Restoration of sediment regimes by modifying dam operations

Abstract

Despite the many positive effects of dams, they can also have several negative impacts on natural systems. One of these impacts is the decrease in sediment inflow towards the coast which can lead to coastal erosion.
A technique that can be applied to improve the situation downstream is modifying the discharge through the dam. Unfortunately, there have not been many studies done yet on such a re-operation regime. This thesis tried to get more understanding of the possible consequences of a modified discharge regime concerning restoration of pre-dam sediment regimes based on the Volta Delta in Ghana as a case study.
In this thesis, the effect of modifying dam operations on both the upstream part of the river (the reservoir) and the downstream part have been investigated. For the upstream section, a simulation was made for a simplified model for a reservoir with similar characteristics as the Volta Lake. According to this model, the modified dam operations are not useful for reservoirs like the Volta Lake. The technique might be useful for shallower reservoirs.
A modified dam operation scenario aims to reintroduce flood pulses to the river. For the downstream part of the river, three aspects of the operation scenarios have been investigated: the discharge peak height during an operation, the duration of such a peak and the extra sediment load that might be brought to the river from the reservoir. The response of the river downstream was measured by two aspects: the sediment load to the ocean and the bed level changes in the river.
To show the response of the river, several simulations were made with different aspects of the operation scenario. At each simulation, only one parameter was changed, e.g., the peak discharge is changed from 1500 to 5000 m3/s in different simulations. By doing so, graphs could be made to show a correlation between the three operation aspects and the two aspects of the river's behavior.
It turns out that by increasing the discharge, more sediment will be transported to the ocean (with a maximum of 16% of pre-dam sediment load). There will be more non-cohesive material carried to the sea than cohesive material. When there is no extra sediment coming to the river, the river bed will degrade. This will happen at equal rates all along the river which will not change the bed slope.
By increasing the sediment load from upstream of the dam without introducing flood pulses, there will be more sediment transported to the ocean. However, most of the sediment will settle at the river bed causing accretion. The accretion of the river bed will not happen at equal rates along the river so the bed slope will change.
Finally, the change in peak duration will affect neither the transport of sediment nor the river bed. The reason for this might be that the river bed adapts to the different flow conditions in such a way that the same amount of sediments is transported towards the coast.