Resource Directed Measures for Protection of Water Resources: River Ecosystems
R18: THE PROCEDURE FOR GENERATING FLUVIAL GEOMORPHOLOGICAL INFORMATION FOR
THE INTERMEDIATE AND ECOLOGICAL RESERVE (QUANTITY)
Senior Author:
R Wadeson, IWR Environmental
Editor:
Heather MacKay, Department of Water Affairs and Forestry
Lizette Guest, Guest Environmental Management
Version:
1.0
Date:
24 September 1999
M:\f_rdm_october\rivers\version 1.0\riv_appR18_version1.0.doc
Department of Water Affairs and Forestry, South Africa
Version 1.0: 24 September 1999
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Resource Directed Measures for Protection of Water Resources: River Ecosystems
Appendix R18:
Role of Geomorphology in Intermediate Ecological Reserve
Determination1
R18.1 Intermediate ecological reserve approach
The time and financial constraints of the IER means that little more than a geomorphological desktop study can
be carried out. This entails the use of readily available data such as 1: 50 000 topographical map sheets, the GIS
data base of WR90, a video of the river where available and aerial photography. The information is used to
provide a general overview of river geomorphology and a broad classification of channel type.
A comprehensive ecological reserve requires the same or similar data where available, but the analysis is more
comprehensive with a detailed classification to the river reach scale.
R18.2 Determination of reference conditions and present
ecological state (Steps 3 and 4).
The determination of present ecological state (geomorphology) forms an essential component of the assessment
of the intermediate ecological reserve. To date there has never been a formal approach to determine the present
state of geomorphology as measured against a pre-determined natural state. The method described here must be
seen as developmental and therefore likely to evolve over time.
For twenty years now geomorphologists have moved away from notions of stability in river systems to those of
metastability, or periods of quasi-stability interrupted by episodes of rapid change, which appear to managers as
challenging demonstrations of instability. This new approach is best summarised in the term thresholds.
An important scenario which needs to be recognised within the threshold concept is the case where a river basin
is affected by artificial developments such as river regulation, land-use change or urbanisation. In this situation
the river basin is unlikely to ever reach a condition of steady state or equilibrium. Instead a series of relatively
stable conditions are interspersed by periods of rapid change as geomorphological thresholds are crossed.
Virtually all catchments within South Africa have been affected by artificial developments. The scale of the
development tends to determine the time scales at which geomorphological change takes place within the river
system. For this reason it is very difficult to determine a pristine or natural state which may be used as a
reference condition for present geomorphological state.
R18.2.1 Reference Condition
In an attempt to determine a reference state for which the present river can be compared, use is made of a zonal
classification system.
Rowntree and Wadeson (1998)2 have developed a zonal classification system for South African rivers modified
from Noble and Hemens (1978). An attempt was made to give each zone a geomorphological definition in
terms of distinctive channel morphological units and reach types. After working in a number of different rivers
around the country it has become clear that channel gradient is a good indicator of channel characteristics and
that probable or expected differences down the profile can be identified from an analysis of gradients
(See Table 1).
1
2
Editor’s note: Consistency of terminology to be checked for version 1.1
Editor’s note: Author to provide references.
Department of Water Affairs and Forestry, South Africa
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Resource Directed Measures for Protection of Water Resources: River Ecosystems
Table 1: Zonal classification system for South African rivers (Rowntree and Wadeson 1998)
1. Source zone
