 16 May, with 70-100 mm of rain recorded over the
previous four days. A further 120-130 mm were recorded on 16th, giving a total storm rainfall
of c. 200 mm. Short-duration rainfalls at the peak of the storm were very high at 40-70 mm
h-1. The recurrence interval for rainfalls up to 6 hours duration exceeded 50 years in the
Wangapeka catchment. The most intense rain fell on an area of hill country that, as at
Pohara-Ligar Bay, is underlain by deeply weathered, highly erodible Separation Point
Granite, and largely planted in plantation forest. Much of the forest here however, had been
recently harvested. The event triggered >1000 shallow landslides which combined with logs,
unrecovered windthrow, logging slash and flood waters to produce debris avalanches, debris
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flows and debris floods. In a study of 1800 ha of the worst affected land, Jelinek (2010)
reported that the landslides were generally small (<20-50 m3), shallow (<0.5m) failures with
long debris tails of highly fluid material, on slopes >25°. Debris flows and debris floods
deeply scoured many ephemeral stream channels, and as much sediment may have been
generated from channel scour as from the landslides. Road and landing failures caused c.
21% of the total sediment generated, but only approximately half of this (10% of the total)
was delivered to gullies and streams. Landslides were most common on recently clear-felled
slopes (<2-5 years), but were also common in mature pine trees with extensive windthrow
canopy gaps. Mid rotation forest (8-25 years), and native forest were least affected by
landsliding. Pasture areas were least affected, primarily because they were on less steep
slopes. Landslide density ranged from 1.9/ha for slopes cleared in the last 2 years to no
landslides in 9-25 year plantations. Sediment generation from landslides was 7 m3/ha, rising
to 9 m3/ha including landslides, and road and landing failures associated with forestry.
The debris flows and debris floods caused flooding and inundation of a number of homes
with debris-laden sediment. Given the rural nature of the locality, these were individual
dwellings, with no subdivisions or concentration of housing. The severity of the event was a
result of a combination of three factors: high and intense rainfall occurring on highly erodible
Separation Point Granite, much of which had been recently clear felled of pine tree following
extensive wind-throw.
1.4.2
Matata
On 18 May 2005, several large debris flows occurred at Matata in the Bay of Plenty. They
were caused by intense rain that triggered possibly hundreds of landslides (debris
avalanches) in the steep erodible catchments behind the town. The storm rainfall totalled 341
mm in 48 hours, with a peak intensity of 30.5 mm in 15 minutes. This equated to about a
500-year recurrence event. The many landslides coalesced in the channels, forming debris
flows. Almost all of the minor channels feeding into tributaries to the main channel carried
debris flows. These then coalesced in the main channel into one or a few large surging flows.
There was no evidence for existing (pre-storm) debris dams having initiated debris flows.
However, channels may have been temporarily dammed by landslides occurring on slopes
destabilised by the passage of the initial debris flows.
Matata is situated on a series of fans that have built over the last 7000 years from sediment
eroded from the steep, forested catchments behind the town. The catchments are underlain
by welded ignimbrite, sandstone/siltstone and andesite, which yield large boulders. The
debris flows and associated debris floods destroyed 27 homes and damaged a further 87
properties. SH2 and the railway line were blocked with debris for many days, road and rail
bridges were damaged, and the total cost was in excess of $20 million. Fortunately no lives
were lost. Historical records indicate that probably four smaller debris flows have occurred
since 1860, and there is geomorphic evidence that equally as large and larger debris flows
have occurred over the last 7000 years.
Recommendations to mitigate future debris flow hazards included sediment detention basins,
debris flood diversion structures, larger size and improved design and alignment of culverts,
and removal of certain dwellings.
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1.4.3
Te Aroha
In 1985 the town of Te Aroha, situated on a fan on the north-western edge of the Kaimai
Range, was affected by a debris flow(s) which killed three people and damaged 50 homes.
While the 24-hour rainfall total of between 150-300 mm was not exceptional, rainfalls in
excess of 100 mm h -1 were.
1.4.4
Stony Creek, Westland
Franz Josef village is a tourist destination located at the apex of an alluvial fan where Stony
Creek flows out of the Southern Alps. Despite it being recognised as a potential debris flow
site (de Scally and Owens 2004), field investigations reporting no evidence of debris flows
and a lack of recent debris flows, has led to significant development. Large boulders exposed
during subsequent excavations have since confirmed the potential for debris flows (Welsh
and Davies 2010).
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2.0
CONDITIONS CONTRIBUTING TO THE 2011 POHARA-LIGAR BAY
DEBRIS FLOWS
2.1
GEOLOGY
The geology of the study area is shown in Figure 2.1. Eight lithological groups are
recognised (Rattenbury et al. 1998). The lithology by catchment is listed in Appendix 1.
2.1.1
Fan deposits (Q2a) (Quaternary)
Fans formed from alluvial and colluvial deposits occur at the base of all the catchments
draining the steep range between Tata Bay and Pohara. They comprise poorly sorted,
angular to rounded, silt- to boulder-sized clasts which are occasionally stratified. They have
well drained, gently sloping, elevated surfaces, making them attractive sites for housing. To
date, significant subdivision development has occurred on two such fans.
2.1.2
Limestone (On) (Oligocene), siltstone (Mb) (Miocene) and sandstone (Eb)
(Eocene)
These rocks form the low hills between Ligar Bay and Pohara. The light grey- brown
siltstones are often well bedded. Although soft and weak, they are not a major source of
sediment due to their relatively low relief. The limestone forms prominent ridges and bluffs
between the siltstones, and was quarried to provide material for the now closed Tarakohe
Cement Works. Several small areas of “Brunner Coal Measures” occur among these low
hills, and comprise sandstone and carbonaceous mudstone with scattered coal seams up to
2.5 m thick.
2.1.3
Separation Point Granite (Ksg) and Granodiorite (Ksd) (Cretaceous)
Separation Point Granite underlies the steep range that runs northeast-southwest from Tata
Bay to Pohara. These plutonic rocks are deeply-weathered and highly erodible. This was the
area that received the highest rainfall during the December 2011 storm, and was the source
of most of the debris flow material deposited on the fans. A small area of granodiorite occurs
in the two southern-most catchments.
2.1.4
Limestone and calcareous mudstone (Oma) (Cambrian-Devonian)
A very small area of hard black limestone and calcareous mudstone (Arthur Marble 2) crops
out on the southern side of the Ellis Creek catchment at the southern end of the study area.
2.1.5
Pikikiruna Schist (Omp) (Late Cambrian-Devonian)
A small area of schist occurs in the two southern-most catchments.
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2.1.6
Faults and earthquakes
Geologic faults in the Takaka area separate major geologic units of different age. These
faults include the Pikikiruna, Pisgah, Golden Bay and Wakamarama faults (Rattenbury et al.,
1998). All these faults juxtapose Miocene rocks against Cretaceous or older basement rocks,
thus they were important as basin-bounding faults during the Tertiary. The major fault in the
area is the Pikikiruna Fault which separates Separation Point Granite from the younger
Tertiary and Quaternary fan sediments. No active faults have been identified locally or are
currently recorded in the GNS Science Active faults database (http://data.gns.cri.nz/af/).
Large historic earthquakes are known within and adjacent to the Tasman region. The largest
events include the M 7.4? Collingwood earthquake and the M 7.8 Murchison earthquake.
The Collingwood earthquake (1868) was centred near Farewell Spit and
has been assigned to the Wakamarama Fault, which may therefore be active
(Anderson et al., 1994). This earthquake caused damage at Tata Beach
(http://info.geonet.org.nz/display/quake/Historical+Earthquakes). The Murchison earthquake
(1929) is reported as being ‘heavily damaging’ at Takaka and Tarakohe. An engineer at the
Tarakohe Cement Works was killed in this event when a limestone boulder was dislodged
from the bluff above the quarry. Despite these events, Takaka occurs in an area of
moderately low seismic hazard according to the NZ National Seismic hazard model (Stirling
et al., 2012). No earthquakes were reported prior to the December 2011 storm which could
have contributed to the erosion and subsequent debris flows and debris floods that occurred.
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Figure 2.1
Geological Map of the study area, showing the distribution of rock units/main lithological groupings
and major faults. From QMap 1:250 000 Geological Map 9 Nelson (Rattenbury et al. 1998). Outline of study
catchments and catchment numbers are shown (see sections 2.2 and 3.0).
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2.2
GEOMORPHOLOGY
The study area comprises a series of catchments that drain the northeast-southwest trending
range of hills between Pohara and Tata Beach on the eastern side of Golden Bay. Twentynine catchments have been identified (numbered from north to south), most of which have
been given local names for easier identification. Where appropriate, the catchments have
been subdivided into hillslope, valley floor and fan components. Six catchments have been
further subdivided into subcatchments where there are significant tributaries that drain
separately onto the same fan (Figure 2.2).
The catchments are small (14-500 ha), steep and with incised streams. The hillslope portions
of the catchments typically have a width:length ratios of 1:3. The steep slopes of the
northern-most ten catchments are underlain by weathered granite, while the remainder also
have varying proportions of limestone, sandstone, siltstone, and schist, especially in the
lower parts of their catchments. These lithologies, when eroded, supply large boulders to
streams. The weathered granite is the source of large quantities of sand and gravel that have
in-filled the narrow valley floors that extend upstream of the fans. This sand and gravel is
also being deposited in the estuary behind Tata Beach. All catchments have developed
gently sloping fans where their streams become unconfined as they exit the steep upper
parts of their catchments. In some instances fans from several catchments have coalesced
into one fan complex. In three cases (catchments 12, 14, 16) the fans are narrow and
elongate, having been confined between low hills of limestone and/or sandstone/mudstone.
The streams that flow across these fans have typically incised only 1-3 m into the fan
surface.
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Figure 2.2
Location of catchments and fans.
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2.3
LAND USE AND SETTLEMENT HISTORY
Prior to European arrival, Māori settlements occurred at Tata Beach, Ligar Bay, and Pohara.
In the late 1800s, European settlers began burning and logging the indigenous vegetation
(Podocarp-Broadleaved forest and scrub), and pastoral farming was soon established. Much
of the following information was provided by Alan Swafford who has farmed in the Ligar Bay
area since the 1950s.
After initial success, low soil fertility and economic conditions led, by the 1930s, to
progressive abandonment of the steep hills, which soon reverted to scrub (Figure 2.3). From
1908-1988 the Golden Bay Cement Works operated at Tarakohe, where a limestone quarry
was established. Many of the company workers and their families lived in Pohara Valley,
which is today the largest settlement on the fans affected by the December 2011 debris
flows. In addition to the Pohara Valley settlement, there is a recently established subdivision
at Nyhane Drive on a fan east of the cement works at Ligar Bay. The Golden Bay Cement
Company bought the steep hills in the 1950s and by the 1960s approximately half the area
had reverted to manuka scrub. In 1983 the cement company burnt the scrub near Tarakohe
and established pine plantations on the hills. Many of these plantations are yet to be
harvested and now have several different owners. Some trees have been harvested (mostly
between 2006 and 2010); above Falconer Rd (Pohara Valley) in Winter Creek, above
Tarakohe Quarry, above Matenga Rd, and above Nyhane Drive.
The vegetation cover (indigenous forest, exotic forest, scrub and pasture) of the study area
at the time of the December 2011 storm was digitised from the orthorectified aerial
photography, and is shown in Figure 2.4. Vegetation cover distribution on a catchment basis
is given in Appendix 2.
Figure 2.3
Matenga Rd area in 1911 showing steep hills in pasture with some areas of scrub. Note old road to
Wainui zigzagging up the hills to the ridge on the sky line (Courtesy of Alan Swafford).
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Figure 2.4
Vegetation cover map of study area in 2011.
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2.4
DECEMBER 2011 STORM
The following information is taken from a Hydrology Summary for the event, compiled by
TDC (Tasman District Council 2012), and subsequent analysis of the rainfall data.
The storm was unusual in that rainfall was highest near the coast. Normally the largest totals
are seen at higher altitudes, usually around twice as much as observed at low altitude. In this
case in the Takaka area, the totals at altitude were only 40% of those seen in Takaka itself.
At TDC’s Kotinga rain gauge by Takaka township, one third of the normal annual rainfall fell
in two days. The maximum 24-hour total was 454 mm, and the maximum 48-hour total was
674 mm. These totals were verified by a manually read check gauge, and compared closely
to the 48-hour totals at the information centre and at a private weather station at Paynes
Ford, which recorded 609mm and 660 mm respectively. Table 2.1 lists the maximum 24, 12,
6, and 3 hour rainfalls at the Kotinga rain gauge.
Table 2.1
Maximum 24, 12, 6 and 3 hour rainfalls at Kotinga Bridge rain gauge during the 13-15 December
2011 rain storm.
