The influence of cohesion on sediment movement in channels of circular cross-section

Abstract

The presence of sediment deposits in sewers causes loss of their hydraulic capacity. This could eventually lead to various operational problems such as surcharging, surface flooding and premature operation of overflows with the consequent increase of pollution of water courses. The present study has covered hydraulics, deposition, erosion and sediment transport in channels of circular cross-section, all with sediment bed. Throughout the programme comparisons between cohesive and non-cohesive sediment results were made. Velocity, turbulence and shear stress distributions obtained for various bed thicknesses showed dependency on the shape effects (bed thickness, bed roughness, flow depth and slope) of the channel. Bed shear stresses predicted using Einstein-Vanoni's separation technique were comparable to the measured values. Initiation of erosion experiments with uniform non-cohesive sediments yielded lower threshold values of mean shear stress than those published for wide channels (i. e., Shields' curve). However, when sand and cohesive additives (china clay, oil, petroleum jelly, etc. ) were used in the experiments a substantial increase of the critical shear stress was observed. This increase was dependent on the amount and concentration of the cohesive additive. A link between laboratory and field (actual sewer sediment behaviour) however, was essential in order to relate the experimental results to sewers. As a result of chemical and rheological studies (Williams and Williams, 1988) of UK sewer samples a synthetic sewer sediment was suggested (Laponite RD clay, sand and water in various proportions) for flume testing. The experimental results showed that for a given clay-gel concentration there is an optimum proportion of sand to clay-gel to achieve maximum resistance to erosion. It was found that only freshly deposited weak sediments (less cohesive Type C sediment) will erode at shear stresses of around 2.5 N/m , whereas slightly consolidated (Type A- Crabtree, 1988) sediment will erode at around 6 to 7 N/m2. Transport experiments using cohesive and non-cohesive sediments resulted in lower shear stresses for non-deposition conditions compared to those corresponding to wide rectangular channels. The study resulted in establishing the hydraulics (though limited) of sewers with deposited beds, erosion thresholds of non-cohesive (uniform and non-uniform mixtures) sediments and cohesive sewer sediment of different degrees of strength. Additionally, it has been possible to establish the transport rates of cohesive sediments (during high flows) over fixed (consolidated) sediment beds likely to be deposited during low flows (DWF). This led to a better understanding of erosion and transport processes of cohesive sediments.