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 ( Smin ) for the design minimum flowrate ( Qmin ) in each sewer reach. Qmin 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 Qmin values, but good estimates of Qmin are crucial for TF design. Design equations and plots have been developed to assist in obtaining Smin values for given Qmin values. As compared to traditional minimum slopes, Smin slopes via the TF method are flatter for sewers carrying typical to larger Qmin values and steeper for sewers carrying smaller Qmin values.