Sediment transport in storm sewers

Literature survey made in this research indicated that the roughness has a significant effect on the design of a self-cleansing sewer. A conceptual model was therefore developed taking into account the effect of roughness in the sediment transport in sewers. This model was later tested against experimental data obtained from a test rig. The rig had a pipe which was specially fabricated in two halves so that it could be divided to coat the interior with uniform sand grains to provide roughness. The experimental data shows a good correlation with the model developed. Two relationships i.e. for smooth and rough pipes, were derived from the results. The effects of volumetric sediment concentration, pipe diameter, sediment size and effective roughness on these relationships were in turn examined. Head loss formulae acquired by past researchers were slightly modified to suit the range covered by the experimental data. The relationships developed in this research were subsequently applied to sewer design. These were later compared to the criterion of 0.76 m/s proposed by the British Standards. In the case of smooth pipes, it shows conclusively that the criterion of 0.76 m/s produces excessive slopes for pipe diameter up to 1.0 metre. However, for rough pipes the criterion gives insufficient slopes to maintain a self-cleansing sewer. This is valid up to a pipe diameter of 0.3 metre. Beyond this diameter the criterion gives high slopes. Comparisons were also made with studies in the wider field of sediment transport on fixed bed. These have set in context the proposed formulae for smooth and rough pipes in the design of storm sewers