24 hours from 19:48 on 13/12/2011 to 19:48 on 14/12/2011
454 mm
12 hours from 7:45 on 14/12/2011 to 19:45 on 14/12/2011
296 mm
6 hours from 10:39 on 14/12/2011 to 16:39 on 14/12/2011
178 mm
3 hours from 13:51 on 14/12/2011 to 16:51 on 14/12/2011
106 mm
Readings from a number of gauges in the area illustrate the rainfall gradient away from the
coast (Table 2.2, Figure 2.5). The 48-hour total at Kotinga is significantly more than anything
previously recorded in that locality over the past 50 years (Figure 2.6), and is estimated as
likely to occur only around once every 600 years.
Table 2.2
Rainfall totals in the Takaka area for the 13-15 December 2011 rain storm.
Distance from
the coast (km)
48 hour
total (mm)
Pohara coast
0.1
405
Rangihaeata Heads
0.5
352
Pohara hill
0.9
470
Puramahoi
2
508
Motupipi
3
529
Kotinga
5
674
Paynes Ford
6
660
Anatoki Valley
10
531
Waingaro Valley
13
410
Little Devil
20
247
Location
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Figure 2.5
Isohyet map of 48 hour rainfall for 13-15 December 2011 rain storm.
Figure 2.6
Histogram of largest storm rainfalls in past 50 years.
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The reasons for the coastal nature of the extreme rainfall were; i) airflow during the storm
was particularly warm and moist at lower levels, and this moisture was available for
conversion to rain immediately as the airstream started to rise and cool over the foothills; ii)
low wind speed.
The main Takaka River did not even reach an annual flood level because of the relatively low
(<100mm) rainfalls over much of its catchment. Even the coastal streams did not experience
exceptionally high flows, because the high rainfall totals were the result of steady rainfall
without periods of high intensity. Where coastal streams did cause flooding it was on account
of being choked with silt, rocks and logs.
Fifteen minute rainfall data from the Takaka@Kotinga site during the debris flow event storm
have been analysed as moving rainfall totals for 1hr, 2hr, 6hr, 12hr, 24hr, and 48hr periods
(Figure 2.7). That is, a 6-hour moving total time stamped at 12:15 is the total rainfall from
06:15 to 12:15, and the 12:30 6-hour moving total is the total for the period 06:30 to 12:30.
The time series for each period was then related to the return period for that duration
(Figure 2.8). That is, for the 48-hour rainfall plot, a point on the graph shows the return period
for the rainfall falling in the 48 hours that ends at the point in question.
The return period is taken from HIRDS (http://hirds.niwa.co.nz/). As HIRDS only gives return
periods up to 100 years, values were extrapolated for longer return periods.
Given that the debris flows were reported to have occurred on the afternoon/early evening of
the 14 December, the 24-hour rainfall of ~450 mm is particularly significant, and has a return
period of ~200 years. Also the 1 hour rainfall intensity peaked between 15:15 and 16:15 at
35.3 mm. Interestingly, the 24-hour rainfall stayed at around a 200-year average recurrence
interval (ARI) for the next 12 hours or so. This is consistent with reports of pulses or waves of
debris flows continuing through the night.
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Figure 2.7
Moving rainfall totals (at 15 minute intervals) for given time periods (1hr, 2hr, 6hr, 12hr, 24hr, 48hr)
plotted against 1 hour actual rainfall, for the December 2011 storm event at the Kotinga rain gauge site.
Figure 2.8
Moving rainfall return periods (at 15 minute intervals) for given time periods (1hr, 2hr, 6hr, 12hr,
24hr, 48hr) plotted against 1 hour actual rainfall, for the December 2011 storm event at the Kotinga rain gauge
site.
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3.0
NATURE OF THE DEBRIS FLOWS AND RESULTING DAMAGE
For description and analysis purposes the catchments in the study area have been
numbered from north to south, together with a locally recognisable name if available.
Catchments have been subdivided into subcatchments where tributaries flow onto a fan at
different locations, and subcatchments are then numbered as 1.1, 1.2, 1.3 etc. (Figure 2.1
and Figure 2.2). Where present, the areas of valley floor and fan for each
catchments/subcatchments were mapped. The valley floor is defined as the area upstream of
the fan, and confined between hills, where sediment has accumulated along the channel.
The fan is immediately downstream of the valley floor where the channel becomes
unconfined. The following is an overview of the December storm erosion and fan deposition
processes (debris flow/debris flood) on a catchment/subcatchment basis.
Catchment 1.1 (Tata Beach)
This is a catchment with regenerating forest at the northern end of the study area that drains
onto a fan on which part of Tata Beach settlement is located. Eleven landslides occurred
within the regenerating forest. Three of these were associated with the road to Wainui Bay
(Abel Tasman Drive). No debris flows of debris floods were generated.
Catchment 1.2
This catchment joins catchment 1.1 just above Tata Beach settlement. Six landslides
occurred within the regenerating forest, five of which were associated with the road. No
debris flows or debris floods were generated.
Catchment 1.3
This very small catchment also drains onto the Tata Beach fan. One landslide occurred
within the regenerating forest, and was associated with the road. No debris flows or debris
floods were generated.
Catchment 2.0
This catchment also drains onto the Tata Beach fan. Several landslides occurred within
regenerating forest/scrub in the lower part of the catchment, and also in the pine forest at the
head of the catchment. No debris flows or debris floods were generated.
Catchments 3.0 (Clemens), 4.0, 5.0, 6.0
These catchments are of similar size, and mainly planted in pine forest. Aerial photography
taken in 1952 shows that the valley floors were infilled with sediment, which would have
taken many years to accumulate. These valley floors were stable at the time of the
December 2011 storm, with 20-30 year old pine trees present. In each catchment several
large landslides occurred within the pine forest on the hill slopes, likely generating debris
flows in the steep, upper catchments and scouring sediment along the channels (Figure 3.1).
The debris flows spread onto the valley floors, depositing sediment and woody debris
(Figure 3.2). It is uncertain to what extent the stream channels had incised into the valley
floors prior to the December storm, but by the time the authors visited six months later the
channels had incised about 1-2 m (Figure 3.3). Where the channels reached the apex of the
fans, the debris flow transformed into debris floods. The deposits on the fans have a high
percentage of coarse sand to fine gravel, and some woody debris which appears to have
floated on the sediment-laden flood water. The debris-flood deposits extend down the fans to
the coastal margin where channels have been infilled. In many places sediment has
overwhelmed culverts and banked up behind Abel Tasman Drive (Figure 3.4).
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Figure 3.1
Channel in upper reaches of Catchment 4.0. Note the channel is steep, with a small width to depth
ratio, and has been has been scoured to bedrock. The channel has a roughly U-shaped form typical of repeated
debris flows (TDC photo).
Figure 3.2
photo).
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Woody debris deposited along the valley floor just upstream of the fan in Catchment 4.0 (TDC
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Figure 3.3
Incision of previously in-filled valley floor of Catchment 4.0 (upstream of Figure 3.2) (TDC photo).
Figure 3.4
Sediment and woody debris deposited by a debris flood onto the fan associated with Catchment
4.0. Abel Tasman Drive is in lower left foreground (Courtesy of Tim Cuff).
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Catchments 7.1, 7.2
These two small catchments are mainly planted in pine forest. They each have several
landslides, and in the case of catchment 7.1 sediment spread onto the valley floor.
Catchments 7.3 (Ligar Bay Villas), 8.0, 9.1
Catchments 7.3 and 8 are planted in pine forest. In catchment 9.1 the upper two thirds are
planted in pine forest, while the lower third of the catchment had been logged in late
2010/early 2011, and was in grass and low shrubs with some slash from logging at the time
of the storm. All three catchments had several large landslides which generated debris flows
(Figure 3.5, Figure 3.6, Figure 3.7), which in the case of 7.3 and 8.0 extended onto the upper
part of the fans and may have extended further. Debris-flood deposits extended to the coast,
and sediment overwhelmed the culverts and banked up behind Abel Tasman Drive
(Figure 3.8, Figure 3.9, Figure 3.10). Much woody debris appears to have floated on the
sediment-laden flood water and been deposited on the fans with the coarse sand and gravel.
All three catchments have infilled valley floors which supplied sediment to the debris flows as
they travelled down the channels. In catchment 7.3 a new channel was formed away from
the existing channel at the top of the fan. Subsequent work was undertaken to return the flow
to the original channel. Catchment 9.1 is a little smaller than the other two, and while the
debris flood extended some distance down the fan, it was supplemented by a debris flood
spilling over from an adjacent catchment (10.1). The volume of sediment and woody debris
from catchment 9.1 may also have been increased by a higher incidence of landsliding in the
area of recently harvested forest (Figure 3.7).
Figure 3.5
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Landslide in pine forest in Catchment 8.0 (TDC photo).
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Figure 3.6
Channel in upper reach of Catchment 8.0 (TDC photo).
Figure 3.7
Landslides on lower hillslopes logged in late 2010/early 2011 in Catchment 9.1 (TDC photo).
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Figure 3.8
Looking down fan of Catchment 8.0. Sediment deposited by debris flood (TDC photo).
Figure 3.9
Looking up fan of Catchment 8.0 from Abel Tasman Drive. Distal end of debris-flood deposit
(TDC photo).
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Figure 3.10 Sediment deposited by a debris flood onto the lower part of the fan associated with Ligar Bay Villas
catchment (7.3). Note sediment from this and other nearby catchments has entered the estuary behind Tata
Beach (Courtesy of Tim Cuff).
Catchment 9.2
This is a small catchment of regenerating forest and planted forest which drains onto the
same fan as catchment 9.1. No landslides occurred during the storm.
Catchment 10.1 (Nyhane Drive)
This is the third largest catchment in the study area. At the time of the storm, the upper third
of the catchment was in pasture with scrub, the middle third in regenerating forest, and the
lower third in pine forest (hill slopes on the true left of the stream had been logged in late
2010/early 2011). Landslides occurred throughout the catchment, generating debris flows in
several tributaries (Figure 3.11, Figure 3.12, Figure 3.13). These coalesced in the main
channel and a large debris flow carrying a large amount of woody debris (Figure 3.14)
spread out onto the fan, severely damaging the first two houses and rendering them
uninhabitable. The debris flow appears not to have made it past these houses, but a debris
flood extended across much of the fan and extended down to the coast (Figure 3.15,
Figure 3.16, Figure 3.17, Figure 3.18, Figure 3.19). The debris flood also flowed across the
fan and onto the adjacent fan of catchment 9.1. Numerous properties were affected by
floodwater and sediment, although no significant structural damage was done to houses.
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Figure 3.11 Large landslide in regenerating forest in Catchment 10.1 (Nyhane Drive). Landslide occurred
across Old Wainui Hill Rd (see Figure 2.3) (TDC photo).
Figure 3.12 Main channel of Catchment 10.1, ~270 m downstream of Figure 3.11. Note large boulder and
channel eroded to bedrock (TDC photo).
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Figure 3.13a Main channel of Catchment 10.1, ~320 m downstream of Figure 3.12. Arrow points to deposit
shown in Figure 3.13b (TDC photo).
Figure 3.13b Enlargement of possible debris-flow deposit shown Figure 3.13a (TDC photo).
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Figure 3.14
Woody debris deposited at the head of the Nyhane Drive fan (TDC photo).
Figure 3.15 Nyhane Drive subdivision showing the path of the debris flood down the fan to the coast. Note a
branch of the debris flood has spread across the fan to link up with a debris flood from the adjacent catchment
(Courtesy of Braeden Lobb).
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Figure 3.16 Upper part (apex) of Nyhane Drive fan (see Figure 3.15) showing houses impacted by a debris
flow. Note large amount of woody debris (Courtesy of Tim Cuff).
Figure 3.17 The two uppermost houses on the Nyhane Drive fan (see Figure 3.16) showing partial burial by
sediment deposited by the debris flow. These houses were subsequently condemned (Courtesy of Tim Cuff).
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Figure 3.18 Debris-flow deposit surrounding house shown in Figure 3.17. Note shed has been moved by debris
flow (TDC photo).
Figure 3.19
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Close up of house at right of Figure 3.17, showing impact of woody debris (TDC photo).
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Catchment 10.2 (Lower Nyhane Drive)
Catchment 10.2 is a small catchment which drains onto the lower part of the same fan as
catchment 10.1, where the majority of the Nyhane Drive houses are situated. The catchment
has a mix of vegetation, and the only landslide was associated with a track. No debris flows
or debris floods were generated.
Catchments 11.1 (Matenga Rd East), 11.2 (Matenga Rd Middle)
Both these catchments are relatively small. Prior to 2009 pine forest covered catchment 11.1
and the upper third of catchment 11.2. At the time of the storm both were mainly in pasture
and scrub, with only small areas of pine forest. Landslides occurred over significant areas of
the steep upper catchments, and debris floods and possibly small debris flows reached the
top of the fan.
