Tractive force design for self-cleansing of sanitary sewers
The new edition of the WEF FD 5/ASCE MOP 60 manual recommends use of the tractive force approach for self-cleansing design of gravity sanitary sewers. The tractive force approach is a major improvement in designing gravity sewers to achieve self-cleansing. This approach results in a self-cleansing slope value, Smin, for the combination of sewer diameter, design particle size, d, and design minimum flowrate, Qmin, for each reach in a sewer system. The fundamental relationship used is τ = γRhS. Design minimum flowrate, Qmin, is defined as the largest one-hour flowrate during the lowest flow week over the design life of the sanitary sewer pipe being designed. A Qmin value for a reach is normally generated by using the relationship Qmin = Qavg1 × P1, where P1 is a peaking factor. In general, past design practice has not required accurate estimation of Qmin values, but they are crucial to tractive force design. Design equations and plots are available that facilitate obtaining Smin. The end result of using the tractive force approach is a design minimum slope tailored for each pipe reach. As compared to traditional minimum slopes, these slopes will be flatter for smaller diameter sewers carrying typical to larger Qmin values and steeper for larger diameter sewers carrying typical to smaller Qmin values.