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City of Morgan Hill
Sanitary Sewer Flow Monitoring and Inflow/Infiltration Study
12-0248 AEG CofMorganHill FM Rpt.docx
Page 19 of 46
Rainfall: Storm Event Classification
It is important to classify the relative size of the major storm event that occurs over the course of a
flow monitoring period
6 .Storm events are classified by intensity and duration. Based on historical
data, frequency contour maps for storm events of given intensity and duration have been developed
by the National Oceanic and Atmospheric Administration (NOAA) for all areas within the continental
United States.
For example, the NOAA Rainfall Frequency Atla
s 7classifies a 10-year, 24-hour storm event in
Morgan Hill (at the coordinates of the RG East rain gauge) as 4.23 inches
(Figure 13). This means
that in any given year, there is a 10% chance that 4.23 inches of rain will fall in any 24-hour period.
Figure 13. NOAA Northern California Rainfall Frequency Map
6
Sanitary sewers are often designed to withstand I/I contribution to sanitary flows for “design” storm events of specific sizes.
7
NOAA Western U.S. Precipitation Frequency Maps Atlas 2, 1973:
http://www.wrcc.dri.edu/pcpnfreq.html