Catchment 11.3 (Matenga Rd West)
Prior to 2009 the upper half of this catchment was in pine forest, but at the time of the storm
was mainly in reverting scrub. A large number of landslides occurred (Figure 3.20), and a
debris flow crossed Nyhane Drive West (Figure 3.21) and flowed through a section of gorge
and onto Matenga Rd. Beyond this point it became a debris flood and joined with sediment
from catchment 11.2 and travelled down the fan and also along drains to houses near the
coast.
Figure 3.20 One of many landslides in Matenga Rd West catchment which supplied sediment and debris to the
stream (TDC photo).
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Figure 3.21 Debris-flow deposit across Nyhane Drive West. Note small diameter culvert under road. Below this
point the debris flow entered a narrow gorge before flowing onto Matenga Rd (TDC photo).
Catchment 12.0 (Quarry)
This is a long narrow catchment which drains into the limestone quarry that was part of the
Golden Bay Cement Works. In the lower part of the catchment the stream flows between
limestone and calcareous siltstone ridges along a narrow sediment-infilled valley floor. Prior
to 2009 the middle third of the catchment was planted in pine forest, but had been logged at
the time of the storm and vegetation cover was a mix of scrub and pasture. A debris flow was
generated by the numerous landslides, which then travelled down the valley and into the
quarry. This debris flow was possibly exacerbated by the failure of an old (but still intact)
forestry road where it crossed the stream. Reportedly the road was built up to 2-3 m above a
culvert of only 300 mm diameter. Presumably the culvert blocked and water and debris built
up behind the road with the structure subsequently failing. Almost directly downstream was a
skid site, and the debris flow eroded the side of the skid, carrying away forestry slash.
Catchments 14.1 (Pohara Valley Rd), 14.3 (Haile Lane)
These catchments each consist of several small tributaries that drain onto the head of a fan.
They are not particularly steep, and are mainly in scrub and indigenous forest, with only a
few landslides. The upper, steeper part of catchment 14.3 appears to drain into a sinkhole,
and where this then drains to is uncertain. No debris flows or debris floods were generated.
Catchment 14.2 (Winter Creek)
Numerous landslides occurred in the wide upper catchment during the storm (Figure 3.22). A
debris flow was generated which travelled down Winter Creek (Figure 3.23) which passes
between a limestone ridge and then down a narrow valley along which Falconer Rd is
located, before reaching the Pohara Valley fan on which Pohara Valley settlement is built.
The debris flow reached and severely damaged a house at 64 Pohara Valley Rd (Udell
property) (Figure 3.24, Figure 3.25). Beyond this property a debris flood continued down the
road and across a number of properties to the coast. At the time of the storm, the upper
catchment had a mix of scrub, pasture and regenerating forest, with a few remnants of a pine
plantation which had been logged around 2008/2009.
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Figure 3.22
Landslides on the steep hillslopes in Catchment 14.2 (Winter Creek) (Courtesy of Braeden Lobb).
Figure 3.23 Section of Winter Creek scoured by a debris flow as it passed downstream. Falconer Rd at bottom
left (Courtesy of Tim Cuff).
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Figure 3.24
Debris flow deposit surrounding property at 64 Pohara Valley Rd (TDC photo).
Figure 3.25
Accumulation of woody debris at 64 Pohara Valley Rd (TDC photo).
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Catchment 14.4 (Haile Lane Creek)
This is the second largest catchment, and like catchment 14.2 it has a wide upper catchment
where most of the landsliding occurred, and the catchment then narrows as the stream
passes between the limestone ridge and down a narrow valley along which Haile Lane is
located, before reaching Pohara Valley settlement. A debris flow was generated which at
least reached 14 Haile Lane (Heath property) and the adjacent water treatment plant
(Figure 3.26). Not far below this point a debris flood continued, to join with the debris flood
from catchment 14.2, and extended to the coast (Figure 3.27). The upper catchment is in
pasture with a little scrub near the limestone ridge.
Figure 3.26
Water Treatment Plant partially buried by debris flood sediment (TDC photo).
Figure 3.27
Tim Cuff).
Debris flood sediment at the junction of Pohara Valley Rd and Abel Tasman Drive (Courtesy of
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Catchment 15.0
This is a small, narrow catchment mainly in regenerating forest. Only a few small landslides
occurred during the storm and no debris flows or debris floods were generated. Bay Vista
Drive has been extended along the upper margin of the catchment, but no houses had been
built at the time of the storm.
Catchment 16.1 (Richmond Rd)
This is a narrow catchment with a number of short tributaries that join the main stream that
flows down an old fan surface on which Richmond Rd is located. A debris flow was
generated from numerous landslides, mainly occurring in pasture in the upper catchment.
The debris flow extended as far as a cluster of karst sink holes immediately upstream of the
junction of Richmond Rd and Bay Vista Drive. A debris flood extended beyond Bay Vista
Drive and spread through a number of properties before reaching the coast.
Catchment 16.2 (Bay Vista Drive)
This comprises several very small catchments which surround the area on which a number
of houses in Bay Vista Drive are built. About five landslides occurred in regenerating forest, a
couple of which affected properties. No debris flows or debris floods were generated.
Catchment 17.1
Like catchment 16.1, this catchment is narrow, and largely in pasture. Only a few landslides
occurred in the steep, upper part of the catchment and no debris flows or debris floods were
generated. The catchment drains onto the lower part of the Ellis Creek fan.
Catchment 17.2 (Ellis Creek)
The Ellis Creek catchment is the largest in the study area. The steep catchment is a mix of
pine forest, regenerating forest and scrub, and pasture. There are a number of smaller
landslides, but the majority of the sediment and debris that generated a debris flow was
derived from a very large landslide originating in granodiorite in an area of scrub between
two stands of pine trees (Figure 3.28). The debris flow travelled down Ellis Creek, and after
the stream passes between limestone ridges, the flow spread out onto the large fan
(Figure 3.29). The debris flow extended at least 0.5 km beyond the hills where a house was
destroyed (Figure 3.30, Figure 3.31). Beyond this point a debris flood spread out up to 0.75
km across the fan and then travelled down channels and drains to the base of the fan behind
the coastal dune sands.
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Figure 3.28 The large landslide that supplied the majority of the sediment that generated the debris flow in the
Ellis Creek catchment (Courtesy of Paul Woperies).
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Figure 3.29 Debris flow and debris flood sediment deposited on the Ellis Creek fan (Courtesy of
Paul Woperies).
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Figure 3.30
Debris flow deposit along Ellis Creek near the head of the fan (TDC photo).
Figure 3.31
Severely damaged house on Ellis Creek fan (TDC photo).
The debris flow was reported to have occurred early in the afternoon of the 15th December
(almost a day later than the debris flow reported at Nyhane Drive). One resident was swept
from his house and carried about 500 metres downstream. The torrent was described as
being 100-metres wide and 10-metres high. Figure 3.32 was taken about an hour later,
looking downstream near the head of the fan, and shows sediment-laden flood waters still
overflowing Ellis Creek. This would appear to be a debris flood surge. Note the roof of the
collapsed shed on the right of the figure with sediment surrounding it, indicating the previous
extent of the flow. Figure 3.33 was taken from the same place the following day (16th
December) and shows Ellis Creek has returned to it channel. Note the large number of
cobbles and boulders deposited adjacent to the banks. Local residents reported seeing a
bright flash around 9 pm on the 14th in the hills. This coincided with a loss of power, and is
presumed to be related to the power lines which cross the landslide. After this, Ellis Creek
seemed “to settle down”. An explanation for these events could be that a slope failure that
occurred just downstream of the large landslide in Figure 3.28 had blocked the channel, and
that this was breached the following day, resulting in the debris flow.
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Figure 3.32 Sediment-laden flood waters (possible debris flood) travelling down Ellis Creek on 15 December
(Courtesy of Braeden Lobb).
Figure 3.33 Ellis Creek on 16 December (cf. Figure 3.32). Numerous cobbles and boulders have been
deposited along the banks (TDC photo).
A smaller debris flow was also generated in Cat Gully Creek, a small tributary that joins Ellis
Creek near the apex of the fan just beyond the limestone ridges. This debris flow was
generated by several landslides in an area of regenerating indigenous forest and scrub.
Local residents report that a large amount of woody debris was carried at least as far as the
old homestead. Beyond the limestone ridge sediment left the channel and it appears a debris
flood spread onto the fan and through an orchard and cow shed, before joining Ellis Creek.
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4.0
ANALYSIS OF STORM DAMAGE
4.1
DATA AND METHOD OF ANALYSIS
4.1.1
Field data
Tasman District Council staff took over 650 aerial and ground photographs in the study area
in the 2-3 days after the storm, documenting the types and extent of damage. Glenn Stevens
(TDC) made numerous visits to the area in the months following the storm to observe the
nature of the sediment deposited on the fans. Between 5-8 June 2012, Mike Page, Rob
Langridge, and Glenn Stevens visited the study area, principally to observe the nature of the
sediment deposits exposed by stream incision in a number of valley floors, and to collect
material for radiocarbon dating.
4.1.2
Aerial photography
Colour aerial orthophotography taken in January 2012 with a 0.4 m GSD was used in ESRI
ArcMap 10.0 to digitise various features (catchment boundaries, landforms, bare ground,
vegetation). Slope and relief-shaded maps were derived from a 10 m DTM, and these
together with geological (QMap) data were overlaid on the aerial photography. Tables were
then generated to provide distributions of the various parameters on a catchment basis.
4.1.3
LiDAR
LiDAR data was flown in January 2010, and again six months after the storm, in April 2012. It
provides only partial coverage of the study area (principally on the fans), and after six months
some fan debris had been cleared. While beyond the scope of this report, future investigation
of the utility of these LiDAR surveys to provide surface elevation changes, debris volumes
and type, and flow pathways on the fans are recommended (Bull et al. 2010).
4.2
RESULTS
4.2.1
Rock type and vegetation cover distribution on a catchment basis
Tables in the appendices list the area and percentage area of rock type by catchment
(Appendix 1), and the area and percentage area of vegetation cover by catchment
(Appendix 2).
4.2.2
Landslide bare ground distribution on a catchment basis
Appendix 3 lists the area and percentage area of landslide bare ground by catchment
contributing area, which is the hillslope area upstream of the fan apex that contributes
sediment to the fan. Bare ground was mapped from the aerial photos and includes both
eroded surfaces and depositional surfaces, as the resolution was insufficient to accurately
differentiate the two. Also bareground in forested areas, especially exotic forest, was
underestimated due to the presence of the tree canopy and the shadow it cast on adjacent
bare ground. Nevertheless this allows a rough comparison of landslide erosion by catchment.
Figure 4.1 is a scatterplot of percentage bare ground against catchment area. Catchments
with debris flows plot above a value of ~4%, with only four non debris flow catchments above
this value.
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Figure 4.1
Scatterplot of percentage hillslope bare ground and catchment area for the 29 study catchments.
Catchments are classified according to debris flow/debris flood occurrence in the December 2011 storm event.
4.2.3
Landslide bare ground distribution in relation to vegetation cover and rock
type
Tables in the appendices list the total area of bare ground in the 29 catchment study area
due to landslides, in relation to both vegetation type (Appendix 4) and rock type
(Appendix 5). These totals are independent of slope. Landslide bare ground density for
grassland was 715 m2/ha, for scrub 628 m2/ha, and for indigenous forest 584 m2/ha. While
exotic forest had a landslide bare ground density of 313 m2/ha, this is an underestimate for
the reasons mentioned in section 4.2.2, and is likely to be close to the density for indigenous
forest. The landslide bare ground density under granite is also an underestimate for the
same reason, as this is the rock type on which most of the exotic forest occurs.
4.2.4
Catchment parameters – the Melton Ratio
The number of debris flows that occurred during the December 2011 storm in Golden Bay,
and also during the Tapawera storm in Motueka in May 2010 has highlighted a hazard that
until now was either not recognised, or regarded as occurring very rarely in the Tasman
District. The majority of these debris flows deposited their sediment onto fan surfaces. The
district has numerous fans which are otherwise attractive sites for housing and other
development, and it can be expected that there will be increased pressure for such
development in the future. For this reason the Tasman District Council want to identify sites
where debris flows can occur, and also the likelihood of occurrence.
A first step to identifying fans and other sites susceptible to debris flows and debris floods is
to identify the catchment parameters under which they can occur. Several such studies have
been carried out both in New Zealand and overseas on a variety of landscapes (Kostaschuk
et al. 1986, Jackson et al. 1987, de Scally et al. 2001, de Scally and Owens, 2004,
Rowbotham et al. 2005). Studies by Wilford et al. 2004 in British Columbia, and by Welsh
and Davies (2010) in New Zealand used i) Melton Ratio (Melton 1965) - an index of
catchment ruggedness and ii) catchment length – the planimetric straight-line distance from
the apex of the fan to the furthest point on the catchment boundary, to identify catchments
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capable of generating debris flows. The Melton ratio (R) is equal to catchment relief divided
by the square root of catchment area:
R = Hb/√Ab
where Hb is catchment relief and Ab is catchment area.
