Impacts of urbanization and climate change on stormwater drainage efficiency: A PCSWMM 7.5 case study in Dilla Town, Ethiopia

• This study found that 55.52% of Dilla town’s stormwater drainage network has deteriorated. • Rapid urbanization raised impervious surfaces from 27.83% (2002) to 65.88% (2022) • Urbanization-Driven Imperviousness Will Reach 91.6% by 2032. • Sub-catchments SC1, SC6, SC7, SC10, SC17, SC25, SC35, and SC37 recorded the highest peak runoff. • Compound Impact of Urbanization and Climate Change Doubles Runoff. Rainfall-induced flooding disrupts social urban life, damages infrastructure, and spreads water-borne diseases, primarily due to rapid urbanisation, changing rainfall patterns, and inadequate drainage systems. This study assesses the combined impacts of climate change and urbanisation on the stormwater drainage networks (SWDN) of Dilla town, Ethiopia, using storm-water management models (EPASWMM 5.2, PCSWMM 7.5), ArcGIS, and climate scenarios RCP4.5 and RCP8.5. The average performance of the SWDN) across six kebeles is poor (55.52 %), due to undersized and insufficient infrastructure. Future projections (2022–2053) indicate increased rainfall intensities of 31.34 mm/hr (RCP4.5) and 35 mm/hr (RCP8.5), exceeding the historical baseline. Over the past two decades, impervious surfaces have increased to 38.05 %, and residential areas expanded to 727.53 ha, rising impervious coverage from 27.83 % to 65.88 %. Sub-catchments SC1, SC6, SC7, SC10, SC17, SC25, SC35, and SC37 exhibit the highest peak runoff due to extensive impervious cover. This study uniquely integrates climate projections, urban growth analysis, and hydraulic modelling using PCSWMM to quantify compound flood risks, representing one of the first such applications in an Ethiopian secondary city. The framework and findings from Dilla are transferable to other rapidly growing sub-Saharan cities facing similar hydrological and infrastructural challenges.