ABSTRACT
This chapter discusses a presentation of the specific hydraulic features of urban drainage systems and the computational techniques for their hydrodynamical modelling. The transition between free-surface and pressurized flow can be modelled by modifying artificially the shape of the pipe so that the propagation properties of the solutions of the Saint Venant equations become identical to those of the solutions of the water-hammer equations for pipe transients. The governing assumptions behind modelling of urban drainage systems are similar to those used in open-channel and closed-pipe systems. Another issue specific to closed-conduit, free-surface flow is the essentially unstable character of the flow when the water rises above a certain threshold level in the pipe. Lateral weirs are often treated as local sink terms along the pipe. If the length of the weir is important, the elevation of the free surface may vary substantially between the upstream and downstream ends of the weir.