The Welsh and Davies (2010) study used 18 catchments in the Coromandel and Kaimai
Ranges, 16 catchments in the Southern Alps and two catchments at Matata, all known to
generate debris flows. They found that all catchments except those at Matata had a Melton
Ratio > 0.5. It must be emphasised that this approach provides a preliminary assessment of
potential debris-flow-prone catchments and fans, and any fans on which development is
proposed require thorough site investigations.
The debris flows that were generated by the December 2011 storm in Golden Bay provided
an opportunity to test the suitability of the Melton Ratio and catchment length for identifying
debris-flow-prone catchments in the Tasman District. In the 7 km between Tata Beach and
Clifton 29 catchments have been identified. All comprise steep hill slopes underlain by
Separation Point Granite, and drain northwest across fans to the coast. Those catchments
south of the Quarry also have small amounts of limestone and calcareous siltstone in their
lower reaches. The size of catchment contributing area (the area that drains to the apex of a
fan) range from 0.0167 km2 (1.67 ha) to 2.251 km2 (225 ha). Vegetation cover ranges from
pine forest to mixes of pasture, scrub, regenerating forest and pine forest.
The 29 catchments were delineated in ArcMap 10.0 (ESRI 2010) on 0.4 m GSD
orthorectified colour aerial photography taken in January 2012. Catchment length was
calculated as the planimetric straight-line distance from the apex of the fan to the furthest
point on the catchment boundary, the catchment area was calculated as the area that drains
to the apex of a fan (referred to as the catchment contributing area), and catchment relief
(altitude range) was calculated from a 10 m DTM of the study area. The three parameters
were then exported into a Microsoft Excel spreadsheet, where Melton ratios were calculated.
Table 4.1 lists the Melton Ratios (R) and catchment lengths (CL) for the study catchments,
together with the occurrence of debris flows and debris floods in the 2011 storm and paleodebris flows identified during field reconnaissance.
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Table 4.1
Melton Ratios (R), catchment lengths (CL) and occurrence of debris flows and debris floods for the
study catchments.
Catchment
length
(CL) (km)
Melton
Ratio (R)
2011
debris
flow
2011
debris
flood
Paleodebris
flow
0.65
0.30
no
no
no?
1.2
0.40
0.48
no
no
no?
1.3
0.21
0.58
no
no
no?
2.0
0.48
0.52
no
no
no?
3.0
0.62
0.56
no
yes
no?
0.74
0.61
no
yes
no
5.0
0.69
0.73
no
yes
no?
6.0
0.67
0.68
no
yes
no?
7.1
0.25
0.43
no
no
no?
7.2
0.31
0.53
no
no
no?
0.63
0.62
yes
yes
no
8.0
0.79
0.52
yes
yes
?
9.1
0.56
0.64
no
yes
?
9.2
0.21
0.47
no
no
no?
Catchment
number
Catchment name
1.1
Tata Beach
4.0
7.3
Clemens Creek
Ligar Bay Villas
10.1
Nyhane Drive
2.03
0.39
yes
yes
yes
10.2
Lower Nyhane Drive
0.60
0.23
no
no
no?
11.1
Matenga Rd East
0.55
0.46
yes
yes
?
11.2
Matenga Rd Middle
0.77
0.59
yes
yes
?
11.3
Matenga Rd West
1.06
0.59
yes
yes
yes
12.0
Quarry
1.77
0.55
yes
yes
?
14.1
Pohara Valley Rd
0.43
0.33
no
no
no?
14.2
Winter Creek
1.79
0.52
yes
yes
yes
14.3
Haile Lane
1.04
0.41
no
no
no?
14.4
Haile Lane Creek
2.30
0.50
yes
yes
?
0.88
0.31
no
no
no?
15.0
16.1
Richmond Rd
1.39
0.58
yes
yes
?
16.2
Bay Vista Drive
0.53
0.24
no
no
no?
1.50
0.28
no
no
no?
2.17
0.38
yes
yes
?
17.1
17.2
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A scatterplot of R against CL for these catchments is shown in Figure 4.2. The scatterplot
shows that catchments which generated debris flows in the December 2011 storm have a
combination of R >0.37 and CL >0.54 km, and those which generated debris floods have a
combination of R >0.56 and CL 0.56 to 0.74 km. It should be pointed out that four debris-flow
catchments (7.3, 8.0, 11.1, 11.2) plot in the same area of the scatter plot as the debris-flood
catchments (i.e. they have similar Melton Ratios and catchment-length values). However, in
each case only small debris flows appear to have reached the upper parts of the fans.
Catchment 14.3 plots in the same area of the scatter plot as the debris-flow catchments,
however no debris flow occurred. This may be due to the upper, steeper part of the
catchment, which is underlain by granite and where landslides occurred, appearing to drain
into a sink hole. The catchment length above this point is only 0.3 km. Therefore, those
catchments where debris flows caused significant fan deposition appear to plot above a
catchment length threshold value of ~1 km.
By comparison, in the study by Welsh and Davies (2010) all debris-flow generating
catchments had R >0.5 except the two Matata catchments which had significantly lower
values. They suggest that other factors such as local topography, where a number of
tributaries may have generated debris flows that coalesced just above the fan, or
lithologically controlled sediment supply conditions may explain why the two Matata
catchments have such low Melton Ratios. Wilford et al. (2004) report that debris-flow
catchments in British Columbia had R >0.6 and catchments lengths <2.7 km, and debrisflood catchments had either R 0.3 to 0.6 or R >0.6 and catchment lengths >2.7 km.
It would appear that while Melton Ratios and catchment length, in combination, are
parameters that can be used to identify catchments prone to debris-flow and debris-flood
processes, the threshold values are somewhat different for different terrains/landscapes.
Within the Separation Point Granite terrain, these threshold values require refinement with
analysis of further catchments and calibration against known debris-flow and debris-flood
records.
Figure 4.2
Scatterplot of Melton Ratio and catchment length for the 29 study catchments. Catchments are
classified according to debris flow/debris flood occurrence in the December 2011 storm event.
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5.0
EVIDENCE FOR PREVIOUS DEBRIS FLOWS
While the Melton Ratio, catchment length and other catchment parameters are indicators of
the potential for debris flows to occur, evidence of actual deposits should be sought,
especially where existing or proposed development put lives and valuable infrastructure at
risk. Given that the December 2011 storm generated debris flows that spread out onto some,
but not all of the fans in the study area, and in the light of the recent development of housing
on the Nyhane Drive fan, several other types of evidence were considered.
5.1
REPORTED POSSIBLE DEBRIS FLOWS
Although it is difficult to confirm the occurrence of debris flows from descriptions of previous
rainstorms and flood events in the area, the following events were among the largest and
may have generated debris flows.
1.
A large flood occurred around 1900 in which a house was reportedly washed out to sea
in Wainui Bay.
2.
The following quote is taken from the book “Wainui Bay” by Maurice Robertson. Around
1920 “a terrific rainstorm hit the Wainui catchment area bringing down millions of tons
of granite and stones. Washouts and slips of this magnitude probably had not
happened for centuries, if ever. The result was the whole length of the river-bed was
filled up about three feet and following floods never shifted one inch of it”.
3.
The Golden Bay Times and Argus for Thursday January 5, 1922 reported a flood in the
Motupipi-Clifton area which was likely the same event as the one referred to at Wainui
Bay. Trees and boulders were carried down Ellis Creek, and the Ellis Winery was
destroyed, with wine barrels washed out to sea. Another article reported that “There
was a bit of a gorge out the back of the winery and the men had felled the bush up
there. The logs had all lumbered down into the gullies and jammed up. They’d dammed
all the water up. So then when this big cloudburst came along, down she all came. I
know all this, saw it all happen. There wasn’t a fence standing around here.
Everywhere you looked, two foot of silt and great heaps of boulders and logs”. This
description appears to be consistent with a debris flow.
4.
In May 1955 heavy rain fell over four days (a newspaper report claims 444.5 mm, while
the rain gauge at Tarakohe recorded 277.6 mm over 4 days). The Tarakohe News,
published by the Golden Bay Cement Company, reported that in the hills behind the
Works there were “extensive slips which dammed up the stream until the volume of
water impounded swept the debris before it and raced down the valley, through the
Works and out to sea, leaving a deposit of about 0.6 m of silt, vegetation and rubbish
right through the Plant”. The newspaper reported that workers at the Plant were
apparently aware of the debris dam up stream and attempted to “reinforce” buildings.
One hundred tons of coal and hundreds of heavy drums were swept out to sea. A
stream suddenly switched course down the main street of the settlement (presumably
Pohara village).
5.
A small amount of debris was deposited near the creek at the apex of the fan on
Nyhane Drive in 1972 (possibly in August when 159.1 mm fell in one day), but a debris
flow did not occur.
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Alan Swafford (local farmer) has made the following comments. He has never seen flooding
on the scale of December 2011 (since the 1950s), and particularly no instances of such large
volumes of sediment and debris being deposited, or the scouring of channels down to
bedrock. He has no recollection of “old timers” talking of floods with the characteristics of
debris flows. However, there were certainly floods with debris (sand and vegetation) (debris
floods?), and while sand has always come down the creeks, a lot more material came down
when the hills were in pasture than after the pine forest had been established. He used the
old Wainui Hill Road (Figure 2.3) in the 1970s, which was in good condition at the time. He
suggests this indicates that there had been no earlier events of sufficient size to cause major
damage (c.f. the December 2011 storm which caused several large landslides that severely
damaged the now abandoned road).
5.2
RAINFALL RECORDS
Daily rainfall records (the 24 hour period from 9 am to 9 am) are available from May 1932 to
December 1988 at Tarakohe, and continuous (15 min interval) readings from December
1985 to 2012 at Kotinga (2 km south of Takaka). The vast majority of rainstorms in the record
are between 100-150 mm over 2-4 days with maximum daily totals of 50-100 mm. However,
the 10 storms with the highest daily totals (>150 mm) have all occurred since 1990. While
the threshold for debris-flow generation is not known, the next highest daily total to
December 2011 (395mm) was in November 1990 (258.7 mm) for which there appears to be
no record of debris flows. The May 1955 rain storm mentioned in section 5.1 is problematic in
that the description of the event is consistent with a debris flow, yet the Tarakohe rain gauge
records only 277.6 mm in four days while a newspaper article reports 444.5 mm.
5.3
GEOMORPHIC AND STRATIGRAPHIC EVIDENCE
5.3.1
Fans and fan morphology
Fans are formed by the successive deposition of sediment eroded from their upstream
catchments. This sediment may be deposited by either floods, debris floods, debris flows or a
combination of all three. The mere presence of a fan is a first indication that there may be a
potential for debris flows. There are a number of morphological features of fans that can
confirm the presence of debris flow deposits. These include:
1.
An uneven fan surface. This may include raised, lobate areas and channel-side levees.
By contrast very smooth, even surfaces tend to be the result of fluvial processes.
2.
Large boulders on the fan surface.
3.
Even-aged vegetation, younger than surrounding growth, and scarring high along
stream sides or on trees.
The fan surfaces in the study area were relatively smooth, although any unevenness
indicating debris flows may have been removed by past cultivation. The fan for Catchment
6.0 did appear to have a lobate feature which may indicate a debris flow or debris flood
deposit. No large boulders were observed on the fans.
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5.3.2
Channel morphology
Debris flows tend to occur where channels are narrow, steep (>5°) and with small width to
depth ratios. These channels often have a distinctive, roughly semi-circular to U-shaped form
scoured to bedrock. This indicates repeated debris flows over a long period of time. Where a
debris flow has not occurred for some time this channel shape may not be apparent if the
channel has been infilled with sediment. A roughly U-shaped channel profile was observed in
the stream in Catchment 4.0.
Paleo-debris flow deposits (C14 dating)
5.3.3
Evidence of earlier debris flows may be buried under the fan surfaces. Such paleo-debris
flows can be recognised when the various layers, or stratigraphy which make up the fan are
exposed by stream incision or by trenching. Sedimentary characteristics of debris flow
deposits include (Figure 5.1):
1.
unstratified deposits with no structure or imbrication
2.
angular to sub-angular clasts
3.
long axes of clasts oriented randomly or parallel to flow
4.
poorly sorted and matrix-supported deposits
a)
Figure 5.1
b)
a) Sedimentary characteristics of debris flows. b) Degree of sorting and imbrication of clasts.
Based on the above criteria, the authors identified several debris-flow deposits in the Nyhane
Drive, Matenga Rd West (Figure 5.2) and Winter Creek catchments that had been exposed
in the banks of streams/channels incised during the December 2011 storm. Material from two
of these deposits in Nyhane Drive catchment, and one from Matenga Rd West catchment,
together with material from a buried soil from Nyhane Drive catchment, two buried soils from
Clemens Creek catchment, and a tree stump exposed in the bed of the stream in Ligar Bay
Villas catchment were collected and carbon dated. In each catchment the debris-flow
deposits were located within the narrow valley floor sediments upstream of the fans, and so
of themselves do not prove that these debris flows reached the fans.
