Long term simulation analysis under two different rainfall regimes as an aid to gully pot management

A roadside gully pot is the most superficial component of the urban drainage system. Being the connection point between the street surface and the network below, involves both hydraulic and water quality aspects, as well as associated management and maintenance problems. Gully pots are supposed to trap solids material washed off the catchment surface, but also to collect and convey the stormwater network. The continuous accumulation of particulate matter brings to a progressive loss of the gully pot hydraulic conveyance, then increasing the probability of urban flooding problems during rainstorm events. This raises questions about maintenance needs and scheduled maintenance timing. Previous laboratory studies have analyzed the settling phenomena inside gully pots, developing and validating also analytical expressions for estimating the trapping efficiency. Based on experimental findings, the present work aims to assess the long-term behaviour of a single gully pot by means of numerical simulation modelling. The analysis made use of two long term rainfall series representative of fairly different hydrological regimes (Milan and Palermo, Italy). The widely known build-up and washoff models included in EPA SWMM 5 have been considered. Their parameters values have been randomly sampled by uniform or non-uniform distributions within the normal ranges reported in literature. From long-term simulation results it is possible to estimate a probability distribution of solids mass annually retained in the gully pot as well as to study the relationship between such mass and the main parameters involved in the build-up and washoff equations.