ABSTRACT
Surface run-off estimation is a common analysis in hydrological study of a watershed based rainfall model. Detailed hydrological studies regarding watersheds especially in real life situation is challenged by lack of adequate data on one hand and the complexity of hydrological systems. On the other hand, the aim of the study was to conduct a rainfall run-off simulation using Ikpoba Catchment Area, Benin City, Edo State, Nigeria, as the case study using HEC-HMS hydrologic model. For the hydrological modelling software HEC-HMS employed for the simulation, the input data were annual maximum daily rainfall data for Benin City for 32yrs period (1990-2021) was used to obtain the peak run-off of 2, 5 10, 25, 50 100 and 200yrs return period. Other input data were; the land use land cover (LULC) map, soil map and the digital elevation model () of the study area which were generated from United States Geological Survey (USGS) Earth Explorer. The annual maximum daily rainfall data was first tested for homogeneity to ascertain the absence of seasonality. To develop the run-off model and compute the peak run-off of 2, 5, 10, 25, 50, 100 and 200yrs return period, information on the watershed characteristics which includes; the lag time, slope, longest flow path and time of concentration were first determined. The initial base flow was assumed to be 0.00, Soil Conservation Systems (SCS) curve number (CN) for direct run-off value was determined from existing standard tables (LULC map) based on the catchment characteristics while the initial abstraction was assumed to be 0.00. The results obtained indicate that for a return period of 2yrs, the peak discharge was 6.40m3 /s, with a direct run-off volume and discharge volume of 44.24mm. For a return period of 5yrs, the peak discharge was 8.60m3 /s, with a direct run-off volume and discharge volume of 58.97mm. For a return period of 10yrs, the peak discharge was 9.90m3 /s, with a direct run-off volume and discharge volume of 68.29mm. For a return period of 25yrs, the peak discharge was 11.6m3 /s, with a direct run-off volume and discharge volume of 79.55mm. For a return period of 50yrs, the peak discharge was 12.7m3 /s, with a direct run-off volume and discharge volume of 87.47mm. For a return period of 100yrs, the peak discharge was 13.90m3 /s, with a direct run-off volume and discharge volume of 95.48mm. While for a return period of 200yrs, the peak discharge was 15.0m3 /s, with a direct run-off volume and discharge volume of 103.57mm. The significance of the results obtained were that the peak run-off determined for the catchment can be employed as a significant data for the design of hydraulic structures such as; dams, and drainage systems.
