A case study on SWMM based evaluation of rain barrel and downspout disconnection scenarios for urban stormwater management

Colombo, the executive and judicial capital of Sri Lanka, is located on the southwestern coast of the country and has experienced rapid urbanization in recent years, driven by infrastructure development, land use changes, and the growing impacts of climate change. These factors have significantly increased the frequency and severity of urban flooding. Therefore, this study explores the stormwater management problems in the 686.3-hectare study catchment, adjacent to the Kelani River in suburban Colombo, and proposes effective stormwater management strategies to avoid flooding in the area. Using GIS-based analysis, the study area was divided into 36 sub-catchments to determine the elevation and imperviousness characteristics of the study area and generate a Digital Elevation Model (DEM). Additionally, land use and land cover (LULC) mapping and change detection analysis were carried out using ArcGIS. The obtained spatial data were used in the EPA-SWMM model to evaluate stormwater responses under different rain barrel-based LID scenarios. Three scenarios were evaluated: the existing condition (no LID), rain barrel with downspout connection (No Disconnect), and rain barrel with downspout disconnection (Disconnect). The runoff coefficients for the baseline scenario were 0.849 and 0.871 for the 2-year and 10-year storms, respectively. The rain barrel with-disconnect scenario reduced these to 0.379 and 0.456, while the no-disconnect scenario resulted in 0.563 and 0.598, respectively. The Disconnect configuration proved most effective in reducing average total runoff by 55% and 48% for the 2-year and 10-year storms, respectively, while more than doubling infiltration rates. In contrast, the No Disconnect configuration provided only moderate runoff reduction through temporary storage, without meaningfully restoring natural infiltration processes. Moreover, Outfall loading was 196.15 and 234.28 L under existing conditions for the 2- and 10-year storms, respectively. The disconnect scenario reduced it to 170.26 and 214.95 L, whereas the no-disconnect scenario increased it to 252.28 and 295.30 L, exceeding the existing condition. The findings indicate that municipal retrofit initiatives must prioritize downspout disconnection rather than just implementing storage rain barrels, focusing initially on the identified low-lying, high-imperviousness sub-catchments to optimize hydrologic advantages per investment unit. The outcomes of this study will give valuable insight to policymakers in incorporating and promoting sustainable climate-resilient structures to overcome stormwater management problems. Future studies should incorporate detailed drainage-network capacity, downstream boundary conditions, and future climate scenarios to further evaluate LID performance.