Rainfall-runoff modelling of the Nyando river basin for flood mitigation
Ajigoh, Elly Okewe
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The study objective was to model the Nyando River basin for flood mitigation. The methods used include the rainfall-runoff relationship peak food frequency Analysis mapping of inundation river plain areas for various return period floods. The study was carried out using available basin measured variables. The main data used for the study were daily rainfall and daily stream flow obtained from the Meteorological Department and Ministry of water Development respectively. The basin rainfall-rainfall relationship was established by correlation and regression analysis. The extreme flood peak magnitudes and their return periods were established by frequency of the type 1 Gumbel distribution. The river hydrologic modeling was carried out using HEC-HMS for simulation of stream flow hydrographs. To delineate flood vulnerable areas on the basin, a digital elevation model of the Nyando river basin was made by GIS Arc View tools. Using TIN, Triangulated irregular network which defined flooded areas corresponding to water levels above river banks at each cross section for the known flood magnitude and return period. The regression result established a direct relationship between basin rainfall and runoff. The frequency analysis showed the floods for return periods of 10, 25, 50,100 years to be 327.9, 397.9, 457, 510.8 m3/s respectively. The mean annual flood was 1983m3/sec the hydrologic modeling showed that the southern sub catchment contributed most to the hydrograph. The study, indicates that the HEC-HMS modeled the flow of Nyando fairly well as depicted by the hydrographs. With any given rainfall input the area under inundation can be determined using the model parameters established for Nyando. This is important for flood mitigation measures. This study has successfully linked the hydrologic model HEC-HMS, Gumbel frequency analysis, and GIS Arc View water surface modeling in flood inundation mapping for the Nyando basin.