Developing Design Storm Hydrographs for Small Tropical Catchments with Limited Data

Abstract

Hydrographs are vital tools in the design and construction of water-control structures in urban and rural systems. The purpose of this study was to explore the development of design storm hydrographs for the small tropical catchment with limited data. In this study, Clark’s Unit Hydrograph method was used to develop synthetic hydrographs for the University of Ilorin Agricultural and Biosystems Engineering field plot. This method was selected for evaluation because the field plot has very limited stream flow data. Kunrun stream, which is the discharge outlet for the watershed, has no gauge stations. Clark's method was evaluated in this study for peak discharge and storm hydrograph analysis of the field plot. The total area of the field plot measures 18.4 ha and is characterized according to its land use: pasture range land (34%), cultivated land (49%), wood or forest land (9%) and 'dirt' (8%). The three necessary inputs needed for the development of the synthetic hydrograph using Clark’s method are: time of concentration, a storage coefficient, and a time-area histogram. The  time of concentration was estimated using the Natural Resources Conservation Service method (NRCS) with the lag time determined from Snyder’s equation. The storage coefficient was estimated at 0.75 hr while the time of concentration was 1.98 hr. Convolution procedures were used in determining the storm hydrograph from the unit hydrograph. Runoff volume and peak discharge from the unit hydrograph were estimated to be 2.4 x 104 m³ and 1.02 m³/s, respectively. According to the 24hour, 100year storm hydrograph, the runoff volume was 5.23 x 104 m³ while the peak discharge was 2.15 m³/s. It was found that Clark's Unit Hydrograph Method was very suitable in the development of design storm hydrograph for small tropical catchment with limited data.

Keywords: Clark’s unit hydrograph, Storm hydrograph, Peak discharge,  ungauged watershed.