Application of Flushing Tanks in Simple Sewer Networks for In-sewer Sediment Erosion and Transport

Reza H.S.M. Shirazi, Patrick Willems and Jean Berlamont

ABSTRACT

Assessing in-sewer sediment accumulation, which causes problems such as loss of hydraulic capacity of sewers and pollution consequences when re-suspended by peak flows, is becoming more in concern and is important in urban drainage design and maintenance. An evaluation based on simulations carried out with InfoWorks CS regarding application of a flushing tank as a tool for eroding deposited sediments from a simple sewer network is presented. The hydrodynamic modeling comprised implementing InfoWorks CS (Wallingford Software, UK) to assess the eroding capability of the generated flush waves regarding sediment removal and transport, and applying the model based on the shear stress estimation in the software (the KUL model developed by Bouteligier et al., 2002).

The simulations were initially done for a simple network composed of one straight conduit partitioned into 5 pipes with lengths equal to 10 m. Various combinations of pipe diameter, pipe slope, sediment characteristics, and DWF in the network was considered. Regarding the dry weather sediment build-up modeling, it was important to reach an equilibrium condition before any implementation of flushing tanks would be considered. Initially, the effect of flush waves emitted from one flushing tank implemented at the most upstream manhole on sediment removal was analysed. Various flushing events were assumed (i.e. 1 flush, 7 flushes with 5 min intervals, and 10 flushes with 10 min intervals).

After the flushing events occurred in the network, their effects on sediment transport were assessed. Whenever the upstream flushing tank was found incapable of generating required criterion for sediment erosion and transport through the network, more flush tanks were proposed to be applied in downstream manholes. Next, having implemented more flushing tanks in potentially effective locations, the results for sediment erosion and transport in the network was analysed and the positive and negative outcomes of such applications were assessed. Afterwards, in the second phase, a few more pipes (as branches) were added to the initial network and the same analyses were repeated to evaluate the overall effects of added parts on sediment build-up and transport.

The final aim was to evaluate whether by implementing flushing tanks in such a simple network, erosion of the settled particles and removing them out of the system could be reached. Whether a flushing tank was efficient in removing and transporting sediments from a part of the sewer network was related to various parameters such as sediment characteristics and the flush interval. In fact, it was understood that the type of sediments and their characteristics (mainly the particle size and density) and other parameters with relevant effects on the overall hydraulic characteristics of the sewer network need to be carefully considered in sediment transport modeling in order to reach to suitable modeling results. In an overall perspective, the capability of such flushing tanks to produce effective forces for removal of the settled particles in sewer pipes is well accepted.


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