1A1: The Need for Low Level Outlets to Combat Siltation
July 13, 2022
Civil Works and Dam Safety
It is inevitable that, sooner or later, most dams will fill with sediment. It is simply a matter of time. Reservoir sedimentation is a serious long-term problem that threatens the long-term viability of storage hydropower schemes. In 2010 global storage capacity was estimated at 6,000,000 km³ but it is projected that 4,000,000 km³ will be lost to sedimentation by 2050. It occurs worldwide at a rate of about 0.8 percent per year, but the sedimentation rate in many regions such as Asia is much higher. Many reservoirs will fill with sediment within the next 100 years or so but some will fill up in a much shorter timeframe. The sediment builds up at the head of the lake and a wall of sediment moves slowly down the lake until it reaches the dam and, eventually, the power intakes. When the sediment reaches the power intakes of a hydro dam, there is a risk of the turbines being destroyed and the power station being abandoned. If this happens the spillway will need to operate continuously and this may lead to spillway failure possibly followed by failure of the dam. Spillways are likely to fail because they are not designed for continuously discharging large amounts of sediment. The concrete and fixed parts will soon be damaged and need to be repaired. Repair is possible only if the spillway is segregated into two or more chutes so that one chute can be isolated and the flow passed down the other chute(s) This paper is intended to draw attention to the problem and to emphasise the need to mitigate or solve the problem by providing a scour intake beneath the turbine intakes. The major problem is designing the upstream gate to operate reliably when finally needed after, possibly, many years with little or no maintenance. A credible solution is proposed but it is recognised that better ones may be found: the objective of this paper is to encourage designers and developers to consider a wide range of solutions and to examine the potential of modern materials to help solve this very serious problem.