Estimation of Design Flood Hydrographs for Osun River at Iwo Control Station in Osun State, Nigeria

Abstract

The unit and storm hydrographs for the catchment of Osun River at Iwo station Osun State, Nigeria were developed. Soil Conservation Service (SCS) and Snyder’s unit hydrograph methods were used to develop synthetic hydrographs for the catchment, while the SCS Curve Number method was used to estimate excess rainfall values from rainfall depth of different return periods. The peak storm flows obtained based on the unit hydrograph ordinates using convolution procedures determined by SCS for rainfall events of 10yr, 20yr, 50yr, 100yr and 200yr return periods for the catchment vary from 819.87 m3/s to 1681.34 m3/s, while those based on Snyder’s method for the catchment vary from 551.41m3/s to 1134.86 m3/s . The statistical analysis at 5% level of significance indicated that there were significant differences in the two methods. The analysis showed that the total peak flows for different return periods obtained from SCS method is higher than that of Snyder’s method by 33.16%. SCS method was recommended for use on the watershed since it incorporates most major hydrological and morphological characteristics of the basins like the watershed area, main channel length, river channel slope and watershed slope.