Evaluation of methods for estimation of design flows in highly urbanised catchments

Abstract

Design flood estimation is essential in design of hydraulic structures like bridges, culverts and side drains. However, in Kampala, the capital city of Uganda, there is a major challenge of lack of flow data as a result of limited stream gauging stations on urban channels. Only two out of the eight major drainage channels in Kampala City have stream gauging stations.

Several flood estimation methods like the Rational method, Transport Road Research (TRRL) Method, Soil Conservation Service (SCS)-Curve Number method among others were developed to help in the estimation of design floods. These methods were developed centuries ago and therefore require update to be in position to capture the current and future changes like climate change and urbanisation.

In this study, a stream gauging station as well as rain gauge were installed in Nalukolongo catchment to measure and record flow data and rainfall for a period of 6 months. Stormwater Management model (SWMM) which is a hydrodynamic model was set up and run using the recorded rainfall and calibrated using observed measured flow data. From calibration, Nash–Sutcliffe model efficiency coefficient (NSE) and Root-Mean-Square Error (RMSE) of 0.89 and 0.051 respectively were computed an indicator that the observed and simulated discharges had a good fit. The peak discharge in the 4 sub-catchments in Nalukolongo channel computed from the calibrated SWMM were compared with the peak discharge values computed using the lumped methods for estimation of design flows from ungauged catchments that is the Rational method, TRRL and SCS methods.

When the peak flows generated by SWMM were compared with those generated by Rational method, TRRL and SCS for 10, 25, 50 and 100 year return periods, the average percentage difference was 16%, 21% and 94% respectively.

The Rational method and TRRL method were found most appropriate for estimation of design floods for Nalukolongo catchment since their peak floods and those obtained using SWMM produced an average difference of 16% and 21% respectively. The small difference was attributed to the fact that the selected model parameters like for land use were representative of the actual characteristics of the study area and additionally storage features within the catchment were not significant.

The Rational method and TRRL method, therefore are the two suitable methods recommended for estimation of design floods in Nalukolongo catchment.