High velocities in sewers - A preliminary appraisal of factors influencing design

Traditionally, sewerage schemes were designed with a set limit for the maximum velocity, because it was believed that velocities higher than the limit would give rise to severe abrasion of the sewers. Although high velocities had to be avoided in the early brick sewers in order to prevent erosion of lime-mortar joints, research has shown that abrasion would not be a problem in modern sewers. Accordingly in 1968 the design recommendations in the Code of Practice on Sewerage were changed and no limit was set for the maximum velocity.

Although removing the limit on maximum velocity enables more economical schemes to be built, factors are introduced into the design that, previously, it was not necessary to consider. This report summarises the research that led to the changes in the design recommendations for maximum velocities, discusses the savings in cost that result, lists other problems that will arise from the use of high velocities, and makes recommendations for further action.