Influence of SUDS Allocated Area on Runoff Reduction in Developing Urban Catchments: a Case Study in San Sebastian (Spain)

Abstract

Despite the increasing use of Sustainable Urban Drainage System (SUDS) techniques, it is still difficult for urban planners to identify how many shall they implement, especially when planning new developments. In addition, most agencies or public authorities limit runoff outflows to the greenfield or undeveloped conditions, but planners do not have the capacity to link this objective to the level of SUDS implementation. Thus, the objective of the present study is to analyse the relation between the implementation level of SUDS and the reduction of urban catchment ouflows (peak flow and volume), compared to greenfield condition. For that purpose, a hydrological model was created with the Storm Water Management Model (SWMM), based on a case study of 3.2 ha in Donostia/San Sebastián (Spain). The greenfield scenario was characterised with a CN equal to 78. Pervious and impervious areas were identified into the urbanised plot, as well as the existing drainage network. Finally, certain area of SUDS was assigned to each subcatchment randomly, and hydraulic variables were compared at the outlet with the ones in the greenfield situation. As the area is a dense urban area, the considered SUDS for this study were green roofs, permeable pavements and bio-retention cells. Four storm scenarios were used, with a 30 minutes duration but different return periods: 2, 5, 10 and 20. Results show how SUDS application is more effective with the peak than with the volume, and that surface with SUDS applied on a 35% of the surface may reduce the outflow peak to its greenfield value.