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a)
b)
Figure 5.2
a) Paleo-debris flow deposits exposed in the stream in Matenga Rd West catchment. b) Note the
unstratified, poorly sorted nature of the deposits, and the matrix-supported angular clasts (GNS Science photos).
Table 5.1 lists the conventional radiocarbon ages and calibrated calendar year ranges for the
seven samples. While this small number of dates is insufficient to establish a comprehensive
record of long-term debris flow occurrence, indications are that recurrence intervals for
events of the size of December 2011 may have been of the order of several hundred years.
Table 5.1
Conventional radiocarbon ages and calibrated calendar year ranges for samples taken from debris
flow deposits, buried soils and tree stumps.
Sample
ID
Lab No
NZA
Location
ND3-2
50978
Nyhane Drive
Description
Reported
CRA
Calibrated Cal year 2
sigma: SHCAL04
calibration curve
wood fragments
4563 ± 21
3349-3096 BC
within buried soil
CL3-2
51087
Clemens Creek
buried soil
816 ± 17
1225-1278 AD
CL3-1
51065
Clemens Creek
charcoal within soil
444 ± 22
1445-1501 AD
ND1-2
51066
Nyhane Drive
buried soil
1319 ± 22
669-813 AD
ND1-1
51088
Nyhane Drive
charred twig
133 ± 17
1808-1949 AD
MR2-1
51089
Matenga Rd
charcoal
7906 ± 25
6804-6599 BC
wood from outside of
1371 ± 18
657-721 AD
West
LV1-1
51090
Ligar Villas
in situ tree stump
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Clemens Creek and Ligar Villas catchments
These catchments are similar in size, steepness, rock type and vegetation to most of the
catchments north of Nyhane Drive catchment. All of these catchments have flat, narrow
valley floors upstream of their fans. These valleys have been infilling with sediment for at
least several hundred years, indicating that significant amounts of sediment eroded from the
surrounding hillslopes, have not reached the fans. Aerial photographs from 1952 and 1965
show that there was little incision by the streams flowing along these valleys. However,
during the December 2011 storm these streams incised by up to 1-2 m into the valley floor,
revealing thick layers of granitic sand and gravel, between which were several buried soils or
paleosols. No debris-flow deposits were visible. Organic material from two of these buried
soils in the Clemens Creek catchment were radiocarbon dated and gave ages of 444 ± 22 yr
BP (60 cm below present ground surface) and 816 ± 17 yr BP (100 cm below present ground
surface). In the Ligar Bay Villas catchment a tree stump exposed in the bed of the stream,
~1.5 m below the present ground surface, was radiocarbon dated at 1371 ± 18 yr BP,
implying burial around this time by sediment.
Nyhane Drive catchment
Nyhane Drive catchment is much larger than the catchments to the north, with 42%
regenerating forest, 32% pine forest, and 20% pasture. It is one of two fans in the study area
that have significant housing, both of which were affected by debris flows and debris floods in
the December 2011 storm. Exposures of inset terraces and the fan upstream of the 2011
debris fan runout provided an opportunity to assess the age of older debris-flow deposits in
this catchment. At a site ~130 m upstream of the fan apex a charred twig (charcoal) within a
grey, gravelly sand layer between angular to sub-angular granite clasts (debris flow deposit)
gave a radiocarbon age of 133 ± 17 yr BP. This debris flow deposit is exposed in the true
right bank of the incised stream, overlain by ~1.5 m of material from the 2011 debris flow and
is 20 cm above current stream level (Figure 5.3). The bank immediately opposite (true left
bank) has exposed ~2.5 m of debris flow-deposit, below the present surface. The clasts have
2-3 cm thick weathering rinds. Beneath this deposit is ~5 cm of cemented sands overlying a
buried soil which in turn overlies another debris-flow deposit. Organic-rich clay from this
paleosol was dated at 1319 ± 22 yr BP (Figure 5.4, Figure 5.5). At a site further upstream
(~230 m from the apex of the fan), a radiocarbon date of 4563 ± 21 yr BP was derived from
wood fragments within a buried soil at the base of the true left bank. Overlying the buried soil
is ~ 2.5 m of gravelly sand grading upwards into brown sands (Figure 5.6).
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Figure 5.3
Exposure in true right bank of Nyhane Drive stream, ~130 m upstream of the fan, showing 2011
debris-flow deposit overlying an older debris-flow deposit with a radiocarbon age of 133 ± 17 yr BP (GNS Science
photo).
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Figure 5.4
Exposure in true left bank of Nyhane Drive stream, ~130 m upstream of the fan, showing ~2.5 m of
debris-flow deposit, below the present surface, beneath which is ~5 cm of cemented sands overlying another
debris-flow deposit which has been dated at 1319 ± 22 yr BP (GNS Science photo).
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Figure 5.5
Close up of lower debris-flow deposit shown in Figure 5.4. Sample for radiocarbon dating was taken
from yellowish brown soil between clasts (GNS Science photo).
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Figure 5.6
True left bank of Nyhane Drive stream, ~230 m from the apex of the fan, showing buried soil
overlain by ~ 2.5 m of gravelly sand grading upwards into brown sands. The buried soil has a carbon date of 4563
± 21 yr BP (GNS Science photo).
In November 2012 a trench and a tank pit were dug on a property near the apex of the
Nyhane Drive fan (Figure 5.7). Both were ~3 m deep, and the trench ~30 m long, and they
were located on a slightly elevated surface of the fan ~3-4 m above that part of the fan onto
which the December 2011 debris flow deposited sediment. On this higher surface the tank pit
exposed ~70 cm of topsoil and sub soil, overlying ~100 cm of light brown, mottled clay with
minor sand, overlying ~40 cm of brown silty granitic sand to fine gravel, with occasional
weathered cobbles, beneath which was a ~70 cm layer comprising granite cobbles and
boulders in a matrix of silty sand (Figure 5.8). These cobbles and boulders were poorly
sorted, irregularly orientated and very weathered (some easily cut with a knife and others
with 2-4 cm weathering rinds). This layer is interpreted as a debris-flow deposit. It extended
to the base of the exposed wall of the trench, and so may be somewhat thicker. Organic
material (root) was extracted from the deposit near the base of the exposure for radiocarbon
dating, but at the time of report writing the age of this deposit is unknown. However, the tank
pit does show that at least one significant debris flow has reached this higher part of the fan
in the past. No buried soils were observed. A debris-flow deposit occurred at a similar depth
in the trench, and is assumed to be the same deposit as the one in the tank pit.
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Figure 5.7
58
Location of tank pit and trench on Nyhane Drive fan.
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Figure 5.8
Tank pit in Nyhane Drive fan. A debris-flow deposit (as yet undated) occurs at the base of the
exposure (TDC photo).
In reports relating to development of the Nyhane Drive subdivision (circa 2001-2003),
reference is made to “big granite stones” occurring in excavated pits and trenches. Granite
boulders were also identified at 41 Nyhane Drive around 2007, during excavations for the
water tank seen in Figures 3.18 and 3.19. These boulders occurred at between 30 and 110
cm depth, and were likely from a debris flow deposit. They were subsequently used as
landscape features (Figure 5.9).
a)
b)
Figure 5.9
House at 41 Nyhane Drive, a) boulders excavated from pit for water tank, b) the boulders used as
landscape features (Courtesy of Hagen Jurke).
Matenga Rd West catchment
This is a moderately long, narrow catchment, largely in scrub, which drains onto the Matenga
Rd fan. Material was carbon dated from a debris-flow deposit in the deeply incised gorge
above Matenga Rd before the stream reaches the fan. There has been recent incision within
the sediments in the gorge, as evidenced by steps and a weir which now sit above the
streambed (Figure 5.10a and b). Several debris-flow deposits are exposed in the gorge, and
the oldest at present stream level has a radiocarbon date of 7906 ± 25 yr BP
(Figure 5.11a and b).
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a)
b)
Figure 5.10 a) Steps and b) Weir perched above present stream level in Matenga Rd West catchment
(GNS Science photos).
a)
b)
Figure 5.11 a) Debris-flow deposit exposed in the gorge of Matenga Rd West catchment, and radiocarbon
dated at 7906 ± 25 yr BP. b) Close up of boxed area in Figure a) (GNS Science photos).
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6.0
LIKELY RESPONSE TO FUTURE RAINFALL EVENTS
6.1
IN THE IMMEDIATE TERM (NEXT FEW YEARS)
Despite the large quantities of sediment deposited on fans and along the coastal margin
during the December 2011 storm, a far greater quantity of uneroded sediment remains on
slopes and in valley floors in the catchments. A small proportion of this has been destabilised
by the storm, where cracking and loose landslide sediment occur on hillslopes, and where
stream incision has occurred in the valley floors. The result is that this looser and more
erodible sediment is vulnerable to transportation downstream in smaller rainfall and run off
events for the next few years. Such events are very unlikely to generate debris flows but may
generate debris floods.
If a storm of similar magnitude to that of December 2011 were to occur in the immediate
future it is likely that there would be a moderate reduction in the amount of landsliding, and
probably in the size of debris flows. This is because the landslides that occurred in the
December 2011 storm removed sediment from sites that were the most susceptible to failure
at that time. On average the sediment that was not eroded now occupies more resistant
sites, and so it will take a rise in the rainfall threshold to trigger a similar level of landsliding to
that in December 2011.
6.2
IN THE SHORT TERM (UP TO TEN YEARS)
The situation described above is likely to continue for the next decade or so. That is debris
flows are still possible, but they are likely to be smaller. Over time, with progressive
weathering of the bedrock and regolith, and without any further large landsliding events,
sediment will build up on slopes, increasing landslide susceptibility again and resulting in a
fall in the rainfall-triggering threshold. This process will continue until another large
landsliding event occurs, and the more time that elapses before the next event, the larger
that event is likely to be. However, within the next ten years it may still require a more
extreme rainfall event to trigger debris flows as large as those of December 2011. However,
it should also be pointed out that storm rainfalls capable of generating debris flows larger
than those experienced in December 2011 are possible at any time.
6.3
IN THE LONG TERM (UP TO 1,000 YEARS)
Based on the historic record and several radiocarbon dated debris-flow deposits, it would
appear that past recurrence intervals for events similar to that of December 2011 may have
been of the order of several hundred years. However, the magnitude and frequency of future
debris flows may change, dependent upon the interplay of rainstorm magnitude and
frequency, weathering rates, and to a lesser extent land use. An increase in the magnitude
and/or frequency of storms over a sustained period could lead to more frequent but ultimately
smaller debris flows as catchment sediment supply is reduced by long-term erosion rates
exceeding weathering rates. Conversely, if storm magnitude and frequency decreases then
debris flows are likely to be larger but less frequent as sufficient time will elapse between
events for the supply of sediment in catchments to be replaced.
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6.4
CLIMATE-CHANGE FORECAST
In addition to natural climate variability, human-induced global warming is forecast to lead to
changes in climate during the 21st century. NIWA have modelled regional changes in annual,
seasonal, and extreme rainfalls for New Zealand (Tait 2011), based on a mid-range scenario
of a 2°C increase in temperature associated with an increase in greenhouse gas emissions
as reported in the Intergovernmental Panel on Climate Change (IPCC) 4th Assessment
Report (IPCC, 2007).
Projected annual precipitation changes for the Takaka district between 1990 and 2090 are
for an increase of 4%, with increases of 6% in summer and winter. Perhaps more importantly
there is an expected increase in the frequency and intensity of extreme rainfall, with the 100year average recurrence interval 24-hour rainfall total projected to increase by around 60-80
mm. What are now 100-yr events are expected to occur about twice as often (i.e. be the
future 50-yr events). It should be noted that the error limits are large compared with the
average change predicted.
Therefore, while there will always be uncertainty around the size of the next storm and when
it will occur, the above projections suggest that the occurrence of potential debris flowgenerating events will increase over the coming century.
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7.0
AREAS AT RISK FROM DEBRIS FLOWS AND DEBRIS FLOODS
7.1
CATCHMENTS LIKELY TO GENERATE DEBRIS FLOWS AND DEBRIS FLOODS
The 29 catchments in the study area can be assessed as to their likelihood of generating
debris flows, based on the following evidence:
1.
occurrence of debris flows in the 2011 storm
2.
historic reports of debris flows
3.
evidence of paleo-debris flow(s) in stratigraphic record
4.
catchment parameters (Melton Ratio and catchment length)
Catchments 1.1, 1.2, 1.3
It appears highly unlikely these catchments are capable of generating debris flows. They did
not do so during the December 2011 event, and in fact only a few landslides occurred, the
majority of which were associated with Abel Tasman Drive. While the valley infills were not
inspected for evidence of paleo-debris flows, combined Melton Ratio and catchment length
values indicate these catchments are below the threshold for debris-flow generation. The
likelihood of debris floods occurring which would extend beyond the valley floors and onto
the fan is low.
