GIS BASED HYDROLOGICAL MODELING FOR WOLDIYAWATERSHED AND HYDRAULIC MODELING OF UPPER SHELLE RIVER

Abstract

This study focused mainly on application of a GIS based semi-distributed watershed model (HEC-HMS) for estimation of peak discharges and run off hydrographs. The Digital Elevation Model (DEM) of the study area was used to extract the physical characteristics of the watershed using Arc-Hydro and Geospatial Hydrologic Model Extension HEC-GeoHMS. HEC-HMS Hydrological Model was used to estimate discharges and Runoff Hydrographs for different return period at different locations of the water shed. Finally determination of water surface profiles for different flow scenarios were made using HEC-RAS and mitigation measures were proposed to alleviate the flood problem in the study area. The peak discharge generated by the HEC-HMS Model at the upstream end of Upper Shelle River is 57.7m 3 /s and this value falls to 34.4m 3 /s when flood is diverted at the upstream of the river. From the HEC-RAS Model analysis, it is observed that ,when there is no diversion of upstream flood, most part of the river is overtopped by flood of 100-year return period and when flood is diverted from upstream junction, 50% of the length of the reach will not be over topped totally and the depth of the overtopping · flood decreased by 50% for the rest of the length of the reach when compared with when there is no diversion of upstream flood. Therefore, diversion of upstream flood or construction of levees has been proposed as alternative remedial measures inboth cases to protect flooding of upper shelle river.