Implementation of Hydrological and Hydraulic Models to Forecast River Flood Risks and Proposition of Management Measures. Case Study of Nyabugogo River Basin in Rwanda
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Nyabugogo River within Nyabugogo catchment in Rwanda is being flooded at every year resulting in human losses and economic losses. For sustainable flood management; flood risk assessment is a useful component as it demonstrates the expected water depth, floodplain area and the expected damage for each event. Therefore, the aim of this study was to forecast Nyabugogo river flood risk and propose mitigation measures which can reduce flood impacts by using HEC-HMS integrated with HEC-GEOHMS and HEC-RAS integrated with HEC-GEORAS. The study focused on assessing the causes and impacts of the flood; assessing the existing flood management measures and describing the geospatial characteristics of the catchment then generating inundation maps and water depth for each event modeled. Floods in Nyabugogo river are mainly due to high topography, soil texture mainly composed of clay; rainfall, informal settlement; urbanization and inappropriate agriculture practices. The results of hydrologic modelling showed the peak discharges of 515.7; 680.2; 761.5 and 875.5 m3/sec for 10,30,50 and 100 return period year respectively and The results of hydraulic modelling demonstrated that flood inundation area increased slightly from the lower event to the high event modelled with 423.35; 426.11; 428.08 and 430.60 ha for 10,30,50 and 100 return period year respectively and also water depth increased slightly as the return period increase where the high water depth was 3.24 m obtained for 100 year return period. For all the event modelled, the more vulnerable land use obtained within the inundations maps were annual cropland, open grassland, open shurbland, settlement, sparse forest and wetland; with 54.804, 53.672, 53.238, 54.804, 139.359, 50.106 ha of inundation area for 100 year return period for each land use respectively. Therefore, some proposed mitigation measures include construction of storage reservoir at the upstream location of the reach; relocation of infrastructures within the flood plain area; buffer zoning around Nyabugogo river, the use of rain water tank for each house and raising public awareness on flood risks. Thus, this study can provide a basic support for decision making and also can help in the planning and management of land use and future probable flood event within the catchment.