Not specified
Low gradient, upland plateau or upland basin able to store water.
Spongy or peaty hydromorphic soils.
2. Mountain
headwater
stream
0.1 - 0.7
A very steep gradient stream dominated by vertical flow over bedrock
with waterfalls and plunge pools. Normally first or second order.
Reach types include bedrock fall and cascades.
3. Mountain
stream
0.01 - 0.1
Steep gradient stream dominated by bedrock and boulders, locally
cobble or coarse gravels in pools. Reach types include cascades,
bedrock fall, step-pool, plane bed, pool-rapid or pool riffle.
Approximate equal distribution of >vertical= and >horizontal= flow
components.
4. Foothills
(cobble bed)
0.005 - 0.01
Moderately steep, cobble-bed or mixed bedrock-cobble bed channel,
with plane bed, pool-riffle, or pool-rapid reach types. Length of pools
and riffles/rapids similar. Narrow flood plain of sand, gravel, or
cobble often present.
5. Foothills
(gravel bed)
0.001 - 0.005
Lower gradient mixed bed alluvial channel with sand and gravel
dominating the bed, locally may be bedrock controlled. Reach types
typically include pool- riffle or pool-rapid, sand bars common in
pools. Pools of significantly greater extent than rapids or riffles.
Flood plain often present.
6. Lowland
sand bed or
Lowland
fllodplain
0.0001- 0.001
Low gradient alluvial sand bed channel, typically regime reach type.
Often confined, but fully developed meandering pattern within a
distinct flood plain develops in unconfined reaches where there is an
increased silt content in bed or banks.
B. Additional zones associated with a rejuvenated profile
7. Rejuvenated
bedrock fall
/ cascades
0.01 - 0.5
Moderate to steep gradient, often confined channel (gorge) resulting
from uplift in the middle to lower reaches of the long profile, limited
lateral development of alluvial features, reach types include bedrock
fall, cascades, and pool-rapid.
8. Rejuvenated
foothills
0.001 - 0.01
Steepened section within middle reaches of the river caused by uplift,
often within or downstream of gorge; characteristics similar to
foothills (gravel/cobble bed rivers with pool-riffle/ pool-rapid
morphology) but of a higher order. A compound channel is often
present with an active channel contained within a macro-channel
activated only during infrequent flood events. A flood plain may be
present between the active and macro-channel.
9. Upland
flood plain
(UFP)
0.0001- 0.001
An upland low gradient channel, often associated with uplifted plateau
areas as occur beneath the eastern escarpment.
Using the 1:50 000 topographic map series a zonal classification must be carried out. The procedure is
described in detail in the geomorphology chapter of the BBM manual (Also see Appendix R2). Quite simply it
requires the measuring of each river length between contours, the capturing of this data to a spread sheet and the
analysis of this data for the percentage change in grade both up and down the river. Results are aggregated for a
zone definition and dissagragated for a reach classification. This information is used to determine zone
boundaries and average gradient and reference conditions down the river channel network. The reference
condition assumes that the river is in a stage of steady state and that no geomorphic thresholds have been
crossed.
The time required for a zonation analysis is approximately one day but is largely dependent on the size of the
river network to be analysed.
The product of this analysis is an annotated diagram of the longitudinal profile of the river clearly showing the
altitude at which the various zones are found an example is given below. The diagram needs to be interpreted
together with the table describing zone characteristics.
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Resource Directed Measures for Protection of Water Resources: River Ecosystems
Figure 1: TUGELA River long profile diagram
R18.2.2 Present Geomorphological State
For the intermediate level of reserve determination present state is assessed using professional judgement based
on a once off site visit. Expert opinion is used to determine a category for each site.
Aspects of river geomorphology that are considered at the river include:







