City of Morgan Hill Sewer System Master Plan 2017

August 2017

3-9

City of Morgan Hill

Sewer System Master Plan

The City’s

2002 master plan included a diurnal curve that was used for peaking dry weather flows.

Similarly, this master plan used 24-hour diurnal patterns for dry weather flows tributary to each

flow monitor, as shown on

Figure 3.1, Figure 3.2

and

Figure 3.3

3.3 WET WEATHER FLOW CRITERIA

The wet weather flow criteria accounts f

or the infiltration and inflows (I&I) that seep into the City’s

sewer system during storm events.

3.3.1

Infiltration and Inflow

Groundwater infiltration and inflow is associated with extraneous water entering the sewer through

defects in pipelines and manholes. Infiltration occurs when groundwater rises or the soil is

saturated due to seasonal factors such as a storm event which causes an increase in flows in the

sewer system. The ground water will enter the sewer system through cracks in the pipes or

deteriorating manholes. Inflow occurs when surface water enters the wastewater collection

system from storm drain cross connections, manhole covers, or roof/footing drains.

Figure 3.4

was developed by King County, Washington and was included in this chapter to illustrate the

typical causes of infiltration and inflow.

There are several accepted methodologies for estimating infiltration and inflows (I&I). These

include:

Methodology 1.

Based on Acreages. In this methodology, factors that may range

between 400 and 1,500 gallons per day (gpd) or more are applied to acreages for

estimating the I&I component.

Methodology 2.

Based on Linear Feet of Pipe. In this methodology, factors that may

range between 12 and 30 or more gallons per day per inch diameter per 100 linear feet

(gpd/inch diameter/100LF) are applied to linear feet of gravity sewers.

Methodology 3

. Based on a percentage of Average Dry Weather Flows. In this

methodology, Infiltration and Inflows (I&I) are calculated based on a percentage of the

average dry weather flow.

Methodology 4

. Based on flow monitoring data. In this methodology, infiltration and

inflows are determined by analyzing flow monitoring data of current and past flow

monitoring efforts.

This capacity analysis and master plan based the infiltration and inflow on specific flow monitoring

data from the Villalobos and Associates (V&A) 2014 Flow Monitoring Program (

Appendix A

Thus, the infiltration and inflows are reasonable and reflect the actual behavior of the sanitary

sewer system.