Optimizing Urban Stormwater Management Using Low Impact Development: Hydrologic Hydraulic Modelling for Flood Mitigation and Water Quality Improvement

Rapid urbanization replaces permeable landscapes with impervious surfaces, intensifying runoff peaks, shortening times of concentration, and degrading receiving-water quality. This paper evaluates the effectiveness of low impact development practices in mitigating pluvial flood risk and improving water quality in a dense Indian urban catchment. A coupled hydrologic–hydraulic modeling workflow was developed using event-based design storms and long-term continuous simulation to capture both peak-flow attenuation and pollutant load reduction. Candidate low impact development controls included bioretention cells, permeable pavements, green roofs, and roadside bioswales. Sites were screened using a multi-criteria suitability index reflecting drainage area, slope, soil hydraulic conductivity, available public right-of-way, retrofit feasibility, and cost per unit treated area. Baseline and with-project scenarios were simulated to quantify changes in peak discharge, runoff volume, hydrograph shape, and event mean concentrations of total suspended solids, biochemical oxygen demand, and nutrients. Results indicate that a distributed retrofit achieving 6 to 8 percent effective impervious area disconnection reduced 10-year peak discharge by 18 to 27 percent and annual runoff volume by 12 to 19 percent. Co-benefits included 35 to 55 percent reductions in total suspended solids and 20 to 40 percent reductions in biochemical oxygen demand, with the largest benefits in subcatchments where storage-based practices were colocated with infiltration opportunities. Sensitivity analysis showed performance was most sensitive to saturated hydraulic conductivity, media depth, and clogging rates, highlighting maintenance as a key determinant of long-term efficacy. The paper proposes a practical planning framework for Indian cities that integrates siting, design, and monitoring, delivering resilient stormwater systems that address both flooding and water quality goals.