14000000038 SOW

• Contract/PO #: 14000000038
• Contractor Name: Applied Weather Associates
• Contract Type: Grant
• Loan Projects - Doc Type: Contract Documents

•
AIs ' #) PO Box 680
'We "' :'; Monument, CO 80132
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[phone]
http://www.appliedweatherassociates.com
,. awaadmin @comcast.net
September 13, 2013
Proposal to Review the Extreme Precipitation Analysis Tool
(EPAT), Phase II: Independent Meteorological Review
Response to Solicitation DQ#: PEA-1402
Overview
The development of an extreme precipitation analysis tool using geographic information system
(GIS)technology is a valid and desirable undertaking. Such a tool can potentially provide
objective analysis results for use in determining extreme rainfall values for individual drainage
basins. The Dam Safety Branch(DSB) of the Colorado State Engineer's Office (SEO) is to be
commended for taking the initiative in 2005 to develop a GIS based tool, subsequently named
the Extreme Precipitation Analysis Tool (EPAT). The intent of this initiative was to objectively
determine extreme rainfall values using procedures applied in the NOAA Hydrometeorological
Reports (HMRs). These procedures have been refined and used in site-specific Probable
Maximum Precipitation(PMP) studies for individual drainage basins. EPAT was developed to
automate the site-specific PMP (SSPMP)methodology.
The DSB has statutory responsibility to set reservoir storage levels and to review and approve
design and construction for dams throughout the state. Design rainfall values from the HMRs
and from SSPMP studies have historically been used for computing the inflow design flood
(IDF). It is imperative that dam spillways are adequate to convey the IDF without causing dam
failure. Hence it is critically important that the design rainfall values be objectively determined
using reliable and reproducible procedures. For a GIS based tool to be used in the determination
of the design rainfall values, it must apply SSPMP determination procedures in an objective,
meteorologically accurate, reliable and reproducible manner. The tool must produce extreme
rainfall values that are consistent with historically observed extreme rainfall events while
calculating extreme rainfall values that can be produced by storms that are physically possible
for a given watershed location. Being"physically possible" is required by the definition of PMP
in the HMRs (e.g. HMR 59). This constraint is also applied to SSPMP studies and hence must
be a constraint incorporated in any GIS tool used to evaluate PMP values. Results produced by
the GIS based tool must be physically possible based on combinations of storm characteristics
that produce the most extreme rainfall values possible while insuring that unrealistic assumptions
or inconsistent combinations of parameters are not allowed. The purpose of the tool is not to
produce the largest rainfall values that can be mathematically calculated but to produce the
largest rainfall values that can be produced by physically possible combinations of storm
characteristics considering the location,meteorology and topography of a drainage basin.