Flow velocities and shear stresses during flushing operations in sewer collectors.

Many relevant problems of drainage systems such as first flush pollution, flow capacity reduction and consequent risks of surcharges are worsened by sediment accumulation in sewer collectors. For the solution of these problems several sediment cleansing methods and techniques, e.g. mechanical methods, traps and flushing devices, have been developed and applied in the last decades. In particular, simulation studies and experimental campaigns have shown the effectiveness of flushing methods as preventive cleansing strategy in sewer collectors. In this paper the results of a numerical investigation for determining flushing waves characteristics both in terms of duration and flow rate are presented. The collector section lengths where flow velocities and average shear stresses determined by flushing waves exceed minimum threshold values are evaluated using a dimensionless approach. The results, related to the operative ranges of the practical applications, are exposed by means of graphs and regressive equations.