Estimating Design Flows for Self Cleansing Sewers

Abstract

The second edition of Gravity Sanitary Sewer — Design and Construction (ASCE MOP 60-WEF MOP FD-5, 2007) recommends use of the tractive force approach for designing self-cleansing sewers. Self-cleansing sewers are desirable because they are expected to better maintain design function and extend service life. A key element of the tractive force method is selection of an appropriate design minimum flow rate. The magnitude of the design minimum flow rate should be sufficient to mobilize and drag a design sediment particle along the sewer invert. The problem of estimating the design minimum flow rate can be investigated using basic principles from the Poisson Rectangular Pulse (PRP) model for residential water demands. According to the PRP hypothesis, residential water use follows a diurnal Poisson arrival process with a time dependent rate parameter. When a water use occurs, it is approximated as a rectangular pulse of random intensity and random duration. This simple premise provides a rich robust picture of residential water use. The objective of this paper is to demonstrate how theoretical results from the PRP model for residential water use lead to reliability-based estimates of the design minimum flow rate needed in the tractive force approach for sizing self-cleansing sanitary sewers. Results from the PRP estimates will be compared against conventional methods.