An Analysis of Strategies for Managing Sediment Inflows to an Off-Stream Reservoir from a Diversion Supply Canal under Conditions of Water Scarcity

In semi-arid environments, rivers are often characterized by intermittent flows that may have high sediment concentrations. In such cases, dams forming storage reservoirs may be developed at off-stream locations having small watersheds, with inflows being provided from a nearby river via a diversion dam and supply canal. This design limits the sediment inflows to only those from the watershed above the reservoir and those carried by the supply canal from the source river, lengthening the life of the reservoir. Sediment concentrations in the supply canal may be reduced by desilting facilities at the diversion dam, and possibly along the canal as well. This paper presents an analysis of strategies that were considered for managing sediment inflows to a supply canal for a proposed off-stream reservoir. An unexpected finding is that a sediment trap basin, which had been proposed to be located along the supply canal, could reduce, rather than improve, the overall sediment removal efficiency of the diversion-supply system. This outcome is due to a lack of sufficient water in the source river to sluice sediment deposits from both the diversion dam pool and the sediment trap basin. This study illustrates how, in cases of scarce water supplies with high sediment loads, there can be tradeoffs between the differing needs for managing water and sediment.