Scouring Stress for Large Solids

Tractive force (stress) design is a valuable tool to assess the ability of a sewer to transport solids. Tractive force theory is based on the assumption that the solids are small (i.e. the solids are smaller than the depth of the flow). However, many real sewer solids are larger than the depth of flow especially in sewers with very little flow. Large solids are often the cause of sewer blockages. The paper presents a method, backed by a large number of experiments, for determining the conditions under which large solids (i.e. solids with a vertical dimension greater than the depth of water) are able to move in a pipe. Criteria are presented to ensure that motion will occur if a sufficient velocity (also reported as a Froude number or modified `solids' Froude number) is exceeded. Flume experiments were used to determine the coefficients to be used in design. Implications for using this work to estimate the slopes and velocities needed to move large solids in sewers with low flow is also presented.