A novel integration of green-grey infrastructure in steep slope micro-watersheds: PCSWMM modeling for non-draining soils in Addis Ababa

Abstract Urban watersheds with steep slopes and poorly draining soils are increasingly prone to flooding and water quality decline under rapid urbanization and climate change. This study assessed the hydrological and water quality performance of integrated green–grey infrastructure (IGGI) using a PCSWMM-based model in a micro-watershed of Addis Ababa, Ethiopia. The analysis considered historical data, projected climate change, and land use change as indicators of future urbanization. A novel IGGI setup was developed by integrating buried rain barrels (RBs) with conventional rain gardens (RGs) within contributing zones—an approach seldom applied in stormwater design. While RBs and RGs are established practices, burying the barrels enhanced stormwater retention by capturing runoff from rooftops and impervious areas and allowing delayed subsurface infiltration. Model calibration and validation showed good performance (NSE = 0.6–0.9; RSR = 0.3–0.6) with acceptable integral square error values. Under baseline conditions, runoff peaked at 3.6 m 3 /s (5,179 m 3 total volume) with pollutant loads of ammonia 18.5 kg, BOD 109.8 kg, COD 1,363.1 kg, nitrate 19.3 kg, and phosphate 21.3 kg. Urbanization increased runoff and pollutant loads by 9–12%, while climate change raised them by 41.5–45%. The combined scenario yielded the highest runoff (7.2 m 3 /s; 9,897 m 3 ) and pollutant loads of ammonia 35.4 kg, BOD 208.8 kg, COD 2,591.4 kg, nitrate 36.8 kg, and phosphate 40.3 kg. Post-IGGI implementation for the baseline, peak runoff and total volume declined by 43.5% and 36.7%, with pollutant loads reduced by 37.2–42.4%. The IGGI framework enhances infiltration, mitigates hydrological and water quality impacts, and supports SDGs 6, 11, and 13 through climate-resilient urban drainage.