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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