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City of Morgan Hill
Sanitary Sewer Flow Monitoring and Inflow/Infiltration Study
12-0248 AEG CofMorganHill FM Rpt.docx
Page 17 of 46
Figure 11shows the rain accumulation plot of the period rainfall, as well as the historical average
rainfall
5in 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.
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
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.html0.0
1.0
2.0
3.0
4.0
5.0
25-Feb
26-Feb
27-Feb
28-Feb
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
Rain (inches)
Historical Average
RG West
RG East
RG South
2.00
4.06
3.34
3.16
∑
=
p
p
d
d w
1
1