FLOOD04822

• County: El Paso
• Community: Colorado Springs
• Stream Name: Plum Creek, Mason Reservoir
• Basin: Arkansas
• Title: Site-Specific Probable Maximum Precipitation for Mason Reservoir
• Date: 1/1/1997
• Prepared For: Black and Veatch
• Prepared By: Henz Meteorological Services
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SUMMARY OF 24 OCTOBER 1996 EXTREME PRECIPITATION STUDY
STORM LIST REVIEW WORKSHOP AT OFFICE OF THE STATE ENGINEER,
DENVER

Opening coments by Alan Pearson

Cnrrent directions in extreme precipitation anal~'sis. US Bureau of Reclamation, Lou Schreiner

· Bireau of Reclamation Dam Safety Department is getting into risk analysis for hydrological structures.
· Bureau will be looking at loading consequences on structures from an assortment of events (e.g., floods,
earthquakes, etc.). Probability of failure and loss of life will lead to structure designs. Bureau "ill need
probabilities for a whole range of floods (from 10,000 year event to 1,000,000 year event to PMF). By
examining historical precipitation records and then computing the 1,000,000 year event, the PMF "ill be
determined. At the present time for the state of Washington, they have assigned the 1,000,000 year event
as equivalent to the PMP for areas west of the Cascades and the 10,000,000 year event for areas east of the
Cascades. Associated PMF probabilities are even more extreme. B.C. Hydro is working on similar risk
analysis for life and property.
· Cost of sophisticated warning systems on the order of$100,000 to $150,000.
· Lou has half of the funding he requested for modelling of extreme precipitation eyents. (It
appears this funding is earmarked fot. the Cotton group at CSU and for NCAR modellers).
* Bureau is tI')ing to get several models up and running on their workstation.

Colorado Extreme Precipitation Data Study, Overview of Accomplishments. Tom McKee, Colorado
Climate Center, Colorado State University

· This Workshop represents the end of Phase 1.
· As a result of Phase I, a much better data set is now available from CCC (more comprehensive than that
used for NOAA Atlas II). A better definition of the 100-)T event is now available.
· Analyses of very short duration events is a problem for NOAA Atlas II.
· Goal of today's Workshop is to identify 15 to 20 design storms for 6 defined regions in Colorado.
These design storms will have potential application for regional and site-specific PMP studies.
· Phase r has resulted in the generation of climatological distributions of variables from upper-air
sOWldings. .
'SummaI')' of Results of Modelling Workshop held at CSU last May:

L Models do appear capable of simulating large storms.
2. A wide variety of models exist.
3. Application of 3-D models is biggest challenge. Large data sets are needed to initialize them.
4. One recommendation for 3-D model use is to use them with a perturbed, fully initialized
stornl. And rather than find a fully initialized historical storm, concensus of Workshop was to
recommend to NWS, NOAA, and others that observations systems (e.g., NEXRAD, satellite) be geared up
to fully capture the initial conditions of future storms as they occur.
· Rapid City storm has been well -simulated.
· A Summary of Modelling Workshop will be included in the CCCICSU Final Report on Phase I.

Comments by Bob Jarrett:

r

· Many people are in\'Olved in rainfall reconstruction for Buffalo Creek.
· Cited USGS/Highways Department coop program for insuumenting small watersheds for rainfall with
5-minute rainfall and streamflow resolution. He.s seen data for 30 watersheds in the Denver area.
Several attendees asked for more info on this program and he promised to provide CCC with more details
later. He did say the program ilses a non-standard raingauge with a 4" orifice. Data in a punched format