Modelling the erosion of mixtures of organic and granular in-sewer sediments

The release of fine-grained organic sediments from sediment deposits can have a detrimental impact on water quality in a number of situations. This paper examines the release of such sediment in the context of the erosion of mixed organic/granular sediment in-sewer deposits. In the European Union, sewer flow quality modeling software uses equations derived from uniform granular sediment studies. Actual sewer sediments are mixtures of organic and granular material and interactions between these fractions may account for the poor performance of current models. Laboratory experiments were carried out using surrogate sewer sediment mixtures. Impaction of the bed surface by saltating granular particles increased the erosion of fine-grained organic sediments. Changes in the composition of the bed surface over the duration of a test resulted in change in the availability of fine-grained sediment. A model that attempted to simulate these mechanisms, using an empirically based correction factor to account for the impaction mechanism and an active bed layer to account for changes in the bed surface composition was developed. The limited success of the simulations indicated that such simple modeling approaches may not be appropriate for organic/granular deposits in which grain sorting occurs.