Tractive Force Design for Self-Cleansing of Sanitary Sewers: Evaluating Design Guidance Based on the Performance of Existing Sewers

The tractive force method is used to design sewers with self-cleansing conditions based on a critical shear stress. The authors have extended the use of the tractive force approach from the design of new sewers to the evaluation of existing sewers under actual conditions. Self-cleansing conditions are assessed by evaluating flow monitor data on a scattergraph using a critical shear stress curve. Existing sewers with adequate selfcleansing conditions are readily identified, as well as those with a potential for silt, sediment, or debris accumulation.Practical examples from flow monitor locations throughout the United States are provided, demonstrating the application of the tractive force method to existing sewers. The authors applied this method to over 200 existing sewers where flow monitor data were available and compared the results with independent silt observations. Based on these results, the general effectiveness of the tractive force approach for the selfcleansing design of sanitary sewers is validated, and the findings support its use for the self-cleansing design of new sewers.