Simulation of artificial flood releases from Skuifraam Dam to limit its impact on the Berg River morphology

摘要

The proposed Skuifraam Dam, South Africa, will be the first major dam on the Berg River and it could have a major impact on the river morphology. The upper reach of the river where the dam is to be constructed is a steep boulder bed river, while 30 km downstream it becomes a sand-bedded stream. A hydrodynamic-morphological study has been carried out to simulate the long-term pre-dam current conditions, the post-dam morphology without any artificial flood releases and the post-dam morphology with artificial flood releases. A one-dimensional model was used with unsteady flow and coupled sediment transport to simulate a representative 10 year period. The hydrodynamic model was calibrated on flow records at two flow gauging stations on the river. The narrowing of the river due to the construction of the dam was taken into account by using an empirical relationship. Analysis indicated that the large boulders would only be transported during major floods and sediment transport simulation therefore focused on graded sand fractions. The effect of the dam was studied by evaluating the simulated total sediment transport balance of the river and by plotting the discharge-sediment load relationship at different locations along the river, for the pre- and post-dam conditions. The simulations considered one annual flood release from the dam during winter in July or August when the reservoir inflow exceeds 50 m~3/s. The artificial flood degrades the upper reach immediately downstream of the dam, but further downstream the sediment transport balance is improved. The simulations carried out in the 73 km river downstream of the dam indicated that artificial floods are required for river maintenance, but that the originally proposed annual flood of 160 m~3/s would be too regular and not effective to limit aggradation of the lower reaches of the river. Alternating artificial floods of 160 m~3/s and 2401 m~3/s have therefore been proposed for river maintenance, which should be released from the dam with freshets and low flow environmental flow requirements. It is proposed that the methodology followed in this investigation is used at any proposed new and existing dams to evaluate the impact of a dam on the downstream river morphology and to design artificial flood releases.