Urban stormwater runoff thermal characteristics and mitigation effect of low impact development measures

Thermal pollution has become a severe environmental problem in China, but studies on thermal characteristics of urban stormwater runoff are scarce. The thermal enrichment of runoff from typical land surfaces was assessed during 2012-2014 in Beijing and Shenzhen, China. The temperature of stormwater runoff from rooftops, grass surfaces and different types of road surfaces was investigated under different rainfall conditions. The mitigation effects of low impact development (LID) measures were also evaluated. Impervious asphalt or concrete surfaces store and transfer heat, and were found to cause thermal enrichment of runoff from the start of a rainfall event. In addition to surface types, pre-event weather conditions and rainfall intensity influenced runoff temperature. The pervious surface of open graded friction course (OGFC) pavement postponed the time of peak runoff temperature. The retention volume of bioretention cells resulted in thermal energy mitigation by directing runoff into the soil and vegetative cover. The grass swales showed effective reduction of runoff temperature by approximately 1-2 degrees C compared to asphalt, concrete and marble pavements. Therefore, LID measures, such as OGFC porous pavements, bioretention cells and grass swales, can mitigate the thermal impacts of urban stormwater runoff and alleviate resulting ecological problems.