Catchments 2.0, 3.0, 4.0, 5.0, 6.0
These five catchments are of similar size, and while small debris flows appear to have been
generated which then spread onto the valley floors during the December 2011 event, they
did not reach the fans. However, all except Catchment 2.0 generated debris floods that
spread out onto the fans. From the exposed stratigraphy in the valley floors in Catchments
3.0, 4.0, and 5.0 there is no evidence of paleo-debris flows during the last ~800 years. Given
the above observations and the Melton Ratio and catchment length values, it is clear that
there is a greater risk of debris floods occurring than debris flows, and that any debris flows
are unlikely to extend far out onto the fans.
Catchments 7.1, 7.2
These two catchments appear to be too small to generate debris flows or debris floods.
Catchments 7.3, 8.0, 9.1
In the December 2011 event, all three catchment generated debris flows which in the case of
7.3 and 8.0 extended onto the upper part of the fans, and may have extended further.
Debris-flood deposits extended to the coast. There was no evidence of paleo-debris flows
during the last ~1400 years in the stratigraphy exposed in the valley floor of Catchment 7.3. It
would appear that while these catchments are more likely to generate debris floods, the
December 2011 storm may have reached the threshold for debris-flow generation.
Catchment 9.2
This is a very small catchment, which like Catchments 7.1 and 7.2 appears to be too small to
generate debris flows or debris floods.
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Catchment 10.1
This catchment generated a debris flow and associated debris flood in the December 2011
storm which damaged several houses in the Nyhane Drive subdivision. Two paleo-debris
flows identified within the valley-floor sediments upstream of the fan have been radiocarbon
dated at 133 and 1319 yr BP. Melton Ratio and catchment length exceed the threshold
values for debris flow generation identified in this study. It would appear that this catchment
has a relatively high probability of generating debris flows, with a return interval of between
200 and 1000 years. Debris floods occurring independently of debris flows will occur more
frequently.
Catchment 10.2
No debris flows or debris floods were generated during the December 2011 storm.
Catchment area and slope are below the threshold for debris-flow generation.
Catchments 11.1, 11.2
Debris floods and possibly small debris flows reached the top of the fan from both
catchments. Both catchments are short and steep, and appear to be near the combined
Melton Ratio and catchment length threshold for debris-flow generation.
Catchment 11.3
A debris flow was generated in the December 2011 storm which flowed through a section of
gorge and onto Matenga Rd, beyond which it became a debris flood. In the gorge several
debris flow deposits are exposed, with the oldest at stream level radiocarbon dated at 7906
yr BP. It is likely that this catchment has a relatively high probability of generating debris
flows, although this cannot be confirmed without further dating of deposits.
Catchment 12.0
In addition to the debris flow generated in the December 2011 storm, it appears that a debris
flow also occurred in 1955 which damaged the Golden Bay Cement Works. This begs the
question whether there were also debris flows in adjacent catchments that went unrecorded.
Unlike the catchments to the north, in this catchment once the stream leaves the steep
granite hills it flows along a long, narrow valley floor confined by two limestone ridges. It
appears that this channel configuration would have continued to the coast, although there
has been much modification by the development of the quarry. The above observations,
together with the Melton Ratio and catchment length parameters indicate that this catchment
has a relatively high probability of generating debris flows.
Catchments 14.1, 14.3
Catchment 14.1 consists of several very small tributaries that drain separately onto the
Pohara Valley fan at the head of Pohara Valley Rd. They may be too small to generate
debris flows. Catchment 14.3 would appear to be above the Melton Ratio and catchment
length thresholds for debris-flow generation. However, the headwaters which generated the
majority of landslides in the December 2011 storm, drain into a sinkhole below a limestone
ridge. Where this then drains to is uncertain, and no debris flow or debris flood occurred
within the lower valley. For these reasons debris flows or debris floods are unlikely to reach
the Pohara Valley fan.
Catchment 14.2
A debris flow generated during the December 2011 storm reached Pohara Valley settlement,
damaging several houses. While no historic records of earlier debris flows were discovered,
evidence of an earlier deposit was exposed by ~1-2 m of stream incision. This deposit was
buried by the 2011 debris flow deposit and is likely of fairly recent age. It is clear that this
catchment has a relatively high probability of generating debris flows.
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Catchment 14.4
This catchment has a similar Melton Ratio and catchment length to Catchment 14.2, and also
generated a debris flow during the December 2011 storm which damaged houses in Haile
Lane in Pohara Valley settlement. Both catchments have a long confined channel between
the steep hillslopes and the fan, from which extra sediment can be eroded by debris flows.
This catchment also has a relatively high probability of generating debris flows.
Catchment 15.0
No debris flows or debris floods were generated during the December 2011 storm.
Catchment area and slope are below the threshold for debris-flow generation.
Catchment 16.1
Melton Ratio and catchment length values show this to be a catchment with a potential for
debris flows. This was borne out by a debris flow which occurred in the December 2011
storm, and extended as far as a cluster of karst sink holes immediately upstream of the
junction of Richmond Rd and Bay Vista Drive.
Catchment 16.2
This catchment is similar to Catchment 14.1 in that it consists of several very small tributaries
that drain separately onto an old (Holocene) fan and colluvial surface on which a number of
houses on Bay Vista Drive are built. These tributaries are too small to generate debris flows.
Catchment 17.1
Although this is a long narrow catchment of moderate size, it only has a small area of steep
slopes prone to landsliding, and a channel which is then confined in a narrow valley between
high, rolling terrace surfaces, before it reaches the coastal plain near the margin of the Ellis
Creek fan. It is therefore unlikely to generate debris flows, and if any occurred they would be
unlikely to reach the coastal plain.
Catchment 17.2
The Ellis Creek catchment is the largest in the study area and has the largest fan surface.
During the December 2011 storm a debris flow travelled down Ellis Creek, and after the
stream passes between limestone ridges, the flow spread out onto the fan, extending at least
0.5 km beyond the hills where a house was destroyed. The report of the Ellis Winery at
Clifton being destroyed by flooding in Ellis Creek in the 1920s, and the washing of wine
barrels out to sea may point to an earlier debris flow. It would therefore appear that this
catchment has a relatively high probability of generating debris flows.
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7.2
SUSCEPTIBLE FAN AREAS
As previously stated, fans develop through deposition of sediment eroded from upstream
catchment areas, and debris flows may be one of these processes. The head or apex of a
fan is always the most vulnerable part of a fan, and building on this part of a fan should be
avoided. Here the debris flow has yet to spread out across the fan, and so its volume and
speed is undissipated. From the evidence and analysis presented in Sections 3, 4 and 5 fans
susceptible to debris flows and debris floods are as follows:
Susceptible to debris flows - fans associated with Catchments 10.1, 11.3, 12.0, 14.2,
14.4, 16.1, 17.2
These fans all experienced significant debris flows and associated debris floods in December
2011. Streams were visited in catchments 10.1, 11.3 and 14.4, and found to have paleodebris flows deposits exposed along their banks.
Possibly susceptible to debris flows - fans associated with Catchments 7.3, 8.0, 11.1,
11.2
These fans all appear to have debris flows which only reached the head of the fans in the
December 2011 storm. This storm may have reached the threshold for debris flow generation
in these catchments. It would be prudent to avoid building on at least the upper parts of these
fans. Debris floods spread onto these fans during the storm and reached to the toes of the
fans and deposited sediment in the estuary.
Susceptible to debris floods - fans associated with Catchments 3.0, 4.0, 5.0, 6.0, 9.1
In these catchments only debris floods were generated in the December 2011 storm. In each
case sediment reached the estuary beyond the toe of the fans. No paleo-debris flow deposits
were observed in the exposed banks of the stream in Catchment 4.0.
The remaining thirteen catchments did not generate debris flows or debris floods.
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8.0
MITIGATION OPTIONS
There is a risk of sediment deposition on all but the smallest fans in the study area. Several
of these have been identified as prone to debris flows, either in the December 2011 event,
from paleo-debris flow deposits, or using catchment parameters. Because of the structural
damage, it is appropriate to consider a higher level of protection for debris flow inundation
than would normally be provided for flood inundation. To match other structurally damaging
hazards such as earthquakes and strong winds, it is appropriate to choose a level of
protection such that there is a 90% probability of the structure lasting 50 years without being
destroyed by a debris flow (an event with 10% probability in 50 years has approximately a
500 year return period). From preliminary investigations, it appears that the current return
period for debris flows of the severity of those in 2011 may be about 200 hundred years,
although this may reduce based climate-change projections for the Tasman District. This
equates to about a 10% probability in 20 years, which is unacceptably high. This indicates
that whatever measures are taken, they ought to be capable of preventing building damage
in events of greater magnitude than experienced in December 2011.
A technique that can be used to manage the risk from debris flows at a site is to spatially
map the annual-individual-fatality-risk (AIFR) on fans. This requires more quantitative data on
the magnitude and frequency of debris flows than has been collected during this study. This
approach has been taken in Christchurch with respect to rock-fall and cliff collapse in the
Port Hills (Massey et al 2012a, 2012b, Taig et al, 2012). The challenge with mapping AIFR is
that it is data intensive, but it allows the raw risk and the adjusted risk, if any engineering
measures are installed, to be calculated. The initial requirement would be for the Tasman
District Council to determine what level of AIFR risk is acceptable to them and their insurers.
The need to determine AIFR for a fan should be determined on a case-by-case basis. This
can be done proactively for green-field sites on debris fans, but must be done reactively for
existing developments.
For those fans where housing development has already occurred, or for fans where
development is being considered, four broad mitigation options may be considered: land use
and management, engineering works, early warning systems, and avoidance.
8.1
LAND USE AND LAND MANAGEMENT
What is the most appropriate land use/land management to minimise the risk of debris
flow/debris floods? There would appear to be three land use options, exotic forestry,
reversion to scrub and ultimately indigenous forest, and pastoral farming (and combinations
of these options).
Both exotic and indigenous forestry will decrease landslide erosion compared with pastoral
farming, and therefore the overall sediment volume of debris flows. Although studies have
not been carried out on the major lithologies/rock types in the study area, studies on several
other lithologies elsewhere indicate that landslide densities are ~10 times greater under
pasture than forest (Hicks et al. 1993, Marden and Rowan 1994, De Rose 1996, Page and
Trustrum 1997).
However, any woody vegetation cover increases the potential for woody debris to be
incorporated into debris flows. This is greatest with an exotic forest cover, and can occur
during and following harvesting when woody debris/slash is left on slopes. Both indigenous
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and exotic forests are vulnerable to wind throw, which can supply woody material through
both breakage and toppling of trees. However, even-aged, mature stands of exotic forest are
particularly vulnerable, given their lack of structural diversity (tall unbranched trees, little
understorey). Tracks and landings can be the source of sediment, especially during
harvesting operations. Woody debris can also block channels, forming debris dams which
can suddenly burst, initiating either sediment surges or debris flows. In the December 2011
storm, debris dams such as the one in Figure 8.1 were probably too small to have
significantly compounded the destructive force of debris flows. The exotic forests planted on
many hillslopes in the study area are ~20-30 years old and are due for harvesting. This will
require careful management where a debris flow or debris flood potential has been identified.
Three articles have recently been published in The New Zealand Journal of Forestry that
discuss the issues associated with the management and harvesting of plantation forests on
erodible steeplands (Phillips et al. 2012, Horner 2012, Bloomberg and Davies 2012).
Horner (2012) summarises a study by Bay of Plenty Regional Council aimed at reducing the
damage from woody debris during heavy rainfall events. The study involved two plantation
forests affected by high intensity rainstorms in 2010-2011 in catchments between Whakatane
and Opotiki. Areas where harvesting had been carried out within the last two years suffered
the greatest erosion and accumulation of woody debris. Sites planted less than five years
were also affected, but to a lesser degree, and forest greater than five years old was least
affected. Recommendations made were:
•
Remove slash – Remove large woody material or deposit it on stable sites. Where
removal is not practical, burning is an option.
•
Poison standing trees – Trees not harvested on steep slopes should be poisoned and
left to break down.
•
Construct slash racks – Use railway iron and wire rope to intercept and trap woody
debris. They need to be cleared when debris builds up, and should not be placed in
streams where they may “burst”. They are smaller, inexpensive versions of the “hard
engineering” debris dams discussed in section 8.2. Standing, managed trees on fans
may also act to hold back woody debris.
While these recommendations relate to physical conditions in Bay of Plenty, they may also
be relevant in other areas such as Tasman District.
The article by Bloomberg and Davies (2012) takes a long time-scale perspective on erosion
and the stabilising effect of forests. They point out that while forests reduce erosion on
human time-scales, on geological time-scales land use is irrelevant as erosion rates are
controlled by tectonic uplift, modified by climate and lithology. While soils tend to be deeper
(more available sediment) under forests, due to less erosion occurring under “normal” rainfall
conditions, this may mean that under extreme rainfall conditions a greater quantity of
material is eroded under forest than pasture.
