The impact of hydropower development on silt and clay loads in the Mekong Delta

Abstract

The hydropower development in the Mekong Basin affects the silt and clay concentrations in the river which has ecological as well as socio-economic effects on the Mekong Delta and its inhabitants. Therefore, the first objective of this research was to obtain a better understanding of the effect of hydropower development on the silt and clay load in the Mekong Delta. Subsequently, the objective was to determine possible sediment management strategies to mitigate the impacts on the silt and clay load in the Mekong Delta.

A conceptual model was developed and applied to analyse these impacts. By means of a sediment balance model with distributed sediment yield and individual trapping efficiencies of dams the annual sediment load has been determined.

This research has shown that the annual silt and clay load downstream has decreased from 140 Megaton ± 40 [Mt] before hydropower development to 26 ± 9 Mt in 2020. Including future hydropower dam projects, this could further decrease to 4-8 ± 3-5 Mt in 2040. Approximately 4% of this sediment load will deposit in the Mekong Delta and therefore it is predicted that the clay and silt deposition will be close to zero in the future.

By 2040, approximately 75% of the total trapped sediment load is trapped in mainstream dams even though they account for 16% of the total dams. The majority (50%) will get trapped in Chinese dams that account for 9% of the total number of dams. This means that almost all sediment coming from China is trapped and will not reach the Mekong Delta. Around 80-85% of the future sediment load that will reach the delta has its source in the 3S region.

This reduction has implications for the sustainability of ecosystems and the productivity of the Mekong Delta. Clay and silt are nutrient rich alluvial sediments that support the ecosystems in the floodplains and delta. These nutrients are crucial for the soil fertility vital for rice cultivation and for various biological processes for fish. Today, nearly 80% of the 20 million people living in the Mekong Delta depend on the river system for their livelihood. However, the rice cultivation is expected to decrease significantly and by 2040, the loss of fisheries could cost close to $23 billion.

Thus, the computed reduction displayed that sediment management strategies are inevitable to maintain the productivity of the Mekong Delta in the future. Therefore, two sediment management strategies were reviewed in this research: cancelling planned dams and reservoir sluicing. Reservoir sluicing is passing incoming sediment-laden water through the reservoir by discharging high flows to enable sediment to move past the dam without depositing.

The analysis of these two strategies showed that the maximum sediment load increase is 1-3 Mt as a result of cancelling planned dams and 2-3 Mt due to reservoir sluicing. This means a very small increase for the deposition in the Mekong Delta (4% deposits). Therefore, the sediment load increase was considered unsatisfactory, suggesting that these strategies are not providing the solution for the Mekong Basin.

The results indicate that effective sediment management strategies are urgently needed and that more research is needed into the effects of other strategies on the silt and clay load in the Mekong Delta.