Flocculation/aggregation of cohesive sediments in the urban continuum: implications for stormwater management

The urban continuum, as it applies to sediments and associated contaminants, represents the area over/through which sediments are conveyed from a depositional or eroded surface to a treatment system and/or receiving water body. This study has focused on the changing physical characteristics of the sediment, with an emphasis on flocculation/aggregation, as it progresses through the urban continuum. The sediments of the urban continuum are found to change from an unflocculated state on the street, to a flocculated state in the surface runoff to a very large floc form in the sewer system. The high organic content in the sewers contributes to the large floc size. The structure of the flocs and the flow regime of the receiving water will dictate the fate of the sediment following a combined sewer overflow. Probability distributions fitted to the distributions of each sediment type (compartment) confirmed significant differences in the sediment population sizes. Bulk and individual particle settling velocity experiments also revealed substantial differences between compartments. Sewer flocs were found to be of low density, with high porosity, water and organic content and with settling velocities which increase with floc size.