Managing reservoir sedimentation through coordinated operation of a transboundary system of reservoirs in the Mekong

Sedimentation is one of the major challenges for the long-term sustainable operation of a hydropower reservoir. Trapping of sediment reduces its storage capacity and consequently diminishes hydropower production. Here first the development of a REServoir Sediment MANagement routine (ResSMan) and its integration into a hy-drological model, the Soil and Water Assessment Tool (SWAT) is presented. ResSMan has functions to determine sediment accumulation in multiple reservoirs, and its impacts on the storage capacity and hydropower pro-duction under user-specified operation policies. It allows to compute the restoration of storage capacity due to the removal of sediment by flushing, i.e. the removal of sediment from a reservoir by passing water and sediment through the low-level outlets, and sluicing, i.e. passing sediment before suspended sediment solids have settled down in the reservoir. The capabilities (flushing and sluicing) of ResSMan were evaluated through a comparison with the Sediment Simulation Screening (SedSim) model, a well-tested sediment management simulation model and the resulting R2 values of 0.99 validated its capabilities. Subsequently, ResSMan was applied to assess and manage reservoir sedimentation using different management strategies in a complex system of 19 reservoirs (12 currently existing) in the Sesan and Srepok (2S) basin of the Mekong River for 2021-2120. The unregulated mean annual sediment yield at the outlet of the 2S basin was estimated as 7.24 million tonnes/year (Mt/y) and it will be reduced to 0.11 Mt/y due to the operation of the 19 reservoirs. In total 924 Mt of sediment will accumulate in these 19 reservoirs over 100 years, resulting in an average trapping efficiency of 74% (ranging from 11% to 97%). System-wide sediment management coordination simulations demonstrated that bi-annually and 5-yearly flushing of alternate reservoirs are effective options for efficiently releasing sediment in the 2S basin. However, the use of frequent flushing (annual or bi-annual) may be more favourable to minimize adverse impacts due to release of higher sediment loads on downstream ecosystems. The analysis is an initial step towards the coor-dination of sediment management plans and policies for the multi-reservoirs system in the Mekong basin.