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August 2017
7-1
City of Morgan Hill
Sewer System Master Plan
2017
City of Morgan Hill
7.0
CHAPTER 7 - EVALUATION AND PROPOSED IMPROVEMENTS
This section presents a summary of the sanitary sewer system capacity evaluation during peak
dry weather flows and peak wet weather flows for the existing and buildout flows. The
recommended sanitary sewer system improvements needed to mitigate capacity deficiencies are
also discussed in this chapter.
7.1 OVERVIEW
The calibrated hydraulic model was used for evaluating the sanitary sewer system for capacity
deficiencies during peak dry weather flows (PDWF) and peak wet weather flows (PWWF). Since
the hydraulic model was calibrated for dynamic modeling, the analysis duration was established at
24 hours for most analyses.
The criteria used for evaluating the capacity adequacy of the wastewater collection system
facilities (gravity mains, force mains, and lift stations) were discussed and summarized in the
System Performance and Design Criteria chapter.
7.2 EXISTING SEWER SYSTEM CAPACITY EVALUATION
The system performance and design criteria summarized, on
Table 3.1
, were used as a basis to
judge the adequacy of capacity for the existing sanitary sewer system. The design flows simulated
in the hydraulic model for existing conditions were summarized on
Table 5.4
and are listed as
follows:
x
Existing PDWF = 5.4 mgd
x
Existing PWWF = 7.7 mgd
During the peak dry weather simulations, the maximum allowable pipe d/D criteria for new pipes
(d/D ratio of 0.75) for was used. For existing pipes, the criteria was relaxed to allow a maximum
d/D ratio of 0.90 (full pipe capacity) to prevent unnecessary pipe replacements. During the peak
wet weather simulations, capacity deficiencies included pipe segments with a hydraulic grade line
(HGL) that rises within three feet of the manhole rim elevation.
In general, the hydraulic model indicated that the sanitary sewer system exhibited acceptable
performance to service the existing customers during both peak dry weather flows (
Figure 7.1
)
and peak wet weather flows (
Figure 7.2
), with exceptions noted in the following sections.
7.2.1
Existing Peak Dry Weather Flows Capacity Evaluation
The existing dry weather flow analysis indicated several areas where pipelines experienced
deficiencies, which are documented on
Figure 7.1
. Additionally, this figure documents pipelines