Evaluating The Functionality of the Existing Natural Wetland as A Nature-Based Solution in Runoff Control in Hai District

Urban flooding has become an increasing challenge in rapidly developing areas where conventional drainage systems are unable to cope with high-intensity rainfall and rising surface runoff. This study evaluated the functionality of Boroti natural wetland in Hai District, Kilimanjaro Region, Tanzania, as a nature-based solution for urban stormwater runoff control. Increasing urbanization, steep topography, and high-intensity rainfall have caused frequent flooding and failure of conventional drainage systems in the area. The study applied EPA-SWMM 5.2 to assess the hydrological performance of the wetland under two scenarios: with the wetland represented as a storage unit and without the storage function, where it was replaced by a junction. Catchment characteristics including sub-catchment area, slope, land use, hydrologic soil groups, curve numbers, and infiltration parameters were generated using ArcGIS 10.4 and standard hydrological methods. Results showed that both scenarios had low runoff continuity error (-0.03%), but the storage unit scenario produced a lower flow routing error (-8.16%) compared to the junction scenario (-16.90%). The findings confirm that natural wetlands significantly improve runoff attenuation and flood mitigation.