City of Morgan Hill Sewer System Master Plan 2017

City of Morgan Hill Sanitary Sewer Flow Monitoring and Inflow/Infiltration Study

Figure 11 shows the rain accumulation plot of the period rainfall, as well as the historical average rainfall 5 in Morgan Hill during this project duration. Rainfall totals for Morgan Hill ranged between 158% and 203% of historical normal levels during this 20-day time period.

Historical Average RG West RG East RG South

5.0

4.06

4.0

3.34 3.16

3.0

2.0

2.00

1.0 Rain (inches)

0.0

1-Mar

2-Mar

3-Mar

4-Mar

5-Mar

6-Mar

7-Mar

8-Mar

9-Mar

10-Mar

11-Mar

12-Mar

13-Mar

14-Mar

15-Mar

16-Mar

17-Mar

25-Feb

26-Feb

27-Feb

28-Feb

Figure 11. Rainfall Accumulation Plot

Rain Gauge Triangulation Distribution The rainfall affecting the sanitary sewer collection system basins must be calculated based on the proximity to the rain gauge locations. The mean precipitation for each site was calculated by taking data from the rain gauges and using the inverse distance weighting (IDW) method. IDW is an interpolation method that assumes the influence of each rain gauge location diminishes with distance. The approximate geographic coordinates of each site were determined and a weighted average was taken of the precipitation data from nearby rain gauge locations.

IDW is performed using the equation

1

p d d w 1 p

∑ =

where the weight, w , depends on the distance, d , from the rain gauge to the monitoring site and p , a user-selected power ( p > 0). The most common choice of p in hydrological studies of watershed areas is 2. Figure 12 illustrates the IDW method with sample data.

5 Historical data taken from the WRCC (Station 043417 in Morgan Hill): http://www.wrcc.dri.edu/summary/climsmnca.html

12-0248 AEG CofMorganHill FM Rpt.docx

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