Ability of Preissmann slot scheme to simulate smooth pressurisation transient in sewers

Urban drainage networks are generally designed to operate in a free-surface flow condition. However, as a consequence of heavy rainfall events or network malfunctioning, filling of sewers (pressurization) and network overflowing may occur. Several softwares are commonly used to simulate floods in drainage networks and their results are usually thought to be reliable and robust. However, no specific studies have been carried out on the behaviour of these softwares during the pressurization transient. Actually, mathematical models based on the momentum equation appear to be more suitable in correctly simulating the pressurization moving jump but models based on the energy equation are much more popular because of their greater simplicity with respect to the formers. Such models make use of Preissmann slot simplification to handle pressurization. In this paper, on the basis of laboratory pipe tests, the reliability of such simplification is studied by means of a popular model: SWMM (Storm Water Management Model). Many numerical tests were carried out by SWMM varying the spatial and time steps as well as the Preissmann slot width in order for the performances of the software to be examined along even wider intervals of these parameters than is usual in practical applications. The comparison between simulated and experimental surges allows one to draw interesting conclusions on the effectiveness of SWMM in simulating pressurization as well as on the choice of the parameters themselves.