In the World Bank we often discuss how important it is to integrate solutions across sectors. In Mombasa, Kenya, we have an example of how a comprehensive sediment management approach will allow the government to lower the environmental impact of a proposed dam and save tens of millions of dollars by reducing the amount of sediment that the dam traps. When too much sediment is trapped in a dam, the lifespan of the dam is shortened considerably so reducing sediment is key for long-term success.
The government was considering a design of a dam to provide drinking water to the city of Mombasa and irrigation and drinking water services to people living near the dam site. The proposed dam was planned for 87m high and It was designed to store almost double the mean annual river flow (105 million cubic meters per year). But about 40% of it was dead storage (water that cannot be drained by gravity through the dam's outlets or spillways) needed mainly to store sediment.
The Bank asked a sediment expert to investigate other ways to handle the sediment to see if such a large amount of dead storage was necessary. He gave options for managing sediment in a more comprehensive manner and for operating the reservoir differently. As a long-term measure, watershed management involving soil conservation and catchment restoration can reduce erosion and sediment inflow to the reservoir. Operational options, such as flushing, sluicing, dredging, and hydro suctioning can reduce sediment deposit in the reservoir. In response to the consultant's guidance, the government and their consultants came up with a design that incorporated catchment management, small dams called check dam to catch sediment, and newly-added bottom outlets to allow sluicing and flushing. The new design also included business opportunities for local people to sell the captured sediment as construction materials.
The new design reduced the amount of dead storage needed considerably. Incorporating the catchment and dam operation options into the project gives the same amount of active storage as the original design from a dam 10m lower. The new design has total storage of 119 million cubic meters (60% of the original design). Only 17% of the dam now needs to be allocated to dead storage. Along with this, the consultant team re-evaluated the type of dam, and what was to be a rock-fill dam could now change to a concrete gravity dam, with a number of advantages: the spillway, which controls the release of flows from a dam, can be incorporated into the concrete dam body, arrangements for flow diversion during construction can be much less complex, local construction materials can be used and the bottom outlets needed for flushing the dam can be added. So overall, that gave the government a smaller dam, smaller area inundated, and the same benefits at lower cost. The figures are still being checked, but current estimates are that these changes have reduced the estimated construction costs by almost 25% or approximately USD 50 million.
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