Evaluating the Performance of Constructed Gravel Trench Systems for Stormwater Quality Improvement in Enugu, South East Nigeria

This study evaluates the pollutant removal performance and hydrological attenuation capacity of constructed subsurface gravel trench systems for stormwater quality improvement in a residential catchment in Enugu, Enugu State, South East Nigeria. Sustainable urban drainage systems, including infiltration trenches, are increasingly promoted as alternatives to conventional piped drainage for managing both the quantity and quality of urban stormwater, but their performance under Nigerian tropical rainfall characteristics and soil conditions has received minimal empirical investigation. Four instrumented gravel trench systems of different specifications were installed in a collaborative arrangement with the Enugu State Urban Development Authority, and their performance was monitored through twenty-seven storm events over one year. Inflow and outflow volumes and pollutant concentrations were measured for total suspended solids, chemical oxygen demand, total nitrogen, total phosphorus, zinc, and faecal coliform. Trench infiltration rate was measured at the start and end of the study period to assess clogging. Results show that gravel trenches achieve mean event pollutant removal efficiencies of 78 percent for total suspended solids, 65 percent for zinc, and 85 percent for faecal coliform. Hydraulic attenuation of the peak runoff rate averages 62 percent for events with antecedent dry periods exceeding 48 hours. Clogging reduces infiltration rates by 38 percent over the study year, primarily driven by fine sediment accumulation in the inlet zone. Performance is reduced for large storm events exceeding 25 millimetres. The study demonstrates technical feasibility and informs maintenance interval recommendations for gravel trench systems in South East Nigeria. Keywords: sustainable urban drainage, gravel trench, stormwater quality, Enugu, infiltration.