Two—Dimensional Modelling of Flow in Rantambe Reservoir for Sediment Management

Abstract

AbstractSediment accumulation in reservoirs reduces reservoir capacities and expected benefits from reservoir projects. This is especially prominent in small reservoirs built across rivers carrying high sediment loads. Rantambe reservoir built across the Mahaweli river just downstream of the Randenigala reservoir is one of the reservoirs severely affected by sedimentation. The original capacity of the reservoir was 11.5 MCM in 1990 and in 2014 the capacity was 6.14 MCM. Randenigala reservoir releases and the flow of the Uma-Oya tributary are the main sources of the Rantambe reservoir, and the Uma-Oya tributary carries large sediment load to the reservoir annually. Flushing of sediment is carried out using the two bottom outlets each with an area of 9.7 m2 during the flood season, though it is not adequate to restore the reservoir capacity. This study focuses on investigating the potential of reducing the sediment entering into the reservoir from the Uma-Oya during the flood season using a sediment bypass tunnel which originates from a low weir built across the Uma-Oya. A two-dimensional hydrodynamic model was set up, flow patterns and bed shear stress were analyzed. For calibration of the model, simulated velocities were compared with the measured velocities. The model was validated with Manning’s roughness coefficient of 0.03. The model was then applied introducing a low weir and a bypass tunnel. The bed shear and velocities increase in Uma-Oya, and the flow is directed towards to the tunnel. There is a 18% increment in the area of high bed shear stress to erode the deposited sediments compared to the condition under current practice of flushing under same conditions. This proposed sediment bypass tunnel is an effective technique to control the sediment management of the Rantambe reservoir.KeywordsReservoir sedimentationSediment bypass tunnelRantambe reservoir