Résumés
Résumé
Le fleuve Orange (Afrique du Sud) est l’un des systèmes fluviaux les plus aménagés au monde : les barrages hydroélectriques ont modifié le volume d’écoulement du fleuve et son débit solide a diminué depuis les années 1930. Plus encore, dans son cours moyen inférieur (secteur d’Upington), le fleuve a été contraint entre des digues de 5 à 10 m de hauteur, des chenaux secondaires ont été remplis pour permettre le développement agricole et plus de 10 déversoirs de bas niveau ont été construits à travers le chenal. Dans des conditions similaires, des changements géomorphologiques visibles et rapides (incision du chenal, alluvionnement, etc.) ont été enregistrés dans divers systèmes fluviaux en région semi-aride. Pourtant, l’étude de cartes anciennes (des années 1920) et de photographies aériennes (à partir de 1937) montre une stabilité remarquable des formes fluviales, et ce, même à petite échelle : des bancs sableux et des seuils rocheux sont facilement reconnaissables dans le chenal principal. L’objectif de cet article est d’explorer les différentes hypothèses expliquant cette stabilité, en utilisant le concept de discontinuité et la théorie de la catastrophe, et de confronter les résultats avec les travaux récents sur le fleuve Orange. Notre recherche est basée sur le calcul des puissances spécifiques des crues vicennales, la reconnaissance de terrain (entre 2000 et 2003) et le prélèvement de sédiments, en utilisant la méthode de la courbe de Passega. Nous avons également effectué des comparaisons entre les photographies aériennes et les cartes anciennes et les photographies aériennes les plus récentes à l’aide des outils SIG (MapInfo). À micro-échelle, la progression et la destruction des roseaux (Phragmites australis et Arundo donax) a été utilisée comme indicateur des changements géomorphologiques du chenal. Les résultats montrent que les concepts de stabilité et d’équilibre, et les changements géomorphologiques fluviaux sont plus complexes qu’une simple comparaison statique.
Abstract
The Orange River (South Africa) is one of the most manipulated water system in the world : power dams have modified the flow volume of the river, and the sediment load has been decreasing since the 1930s. Moreover, in the lower middle reaches of the river (Upington region), the river has been constrained between 5 to 10 m high levees, secondary channels have been filled for agricultural development, and more than 10 low level weirs have been built across the channel. In comparable conditions, visible and rapid geomorphological changes (channel incision, aggradation, etc.) have been monitored in various water systems in semi-arid regions. Yet, the comparison between old maps (from the 1920s) and aerial photographs (from 1937 onwards) shows a remarkable stability of fluvial patterns, even at a small scale : sandy banks and rocky outcrops are easily recognizable in the main channel. The aim of this paper is to explore several hypotheses about this stability, using the Serial Discontinuity Concept and the Catastrophe Theory, and confronting the results with recent works on the Orange River. Our research is based on unit stream power calculation for 20-year return floods, field recognition (from 2000 to 2003) and sediment sampling, sorted using the Passega curve methodology. We also made comparisons between old maps and aerial photographs and the most recent aerial photographs using GIS tools (MapInfo). At the micro-scale level, the progression and the destruction of reeds (Phragmites australis and Arundo donax) has been used as an indicator of geomorphological changes in the river channel. The results show that the notion of stability and equilibrium, and fluvial morphological changes are much more complex that the one given by a simple static comparison.
Parties annexes
Références
- Baker, V.R. et Kochel, R.C., 1988. Flood sedimentation in bedrock fluvial systems, p. 123-137. In V.R. Baker, R.C. Kochel et P.C. Patton, édit., Flood Geomorphology. John Wiley and Sons, Chichester, 528 p.
- Benade, C., 1993. Management proposals for the regulated Orange River system within the borders of the Cape Province. Northern Cape Scientific Services, Cape Nature Conservation and Museums, Kimberley.
- Basson, M.S., 1999. Main Report, Orange River development project re-planning study. Department of Water Affair and Forestry, Pretoria.
- Bravard. J.-P., 1987. Le Rhône du Léman à Lyon. La Manufacture, Lyon, 451 p.
- Bravard, J.-P. et Peiry, J.-L., 1999. The CM pattern as a tool for the classification of alluvial suites and floodplains along the river continuum, p. 259-268. In S.B. Marriott et J. Alexander, édit., Floodplains : Interdisciplinary Approaches (no. 163). London Geological Society, Londres, 284 p.
- Davies, B.R., 1979. Stream regulation in Africa : a review, p. 113-142. In J.V. Ward et J.A. Stanford, édit., The Ecology of Regulated Streams. Plenum Press, New York.
- DWAF, 1992. Lower Orange River : Hydrology. Orange System Analysis. Department of Water Affairs and Forestry, Pretoria, PD000/00/0789, WPAM6417.
