Assessment of urban stormwater quality by informing a hydrologichydraulic model with microscopic traffic simulations

As urbanization continues to increase and transportation networks expand with many more vehicles on the roads, cities are facing growing challenges, including longer commuting distances, higher rates of traffic accidents, increased pollution from traffic along with reduction of green spaces. When a rainfall event occurs after a dry period, the so-called first flush phenomenon generates (Sartor and Boyd, 1972). It indicates the effect of the first and faster stormwater runoff, which, by washing over impervious surfaces such as roads, gather together pollutants, like particulate matter, nutrients, bacteria and heavy metals. Without proper water treatment measures, this contaminated flow represents one of the major causes of drainage systems and water bodies quality deterioration (e.g., Barbosa and David, 2012). Several studies pointed out a clear correlation between the increase of the contamination level of stormwater runoff from roads and the number of vehicles (e.g., Barrett et al., 1998) or the average daily traffic density (Hares and Ward, 1999). However, the lack of reliable data of stormwater quality together with the costs and difficulties in performing regular monitoring campaigns of traffic in different locations and for different pollutants (i.e., heavy metals, particulate matter, total suspended solids, nitrogen oxides, etc.), have hindered the possibility to quantify the level of contamination. To overcome some of these obstacles, this study combines two modelling procedures that simulates the traffic related emissions by using AIMSUN (Advanced Interactive Microscopic Simulator for Urban and Non-Urban Networks, AIMSUN 2024), a well-known integrated traffic and emission model, and the relative build-up, wash-off and transport throughout the drainage system by using the Storm Water Management Model (SWMM, Huber 1985). AIMSUN and SWMM were used to characterize the wash-off and fate of particulate matter generated by traffic (hereafter PM) and its possible reduction by the introduction of a first-flush tank in a roundabout of the city of Trento (Italy). The study also explores several alternative layouts to an existing roundabout in order to identify the optimal one from a combined environmental and traffic management perspective.