Sediment transport in sewers - a step towards the design of sewers to control sediment problems

It is the purpose of combined sewerage systems to convey a variety of solids, and associated liquid wastes, for treatment and return to the environment. The movement of the solids in sewers has come under increased scrutiny in recent years, due to the pollution and maintenance concerns associated with this material. The real or perceived physical, biochemical, ecological and aesthetic impacts of sewage spills into the environment, via separate storm sewers, combined sewer overflow (CSO) structures and treatment plants, have recently entered more prominently into public awareness and perception. Worldwide, engineers have become increasingly aware that the methodologies currently in use to design sewers to prevent solids deposition and minimize the hydraulic and other effects, are inadequate. A modified design approach, recently proposed by the Construction Industry Research and Information Association (CIRIA), aims to address these problems by providing a standardized procedure to minimize sedimentation in sewers. This approach relies substantially on laboratory-based work for calibrating the sediment transport, and to a lesser extent, hydraulic resistance relationships. This paper examines the basis of the CIRIA design methodology, and compares the laboratory work with real sewer conditions (based on real sewer sediment data collected in the UK, and other European sewers). The aim is to provide sewerage designers, managers and planners with information about the general applicability of current methodologies regarding the design of sewers, to manage sediment problems. This information is presented in the context of the current understanding of the physical processes which control the movement and deposition of real sewer sediments, rather than those studied in laboratories.