BOARD01370

• Board Meeting Date: 11/23/1998
• Description: Flood Section - Floodplain Designations and Approvals - Existing Conditions and Future Conditions Floodplain Delineations and Designations
• Board Meetings - Doc Type: Memo

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Colorado Water Conservation Board
Department of Natural Resources
721 Centennial Building
1313 Sherman Street
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]

STATE OF COLOAADO

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MEMORANDUM

Roy Romer
Governor
James S. Lochhead
Executive Director, DNR
DariesC. Lile, P.E.
Director, (WeB
Peter H. Evans
Acting Director, eweB

TO:

Colorado Water Conservation Board Members

FROM:

Larry Lang RU
Brian Hyde \13 L1

DATE:

November 12,1998

SUBJECT: Agenda Item 4.h.., November 16-17, 1998, Board Meeting-
Floodplain Designations and Approvals-Existing Conditions and Future
Conditions Floodplain Delineations and Designations

EXECUTIVE SUMMARY

The staff wishes to present to the Board a flood hydrology issue that is directly related to Agenda
Items 4,a, b, and c. During the technical review for those three items (Floodplain Resolutions Nos.
98-472, 98-473, and 98-474) this hydrology issue was discovered, The 3 studies are all Flood
Insurance Studies (FIS') for the City of Westminster (1988 FIS, 1991 revised FIS, 1993 revised
FIS), They are all based on a flood hydrology analysis which assumes existing watershed land use
conditions. The previously designated and approved Flood Hazard Area Delineation (FHAD) for
Westminster, prepared by the Urban Drainage & Flood Control District (UD&FCD), is based on
different flood hydrology which assumes future watershed land use conditions, as do all of the
UD&FCD floodplain studies. Within the CWCB's Rules and Regulationsfor the Designation and
Approval of Floodplains and of Storm or Floodwater Runoff Channels in Colorado the concept of
"future conditions" flood hydrology is addressed. The proposed CWCB action for these three FIS'
would result in the designation and approval of two sets of floodplain delineations for the
same stream reaches. A summary and discussion of the proposed actions is presented below,

DISCUSSION

Back2round
Agenda Items 4,a, 4,b" and 4.c, include the proposed designation of the 1988 FIS for the City of
Westminster and two subsequent revised FIS' (one prepared in 1991 and one prepared in 1993), The
proposed designations require that we confront the question of having two detailed floodplain
delineations for the same set of streams. There is a "future conditions" floodplain study and there is