City of Morgan Hill Sewer System Master Plan 2017

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: x 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. x 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. x 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. x 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.

August 2017

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City of Morgan Hill Sewer System Master Plan