City of Morgan Hill Sewer System Master Plan 2017

August 2017

3-14

City of Morgan Hill

Sewer System Master Plan

3.3.2

Sewer System Flow Monitoring

In 2014

V&A’s services were used for a temporary flow monitoring program to c

apture 10 sites

during dry and wet weather flows. In addition to temporary flow monitors, flow monitored data from

Harding meter and SCRWA was also incorporated into the analysis. The V&A flow monitored

locations are shown on

Figure 3.5

The rain gauge data for the V&A flow monitoring period was obtained from V&A. There were three

rain gauges used for the wet weather analysis. The V&A rain gauges, were located in the East,

West and South portion of the City. The east rain gauge was located near Tennant Avenue and

Foothill Avenue, the west rain gauge was located near West Dunne Avenue and Dewitt Avenue,

and the south rain gauge was located near Santa Teresa Boulevard and West Middle Avenue.

The flow monitoring and rain data was used in this analysis to calibrate the computer hydraulic

model to average dry weather flow and wet weather flow conditions.

3.3.3

10-Year 24-Hour Design Storm

A synthetic design storm is typically used to evaluate the sewer collection system’s response

during wet weather flow conditions. The design storm information was collected from the National

Oceanic and Atmospheric Administration (NOAA) Atlas 14 Volume 6 (

Table 3.4

10-Year Frequency.

Industry standards include design storms that range between 5-year

and 20-year events. Based on current regulatory trends, a 10-year storm event was

chosen for the City to evaluate the capacity adequacy of the sanitary sewer system.

24-Hour Duration.

Peak flows from a storm event are usually cause by brief intense

rains, that can happen as part of an individual event or as a portion of a larger storm. The

24-hour storm duration is longer than needed to determine peak flow but aids in identifying

infiltration and inflows a sewer system may experience during a storm event.

Balanced Rainfall Centered Distribution.

The National Resources Conservation

Service, previously known as the Soil Conservation Service, has developed rainfall

distributions for wide geographic regions based on traditional Depth-Duration-Frequency

(DDF) rainfall data. In this methodology, the highest rainfall intensity is placed at the

center of the storm. Incrementally lower intensities are placed on alternating sides of the

peak.

Thus, the NOAA Atlas 14 Depth Duration Frequency (DDF), 10-year 24-hour (10yr-24hr) design

storm, with a balanced rainfall distribution, was used to evaluate the capacity adequacy of the

City’s sanitary sewer system during wet weather flow conditions.

The selected 10-year 24-hour design storm was further compared to historical storm events,

between February 2014 and March 2014, as shown on

Table 3.5

. The table lists the total rainfall

volume, duration, peak hour intensity, and total monthly rainfall (if available) for each storm event.