Particulate organic matter concentrations determined at the outlet of a low-sloped combined sewer are observed to increase significantly in the very early stage of storm runoff events. It is found that this is not a result from scouring of actual deposited sediments in the sewer (which are granular), but from the dynamic behaviour of a distinct component of the sewage flow itself. During low flows in dry weather, a part of the coarsest material present in normal sanitary sewage (primarily organic debris such as human and food wastes, of about millimetric size and larger) settles and leaves the main body of the flow, but does not reach the sewer bottom to deposit there. The lowest part of the flowing sewage acts in this case as a temporary storage of highly polluting matter, as its longitudinal velocity is lower than the one of the remaining flow. An explanation for the building of these ‘dense undercurrents’ is formulated, based on a difference between free settling in the main body of flow and hindered settling near the bottom. Although this still merits further confirmation, it is likely that the process which maintains coarse solids in suspension in the undercurrent is a lift force induced by shear in the near-bed zone.

The collapse of the undercurrent and the subsequent release of polluting material into the whole water column has been observed in the considered sewer to occur when bed shear stress exceeds about 1.1-1.2 N/m2. This low value can explain flushing effects occurring frequently during rainy period, or even at peak dry weather flows. On the basis of properties illustrated in this study, a number of management options can be considered for revised day-to-day operation of low-sloped interceptor sewers where dense undercurrents are likely to develop.

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