Designing Sewers to Control Sediment Problems

Abstract

The need for sewers to carry sediment has been recognised for many years. Traditionally a minimum `self-cleansing' velocity was specified in the UK and, although this approach had been successful in many cases, it was appreciated that a minimum velocity — unrelated to the characteristics and concentration of the sediment or to other aspects of the hydraulic behaviour of the sewer — could not properly represent the ability of sewer flows to transport sediments. In particular, it was already known that a higher flow velocity would needed to transport a greater concentration of sediment, or to transport a given concentration of sediment in a large sewer than in a small sewer. During the 1980s, sediment transport theory had been increasingly applied to the design of sewers, particularly in major interceptor sewer schemes. But, in the absence of any universally recognised guidance, the design methodologies and criteria adopted were developed on a project-by-project basis, building on the designers' increasing experience and understanding of the subject. In recognition of this, a research project was initiated by CIRIA (Construction Industry Research and Information Association) in 1992, to develop a new design methodology for sewers, which would take advantage of the available knowledge on sediment mobility and the effects of sediment deposition on the hydraulic performance of sewers. The project culminated with the issue of CIRIA Report 141. This paper describes the main findings of the project and summarises the design guidance contained in the report.