Morphology and habitat - riffles, pools, rapids, bars etc
Boundary condition - erosion, vegetation, stability
Instream disturbance - bridges, weirs, pumps
Valley form - confined, unconfined
Pattern - single thread, anabranching, meandering
Morphometry - width, depth (incision)
Sediment - bars, suspended load etc.
Department of Water Affairs and Forestry, South Africa
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Resource Directed Measures for Protection of Water Resources: River Ecosystems
A categorisation of present geomorphological state uses 6 categories A-F. A description of these classes
follows:
CATEGORY
DESCRIPTION
A

Unmodified, natural;
B

Largely natural with few modification;

A small change in geomorphology and natural habitats

Moderately modified;

A change in geomorphology and in -stream habitat but geomorphic thresholds do not
appear to have not been crossed.

Largely modified;

Large changes in geomorphology and in -stream habitat, geomorphic thresholds appear
to have been crossed with the river moving towards a new equilibrium.

Seriously modified;

The loss of natural in -stream habitat is extensive. The system appears to be extremely
unstable.

Critically modified;

Channel Modifications have reached a critical level with an almost total loss of natural in
- stream habitat. Geomorphological changes are virtually irreversible.
C
D
E
F
The time required for the assessment of present state is approximately 1.5 hours on site and is carried out as part
of the general geomorphological analysis.
R18.3 Setting the intermediate ecological reserve (Step 6)
R18.3.1 Preparatory work
Prior to arrival at a selected IER field site a geomorphological classification needs to be completed. The
detailed method for this is outlined in the Building Block Methodology (BBM) manual. A brief description of
the levels of geomorphological analysis is given below.
Segment analysis (section 12.4.1 BBM manual):
Using maps and data from the WR90 database, runoff and sediment yield maps for the catchment are produced.
A segment is defined as a length of channel along which there is no significant change in the imposed flow
discharge or sediment load. Segments can be delineated by overlaying maps of runoff and sediment yield with
the channel network so as to identify major changes in runoff and or sediment along the length of the channel.
Segment boundaries should be co-incident with major tributary junctions and/or a changes in stream order.
Zonal analysis (section 12.4.2 BBM manual):
This utilises information from the digitised 1: 50 000 topographical maps. The product of this is the long profile
as described previously in this document.
Assessment of long term channel change (section 12.4.5 of the BBM manual):
Aerial photographs are a useful tool to aid the assessment of past changes in channel morphology. The earliest
cover, available for most of the country, was taken in the late 1930s at a scale of 1: 25 000. Aerial photography
was taken thereafter at approximately ten-year intervals at scales ranging from 1: 20 000 to 1: 50 000. Channel
plan features and channel morphology can only be distinguished on large-scale photography (1: 30 000 or
greater), and then only for wider channels lacking overhead vegetation. The 1: 50 000 cover of 1975 is of
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Resource Directed Measures for Protection of Water Resources: River Ecosystems
limited value. Large-scale aerial photography at 1: 10 000 is available for selected rivers such as the lower Sabie
and the lower Olifants. Time and financial budgets should allow for the purchase and analysis of sequential
aerial photographs at least for the reaches in which IER sites are located.
Assessment of long term change must take into account the features of both the reaches of interest and their
position in the channel network relative to the planned developments and sediment source areas. For example,
if the main source of present day sediments is upstream of a planned reservoir, the downstream channels may
change from being transport limited to supply limited. The extent of bedrock sections would be likely to
increase. In contrast, if there are significant sediment inputs via tributaries downstream of the proposed dam,
the channel is likely to become transport limited due to the concomitant reduction in the frequency of flood
events. In this situation reaches would be prone to aggradation and a build up of sand and gravel bars. Such
aggradation would be greatest in reaches with a moderate to low gradient. Steeper reaches would be more likely
to maintain their present form.
The presentation of results for the assessment of channel change will be in the form of a verbal report and hand
traced maps of channel morphology. This should be accompanied by reproductions of sequential photographs
where appropriate. Photographs should be available at the workshop.
Time required for the IER preparatory work is approximately three days depending on the size of the catchment.
R18.3.2 Specialist meeting
The specialist meeting for the IER procedure is one that normally occurs on site i.e. next to the river. A detailed
description of the full procedure for the setting of a comprehensive reserve is given in section 12.4.4 of the
BBM manual. It is important to note here that the IER procedure requires considerably less data collection at
the site.
The geomorphological input for this meeting requires a full visual survey of the site using the forms that have
been prepared by Rowntree and Ziervogel (1999) for the National Aquatic Ecosystem Biomonitoring
Programme.
The following information must be recorded:

It is important to assign a reach type to the site to assist extrapolation of results at a later date.

Detailed field notes should be made of the cross-section morphology. This information can later be
transferred to the surveyed cross-sections when they become available. Evidence of past flood levels should
be noted and the position of debris lines included in the cross-section surveys.

An assessment of the advantages and disadvantages of the site with respect to the geomorphology should be
made whilst in the field (see Appendix R22). Particular areas of interest include the degree of stability at a
site, the type of sediment found on the bed and its mobility, evidence of an annual flood height (bankfull
terraces etc), habitat and morphological diversity.
The time required for an on site analysis is approximately 2 hours, a considerable degree of expert judgement is
used in this IER procedure due to the lack of time and resources at this level of analysis.
After the on site analysis the geomorphologist should have a broad understanding of the flow requirements
necessary to carry out the various geomorphological tasks within a reach such as local scour and deposition and
bed and bank maintenance. This understanding can then be formalised and quantified utilising data provided by
the hydraulic engineer i.e. depth/velocity/discharge relationships. The justification for required flows are given
in the form of motivation sheets utilising the above information.
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R18.4 Specialist training
There is a high level of expertise required by a geomorphologist for the completion of an IER due to the fact that
there is limited time and resources available and therefore a number of decisions are made based on expert
opinion rather than data collection. Of particular significance is the fact that reasonable results are not so much
dependant on the level of formal geomorphological training but rather a broad understanding of South African
river conditions. The most important requirement then is extensive experience of fluvial environments within
South Africa.
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