- Graf, W.L., 1979. Catastrophe theory as a model for change in fluvial systems, p. 13-32. In D.D. Rhodes et G.P. Williams, édit., Adjustment of the Fluvial System. Unwin Hyman, Londres, 372 p.
- Graf, W.L., 1988. Fluvial Processes in Dryland Rivers. Springer Verlag, Berlin, 343 p.
- Jolly, I.D., 1996. The effects of river management on the hydrology and hydroecology of arid and semi-arid floodplains, p. 577-609. In M. Anderson, D.E. Walling et P. Bates, édit., Floodplain Processes. John Wiley and Sons Australia, New South Whales, 668 p.
- Kovacs, Z., 1988. Regional maximum flood peaks in Southern Africa. Department of Water Affairs, Pretoria, Technical Report TR137.
- Lagasse, P.F., 1994. Variable response of the Rio Grande Dam Construction, p. 285-312. In R.A. Schumm et B.R. Winkley, édit., The Variability of Large Alluvial Rivers. American Society of Civil Engineers, New York, 467 p.
- Mackenzie, R.S. et Craig, A.R., 1999. Evaporation losses from south african rivers. South African Water Research Commission, Pretoria, WRC Report 638/1/99.
- Passega, R., 1957. Texture as characteristic of clastic deposition. Bulletin of the American Association of Petroleum Geologists, 41 : 1954-1984.
- Palmer, R.W., 1997. Principles of integrated control of blackflies in South Africa. South African Water Research Commission, Pretoria, WRC Report 650/1/97.
- Petts, G.E., 1984. Impounded Rivers : Perspectives for Ecological Management. John Wiley and Sons, Chichester, 326 p.
- Poff, N.L. et Ward, J.V., 1992. Physical habitat template of lotic systems : recovery in the context of historical pattern of spatiotemporal heterogeneity. Environmental Management, 14 : 629-645.
- Rooseboom, A. et Van Vuuren, S.J., 1988. Regime changes in the Caledon River associated with sediment deposition upstream of Welbedacht barrage, p. 365-369. In W.R. White, édit., International Conference on River Regime. John Wiley and Sons, Chichester, 456 p.
- Springer, G.S., Tooth, S. et Whol, E.E., 2006. Theoretical modeling of stream potholes based upon empirical observations of the Orange River, Republic of South Africa. Geomorphology, 82 : 160-176.
- Standford, J.V. et Ward, J.A., 1983. The serial discontinuity concept of lotic ecosystem, p. 29-42. In T.D. Fontaine III et S.M. Bartell, édit., Dynamics of Lotic Ecosystems. Ann Arbor Science, Ann Arbor, 494 p.
- Standford J.V. et Ward, J.A., 2001. Revisiting the serial discontinuity concept. Regulated Rivers, 17 : 303-310.
- Thoms, M.C. et Walker, K.F., 1991. Channels changes related to low level weirs, p. 235-249. In P.A. Carling et G.E. Petts, édit., Lowlands Floodplains Rivers : Geomorphological Perspectives, John Wiley and Sons, Chichester, 302 p.
- Tinkler, K. et Whol, E., édit., 1998. Rivers over Rock. Fluvial Processes in Bedrock Channels. American Geophysical Union, Washington D.C., Geophysical Monographs Series 107, 323 p.
- Tooth, S. et MacCarthy, T., 2004. Anabranching in mixed bedrock alluvial rivers : the example of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Geomorphology, 57 : 235-262.
- Tricart, J., 1956. Types de fleuves et systèmes morphogénétiques en Afrique occidentale, p. 303-345. Com. Travaux Hist. et Sc., Section de géographie.
- UNEP, 1995. Global Environment Monitoring System : Water Quality of World River Basin. Nairobi, Unep Environmental Library no 14, 40 p.
- Vannote, R.L., Minshall, G.W., Cummins, K.W., Sedell, J.R. et Cushing, C.E., 1980. The river continuum concept. Canadian Journal of Aquatic Sciences, 37 : 130-137.
- Walker, K.F., 1992. A semi-arid lowland river : River Murray, South Australia, p. 472-492. In P.A. Calow et P.E. Petts, édit., The Rivers Handbook, vol. 1. Blackwell Scientific, Oxford.
- Whol, E., 1998. Bedrock channel morphology in relation to erosional processes, p. 133-151. In K. Tinkler et E. Whol, édit., Rivers over Rock. Fluvial Processes in Bedrock Channels. American Geophysical Union, Washington D.C., Geophysical Monographs Series 107, 323 p.
- Zawada, P.K., 1997. Palaeoflood hydrology : method and application in flood-prone Southern Africa. South African Journal of Science, 93 : 131-132.