Drainage efficiency in urban environment

Urban drainage systems may prove locally inefficient as a consequence of an unexpected amount of water failing to reach a certain number of inlets in the network. Local failures are indeed observed as a consequence of precipitation events with even lower intensities than the design rainfall, when inlets are requested to drain contributing areas that are larger than the design ones. Micro-topography effects are usually responsible for local failures of the urban drainage system, and their potential in terms of local flooding can be investigated by taking a proper degree of topographic details into consideration. The flooding potential associated with the failure of a number of inlets in the system is investigated in this paper by means of a flood propagation model specifically designed for areas with complex topography and applied herein to the urban environment. The model involves a hyperbolic two-dimensional solution of the De Saint Venant equations that is obtained over a finite element representation of the spatial domain with an unstructured triangular network. This approach allows detailed description of the flow patterns associated with stormwater volumes along the surface drainage network according to the actual performance of the existing inlets. The results presented refer to the application of the model to the case study of an urban catchment within the Historic Centre of the town of Genoa. The simulations confirm that micro-topography effects have the potential to produce local flooding with significant water depth and velocity in zones of flow concentration, and that local topography may significantly affect the behaviour of the urban drainage system as a whole.