Calibration and Validation of SWMM Model in a Sub Catchment in Eldoret Town, Kenya

Flooding in Eldoret town has been occurring during heavy downpours resulting in loss of property, live and interruption of transportation systems. This study aimed at determining the catchment characteristics in Eldoret town for the calibration and validation of SWMM5 model. This model was developed by US Environmental Protection Agency. The model has been evolving from Version 1 in 1971 to Version 5 of 2005 called SWMM5. It can simulate runoff quantity in any given catchment. Rainfall was measured using rain gauge while discharge was measured using current meter. Digital Elevation Model of the study area was also obtained. The results showed that the catchment drained an area of approximately 696.5 hectares with a total of 23 sub catchments. The average slope was found to be 2.57% and the mean average imperviousness was 25.72%. The drain base flow was found to be 0.002 m³/s. However, during prolonged rainfall, the discharge increased up to 32.4 m³/s. The maximum rainfall observed during the study period was 32.4 mm which resulted in the maximum average discharge of 0.131m³/s and resulted in overflow in the drain.  Calibrated model had N-Imperv of 0.45, Dstore-Imperv of 2.5 and Dstore-Perv of 8. ISE values of 1.9 and 1.4 were observed for calibration and validation, respectively.  NSE values of 0.97 and 0.99 were observed for calibration and validation, respectively. This meant that model excellently simulated rainfall-discharge relationship in the study area and can be used for final design by the Eldoret town engineers. Future study is required to calibrate the model based on more measurements of rainfall and their discharges in the catchment.