BOARD02543

• Board Meeting Date: 3/21/2002
• Description: Flood Section - Extreme Precipitation Study Update
• Board Meetings - Doc Type: Memo

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STATE OF COLORADO

Colorado Water Conservation Board

Department of Natural Resources
1313 Sherman Street, Room 721
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
www.cwcb.state.co.us

MEMORANDUM

Bill Owens
Governor

TO:
FROM:,
DATE:
RE:

Colorado Water Conservation Board Members 4-/
Larry Lang, Chief, Flood Protection Section lrr
March 11, 2002
Agenda Item 23e. , March 21-22, 2002 CWCB Meeting
Extreme Precipitation Study Update

Dan McAuliffe
Deputy Director

Greg E, Walche,
Executive Director

Rod Kuharich
CWCB Director

Introduction
The purpose of this memo it to provide a progress report on the joint CWCB and the ~vision of
Water Resources (DWR) Extreme Precipitation Investigation Project. Phase II of the project is
under contract with Colorado State University (CSU) Department of Atomospheric Sciences in
Fort Collins, Colorado. The Phase II objective is to research and develop a three-dimensional
convective storm model for estimating extreme precipitation events above 5,000 feet (msl) as
they apply to design of dams, spillways, and other structures in Colorado.

Discussion
Mountainous regions such as Colorado are notorious for being susceptible to flash floods and
extreme precipitation events. These events can occur when moisture-rich lower tropospheric air
flows into high terrain by either large-scale flow patterns and/or solenoidal circulations resulting
from heating of elevated terrain. As this air flows into high terrain, it undergoes orographic
ascent, cools to saturation, and often leads to convective instabilities, Extreme precipitation can
then occur as a' result of localized terrain-induced heavy precipitation in widespread storm
systems, and/or from strong convective storms. Moreover, complex mountainous terrain includes
deep canyon systems that can channel rainfall runoff producing deadly and damaging flash
floods.

Current methodologies of probable maximum precipitation (pMP) and flood estimation have
been shown to be incompatible with paleohydrographic results when applied to high
mountainous terrain. Using stratographic and geomorphic evidence it is concluded that there is
little evidence of extreme rainfall and floods above 7,500 ft elevation, These studies suggest that
current PMP estimates may overestimate high elevation rainfall potential by a factor of two, This
overestimate, if supported by further studies, has major economic consequences as it effects the
design and construction of dams at elevations above 7500 ft. The plan is to apply a three-
dimensional (3D) convective storm model to develop a new method of estimating extreme
precipitation and to develop concepts of how extreme precipitation varies with altitude in
Colorado.

Flood Protection. Water Supply Planning and Finance. Stream and Lake Protection
Water Supply Protection. Conservation and Drought ~lanning