Abstract

As South African ecologists become increasingly aware of the possible long term impacts of river regulation schemes on channel structure and associated aquatic habitats, fluvial geomorphologists are being called upon to assist in recommending instream flow requirements for future river regulation schemes. In South Africa, the standard procedure for the assessment of Instream Flow Requirements (IFR) is the Building Block Methodology (BBM). Central to the BBM is the assumption that the flow regime can be divided into three groups of flow: the low flows, freshes and floods. Each group has a particular ecological (and geomorphological) significance. The IFR assessment are applied at selected sites, chosen to represent the river reaches downstream of the dam site. This paper presents a geomorphological framework developed as a component of the BBM. This framework recognises five important geomorphological issues which are addressed within an IFR exercise: (1) A general assessment is made of the catchment condition to determine the potential for morphological change within the river. (2) An evaluation of the geomorphological characteristics of the river network is carried out to aid the selection of IFR sites within representative reaches. (3) At each IFR site an assessment is made of the relationship between hydraulic diversity and discharge for the different morphological units present at the site. This assessment relates in particular to the low flow component of the BBM. (4) The freshes and floods required to maintain channel form and bed condition are determined at each IFR site. An assessment is also made of the likely pattern and direction of morphological change that are likely to result from the recommended flow regime. (5) In the case of interbasin transfers, an assessment should be made of the potential impact of transferred water on the receiving channel. To illustrate the practical application of the geomorphological framework to an IFR procedure, an example is given from a recently completed study carried out in the Tugela River in KwaZulu-Natal, commissioned by the Department of Water Affairs and Forestry. The underlying theoretical principles upon which decisions were made are outlined in this paper.

