Downstream changes in river morphology as a result of dam developments

[摘要] ENGLISH ABSTRACT:A dam can have a significant impact on the downstream river morphology by alteringboth the flow regime and the sediment load. The effect of a dam is dependent onfactors such as the storage capacity relative to the mean annual runoff (MAR), theoperation of the reservoir and the sediment yield of the catchment. Changes in theriver morphology include the degradation and coarsening of the riverbed, generallycloser to the dam, and aggradation further downstream where the sediment deliveredby tributaries cannot be carried through because of the reduced sediment transportcapacity of the river. The impact of a dam can stretch over several hundreds ofkilometres.The main objective of this thesis was to gain a better understanding of the impactsthat dams can have on the downstream river morphology. This was done bydeveloping equations that can describe the channel geometry, investigating the effectof the presence of clay and silt on the sediment transport behaviour of sediments, aswell as detailed evaluation of simulations carried out with a one-dimensionalmathematical river model (MIKE 11).The calibrated regime equations were found to be comparable to other internationallydeveloped regime equations and to be suitable for natural rivers. It was found,however, that these regime equations are not applicable to rivers downstream of damsthat have highly unnatural release patterns. Further research is needed in this regard.By investigating the effect of cohesive sediments on the sediment transport behaviourof mixed sediments it was found that as little as 7% clay and silt in the bed couldaffect their sediment transport characteristics. A methodology was also developed bywhich the critical conditions for mass erosion of cohesive sediments can be describedin terms of the applied stream power. Sediment transport equations were calibratedand verified in terms of the unit input stream power for fine and non-cohesivesediments. The sediment transport equation for fine sediments was implemented inMIKE 11.The simulations over a 40 km reach of the Pongola River downstream ofPongolapoort Dam, have shown that even when a large demand is placed on thestored water, and most of the smaller floods are therefore absorbed by the dam, thedownstream impact can still be considerable, with as much as 5 m deep erosion inplaces. The sediment loads are generally reduced (by as much as 35%), but theeffective catchment area downstream of the dam has been reduced by as much as90%, indicating that substantial erosion had to have taken place in the river.Coarsening of the riverbed was also observed during the simulations.

[发布机构] Stellenbosch University