WSPC07654

• File Number: 8200.300.20
• Description: Colorado River-Colorado River Basin-Colorado River Basin Legislation-Law-Federal
• State: CO
• Water Division: 5
• Date: 8/1/1989
• Title: CR Colorado River Floodway Protection Act-Corres Reports etc-1987-89-Report of Colorado River Floodway Task Force
• Water Supply Pro - Doc Type: Report/Study

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002759

CHAPTER V

DRAFT
August 1, 1989

RECOMMENDATIONS ON THE DEVELOPMENT OF SPECIFIC DES IGN
CRITERIA FOR THE CREATION OF THE FLOODWAY BOUNDARIES.

The boundaries of the Floodway will be determined by the
size of the river flow that must be accommodated. The river
flow is to be as determined by the combination of releases
from the mainstream dams (HOOver, Davis and Parker) and side
inflow from tributar ies and washes which represent a
realistic one-in-one hundred year river flow or 40,000 cfs,
whichever is greater. These requirements are stated in
Section 5(b) (1) of the Colorado River Floodway Protection v
Act and the accompanying legislative history. 0'

The vast volume of reservoir storage space at and above
Hoover Dam and river runoff above Hoover Dam dominate the
river flow patterns. Tributary runoff below Davis Dam occurs
from normally dry washes as a result of localized, intense
thunderstorms. While these flows may be of significant size
locally, they are usually of short duration and will
dissipate within the river channel over a relatively short
distance and time span. Historic river flow records are
quite good regarding normal flows in the river and releases
from the dams. However, there are very few good records on
rainfall events or flood events in the normally dry tributary
area.

A major problem faced by the Task Force was to develop a
technically sound process to evaluate tr ibutary flood flows
considering the lack of available data. A smaller Technical
Subcommittee, composed of Task Force members and personnel
from Bureau of Reclamation and the Corps of Engineers, and
private consultants, was formed to develop a recommended
process to determine tributary flood flows and recommended
specific design criteria for the creation of the F100dway .
boundaries. The final recommended design criteria and the \&~1
recommended tributary flood flow determination process (&j~.
developed by the Subcommittee were documented in a draft ),l \ .. ~
report of May 20, 1988 entitled Methodology for Determining~ 9 ,,'s ~ )~
the Floodway Flows and Boundaries on the Colorado River. ~~ \~
This report was accepted by the full Task Force at it I"
October 25, 1988 meeting. The final version of this
Methodology report is included as Appendix F of this report.

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