Water attracts water. When rivers are reduced to token gesture flow due to upstream damming, they no more attract rain. When rivers are impotent to attract rain, tropical rain storms change their patterns and behave randomly, ignoring river.
When there is no rain in tropic, there is no flood-flush effect which can clean mud and sediments into delta area. This results into increase sea level against land.
One in-dept research points it out.
The world’s rivers deliver 19 billion tonnes of sediment to the coastal zone annually1, with a considerable fraction being sequestered in large deltas, home to over 500 million people. Most (more than 70 per cent) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping.
Research by the University of Southampton shows that a change in the patterns of tropical storms is threatening the future of the Mekong River delta in Vietnam, indicating a similar risk to other deltas around the world.
Deltas are landforms made from sediment washed into rivers and carried downstream. The sediment builds up where the river meets slow moving or still water, such as seas or lakes. Deltas naturally subside under their own weight, so a constant flow of new deposits is vital to offset these changes and prevent flooding which could be disastrous to agriculture and the environment.
Their data shows that of all the sediment transported to the delta, one third is due to tropical cyclones. It also shows that the Mekong’s sediment load has declined markedly in recent years – largely due to changes in the location and intensity of storms tracking across the upstream rivers that feed the delta.
Sand mining is already reducing the sediment being delivered to the Mekong delta and further reductions are anticipated as a result of future damming upstream. Therefore, if the storm projections are correct and even less sediment is washed downstream, the delta’s prospects look bleak.
Our study is the first to show the significant role tropical storms can have in the delivery of sediment to large river deltas. This has implications for a range of other major rivers, such as the Ganges in Bangladesh, the Yangtze in China, and the Mississippi in the US. All of these have catchments that are regularly struck by tropical storms. Some 500 million people live and work in the world’s major river deltas – and our work shows we can’t evaluate their future vulnerability to sea-level rise without also considering changes in the storms that feed the deltas.
Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity
Here we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones in transmitting suspended sediment to one of the world’s great deltas. We demonstrate that spatial variations in the Mekong’s suspended sediment load are correlated (r = 0.765, P < 0.1) with observed variations in tropical-cyclone climatology, and that a substantial portion (32 per cent) of the suspended sediment load reaching the delta is delivered by runoff generated by rainfall associated with tropical cyclones. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 megatonnes over recent years (1981–2005), of which 33.0 ± 7.1 megatonnes is due to a shift in tropical-cyclone climatology. Consequently, tropical cyclones have a key role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast. It is likely that anthropogenic sediment trapping in upstream reservoirs is a dominant factor in explaining past5, 6, 7, and anticipating future8, 9, declines in suspended sediment loads reaching the world’s major deltas. However, our study shows that changes in tropical-cyclone climatology affect trends in fluvial suspended sediment loads and thus are also key to fully assessing the risk posed to vulnerable coastal systems.
Sediment-related impacts due to upstream reservoir trapping, the Lower Mekong River
A sharp decrease in total suspended solids (TSS) concentration has occurred in the Mekong River after the closure of the Manwan Dam in China in 1993, the first of a planned cascade of eight dams. This paper describes the upstream developments on the Mekong River, concentrating on the effects of hydropower dams and reservoirs. The reservoir-related changes in total suspended solids, suspended sediment concentration (SSC), and hydrology have been analyzed, and the impacts of such possible changes on the Lower Mekong Basin discussed. The theoretical trapping efficiency of the proposed dams has been computed and the amount of sediment to be trapped in the reservoirs estimated. The reservoir trapping of sediments and the changing of natural flow patterns will impact the countries downstream in this international river basin. Both positive and negative possible effects of such impacts have been reviewed, based on the available data from the Mekong and studies on other basins.