FLOOD08539

• County: Statewide
• Basin: Statewide
• Title: Floodplain Designations and Approvals
• Date: 2/23/1998
• Prepared For: State of Colorado
• Prepared By: CWCB
• Floodplain - Doc Type: Community File

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Colorado Water Conservation Board
Department of Natural Resources
721 Centennial Building
131 3 Sherman Street
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]

STATE OF COLORADO

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February 23, 1998

Roy Romer
Governor

Michael Buckley, Chief
Hazard Identification Branch, FEMA
Federal Center Plaza
500 C Street, South West
Washington, D,C, 20472

J.lmes S. Lochhead
Executive Oirecror, DNR

Daries C. Ule, P.E.
Director, ewcs

Dear Mike:

In an effort to better understand the FEMA review process and criteria for evaluating technical
information related to floodplain studies, the Colorado Water Conservation Board (CWCB) is seeking
guidance from your office. The CWCB has adopted "Rules and Regulations for the Designation and
Approval of Floodplains and of Storm or Floodwater Runoff Channels in Colorado," dated December 11,
1987. Included within these rules and regulations are criteria for evaluating floodflow-frequency
information in cases where new hydrology studies produce peak flow results that are different from those
in an existing study for the same stream reach, The specific language that I would like for you to review is
presented below,

"Proposed jlood discharges shall be compatible with those used in previously completed studies
on the same watercourse, The results of a later jloodjlow-jrequency analysis shall be considered
where [the results] disagree with discharges used in completed studies, only when they can be
shown to be significantly different. statistically, than the previously used discharges, The test for
significance shall be based on the conjldence limits of the latest analysis. as described below,

Where a later jloodjlow-frequency analysis provides discharges that differ from those established
previously in studies on the same stream, the latest discharges shall be adopted if the previously
established discharges do not fall within the 95 and 5 percent confidence limits (90 percent
confidence interval) of the most recent estimates, The previously established discharges shall be
adopted if they fall within the 75 and 25 percent confidence limits (50 percent confidence
interval) of the most recent estimates, Where the previously established discharges fall between
the 50 and 90 confidence intervals of the most recent estimates, the situation shall be presented to
the Board for resolution. "

The CWCB requires that a qualified engineer using accepted engineering practices and techniques has
appropriately analyzed a new hydrologiclfloodplain study, Once the new study has been reviewed and
accepted by local and state officials, the above quoted rule should be applied if a comparison is being
made to an old study,

Admittedly, the above language is not very easy to follow and may require several readings to fully
comprehend it. According to the passage, the new hydrologic study is considered to be the benchmark
against which the old study should be compared. The key points ofthe second paragraph of the passage
have been summarized below to the best of my understanding,