The creation of a reservoir in the White Volta River, Ghana

Abstract

There are plans for the construction of a multi-purpose dam in the White Volta River near Pwalugu in northern Ghana. The reservoir that will be formed should be beneficial for hydropower generation, irrigation and fishery. Up to now there has been no research into the morphological consequences of the creation of this reservoir. In this study the rate and location of sedimentation in the reservoir and the erosion of the riverbed downstream of the dam are investigated. Sedimentation could lead to a loss of storage capacity having less water available for hydropower generation and irrigation purposes. Delta deposition (i.e. sedimentation of coarse sediment in the upstream part of the reservoir) could lead to higher water levels in the river upstream, causing an increased risk of flooding. Erosion of the riverbed downstream of the dam could lead to decreased water levels in the downstream reach. This could provoke riverbank collapse and have negative effects on agriculture due to lowering of groundwater levels. Possibly bridges downstream could be damaged by scouring of the foundations. The aim of this research is to understand the large-scale morphological processes and to give first order estimations of the morphological changes. Additionally, the sediment balance of the White Volta catchment is investigated to estimate landscape erosion. The method consists of a literature study, a field study and a morphological model (SOBEK) of the White Volta River. The total sedimentation in the intended reservoir has been analysed by determining the yearly sediment transport of the White Volta River. The estimation is based on field measurements, conducted in September 2011 at a location close to the proposed dam site, supplemented with a literature study and theoretical transport formulas. The yearly average sediment inflow appeared to be ± 1 million ton/year. The theoretical trap efficiency of the reservoir is nearly 100%, so it is assumed that all sediment will be deposited. If all sediment settles in the active storage of the reservoir, this would mean a storage loss of 3% per 100 years. The impact of this storage loss on the functions of the reservoir is considered negligible. The delta deposition (i.e. sedimentation of coarse material) and the erosion downstream of the dam have been estimated using SOBEK RE, a 1D software package capable of solving the hydrodynamic, sediment transport and morphological equations. The model parameters followed from the field research. The delta deposition depends on the water level variation in the reservoir. If the water level varies between maximum and minimum operative level, the reservoir will vary in length over 40 km and most of the delta deposition will occur in this region. The delta deposition is propagating in both upstream and downstream direction. Due to upstream propagation the riverbed level upstream of the reservoir will also increase. After 50 years, the effect is restricted to ± 30 cm just upstream of the lake and has damped out at the border of Burkina Faso. The water levels will slightly increase during high flows. More research is needed to analyse the increased risk of flooding. Erosion of the riverbed downstream of the dam will occur. The released flow through the dam is expected to be sediment-free. As the sediment concentration is lower than the sediment carrying capacity, the river will take up sediment and the bed will experience erosion. The riverbed level could decrease by 20-30 m in 50 years just downstream of the dam. The erosion rate is reducing in downstream direction, but propagating in time. After ± 30 years a bridge near Pwalugu could be affected. The erosion could be limited locally if any coarse layers are present beneath the riverbed. The erosion rate depends on the released flow through the dam, which is determined by the operational strategy. Spillage could occur when the reservoir gets completely filled. These high flows will have a substantial negative effect on the erosion. Spillage cannot be completely prevented, but the risk could be reduced by ensuring that the lake is at minimum level at the start of the rain season. The drawback of this solution is a decreased energy generation, as the lake will not fill up completely during dry years. The sediment balance of the White Volta catchment is based on sediment deposition and erosion rates. During the dry season there is a supply of Saharan dust into the catchment by so-called Harmattan winds. During the rain season eroded material is flowing out of the catchment as suspended sediment in the White Volta River. The Harmattan dust deposition rate has been derived from literature. The sediment deposition and erosion rate are equal (± 15 mm/1000 years), therefore there is no net landscape erosion in the White Volta catchment. Some parts might be eroding and some parts accumulating, but on average there is equilibrium. To analyse if there are areas more prone to erosion, the origin of deposited sediment on the riverbanks was analysed. This sediment has been collected during the field research in September 2011 and subsequently the texture and mineralogy have been analysed. This has been compared to the composition of the soil in the catchment and the composition of Harmattan dust. Both compositions were derived from literature. The texture of the deposited sediment on the riverbanks appeared to be mainly silt and clay. The mineralogy of the sediment is quartz, feldspar and kaolinite. As these minerals are abundant in the soil as well as in Harmattan dust, no definite conclusions can be drawn on the origin of the sediment. The general conclusion is that the creation of a reservoir will have morphological effects in the reservoir and downstream of the dam. In the intended reservoir sedimentation will occur, but the effect on the functions of the reservoir is negligible. Delta deposition will mainly occur within the reservoir and partly in the upstream riverbed. As the sand will be spread out over a large distance the riverbed increase will be limited. The effect on the water levels in the river should be further investigated. There will be significant erosion of the riverbed close to the dam. In the future the riverbed erosion could damage Pwalugu bridge. Other negative consequences such as bank collapse and the draw down of groundwater levels could occur locally. Applying sediment management strategies could reduce the impact on the river morphology. Possible measures to reduce the erosion and sedimentation rates include dredging or sluicing of high-turbidity currents. The factors that are most determining the results of this study are the riverbed composition and the suspended sediment load. In order to obtain more accurate results, model refinement should start with collecting more data in the field.