6 SANITARY SEWER CAPACITY ANALYSIS

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.

6.2 COLLECTION SYSTEM PERFORMANCE

The City's collection system was analyzed using projected loads. The capacity of the collection system was assessed for both the existing condition and buildout condition under the current West Jordan Land Use Plan. As mentioned previously, when flow in a pipeline reaches a d/D ratio of 0.75, the pipeline is considered to be at full capacity. New sewers (parallel or replacement) are recommended to relieve these deficient pipelines.

The following scenarios were modeled to evaluate the system d/D capacity of every pipeline in the City:

   •   Existing System Analysis (Figure 6-1)

      o   100 gpcd loading with Inflow and Infiltration.

   •   Buildout System Analysis

      o   Current West Jordan land use at the time this report was written with the existing pipe system along with potential future new pipes and associated loading.

Additional scenarios were also completed in the process of this master plan update. These were provided as a planning tool for City staff to better understand the effect on the overall collection system of various growth patterns. These scenarios ranged from increased density options, specific developments, industrial expansion, and other alternatives. Figures were created as a brief summary of these scenarios and are provided in Appendix C for future reference. The buildout system analysis did not include any of these scenarios but instead only included the approved land use at the time of this update.