Abstract

Sedimentation in main sewers takes place where the sediment transport capacity of the flow is exceeded and is caused by the combination of low dry weather flows, large pipe diameters and low longitudinal slopes. Then depositions will build up and consolidate at the sewer bottom during dry weather periods. These deposits may not be removed by the next stormwater runoff which leads to a need for cleaning the affected sewer stretch (Ashley et al., 2004; Ashley et al., 2005).

To clean main sewers by flushing, the dry weather runoff is usually stored behind a flushing shield and the necessary flushing volume depends on the length of sewer reach, the diameter and the slope. The storage time of the flushing volume depends on the flow rate of the dry weather runoff. During the storage period, when the flushing shield blocks the sewer profile, the remaining dry weather runoff downstream of the flushing shield is slowly drained due to the sewer geometry. In many cases the sewer reach that has to be cleaned by flushing, is not totally drained before the necessary flushing volume is reached and the flush wave is released. This means that the flush wave runs into a water body of dry weather runoff, which affects the power and cleaning effectiveness of the wave considerably. This hydraulic behaviour of the dry weather runoff is usually not taken into account for the design of flushing devices, either in scientific investigation or in practical applications.

Many numerical and practical investigations on sewer flushing were carried out over recent years with respect to flush waves which act like dam-break waves along a sewer reach. The main objective was to analyse the behaviour and cleaning efficiency of the flush waves created by different flushing devices. Thus the focus was on the creation of the needed critical bottom shear stresses required to lift deposits from the sewer bottom and transport them to sewer reaches with a higher transport capacity or to the treatment plant. An extensive summary can be found in Schaffner (2008).

The usual assumption in these investigations was of a dry sewer bottom downstream of the flushing devices. This initial condition is usually found in stormwater tanks or sub-main sewers which are totally drained before they are flushed, but unfortunately this initial condition is seldom found in main sewer systems.

The intention of the present investigation was to analyze the behaviour and influence of different downstream levels of the remaining dry weather runoff on the bottom shear stresses of different flush waves. Therefore typical sewer geometry with an adequate dry weather runoff was created in a one-dimensional numerical model. In seven scenarios the movement of the dry weather runoff was modeled for the storage time of the flushing volume and the assumed initial conditions for the modeling of the flush waves. The investigations of the calculated bottom shear stresses gave a good estimate on the potential cleaning efficiency of the waves. In the next step the results of the flush waves, modeled with realistic downstream levels, and were then compared to flush waves modeled with a dry sewer bottom to show the differences of the two approaches.

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