Validation of existing bed load transport formulae using in-sewer sediment

The presence of a granular layer in the inverts of pipes has been reported in a number of sewer systems in France, the United Kingdom, Belgium, and Germany. Given the typical range of flow conditions found in combined sewers and typical particle characteristics of inorganic in-sewer solids the mode of transport of many of these mineral sediments may normally be considered as traditional bed load transport. Current commercial software packages modelling the erosion of sediments and the transport of pollutants in sewer pipes still utilise well-known, or modified versions of, bed load or total load transport equations that were derived for transport of non-cohesive material in alluvial streams. In this paper the predictive performances of the equation of Ackers and White (originally developed for the transport of sediment in rivers) and the equation of May (a formula derived from laboratory pipe experiments) are examined against two separate data sets. The first data set is obtained from laboratory erosion experiments on sewer sediment at the Hydraulics Laboratory of the University of Ghent. This sediment was collected from the experimental catchment "Le Marais" in Paris. A second data set is derived from bed load transport rate measurements recorded in the inlet pipe at the National CSO test facility in Wigan, United Kingdom. These formulae were selected for study because of their widespread use in the prediction of in-sewer sediment transport both in commercial software and in the latest UK design guidance for new sewers. It was seen that neither formula performed well. It was seen that each of the traditional formulae have significant difficulties in predicting the erosion thresholds and transport rates for individual grain size fractions from such non-uniformly sized in-sewer deposits.