Assessment of the overflow reduction potential of a plan to install extensive rooftop SUDS on Cartagena’s buildings

Resumen

Introduction: Spanish cities are starting to analyse the possibilities of reducing water bodies pollution due to combined sewer overflows through the implementation of Sustainable Urban Drainage Systems (SUDS). Cartagena is a mid-sized city of about 200 000 inhabitants located in a semiarid region. The city is mostly flat, although is surrounded by steeped hills and mountains. Frequent high groundwater phreatic levels limit the possibilities of infiltration. Nevertheless, rooftop SUDS can be an interesting possibility, as most people live in apartments with flat roofs. In this work, an extensive preliminary assessment is carried out to estimate the possible impacts of the application of this technique on different ratios of rooftop SUDS penetration and for different types of events.Methods and data: Data from buildings surfaces is obtained from OpenStreetMap, due to its high quality in the polygonal representation of the buildings. It is assumed that the potential surface of rooftops is limited by the typical roof area ratio of the buildings according to typical residential morphologies [1]. The sewer system has been modelled through EPA SWMM [2]. The rooftops have been based on the model and parameters assessed in [3], being replaced a ratio of each building surface belonging to a subcatchment by the model using for that purpose the library swmmio [4]. Extensive simulations have been done for the different penetration ratios, evaluating the influence for different types of events (registered or historical and design events). The water quality improvements caused by the rooftops has been neglected, being the analysis focused only on the sewer overflow reduction, and specially the overflows to the harbour.Results and discussion: The effect can be quite relevant in lower rains, and negligible in greater events. Further analysis can be carried out to evaluate which areas of the city have a greater influence. This kind of analysis is relevant and should be a requirement before elaborating a SUDS city plan.