6.1 INTRODUCTION

A capacity analysis of the modeled collection system was performed after completion of the calibration and establishment of the LOS. The capacity analysis identifies areas in the collection system where flow restrictions occur or where pipe capacity is insufficient to pass the peak flows. Pipes that do not have enough capacity to pass the peak flows can produce backwater effects in the collection system and potentially cause unwanted sanitary sewer overflows (SSOs). Maximum flow over full pipe flow or depth of flow over diameter of pipe (d/D) are accurate methods of predicting how much pipe capacity is remaining in the system, and if a pipe needs to be upgraded. A full pipe has a d/D value of 1.0 and has no reserve capacity, while a pipe flowing half full has a d/D value of 0.5 and has some reserve capacity. Table 6.1 provides the recommended d/D values and minimum pipe slopes for pipe design.

Table 6-1:
Design Slope Parameters

Pipe Diameter (inches)	Maximum Allowed depth/Diameter d/D	Minimum Pipe Slope
8	0.50	0.40 %
10	0.55	0.28 %
12	0.60	0.22 %
15	0.65	0.15 %
18	0.75	0.12 %
21	0.75	0.10 %
24	0.75	0.10 %
27	0.75	0.10 %
30	0.75	0.10 %
36	0.80	0.10 %

 

Ten State Standards allows for pipe slopes less than 0.10% (S=0.0010). However, practical construction tolerances do not warrant lesser slopes.

For the update of this report, it was determined that the City would allow a maximum d/D of 0.75 in the existing system. This is often referred to as the LOS that the City will provide. Once an existing pipe exceeds this LOS, it is considered deficient and may need to be replaced.