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References

Andrews, E.D.
1980
.
Effective and bankfull discharge of streams in the Yampa Basin, Colorado and Wyoming
.
Journal of Hydrology
,
46
:
311
330
.
Bathurst, J.C.
1987
. “
Measurement and modelling bedload transport in channels with coarse bed material
”. In
River Channels: Environment and Process
, Edited by: Richards, K.
272
290
.
Oxford
:
Blackwell
.
In
Carling, P.A.
1988a
.
Channel changes and sediment transport in regulated UK rivers
.
Regulated Rivers: Research and Management
,
2
:
369
387
.
Carling, P.A.
1988b
.
The concept of dominant discharge applied to two gravel bed stream in relation to channel stability thresholds
.
Earth Surface Processes and Landforms
,
13
:
355
367
.
DWAF (Department of Water Affairs and Forestry)
.
Starter Document, Tugela River IFR Workshop
.
September 26-30 1995
.
Pretoria
:
Montello Lodge
.
Erskine, W.D.
1985
.
Downstream geomorphic impacts of large dams: the case of Glenbawn Dam
.
NSW. Applied Geography
,
5
:
195
210
.
Estes, C.E. and Osborn, J.F.
1986
.
Review and analysis of methods for quantifying instream flow requirements
.
Water Resources Bulletin
,
22
(
3
):
389
398
.
Frissell, C.A., Liss, W.J., Warren, C.E. and Hurley, M.D.
1986
.
A hierarchical framework for stream classification: viewing streams in a watershed context
.
Environmental Management
,
10
:
199
214
.
Graf, W.L.
1988
.
Fluvial Processes in Dryland Rivers
,
New York
:
Springer Verlag
.
Gregory, K.J. and Park, C.C.
1974
.
Adjustment of river channel capacity downstream from a reservoir
.
Water Resources Research
,
10
:
870
873
.
Hammad, Y.H.
1972
.
River bed degradation after closure of dams
.
Journal of Hydraulics Division
,
98
:
591
607
.
ASCE, (HY4)
Harvey, A.M.
1969
.
Channel capacity and adjustment of streams to hydrologic regime
.
Journal of Hydrology
,
8
:
82
98
.
Hjulström, F.
1935
.
Studies of the morphological activities of rivers as illustrated by the River Fyris
.
Bulletin Geological Institute University of Uppsala
,
25
:
221
527
.
Hughes, D.
1994
.
Amatola Regional water Supply Scheme: Nahoon River Release
,
DWAF Report No. VS/600/08/E001
Pretoria
:
DWAF
.
Kellerhals, R. and Gill, D.
Observations and potential downstream effects of large storage projects in Northern Canada
.
Proceedings of the 11th Congress of the International Commission on Large Dams
.
Madrid, Spain.
pp.
731
754
.
King, J., Tharme, R., Bruwer, C. and Louw, D.
August
1993
.
Explanation of the building block methodology
,
August
,
Pretoria
:
DWAF
.
Olifants River Rosendal Dam IFR Worksession, 24–26
King, J. and Louw, D.
1998
.
Instream flow requirements for regulated rivers in South Africa using the building block methodology
.
Aquatic Ecosystem Health and Management
,
1
:
109
124
.
Kochel, R.C.
1988
. “
Geomorphic impact of large floods: review and new perspectives on magnitude and frequency
”. In
Flood Geomorphology
, Edited by: Baker, V.D.
169
187
.
New York
:
Wiley
.
In
Naiman, R.J., Ionzarich, J.D., Beechie, T.J. and Ralph, S.C.
1992
. “
General principles of classification and the assessment of conservation potential in rivers
”. In
River Conservation and Management
, Edited by: Boon, P.J., Calow, P. and Petts, G.E.
93
123
.
Chichester
:
Wiley
.
In
Petts, G.E.
1979
.
Complex response of river channel morphology subsequent to reservoir construction
.
Progress in Physical Geography
,
3
:
329
362
.
Petts, G.E.
1980
.
Long term consequences of upstream impoundment
.
Environmental Conservation
,
7
(
4
):
325
332
.
Pickup, G. and Warner, R.F.
1976
.
Effects of hydrologic regime on magnitude and frequency of dominant discharge
.
J. Hydro.
,
29
:
51
75
.
Rowntree, K.M. and Wadeson, R.A.
Translating channel morphology into hydraulic habitat: application of the hydraulic biotope concept to an assessment of discharge related habitat changes
.
Proceedings 2nd IAHR International Symposium on Hydraulics and Habitats
. pp.
A281
A292
.
Sherrard, J.J. and Erskine, W.D.
1991
.
Complex response of a sand bed stream to upstream impoundment
.
Regulated Rivers: Research and Management
,
6
:
53
70
.
Van Niekerk, A.W., Heritage, G.L. and Moon, B.P.
1995
.
River classification for management: the geomorphology of the Sabie River in the Eastern Transvaal
.
South African Geographical Journal
,
77
(
2
):
68
76
.
Wadeson, R.A.
1994
.
A geomorphological approach to the identification and classification of instream flow environments
.
South African Journal of Aquatic Science
,
20
(
1–2
):
38
61
.
Wadeson, R.A.
1996
.
The development of the hydraulic biotope concept within a hierarchical geomorphological model
,
South Africa
:
Rhodes University Grahamstown
.
unpublished Ph.D. thesis
Wadeson, R.A. and Rowntree, K.M.
1994
. “
A hierarchical geomorphological model for the classification of South African river systems
”. In
Classification of rivers and environmental health indicators. Proceedings of a joint SA-Australian workshop, 7–14 February, 1994, Cape Town, South Africa
,
WRC Report no. TT63/94
Edited by: Uys, M.C.
49
67
.
Pretoria
:
Water Research Commission
.
In
Wadeson, R.A. and Rowntree, K.M.
1998
.
Application of the hydraulic biotope concept to the classification of instream habitats
.
Aquatic Ecosystem Health and Management
,
1
:
143
157
.
Williams, G.P. and Wolman, M.G.
1984
.
Downstream effects of dams on alluvial rivers
,
US Geological Survey Professional Paper 1286
Washington DC
:
USGS
.
Williams, G.P.
1978
.
Bankfull discharge of rivers
.
Water Resources Research
,
14
:
1141
1154
.
Wolman, G. and Miller, J.
1960
.
Magnitude and frequency of forces in geomorphic processes
.
J. Geology.
,
68
(
1
):
54
74
.