Phillips et al. (2012) review the state of knowledge regarding the impacts of plantation forest
harvesting on erosion in New Zealand, and identify knowledge gaps. They suggest that
understanding and management of the risks associated with high intensity, low frequency
storms (such as the one in 2011 which caused the debris flows in the Pohara- Ligar Bay
area) is an area that requires further study. They identify some of the ways of managing the
risks as; retirement of areas of high erosion risk, longer-rotation species, increased stocking
rates, planned reversion, and larger riparian buffer zones.
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It is likely that a mix of all three land uses will continue to occur within the study area in the
future. Avoiding having large areas of even-aged exotic forest could potentially reduce the
supply of woody debris if storms were to coincide with times when management-induced
disturbance to the forest is greatest. Allowing some areas to revert to scrub and ultimately
indigenous forest may be the best option solely from a hazard perspective. The challenge is
to develop a mix which achieves both economic and hazard-mitigation benefits, and this will
require detailed consultations and assessments.
Figure 8.1
Small debris dam, largely consisting of woody debris, blocking a channel in Catchment 8.0
(TDC photo).
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8.2
ENGINEERING WORKS
Engineering works can be used to prevent debris reaching built areas of the fan, either by
retaining debris usually before it reaches the fan, or by controlling the predicted debris paths
across the fan. Debris retention structures range from earthen bunds to collect or deflect
sediment into designated ponding areas, to rock, wire and concrete dams across streams
which retain coarse debris while allowing through flow of finer sediment and water
(Figure 8.2, Figure 8.3). High risk areas of the fan such as recent debris flow deposits,
existing channels or low lying areas can be designated as future debris flow paths. Existing
channels can be modified by deepening, widening or straightening to increase their transport
capacity and reduce the risk of overbanking. This may involve just earth works, or channels
may be lined with concrete or other material (Figure 5.10). Embankments can be constructed
around low-lying areas or areas of recent debris-flow deposits, allowing future flows to
spread out within a confined area as they move down the fan.
While these options can significantly reduce risk they require continual monitoring,
maintenance and emptying of material to maintain their effectiveness. They are often
employed in other countries where debris-transport events occur frequently, and where the
protection of significant assets easily justifies the expense of construction and maintenance.
A reservation about such engineering works is that once installed they can engender an
illusion of safety if they are not designed to cope with the largest possible event. This can
lead to more intensive development, which when the structures are overwhelmed, will result
in greater damage. Identifying what is the largest possible event can be very difficult. It is
worth remembering that it is natural for streams to migrate across depositional surfaces (that
is how they develop), and attempts to confine them are ultimately likely to fail.
The two fans where such engineering structures may be appropriate are those on which the
Nyhane Drive subdivision and the Pohara Valley houses are sited.
8.2.1
Nyhane Drive fan
The Nyhane Drive fan has the characteristic shape of most fans, narrow at its head or apex
and then widening downslope to a relatively broad toe area. The natural (slightly modified)
stream channel flows down the eastern side of the fan. The Nyhane Drive subdivision is
located to the west of the stream, with several houses within a few meters of the stream
(Figure 3.15). Near the apex of the fan there is an older slightly elevated surface ~4-5 m
above the rest of the fan and ~50-80 m back from the channel. On reaching the fan the 2011
debris flow left the channel and spread onto the fan but did not reach this higher surface. It
would be prudent to avoid further development of the lower surface near the channel, at least
as far down the fan as the first two houses that were severely damaged by the debris flow.
Consideration should be given to the removal (if still habitable) of these two houses. The
upper surface is beyond the reach of debris flows of the size experienced in 2011, and as
such is a safer location for houses. An option for the two properties with severely damaged
houses on the lower surface , and also the as yet undeveloped upstream sections, could be
to extend this upper surface out away from Nyhane Drive to create sufficient area to build on.
This upper surface may still be at risk of significantly larger debris flows, and it is
recommended that trenching of the surface be carried out to identify if any debris-flow
deposits are present. With some modifications (including an earthen bund), the lower surface
at this point on the fan could then act as an area for the collection and containment of debris
flow and debris flood sediment and debris.
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Another option is modification of the existing stream channel. This would involve deepening,
widening and possibly lining the channel as in Figure 8.4. Consideration could also be given
to moving the channel further to the east, away from the existing housing so that it flows onto
the adjacent fan from Catchment 9.1. This fan has not been developed for housing and
debris flow or debris flood material overflowing the channel could be safely deposited onto
this surface. This would appear to be a feasible option, as in the 2011 event some of the
sediment from the Nyhane Drive stream did spill out of its channel and flow onto this fan
(Figure 3.15).
8.2.2
Pohara Valley fan
The fan on which Pohara Valley settlement is situated does not have the typical “fan” shape.
Rather, it is a long narrow fan which has been confined in a valley between two rows of hills,
and is feed by several tributaries that reach the fan at different locations. This, together with
the fact that almost the entire fan has been developed for housing, makes the options of
creating sediment retention areas or moving the stream channels on the fan (as suggested
for Nyhane Drive) impractical. However, because of the greater number of houses, and
therefore people at risk, the alternative of more expensive mitigation options may be justified.
This could include modifications to the existing channels to increase the capacity to carry
sediment, debris and flood waters, primarily by deepening, widening, realigning and possibly
lining vulnerable sections of the channel. Culverts should also be significantly enlarged.
Consideration should also be given to the use of debris-retention structures in Catchments
14.2 (Winter Creek) and 14.4 (Haile Lane Creek). Nearly all the landslide-generated
sediment, and much of the woody debris was derived from the upper, granite areas of these
catchments. In both catchments the streams pass between a limestone ridge before
travelling down narrow valley floors to the Pohara Valley fan (Figure 8.5). Debris retention
structures similar to those in Figure 8.2 and Figure 8.3 could be located just upstream of the
limestone ridge in both catchments.
Figure 8.2
Debris retention structure in Canada, which retains coarse debris (boulders and logs), while
allowing through-flow of finer sediment and flood water.
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Figure 8.3
Debris flow retention net in Japan.
Figure 8.4
Mitigation works on a steep debris flow fan in Italy. The structure confines the passage of debris
flow and debris flood material across the apex of the fan (high hazard zone).
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Figure 8.5
Possible sites for debris retention structures in Winter Creek (Catchment 14.2) and Haile Lane
Creek (Catchment 14.4), above the Pohara Valley settlement.
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8.3
EARLY WARNING SYSTEMS
Where a known high risk exists, but for social or economic reasons other mitigation options
are not suitable, an alternative could be to develop an early warning system. This would
allow evacuation, but would not protect buildings and other infrastructure. However, this
approach is not currently practicable for the study area. While Tasman District Council
operate telemetered rain gauges and river-monitoring stations which allow early warning for
floods, they are not sufficient for debris flows. Debris flows require a specific set of
conditions, and rainfall is only one, albeit important, factor. Due to the highly localised cells of
intense rainfall that characterise storms in the district, telemetered rain gauges would need to
be sited within or very close to susceptible catchments. Further, because of the speed of
debris flows and the short length of the channels down which they travel, gauged rises in
stream flow or the use of trip wires do not provide sufficient warning time.
Weather radar can forewarn of severe rainfall, however this does not guarantee that the
forecasted amount of rain will fall, exactly where it will fall, or more importantly whether a
debris flow will be generated. Currently any warnings issued would be precautionary at best,
and this would likely lead to public apathy after several “false alarms”.
8.4
AVOIDANCE
Perhaps the most prudent and cost-effective option where a high debris-flow risk has been
identified is to avoid development or limit further development. In the study area these option
are still available on most fans where an unacceptably high risk is identified. Where
development has already occurred, principally on the Nyhane Drive and Pohara Valley fans,
there may be key areas such as the apex of the Nyhane Drive fan, where the best option is
to remove buildings or infrastructure. This choice would need to be based on detailed site
assessment.
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9.0
CONCLUSIONS
•
In December 2011 the Pohara-Ligar Bay area in Golden Bay was hit by an extreme
rain storm which caused severe flooding, landsliding, and more unexpectedly, debris
flows and debris floods. Several of these debris flows and debris floods severely
damaged houses on fans at Nyhane Drive and in the Pohara Valley.
•
Debris flows and debris floods are natural processes that erode, transport and deposit
sediment and other debris. Deposition commonly occurs on fans, and some fans are
largely formed by these processes.
•
Analysis of rainfall totals and intensities estimate that the storm has a return period of
~200 years. Radiocarbon dating, albeit of only several deposits, suggest a recurrence
interval of 200+ years for debris flows. However, debris flow occurrence may increase
in the future, given the projection for an increase in the frequency and intensity of
extreme rainfalls in the district in the coming decades.
•
The debris flows were initiated in the steep, upper catchments by landslides that
delivered soil, rocks and vegetation into streams, where they coalesced to form
extremely rapid flows of water-saturated debris which further increased in volume as
they scoured material in the floors of valleys.
•
Where the streams become unconfined the debris flows deposited this fast moving
sediment, typically onto the surface of fans. The coarsest debris was deposited near
the head on the fans, causing the greatest structural damage to buildings. As the flow
travelled down fan, velocity, debris size and sediment concentration reduced, and the
process became a debris flood. While debris floods are not as dangerous as debris
flows and cause less structural damage to buildings, a greater number of houses in
Nyhane Drive and Pohara Valley were affected by sediment deposition from debris
floods. Sediment transported by debris floods reached the estuary behind Tata Beach.
•
Of the twenty nine catchments assessed between Tata Beach and Pohara, some
generated debris flows and debris floods which deposited sediment over much of their
fans, in some catchments only debris floods reached their fans, while in others no
debris flows or floods were generated.
•
Within the study area, debris flows and debris floods occurred in catchments where
exotic forest was the dominant vegetation, and also in catchments dominated by
grassland. In the adjacent Wainui catchment, debris flows and debris floods also
occurred under indigenous forest.
•
The twenty nine catchments were evaluated for two catchment parameters, the Melton
Ratio and catchment length. Threshold values for these parameters, in combination,
were able to identify those catchments which generated debris flows and debris floods.
•
Melton Ratio and catchment length parameters only provide a reconnaissance or high
level assessment, but may prove useful for a district-wide assessment where many
fans are involved. As parameter data is gathered for more and more catchments, and
can be calibrated against known debris flow/debris flood occurrence, thresholds can be
more closely defined. However, site investigations are always necessary where
development is proposed.
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•
Given the risks associated with future debris flow and debris flood occurrence on the
Nyhane Drive and Pohara Valley fans, several mitigation options should be considered.
On the Nyhane Drive fan, where areas of the fan are still undeveloped, the use of
bunds to collect and contain debris, and channel relocation and modification are
options. On the more highly developed Pohara Valley fan, stream modification and
construction of sediment retention dams are options.
•
A question remains as to the mix of land uses and land management required to
optimise the reduction in the volume of both sediment and woody debris that will be
delivered to the fans by future debris flows.
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10.0
RECOMMENDATIONS
•
Investigations into debris flow/debris flood hazards should be carried out on any fans
on which housing or other infrastructure developments are being considered, not only
within the study area, but also throughout Tasman District.
•
Investigations should include:
˗
initial analysis of catchment parameters (Melton Ratio and catchment length) to
identify catchment potential for debris flow/debris flood generation.
˗
identification of historic debris flow/debris flood events from reports and rainfall
records.
˗
interpretation of fan geomorphic and topographic features.
˗
trenching across sections of the fan to identify and date paleo-debris flows/debris
floods.
•
On a fan where debris flow/debris flood hazards have been identified, it is prudent to
avoid building near the head of the fan (apex), in low areas which may become debris
flow/debris flood paths, and near existing channels.
•
On the Nyhane Drive fan it would be prudent to avoid further development of the lower
surfaces near the head of the fan and to possibly remove any badly damaged
buildings. With some modifications (including an earthen bund), these areas could then
be used to collect and contain debris that overflows the stream channel in future storm
events.
•
The existing stream channel on the Nyhane Drive fan should be enlarged and lined to
increase its transport capacity, and consideration given to moving it eastwards to allow
any future uncontained debris flows and debris floods to spill onto the adjacent fan
which has no housing development.
•
On the Pohara Valley fan stream channels should be enlarged to increase their
capacity to contain flood waters and debris, and also lined to prevent bank erosion.
Culverts should be similarly enlarged.
•
Given the number of people and assets that occupy the Pohara Valley fan, and the
limited ability to constrain debris flows and debris floods as they pass through the
settlement, consideration should be given to construction of debris retention dams
higher in the catchment.
•
A detailed assessment of debris flow and debris flood pathways and debris distribution
be carried out on the Nyhane Drive and Pohara Valley fans, using differencing of preand post- 2011 event LiDAR surveys. This would provide valuable information for the
design of any engineered mitigation works.
