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Rehabilitation of the former Northern Swamp Lake Naivasha - Kenya : On the modeling of the sediment trapping efficacy for two rehabilitation alternatives

Cornelissen, M.A.J.M. (2014) Rehabilitation of the former Northern Swamp Lake Naivasha - Kenya : On the modeling of the sediment trapping efficacy for two rehabilitation alternatives.

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Abstract:Erosion, induced by natural processes such as wind and rainfall and enhanced by anthropogenic activities such as agribusiness and deforestation, produces sediments that are carried downstream by rivers. The deposition of the sediments in the downstream areas creates new landmass for various life forms to live. However, the downstream deposition also causes problems such as sediment accumulation in delta regions and near boat ramps hindering navigation, and altering of the species composition affecting the ecological state. Erosion and the related problems also occur in the Lake Naivasha Basin, which is situated 80 km northwest of Nairobi in Kenya. The siltation affects the turbidity of the water with indirect influences on water use for human activities, fisheries, tourism and agriculture such as the flower business. The higher erosion rates which cause increasing sedimentation of the lake also cause greater inputs of nutrients and pollutants to the lake. These nutrients and pollutants threaten the ecological state of the lake, i.e. by an increased nutrient concentration. A possible solution to decrease the siltation of the lake is to rehabilitate the former Northern Swamp, a former wetland of about 4 km² north of Lake Naivasha to retain sediments and prevent them from ending up in the lake. In 2009, Marula Estates, a large commercialized agricultural landholding bordering the former Northern Swamp, on its own initiative performed a study on the current state of the wetland and created a plan to rehabilitate the former Northern Swamp. The first phase of this plan was implemented already in 2009 and because the results are satisfying, the second phase of the project, which consists of the construction of a dam to buffer water from the Malewa River, will be implemented. The implementation of the second phase however is currently on hold due to the high lake water level, which needs to drop sufficiently to provide access for machinery to execute the necessary works. This forced break gives the opportunity to review the design from a Water Engineering perspective, which led to the research objective of this study: “To evaluate the functioning of two alternative rehabilitation alternatives, with in particular the sediment trapping efficacy, by modeling the sediment transport processes for various Malewa River discharges using a one-dimensional modeling approach”. The first rehabilitation alternative is a wetland through which the water flows from the Malewa River via a spillway channel and wetland into Lake Naivasha, based on the design as presented by Marula Estates (2013). The second alternative consists of the same spillway channel to divert the water from the Malewa River into a meandering channel flowing into Lake Naivasha. To achieve this research objective, a literature study and field work, in May – June 2013, are done to get insight in the former Northern Swamp and the rehabilitation plan. To model the sediment transport processes an empirical formula that relates sediment load and discharge is used together with an integrated software package for river management called SOBEK. With this model it is possible to make an estimation of the amounts of sediments entering through the inlet construction, and it is possible to get insight in the sediment transport processes within the two rehabilitation alternatives. The Wetland alternative turns out to be the best alternative when considering sediment trapping efficacy. For the year 2010, when taking into account the maximum discharge entering the wetland of 25 m³/s to prevent flooding of the spillway channel, the sediment inflow into Lake Naivasha could have been reduced with 15%. Due to the current wetland design specifications, based on discharge data from 1960 till 2010, on average only 1.25% of the time water can be diverted from the Malewa River through the spillway channel into the wetland. For 2010 however, water could have been diverted for almost 7% of the time and therefore, the reduction in sediment inflow into Lake Naivasha is likely to be lower for other years. Also, due to the limited possibility of utilization of the wetland, rehabilitation of the former Northern Swamp is going to be difficult.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:38 earth sciences, 56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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