Manhole Incidence on Transient Flow Modeling during Storm Sewer Flooding Event

Formerly urban drainage systems were sized to quickly convey rainwater from urban area to areas of natural flow (rivers, streams, lakes, etc). Hence, they were sized on the assumption of permanent regime and free surface flow. With the frequent flooding of storm sewer and accordingly accidents recorded, numerical modeling of transient flow has now become unavoidable. Several models of transient flows are now available, each with its advantages and limitations. This paper examines the possible impact of the inclusion of Manhole size on the flooding speed of urban drainage systems. Indeed, we assumed that the pressurisation of the drainage network starts with the rapid filling of manhole, ie as soon as the water level in the manhole reached the crown of the pipe being filled.

Based on this assumption, a filling model is built to analyze the impact of manhole size on the propagation of the filling or emptying wave. This model uses the method of characteristics within the conduct and an innovative method based on intumescences technique at the system limits. Changing the manhole size makes it possible to quantify the storm sewer filling speed during a precipitation event. The manhole filling processes is found to be more or less important and the sewer pressurisation a greater or lesser degree in intensity and duration depending on the manhole size and its height.

In addition, because of its simplicity, the method proposed in this paper can be integrated in various numerical methods for solving transient flow (finite difference implicit or explicit, finite volume method, finite elements), and could even allow the combination of several solving techniques in a dynamic calculation of the network. A comparative analysis base on a theoretical study case makes it possible to compare different existing techniques and practices.