Laboratory experiments and numerical modelling of the scouring effects of flushing waves on sediment beds.

This paper presents an investigation on the scouring effects of flushing waves on sewer sediment deposits in order to derive indications on the design and set-up of flushing devices in sewer systems and in stormwater storage tanks. The investigation concerns both laboratory experiments and numerical modelling. The experimental campaign was carried out on a channel at the laboratory of the Department of Hydraulic and Environmental Engineering of the University of Pavia adopting a simple flushing device. The first set of experiments is developed without sediment and the water levels during the flushing processes are monitored by a digital video-camera. The next tests are carried out with uniform layers of sediment at the bottom of the channel and the effects on sediment beds produced by flushes are analysed. The numerical model solves the De Saint Venant equations describing unsteady flow in open channel and the continuity equation of Exner for the conservation of the sediment mass. For this purpose, the MacCormack explicit finite difference scheme is introduced. A sediment erosion relationship linking the rate of erosion to an excess of shear stress at the bottom is included in the model. The comparison between the experimental and computed water depths shows a good agreement for different flushing conditions and longitudinal slope of the channel. The tests with different sediment beds allow the calibration of the parameters that globally describe the erosion process produced by flushes.