Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Rainfall
2.2 RAINFALL DEPTH
The most commonly used descriptor of rainfall is the rainfall depth; however, for modeling purposes,
two other rainfall descriptors must be defined. First, the rainfall duration and frequency of
occurrence of rainfall depth for that duration must be assigned. Second, since the rainfall depth
is a descriptor of the rainfall occurrence at a point in space, both the spatial and the temporal distribution
of the rainfall depth must be defined. In this section, the rainfall depth-duration-frequency
statistics for use in Maricopa County are described. Subsequent sections describe the
spatial and temporal distributions that are to be applied for the 6-hour local storm, the 24-hour
general storm, and the temporal distribution for the 100-year, 2-hour storm.
2.2.1 Data Source
The most comprehensive and available source of rainfall data analysis for Maricopa County is
the NOAA Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 1: Semiarid
Southwest (NOAA Atlas 14) (Arizona, Southeast California, Nevada, New Mexico, Utah) (Bonnin
et al, 2004). The NOAA Atlas 14 is to be used for all drainage design purposes in Maricopa
County. The District has elected to use the mean partial duration time series point precipitation
values from NOAA Atlas 14 rather than the values for the upper or lower bound of the 90 percent
confidence intervals. For critical projects that can significantly affect public safety, health and
welfare, including floodplain delineation and dam safety studies, the engineer/hydrologist should
check model results against indirect methods as defined in Chapter 8. These analyses should
include performance of parameter sensitivity analyses, including the use of the upper bound of
the 90 percent confidence interval point precipitation data, to ensure the model results are reasonable
in comparison with available historic gage data for the watershed or hydrologically similar
watersheds. As a result of such analyses, the engineer/hydrologist may elect to use the point
precipitation values from the upper bound of the 90 percent confidence interval instead of the
mean values, in order to better conform with available appropriate gage data. This application
will be acceptable to the District. Use of the values for lower bound of the 90 percent confidence
interval is not recommended.
The NOAA Atlas 14 data available through the NOAA Atlas 14 web site are not to be used for
studies in Maricopa County. Instead, the NOAA Atlas 14 maps in Appendix A.1, the ESRI ASCII
Grid data files available on the District’s web site, or the data supplied with the District’s DDMSW
computer program are to be used. This data was taken from NOAA Atlas 14, Volume 1, Version
4.0, dated June 19, 2006. This is the version the District has reviewed and accepted for use in
Maricopa County. Subsequent versions published by NOAA shall not be used until the District
has reviewed the data, formally adopted its use by revising this document, or issued an addendum
to this document, posted the new version on the District web site, and updated DDMSW.
2-4 August 15, 2013