Colorado Division of Water Resources (CDWR), CDWR Extreme Precipitation Analysis Tool (EPAT), CO

Extreme precipitation events present a serious design challenge to designing or rehabilitating dams and levees. Developing a solid scientific methodology to predict extreme weather has been problematic. Until recently, the best available data came from a series of hydro-meteorological reports developed jointly by the U.S. Army Corps of Engineers (USACE), the National Weather Service (NWS) and the Bureau of Reclamation (BOR) in the 1980s. These reports established a common standard to estimate a probable maximum precipitation or PMP event but were not refined enough to provide data for a specific geographic location.  The most recent definition of Probable Maximum Precipitation is given in Hydro-Meteorological Report (HMR) 55A (1988) as:

 

 

“Theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given storm size area at a particular geographic location at a certain time of year.”

 

The generalized PMP approach of the federally produced Hydro-Meteorological Report (HMR) series provides a generalized standard of practice for determining a regional PMP value for design. The HMR-PMP process does not include basin-specific impacts due to topography on precipitation nor is it a storm-based process.  Over the past two decades Site-Specific Probable Maximum Precipitation (SSPMP) studies have become an alternative method for computing PMP. The SSPMP relies on a storm-based maximization and transposition approach with incorporated basin-specific topographic influences.  Taylor (2003) identified the following differences between SSPMP and HMR-PMP:
 

“Site-specific PMP is lower due to a much-reduced orographic component, and better evaluation of natural barriers that limit moisture inflow to the sites. Over-estimation of HMR PMP seems generally to increase with elevation.”

 

However, SSPMP studies have no designated standard of practice when it comes to the methodologies used and the choice and placement of extreme events over the basin being studied.  This practice led to inconsistencies in determining SSPMP across the State of Colorado. 

 

The National Research Council (1994) report provides an evaluation on the options of estimating bounds on extreme precipitation events for PMP studies.  Among the recommendations by this NRC study were the following:
 

·    Improvements to PMP estimates are most critical for small-area basins during short events.  Time periods of less than 6 hours and drainage areas from 25 to 1000 kilometers are of particular interest.  The Committee recommends a concerted effort to make such improvements.

 

·   The Committee recommends that the NWS work to establish the accuracy of WSR-88D rainfall estimates, especially for heavy rainfall events and develop formal plans for integrating WSR-88D data into a PMP storm catalog.

 

·   The Committee recommends that strategies be investigated for estimating probabilities of extreme rainfalls, using the best available concepts and methods of meteorology and statistics.
 

·   The Committee further recommends that attention be focused on storm-based analyses of extreme rainfall.

 

In addition, the 1994 NRC assessment supports that:

 

“Radar also offers great potential for transforming station-based precipitation statistics into storm-based statistics for introduction into the storm-based PMP procedures.  The dynamics and thermodynamics of extreme events play an important role that may not be adequately reflected in PMP procedures.

 

Having a source of objective, basin-specific extreme precipitation data available for direct input into hydrologic models is necessary but hard to obtain to develop calculations for Probable Maximum Flood (PMF) events. HDR developed an innovative Extreme Precipitation Analysis Tool (EPAT) to meet this need.

Working with the Colorado Division of Water Resources Dam Safety Branch, HDR developed EPAT using GIS technology to solve the long-standing dilemma of providing basin-specific extreme precipitation data. The EPAT software developed by HDR requires historical storms that developed in the same geographic region to be used when determining the PMP for basins that reside in those regions.  The storms are placed over the dam’s basin in a manner that makes the storm’s spatial coverage consistent with what would occur in nature.  The automated process follows certain calculation steps considered important in HMRs.  These steps are translated into Geographic Information System (GIS) processes for maximizing and for transposing the storm to the new location. 

By creating reproducible results, the EPAT essentially takes the guesswork out of a PMP and provides watershed peak rainfall amounts, total volumes, and temporal distributions for local and general storm events. The EPAT embodies techniques from HMR-49, HMR-55A HMR-52 and is an automated GIS product that creates scientifically reproducible results for locations in Colorado.  EPAT has established a new Colorado standard of practice for dam safety analysis and replacing Site-Specific Probable Maximum Precipitation SSPMP events for locations above 5500 feet.

 

Project Highlights