Assessing the performance of Sustainable Drainage Systems (SuDS) in urban context using SWMM5 modelling scenarios: the example of a typical industrial area in Lombardia Region, northern Italy

Recently, particularly invasive urbanization dynamics, resulted into a substantial increase in the urban impervious surface that forced the administrations to deal more frequently with the inability of the traditional drainage systems to manage stormwater in a sustainable and effective manner. Worldwide, integrated approaches, such as Sustainable Drainage Systems (SuDS), whose basic principle is the management of rainwater at source through the implementation of prevention, mitigation and treatment strategies, are increasingly being developed.

The project aims to assess the benefits, in terms of reduction of floods, deriving from the widespread implementation of SuDS in an industrial area of about 300 ha in northern Italy and to analyse their behaviour under local climatic conditions. For this purpose, in absence of rain gauges in the case study area, analyses were carried out to obtain reliable and continuous rainfall data from all weather stations closest to the basin. Therefore, 10 years of rainfall data (2009-2018), recorded at 15 minutes timesteps from 10 station, have been acquired by the Regional Agency for Environmental Protection of the Lombardia Region and Inverse Distance Weighting has been used as a methodology of interpolation to obtain precipitation for the area of interest.

Critical precipitation scenarios, both annual and event scale, have been identified to evaluate the performance of SuDS during significant rainfall periods or events. For this reason, it was considered appropriate to extract from the complete dataset the year characterized by the maximum precipitation amount (1515.57 mm), the rain events with the maximum intensity in an hour (5.23 mm/h), with the maximum overall intensity (7.36 mm/h) and with the highest return period (5 years with a 6.87 mm/h intensity).

SWMM5 modelling allowed to compare the performance of the sewer system of the basin (overall 1148 nodes, 1141 pipelines for a total of 36 km of network) in a “traditional” scenario, without integrated strategies, and after the implementation of  green infrastructures (about 10% surface area and located in the basin in accordance with the current structure of the urban agglomeration).

The results, assessed in terms of reduction of different parameters such as runoff coefficient (on average 12% for the year and 39% for the event analysis), maximum flow in the pipelines (on average 3% and 31% respectively), maximum total inflow in the outfalls (on average 7% and 40% respectively) and node flooded (on average 23% and 57% respectively) following the implementation of SuDS, suggest in the first instance that these systems can give their contribution in the mitigation of the effects of flooding in urban areas. Indeed, analyses aimed at investigating punctually over time flow and volume in the outfalls conducted so far, brought about no extremely positive results and the performance of SuDS seems to be particularly challenged by severity of rainfall events.  As future aspects, this research strives to assess the performance of sustainable drainage systems under common rainfall scenarios and to establish, through an analysis of the climate change effects and the creation of rainfall data projections, the performance of these systems also over time.