Browsing by Author "Mohamed, Hussein Gadain"

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    Development and application of decision support tools for lake Victoria sub-catchments
    (University of Dar es Salaam, 2013) Mohamed, Hussein Gadain
    Lake Victoria is an important hydrological system that serves its bordering countries and the Nile basin. Its sub-catchments are one of the most densely populated areas in East Africa. This has led to mounting pressure on the available land and water resources, resulting into increased water demands, lake level fluctuations, forest degradation and floods. Pressure on these resources and others is expected to increase even further. The objective of this study was to develop and apply decision support tools for the Lake Victoria sub-catchments. Hydrological models have received increasing recognition in recent years as effective tools in supporting water resources management decisions. Four models that operate independently, and an impact assessment model have been developed in the form of decision support tools using results of remote sensing analyses, GIS technology and hydro-meteorological data. These are: LVIC_SRM (Lake Victoria Spatial Rainfall Model), LVIC_RRM (Lake Victoria Rainfall-Runoff Model), LVIC_IFS (Lake Victoria Inflow Forecasting System), LVIC_WBAL (Lake Victoria Water Balance Model), and LVIC_IAM (Lake Victoria Impact Assessment Model). Various hydrological analyses were performed and outputs obtained using these models. Using LVIC_SRM by incorporating satellite and observed rain gauge data into the krigging models improved rainfall estimation when compared to single variable krigging. A relation between rainfall over the lake and its basin was built and has improved significantly with use of satellite data. LVIC_WBAL model established that rainfall constitutes 85% of the total lake inflow making the water level highly sensitive to changes in rainfall. Earlier and recent level rise were captured with reasonable accuracy and were attributed mainly to increase in rainfall in the area. The future behaviour of the lake was predicted using a stochastic data generator, and that future lake level will not follow a particular trend. Using satellite images the study demonstrated that, stream flow in the Mara catchment has changed leading to increased peak flows and there is likelihood of occurrence of flash floods during wet season and lowered flows during dry season. The decline in natural vegetation has been attributed to deforestation and increased agriculture among others. With the help of the inflow forecasting system, potential flood areas in Nyando river can be delineated and at risk population be alerted in advance and evacuated. Further, the impact on lake hydrology was assessed using development scenarios of agriculture, water supply and increased outflow. Through LVIC_IAM the study concluded that, irrigating 3% of arable lands in the lake basin has higher impacts on lake hydrology when compared to water withdrawal for domestic purposes in Tanzania. Provided reasonable data availability, the impact model could be useful tool for water development in the lake basin. The models are specific decision support tools for Lake Victoria sub-catchments and the study recommends their wider application in other tropical catchments with similar characteristics.
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    A one -dimensional numerical simulation of dam break flows
    (University of Dar es Salaam, 1994) Mohamed, Hussein Gadain
    The unsteady flow, produced by a dam breaking across its entire length, is modelled as one-dimensional flows. Such flows are described by a set of quasi-linear, hyperbolic partial differential equations, called the Saint-Venant equations. In this research work the movement of flood waves resulting from a dam break and sudden closure of a gate was investigated based on one-dimensional numerical solution of Saint-Venant equations. Two explicit finite difference numerical schemes with second order accuracy were selected to solve the governing equations written in two different forms. In the first case the MacCormack numerical scheme was employed to solve the system of flow equations written in a conservation form. The actual upstream and downstream boundary conditions of the flow field, i.e., a depth of flow equal to zero at these boundaries and beyond were used for the solution of the flow equations. In the second case the governing equations were arranged in a non-conservative form, known as the characteristic form and compared for the analysis of unsteady free-surface flows. The problem was simulated to an instantaneous closure of a gate in a rectangular channel. The Lambda numerical scheme was used to split the flux vector into positive and negative parts, each of which corresponds to the direction of characteristic, thereby allowing use of proper finite differences for the space derivatives. The depth of flow at the upstream boundary was specified, resembling a constant head reservoir. In both cases the numerical solutions obtained were compared with the experimental data and the analytical solution was found to be satisfactory.