WSP04003

• File Number: 8220.200.05.P
• Description: Hoover Dam/Lake Mead/Boulder Canyon Project
• Basin: Colorado Mainstem
• Date: 7/16/1983
• Author: Region IX IFHMT
• Title: Interagency Flood Hazard Mitigation Report in Response to the July 1 1983 Disaster Declarations
• Water Supply Pro - Doc Type: Report/Study

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Federal Emergency Management Agency

Region IX Building 105
Presidio of San Francisco, CA 94-129
July 13, 1983

MEMORANDUM FOR: Robert J. Adamcik
Disaster Recovery Manager

FROM: The Flood Impact Task Force
SUBJECT: The Flood Impact Task Force Report

The Flood Impact Task Force convened at the FEMA Field Office
in Lake Havasu City, Arizona to address the potential flash flood-
ing impact on the Lower Colorado River. Members of the Task Force
involved in this analysis and in the preparation of this document
include Mike Mulvihill, U.S. Army Corps of Engineers (L.A. District);
Maurice Roos, California Department of Water Resources; Dave
Westnedge and Tom Bowers, National Weather Service (Salt Lake City);
Claire Jensen, National Weather Service (Phoenix); Pat Deschamps,
Arizona Department of Water Resources; and Ed Richardson, Bureau
of Reclamation (Parker/Boulder City).

Potential Flood Analysis-
The analysis of four distinct cases of hydrological events and
the subsequent impact of these events on the Colorado River below
Hoover Dam are presented in the following paragraphs.

A. Isolated Flash Flood
The impact of an isolated flash flood may be substantial at
and immediately downstream of the confluence of the mouth of a
wash with the Colorado River. This impact depends upon the
size of the drainage area, the intensity and amount of precipitation,
and the water level of the Colorado River.
For a worst case example, a flash flood from the Silver Creek
wash drainage area near Bullhead City, which is 35.7 square miles,
could produce an estimated 100 year peak discharge of approxi-
mately 15,000 cfs. This peak, in turn, could cause a possible
local rise of up to 2.5 feet in the Colorado River. For wider river
reaches, the local rise would be less. This flood wave would at-
tenuate rapidly and its effect would almost be dissipated approxi-
mately 20 miles downstream. However, this attenuation requires
further study.

As a second example, "Tributary H" (4.61 square miles of drain-)
age area) in the Parker Strip area could have a 100 year flow rate
of abo~~t, which would attenuate rapidly with little impact
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dO)l7I1stream.

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B. Multiple Basin Storm
The likelihood of this type of event is less than the isolated

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