Abstract
Abstract. Since Water Framework Directive (WFD) was passed in year 2000, the conservation of water bodies in the EU must be understood in a completely different way. Regarding to combined sewer overflows (CSOs) from urban drainage networks, the WFD implies that we cannot accept CSOs because of their intrinsic features, but they must be assessed for their impact on the receiving water bodies in agreement with specific environmental aims. Consequently, both, urban system and the receiving water body must be jointly analysed to evaluate the environmental impact generated on the latter. In this context, a coupled scheme is presented in this paper to assess the CSOs impact on a river system in Torrelavega (Spain). First, a urban model is developed to statistically characterise the CSOs frequency, volume and duration. The main feature of this first model is the fact of being event-based: the system is modelled with some built synthetic storms which cover adequately the probability range of the main rainfall descriptors, i.e., rainfall event volume and peak intensity. Thus, CSOs are characterised in terms of their occurrence probability. Secondly, a continuous and distributed basin model is built to assess river response at different points in the river network. This model was calibrated initially on a daily scale and downscaled later to hourly scale. The main objective of this second element of the scheme is to provide the most likely state of the receiving river when a CSO occurs. By combining results of both models, CSO and river flows are homogeneously characterised from a statistical point of view. Finally, results from both models were coupled to estimate the final concentration of some analysed pollutants (biochemical oxygen demand, BOD, and total ammonium, NH4+), within the river just after the spills.
Highlights
Control of waste and storm water generated from urban areas has been an issue since the times of the earliest civilizations
When a spill from urban runoff occurs, river biochemical oxygen demand (BOD) rises to values around 10 mg/l, which do not comply with the quality values required
According to the EU Water Framework Directive, it is at present necessary to analyse the impact these combined sewer overflows (CSOs) generate into the environment
Summary
Control of waste and storm water generated from urban areas has been an issue since the times of the earliest civilizations. Continuous and rapid growth of urban areas and the above mentioned legal requirements have favoured new alternative approaches to storm water management These measures are generally called best management practices (BMP), sustainable urban drainage systems (SUDS) or low impact developments (LID) (Freni et al, 2010a). CSO effects into the river will be studied at these locations, to find out the present situation and its influence on river water quality In this case, continuous simulation of the rainfall-runoff process is an essential tool to be able to estimate the initial moisture state and base flow prior to any storm event causing the streamflow to rise. Models are coupled to establish the existing relationship between overflows, produced in drainage systems during storm events, and their impact on the receiving river
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