The trapping efficiency of roadside gully pots during rainfall events: an experimental laboratory study

Roadside gully pots act as a link between surface runoff and drainage system, therefore they have to be considered as a relevant part and parcel of urban drainage systems. It is a well-established opinion that their main function is to protect downstream drainage facilities, treatment plants as well as receiving waters from excessive sediment loads. Anyway a continuous supply of solids to a device supposed to trap them, leads unavoidably to gradual silting and eventually to clogging problems. These factors appear quite relevant in Italian cities, mainly served by combined sewer systems where trapped gullies are required to prevent the escape of bad odour, being then particularly subject to silting and clogging problems. This study has therefore the objective to determine which variables influence the gully pot capability of retaining solids (efficiency). Several laboratory tests have been performed on a simple plastic gully pot (40 x 40 cm), varying the characteristics of the rainfall events (inflow rate) and of the influent solids (diameter, particle size distribution and specific gravity). Results obtained don’t seem to contradict previous research, but usefully extend their substantial validity also to contexts and conditions for which they had not been directly verified. Furthermore the role of particles settling velocity in efficiency determination has been investigated. Efficiency has been calculated adopting different settling velocity formulas proposed by various authors and eventually compared to experimental data.