Self-cleansing sewer design based on sediment transport principles

The need for sewers to carry sediment has been recognized for many years. Traditionally, a minimum "self-cleansing" velocity was specified 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 behavior of the sewer-could not properly represent the ability of sewer flows to transport sediments. During the 1980's, 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 recognized guidance, the design methodologies and criteria adopted were developed on a project-by-project basis, building on the increasing experience and understanding of the subject of the designer. In recognition of this, a research project was initiated by the U.K.'s Construction Industry Research and Information Association to develop a new design methodology for sewers, which would take advantage of the available knowledge (mostly laboratory derived) on sediment mobility and the effects of sediment deposition on the hydraulic performance of sewers. This paper describes the main findings of the project and summarizes the recommended design guidance.