Tractive Force Design for Sanitary Sewer Self-Cleansing

The 2007 edition of the ASCE MOP 60-WEF FD 5 Manual recommends the tractive force (TF) method for self-cleansing design for gravity sanitary sewers. TF design is a major improvement over traditional methods to achieve self-cleansing in gravity sewers. This approach results in a self-cleansing pipe slope value $\left(S_{\text{min}}\right)$ for the design minimum flowrate $\left(Q_{\text{min}}\right)$ in each sewer reach. $Q_{\text{min}}$ is the predicted largest 1-h flowrate in the reach during the lowest flow week over the sewer design life. Past design practices seldom included accurate estimation of $Q_{\text{min}}$ values, but good estimates of $Q_{\text{min}}$ are crucial for TF design. Design equations and plots have been developed to assist in obtaining $S_{\text{min}}$ values for given $Q_{\text{min}}$ values. As compared to traditional minimum slopes, $S_{\text{min}}$ slopes via the TF method are flatter for sewers carrying typical to larger $Q_{\text{min}}$ values and steeper for sewers carrying smaller $Q_{\text{min}}$ values.