Sediment transport over deposited beds in sewers

Abstract

In sewer networks deposition of solids can occur from time to time, due to the intermittent nature of flow. The longer the deposits remain in sewer systems the more likely it is that the sediment properties will change. Eventually these depositions can become cemented (consolidated) especially during dry weather flow (DWF) when the boundary shear stress values are lower than the critical values and the velocity is not enough to carry the sediment along the sewers. The main objective of the present study was to highlight and cover the shortage of methods and approaches in understanding the nature of sediment transport problems in sewers with a build up of permanent deposits. Extensive experiments were carried out in a circular cross section channel (D=305 mm) with various fixed bed thicknesses namely 47 mm, 77 mm and 120 mm, and three different bed roughnesses (0.0 <k₈(mm)<1.40). In the first part of the study, the characteristics of flow in a circular cross section channel with flat bed was studied, the object being to investigate how the deposited bed affects velocity and bed shear stress distributions in the channel. The measurements showed a very strong dependency on the bed thickness, flow depth and bed roughness. The turbulence characteristics of such a channel were also investigated with a Laser Doppler Anemometer (LDA). An empirical method of determining the flow friction factor (Darcy-Weisbach's factor) for circular cross section channels with sediment beds was developed and compared with previous methods. It has been shown (Ackers, 1984, May, 1989) that the presence of stable deposits in the invert of sewers increases the sediment capacity of the channels and consequently, reduces the required gradient along which the channel will be laid. Due to incomplete information concerning the behaviour and the mechanism of sediment transport in circular channels with flat beds, the second part of this study was devoted to a comprehensive investigation of: (a) The incipient motion of grouped touching particles resting on the channel bed. The investigation led to the proposition of predictive equations for the critical values of shear stress and velocity at threshold of particle motion. The results were compared with past research results. (b) Bed load transport of non-cohesive sediments without deposition. Six different sizes of particles were used ranging from O.53<d₅₀(mm)< 8.4. Equations were developed to predict the sediment transport in circular cross section channels with different flat bed thicknesses. These equations can be used for designing deposit free sewers with flat beds and for identifying sewers suffering from a build up of deposits in the existing system.