Evaluating green roof hydrological performance on varying slopes using EPA SWMM

Janice Lynn Ayog; Venus Tan.

Urbanization and climate change have increased the frequency and intensity of urban flooding, particularly in tropical cities such as those in Malaysia. Green roofs offer a promising low-impact development (LID) strategy for stormwater management by reducing runoff volume and peak discharge; however, there remains limited understanding of how roof slope influences green roof hydrological performance under tropical rainfall conditions. This study evaluates the effectiveness of green roofs with different slopes in reducing peak runoff and assesses the capability of the EPA Storm Water Management Model (SWMM) to simulate slope-dependent runoff responses. Experimental runoff data were obtained from a laboratory-scale green roof model with slopes of 2% and 6%, subjected to controlled simulated rainfall events, and were used for calibration and validation of EPA SWMM, focusing on key indicators including peak discharge and time to peak. Results show that green roofs significantly reduced peak discharge compared to conventional roofs, with reductions of 97.2% at 2% slope and 95.4% at 6% slope, with the shallower slope exhibiting greater runoff attenuation associated with delayed runoff response. SWMM simulations demonstrated satisfactory agreement with observed data, with Nash–Sutcliffe Efficiency (NSE) values between 0.50 and 0.65 and RMSE–observations standard deviation ratio (RSR) values between 0.58 and 0.68. Overall, the findings indicate that green roofs, including those on steeper slopes, are effective in attenuating stormwater runoff, and that EPA SWMM is a suitable tool for comparative modelling of green roof hydrology under tropical conditions. These insights support the integration of green roofs into urban stormwater planning in Malaysia and similar environments.

KEYWORDS: Green roof; EPA SWMM; hydrological modelling; Low impact development; Model performance.



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