Sediment transport in sewers

Abstract

Sewers have been designed on the concept of self-cleansing where sediments are expected to move continuously without deposition. Due to the intermittent nature of the flow, deposition of solids in sewers could still occur especially at low flows such as during the receding flow or dry weather flow. The study of sediment movement in sewers will therefore need to cover both rigid (no-deposition) and loose (some deposition) boundary conditions. The present study extended the available data in rigid boundary conditions (clean pipes) to include the effects of surface roughness and pipe size. A complimentary study on the effect of sediment deposits (pipes with deposited beds) was also carried out. Extensive experiments on bed load transport of non-cohesive sediments without deposition were carried out in pipe channels of 154mm, 305mm and 450mm dia. covering wide -ranges of flow depths (0.15 < ya/D < 0.80), sediments (0.46 < dso (mm) < 8.3) and three different bed roughness values (0.0 < ko (mm) < 1.34). Supplementary data on transport over loose beds were collected in a 450mm dia. channel with various bed thicknesses up to 23% of pipe diameter. New transport equations based on all variables involved in the process were derived. Extensive uses of data from other relevant studies were made. The combination of the present and other data for both rigid and loose boundary conditions in pipes produced equations which could be applicable over wide range of conditions in sewers. A complimentary study on the rigid bed rectangular channels was also carried out. Using the newly derived equations, appraisals of the traditional concept of constant velocity criterion were made. The results show the inadequacy of the present design practice for diameters, pipe larger than 300mm. The comparisons made between the newly derived equations for rigid and loose boundaries in pipes suggest that sewers can be designed with clean inverts for diameters up to 1.0m while sewers with larger diameters should be designed allowing for an "optimum" depth of sediment deposits. Design charts based on the newly derived equations were devised.