THE EFFECTS OF DEVELOPMENT ACTIVITIES ON THE OCCURRENCE OF FLOODS IN THE MBABANECITY SWAZILAND

:Urbanization and expansion of structural developments within the urban area of Mbabane are challenges that require a rapid response in terms of flood-damage mitigation measures. There have been several cases of flash floods in Mbabane mostly resulting from heavy downpour and a poor drainage system, thus causing in loss of property and goods. In this study, the effects of development activities on the occurrence of floods in Mbabane City were investigated using two set of orthophotomaps one for 1972 (Scenario 1) and the other for 2003 (Scenario 2). Impervious areas were computed and quantified to determine the hydrological response of the catchment due to development activities. The study used two approaches, where the amount of runoff in two scenarios were estimated (i) using the runoff coefficient method and (ii) a runoff simulation model (SWMM). Before the simulation was run, some parameters of the model were computed using physical characteristics while others were estimated using secondary data. Results thus obtained indicate that increasing development activities were responsible for the occurrence of flash floods in Mbabane City and primarily due to percent increase of imperviousness which had increased in 2003 compared to 1972. It was also determined that the generated amount of volume (of runoff) from Scenario 2 far exceeded that from Scenario 1.Therefore, it is recommended that the drainage system in the Mbabane city be upgraded, properly re-routed and cleaned regularly while the Mbabane River should be channelized to accommodate excess runoff or alternatively a gauging station together with a flood-warning system be installed to reduce loss of life and property in the city