A tailored green-approach for managing CSOS in high density urban areas

This study was aimed at providing evidence about the possibility to adopt low-cost “green-approaches” for mitigating peak flow, volume and pollution loads of CSO in areas where high density of urbanization occur. In particular, the proposed approach exploits detention, infiltration and self-depuration capacities of natural systems already existing in peri-urban areas, in order to achieve a tailored solution that is suited to the specific context. The findings presented in this work show that combining first-flush tank (FFT) with a constructed wetland system (CW) and finally managing the flows in the receiving water body (RWB) about 90% of pollutant load abatement can be achieved. Therefore, the system can be simultaneously designed to pursue flood risk reduction (abatement of flow peak) and improvement of receiving water quality objectives. Moreover, each component of the proposed system has some peculiarities. For example, CW performs a dual function, i.e. on one hand it cuts the peak of flow thanks to its storage capacity, while on the other hand it reduces the volume thanks to its infiltration capacity. This features are most important especially in the context of “hydraulic-hydrologic” invariance measures (that are becoming more and more present at local scale) where the reduction of peak discharge has necessarily to be accompanied by a reduction of volumes. Further improvements can be obtained through the control of flow in the RWB aimed to maintaining a correct ratio between upstream accumulation and downstream flow control according (i) the variability of flow in input to the RWB and (ii) the downstream canal hydraulic capacity. Flow control can be achieved by installing smart gates that operate automatically based on flow sensors and software-based actuators. Finally, the additional ecosystem services that can be provided by the green components of the system combined with the relatively low-cost of the interventions make the approach particularly attractive for small municipalities where large investments are seldom possible.