RESEARCH ARTICLE Proposal of a semi-probabilistic approach for storage facility design B. Bacchi*, M. Balistrocchi and G. Grossi DICATA, University of Brescia, via Branze n8 43, 25123 Brescia, Italy (Received May 2006; final version received February 2008) Storage facilities are key devices in mitigating the urban drainage impact on receiving water bodies, but their design is still affected by high uncertainty. The analytical-probabilistic approach has recently raised interest, because the facility performances are directly related to probability. Starting from statistically independent storm events, distributions of the meteorological variables must be fitted. Rainfall series, recorded in three Italian raingauges, were examined for appraising two main concerns: the choice of proper probability distributions for rainfall volume and the sample sensitivity with respect to the analysis criterion. The analytical derivation of the model is then finally discussed. Keywords: overflow control; rainfall time series; statistical analysis; sustainable drainage systems 1. Introduction The ability of controlling the wet weather overflows in combined or storm sewer networks is a primary target for establishing a sustainable drainage system in urban catchments. Among the several available techniques, storage facilities are structural practices that may be used for the mitigation of the urban drainage impact in very different manners. In Italy, where very intensive urbanisation settlements are present, they traditionally consist of detention reservoirs built in most cases at the ground level. They are usually connected to the sewer system upstream a spillway device or a final outlet and therefore their contributing area can be quite extended (Urbonas and Stahre 1993, Walesh 1989). Traditional methods for a detention storage capacity design are based on the concept of the design hydrograph derived from intensity-duration-frequency (IDF) curves, as- suming a uniform spatial distribution: firstly the time distribution of precipitation is derived from the IDF curves for a fixed return period and then the design hydrograph is obtained by routing the hyetograph through a mathematical rainfall-runoff model. This approach is widely used in the Italian engineering practice, but it does not correctly quantify the hydrological uncertainty of the computed storage. Nevertheless, assuming a frequent occurrence of the so called first flush phenomena (Martin 1999, ASCE WEF 1998), the employment of a storage detention facility can be also aimed at capturing the first high polluted fraction of runoff. Therefore, when the over- flow process begins, only the less contaminated runoff is spilled out of the sewer network into the receiving water body. The stored volume is not treated in the device itself, but it is drained into the waste water treatment plant after the end of the storm. In combined sewer systems it is discharged into the downstream pipes, while in separate systems it is diverted into the sanitary network. Despite the primary purpose, which is the overflow pollution load reduction, the decrease of the number and the volume of the overflows are also further consequences. The design of such a quality control device involves a lot of unsatisfactorily under- stood aspects: the device capacity estimation, the connection scheme between the device and the sewer network and the management criteria during wet weather conditions and the subsequent dry periods. Simple rules recently developed in some Italian Regions for the definition of the so called ‘water quality volume’ are definitely not representative of the complexity of the problem. In fact only a fixed value of rainfall over the impervious area of industrial settlements is considered as polluted rain and no information about the manage- ment of the captured volume is provided (Lombardy Region 1985). On the other hand, more sophisticated approaches like water quantity and quality model simulations, need a strong conceptualisation and simplification of the involved physical phenomena (not really different from those used in traditional design methods), as well *Corresponding author. Email: baldassare.bacchi@unibs.it Urban Water Journal Vol. 5, No. 3, September 2008, 195–208 ISSN 1573-062X print/ISSN 1744-9006 online Ó 2008 Taylor & Francis DOI: 10.1080/15730620801980723 http://www.informaworld.com Downloaded By: [Bacchi, B.] At: 16:21 17 September 2008