•
Improve land management of the upper catchment to reduce the supply of sediment
and woody debris to streams. This will require detailed assessment be carried out on
the effects of land use and land management on the supply of both sediment and
woody debris in the upper catchments.
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•
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It is likely that a mix of all three land uses will continue to occur within the study area in
the future. Avoiding having large areas of even-aged exotic forest could potentially
reduce the supply of woody debris if storms were to coincide with times when
management-induced disturbance to the forest is greatest. Allowing some areas to
revert to scrub and ultimately indigenous forest may be the best option solely from a
hazard perspective. The challenge is to develop a mix which achieves both economic
and hazard-mitigation benefits, and this will require detailed consultations and
assessments.
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11.0
ACKNOWLEDGEMENTS
The funding for this study was provided by Tasman District Council. Tasman District Council
staff provided the following assistance: hydrological information on the storm (Martin Doyle
and Monique Harvey), analysis of rainfall data (Matt McLarin), GIS analysis (Terry
O’Donnell). The Council also provided the orthorectified aerial photographs. Mauri
McSaveney and Grant Dellow (GNS Science) reviewed the report. We would like to thank
the land owners in the study area who answered our questions and allowed us to wander
freely on their properties.
12.0
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Reyners, M., Bradley, B., Rhoades, D., Smith, W., Nicol, A., Pettinga, J., Clark, K.,
Jacobs, K. 2012. National seismic hazard model for New Zealand: 2010 update.
Bulletin of the Seismological Society of America, 102 (4), 1514-1542,
doi:10.1785/0120110170.
Taig, T.; Massey, C.I.; Webb, T.H. 2012 Canterbury earthquakes 2010/11 Port Hills slope
stability: principles and criteria for the assessment of risk from slope instability in the
Port Hills, Christchurch. GNS Science consultancy report 2011/319. 45 p.
Tait, A. 2011. Climate change projections for New Zealand – A literature review. NIWA client
report WLG2011-48, 43p.
Tasman District Council. 2012. Report summary – December 2011 Rain Event. Report No:
RCN12-01-10.
Welsh, A., Davies, T. 2010. Identification of alluvial fans susceptible to debris flow hazards.
Landslides, online publication, doi:10.1007/s10346-010-0238-4.
Wilford, D.J., Sakals, M.E., Innes, J.L., Sidle, R.C., Bergerud, W.A. 2004. Recognition of
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Landslides, 1, 61-66, doi:10.1007/s10346-003-0002-0.
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13.0
GLOSSARY
Bund: an embankment to control the flow of water or sediment.
Clast: a rock fragment or grain resulting from the breakdown of larger rocks.
Colluvium: a general term for soil and/or rock material transported by sheetwash or gravity,
and deposited on gentle slopes at the base of hillslopes.
Debris avalanche: a very rapid to extremely rapid (5-~20 m/s, 15-60 km/hr), shallow slide or
flow of partially or fully water-saturated debris on a steep slope, which is not confined within
an established channel.
Debris flood: a very rapid (up to ~5 m/s), surging flow of water, heavily charged with debris,
in a steep channel. A debris flood almost always occurs as a continuation downstream of a
debris flow, but can occur in the absence of a debris flow.
Debris flow: a very rapid to extremely rapid (5-10 m/s, 15-30 km/hr) flow of water-saturated,
non-plastic (granular) debris in a steep channel. Speeds are often faster than a fit human can
run. The sediment has a consistency of wet concrete, with sediment concentrations often in
excess of 60% by volume (80% by weight) compared to flood waters, where sediment
concentrations are generally <4% by volume (10% by weight).
Imbrication: an arrangement of clasts where they overlap one another in a consistent
fashion, such as roofing tiles, fish scales.
Landslide: a generic term in scientific and geotechnical literature for the movement of a
mass of rock, earth or debris down a slope under the influence of gravity. It covers a wide
variety of mass movement types and usually involves rapid failure along a slip plane at the
contact between more permeable material and underlying less permeable material.
Landslides occur on a wide variety of rock types and can be of any size.
LiDAR: an acronym for Light Detection And Ranging, which is an method of measuring the
distance to objects or surfaces by recording the time it takes for pulsed laser light to be
reflected back from the target to the emitting device.
Matrix: finer-grained sediment in which larger clasts are embedded.
Melton Ratio: an index of catchment ruggedness used to identify debris flow- and debris
flood-prone catchments. It is equal to catchment relief divided by the square root of
catchment area.
Orthorectified: images such as aerial photos that have been “rectified” or corrected for the
effects of distortion due to perspective and terrain relief, thus creating a planimetrically
correct image where features are represented in their true positions, and allowing accurate
measurements to be made.
Paleo-debris flow: a debris flow that occurred in pre-historic times, or before such events
were recorded.
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APPENDICES
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APPENDIX 1: CATCHMENT
UPSTREAM OF FAN APEX)
TYPES
(FOR
CATCHMENT
Catchment
Number
Rock Type
1.1
granite
19.5
100
1.2
granite
7.4
100
1.3
granite
1.9
100
2.0
granite
9.6
100
3.0
granite
13.1
100
4.0
granite
16.6
100
5.0
granite
12.7
100
6.0
granite
12.3
100
7.1
granite
2.8
65
gravel
1.4
33
Area (ha)
sandstone
0.1
2
granite
4.8
100
7.3
granite
14.3
100
8.0
granite
21.9
99
gravel
0.4
1
granite
10.0
99
gravel
0.1
1
granite
0.9
56
gravel
0.7
44
granite
102.0
98
gravel
1.6
2
granite
3.7
54
gravel
3.1
46
11.1
granite
18.7
100
11.2
granite
12.4
99
gravel
0.1
1
granite
23.2
84
limestone
4.5
16
granite
31.7
66
limestone
14.7
31
siltstone
1.6
3
siltstone
13.7
93
limestone
1.0
7
granite
64.4
80
limestone
9.3
11
siltstone
7.3
9
siltstone
19.4
67
limestone
8.1
28
9.2
10.1
10.2
11.3
12.0
14.1
14.2
14.3
14.4
AREA
Area (%)
7.2
9.1
84
ROCK
granite
1.4
5
granite
95.9
80
siltstone
14.0
12
limestone
9.9
8
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Catchment
Number
Rock Type
15.0
limestone
16.1
16.2
17.1
17.2
Area (ha)
Area (%)
8.7
58
siltstone
6.3
42
granodiorite
16.9
35
limestone
10.4
22
siltstone
10.3
21
granite
9.7
20
gravel
0.8
2
schist
0.1
0
siltstone
6.1
52
limestone
5.7
48
limestone
22.9
43
gravel
13.0
25
siltstone
11.6
22
schist
4.5
8
granodiorite
0.5
1
sand
0.3
1
granodiorite
83.5
38
granite
56.8
26
schist
46.2
21
limestone
31.8
15
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APPENDIX 2: CATCHMENT VEGETATION COVER (FOR CATCHMENT AREA
UPSTREAM OF FAN APEX).
86
Catchment
Number
Vegetation Cover
1.1
Area (ha)
Area %
% Forest
Indigenous forest
19.6
100
100
1.2
Indigenous forest
7.3
100
100
1.3
Indigenous forest
2.1
100
100
2.0
Exotic forest
Indigenous forest
Grassland
5.5
3.7
0.4
57
39
4
96
3.0
Exotic forest
Indigenous forest
Grassland
9.8
2.9
0.4
75
22
3
97
4.0
Exotic forest
Indigenous forest
15.5
1.1
93
7
100
5.0
Exotic forest
12.8
100
100
6.0
Exotic forest
12.4
100
100
7.1
Exotic forest
Grassland
Indigenous forest
2.2
1.4
0.8
50
32
18
68
7.2
Exotic forest
Indigenous forest
4.3
0.5
90
10
100
7.3
Exotic forest
14.3
100
100
8.0
Exotic forest
Indigenous forest
Grassland
21.7
0.7
0.1
96
3
1
99
9.1
Exotic forest
Grassland
6.2
3.9
61
39
61
9.2
Indigenous forest
Exotic forest
1.2
0.5
71
29
100
10.1
Indigenous forest
Exotic Forest
Grassland
Scrub
42.6
31.7
20.0
6.2
42
32
20
6
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Catchment
Number
Vegetation Cover
Area (ha)
Area %
% Forest
10.2
Indigenous forest
Grassland
4.5
2.3
66
34
66
11.1
Scrub
Grassland
Exotic forest
12.8
2.8
0.6
79
17
4
4
11.2
Grassland
Scrub
Exotic forest
7.6
4.4
1.3
57
33
10
10
11.3
Scrub
27.7
100
0
12.0
Scrub
Grassland
33.6
14.4
70
30
0
14.1
Scrub
14.7
100
0
14.2
Indigenous forest
Grassland
Scrub
31.3
29.3
13.0
42
40
18
42
14.3
Grassland
Indigenous forest
17.8
11.2
61
39
39
14.4
Grassland
Indigenous forest
103.3
16.5
86
14
14
15.0
Indigenous forest
Grassland
10.1
4.9
67
33
67
16.1
Grassland
Indigenous forest
Scrub
Exotic forest
29.9
12.9
4.2
1.1
62
27
9
2
29
16.2
Indigenous forest
11.8
100
100
17.1
Grassland
Indigenous forest
Exotic forest
36.8
13.9
2.0
70
26
4
30
17.2
Exotic forest
Indigenous forest
Grassland
Scrub
83.3
83.0
40.0
18.9
37
37
18
8
74
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APPENDIX 3: CATCHMENT BARE GROUND FOR DECEMBER 2011 STORM
(FOR CATCHMENT AREA UPSTREAM OF FAN APEX).
Catchment
number
1.1
1.2
1.3
2.0
3.0
4.0
5.0
6.0
7.1
7.2
7.3
8.0
9.1
9.2
88
2
Area bare ground (m )
% catchment area
Hill slopes
4317
2.5
Valley floor
2074
10.5
Hill slopes
1210
1.9
Valley floor
298
3.2
Hill slopes
949
5.1
Valley floor
295
14.3
Hill slopes
585
0.7
Valley floor
102
1.3
Hill slopes
2866
2.4
Valley floor
1906
18.1
Hill slopes
4771
3.0
Valley floor
1525
17.3
Hill slopes
1565
1.3
Valley floor
387
5.1
Hill slopes
4311
3.8
Valley floor
4202
46.9
Hill slopes
748
2.5
Valley floor
2533
17.9
Hill slopes
779
1.8
Valley floor
0
0
Hill slopes
6218
4.7
Valley floor
2777
27.6
Hill slopes
9526
4.3
Valley floor
411
70.6
Hill slopes
9606
9.9
Valley floor
3312
91.3
Hill slopes
0
0
Valley floor
0
0
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Catchment
number
10.1
10.2
11.1
11.2
11.3
12.0
14.1
14.2
14.3
14.4
15.0
16.1
16.2
17.1
17.2
2
Area bare ground (m )
% catchment area
Hill slopes
39084
3.9
Valley floor
1672
55.4
Hill slopes
1121
1.8
Valley floor
116
2.4
Hill slopes
14587
9.0
Valley floor
-
-
Hill slopes
19983
15.1
Valley floor
-
-
Hill slopes
49369
17.8
Valley floor
-
-
Hill slopes
44824
9.7
Valley floor
11250
58.3
Hill slopes
7240
4.9
Valley floor
-
-
Hill slopes
67106
9.2
Valley floor
2019
19.2
Hill slopes
8022
3.5
Valley floor
174
0.3
Hill slopes
68720
6.0
Valley floor
2595
5.3
Hill slopes
2009
1.6
Valley floor
58
0.3
Hill slopes
46958
10.0
Valley floor
7457
73.6
Hill slopes
2184
1.9
Valley floor
-
-
Hill slopes
21590
4.3
Valley floor
0
0
Hill slopes
103371
4.8
Valley floor
32006
34.2
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APPENDIX 4: LANDSLIDE BARE GROUND DISTRIBUTION IN RELATION TO
VEGETATION COVER
Landslide bare
2
ground area (m )
Area of vegetation
(ha)
Landslide bare
2
ground (m /ha)
Grassland
225,312
315
715
Scrub
85,437
136
628
Indigenous forest
162,335
278
584
Exotic forest*
70,525
225
313
Vegetation type
* The area of landslide bare ground in exotic forest has been under estimated – see section 4.2.2
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APPENDIX 5: LANDSLIDE BARE GROUND DISTRIBUTION IN RELATION TO
ROCK TYPE
Landslide bare
2
ground area (m )
Area of rock type (ha)
Landslide bare
2
ground (m /ha)
Granite
322,618
568
568
Granodiorite
60,497
101
599
Limestone
92,088
127
725
Siltstone
33,271
90
370
Schist
32,171
51
631
Gravel
2,905
21
138
Rock type
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