Author(s): De Paola Francesco; Galdiero Enzo; Gualtieri Carlo; Pugliese Francesco; Giugni Maurizio
Keywords: Best Management Practices; Urban Hydrology; Storm-water Tank; Probabilistic approach
Abstract: Stormwater detention tanks are frequently used as a structural measure for mitigating impacts of combined and separated sewer overflows from urban catchments into receiving water bodies. The dimension of these capacities depends on the climate pattern and the sewer system behaviour, and it can be estimated by using continuous simulation or probabilistic approaches. The latter are generally based on a stochastic model which is able to fit the statistical pattern of observed rainfall records and on an urban hydrology model that transforms rainfall in sewer discharge. A key issue is the identification of the optimal structure of the stochastic rainfall model. In this paper a conceptual stochastic model of rainfall is proposed in which storms occur in a stochastic process, where each storm has a random lifetime. Different events are identified through the definition of an Inter-Event Time Duration (IETD) ,that is the duration of the dry period between two consecutive storms. Point processes are frequently applied where rainfall events are represented through the occurrence of rectangular pulses, which are governed by specific descriptors (volume, duration and IETD) .The paper focuses on the analytical derivation of the Probability Density Function (PDF) of the number of overflows as a function of the storm-water capacity. This is performed by using the derived probability distribution approach. The number of overflows is used as a performance indicator of the tank effectiveness in mitigating the environmental impacts on natural water bodies (sea, rivers, lakes etc. )in terms of both quantity and quality. The proposed approach is applied to two different sites located in Campania Region, in Southern Italy. For both locations the Pareto-2 and the Gamma PDFs show good results in fitting the rainfall depth and duration at varying the IETD, and they perform better than the Weibull and the Exponential model, which were widely used in past studies. Furthermore, it turned out that Fatigue Life or the Log-Gamma PDFs are able to provide the best fit for the IETD. Finally, the extension of the proposed model to different hydrological conditions is discussed.