Application of Flushing Tanks in Simple Sewer Networks for In-Sewer Sediment Erosion and Transport
The principal objective of urban drainage systems is to transport the aqueous and solid wastes emanating from domestic, industrial and storm sources for treatment and disposal. The solids that enter the system are often a cause for concern as they may settle out to form permanent or semi-permanent deposits on sewer inverts which can generate problems such as hydraulic restrictions, reduction in the design flow capacity of the sewers, and increase of the risks of surcharging during storm events. Generally, in-pipe deposits result from accumulation of sediments during dry weather flow (DWF) periods and during the recession of storms (Skipworth et al., 2001). Solid materials largely originate from domestic and industrial wastewater and from unpaved catchment areas (Dettmar et al., 2002). The sediment deposition tendency is different depending on the location of a sewer in a network and the characteristics of the conduit such as size, gradient and shape. Deposition will occur at a rate depending upon the flow characteristics, the nature of the particles and their concentration in suspension near the bed (Fraser & Ashley, 1999).
To account for the effects of sediments in sewer systems, an appropriate sediment transport modeling should be carried out which needs to consider the following (Fraser et al., 2005):
- To produce rapid, detailed, continuous hydraulic simula-tions for sewer flows with long-term durations;
- To determine the most likely locations of sediment deposition using a pipe-by-pipe analysis;
- To predict approximate depths of sedimentation at loca-tions identified previously so as to rank potentially problematic locations; and
- To predict sediment concentrations during dry weather and storm events allowing for potential erosion volumes from deposited beds.
The major requirement in urban drainage design is to ensure that sediment would be transported through sewer pipes at the same rate that it approaches the sewer network without any long-term build-up of sediment deposits (May, 2001). It is important to note that since flow rates and sediment loads in sewer systems can vary considerably with time, it is unrealistic to expect to be able to design a sewer network so that no deposition would occur under various flow conditions. Accordingly, sewers should be designed to transport sediment at a rate sufficient to limit the depth of deposition to a specified proportion of the pipe section to maintain the required hydraulic characteristics of the conduit. According to Bertrand-Krajewski (2002), two important issues regarding sewer system design are:
- How to avoid or at least limit deposition in new sewers by means of appropriate rules based on sewer shapes, slopes and flow velocities; and
- How to cleanse deposits by means of appropriate flushing and mechanical devices.
In fact, avoiding deposition is not always promising, particularly in flat regions, where the necessary slopes for sewer pipes to be self-cleansing are not available due to the costs of deep excavations and pumping systems (especially in the most upstream parts of the net-work). Nevertheless, the deposited particles (see Figure 11.1) may be re-entrained later by means of higher flows in the network. The magnitude of erosion varies in response to the time varying hydraulics. Along with sufficient flow velocity and bottom shear stress, successive occurrence of high flow conditions, which would force deposited particles to unhinge, has a substantial effect on re-entrainment of the deposits. In this regard, the shear stress is the key parameter responsible for the start of sediment transport when its critical value for certain sediment characteristics is exceeded.
In addition, given that designing sewer systems to be self-cleansing is not always possible, particularly in flat regions, the use of flush tanks that generate controlled flush waves into the connected sewer system could be suitable. Flushing is able to realize a preventive strategy under economical and ecological conditions by generating flush waves continuously or quasi-continuously (Dettmar & Staufer, 2005). The operation of the flushing devices is usually based on the storage and successive release of flushing volumes, able to scour deposited sediments and to transport them downstream towards steeper sewer sections (Campisano et al., 2004).
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