Runoff generation of impermeable and permeable surface

Abstract

Urbanisation has drastically altered the hydrological behaviour of landscapes, primarily through the expansion of impervious surfaces such as asphalt and concrete. This transformation leads to increased surface runoff, reduced infiltration, and a higher risk of urban flooding particularly in regions like Malta, where short, high-intensity rainfall events are becoming more frequent. This study investigates the runoff characteristics of three urban surface types: asphalt, concrete, and grasscrete, under two common slope conditions (3° and 6°), using a custom-built, portable rainfall simulator. Runoff volumes and time to runoff generation were measured across repeated trials, with data used to calculate runoff coefficients and assess infiltration behaviour. Results demonstrated that grasscrete significantly reduced runoff volumes and delayed the onset of surface runoff compared to the impervious alternatives. On average, grasscrete generated 40% less runoff than concrete and 27% less than asphalt. Statistical analysis confirmed these differences to be significant. Impervious surfaces showed almost immediate runoff response, while grasscrete showed delayed runoff of up to five times longer under both slope conditions. Despite limitations such as wind interference and collection inefficiencies, the results aligned well with literature. These findings confirm that grasscrete offers tangible hydrological benefits and should be more widely implemented as part of Sustainable Urban Drainage Systems across Malta and Gozo. As the island faces increasingly erratic and intense rainfall, alternative paving materials like grasscrete offer a scalable and effective solution for runoff mitigation, flood risk reduction, and potential groundwater recharge.