BOARD02642

• Board Meeting Date: 1/12/1998
• Description: Flood Section - Floodplain 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 COLORADO

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Roy Romer
Goyernor

MEMORANDUM

J;lmes S. Lochhead
Executive Director, DNR

DariesC. Lile, P.E.
Director, CWeB

TO: Colorado Water Conservation Board

FROM: Larry Lang

DATE: November 17, 1997

SUBJECT: Agenda Item 5a.i., November 24-25, 1997 Board Meeting--
Floodplain Designations

INFORMATION

The Colorado Water Conservation Board at its May 12, 1997 meeting designated and approved a
Floodplain Infonnation Report entitled "City of Sterling, Floodplain Analysis, Pawnee Creek
Overflows, prepared by Icon Engineering, Inc. dated February 3, 1997. The Board's approval
and designation action of the 1 OO-year floodplain infonnation in said report was by Floodplain
Resolution No. 97-456. This action was at the request of the City of Sterling, Colorado.

During the Board's staff review, the hydrology and hydraulic detenninations were found to be in
compliance with the CWCB technical standards. The hydrology and hydraulic detenninations for
Resolution 97-456 were based on previously designated floodplain studies. The infonnation was
approved and designated as detailed infonnation under the Board's "Rules and Regulations for
the Designation and Approval of floodplains and of Stonn or Floodwater Runoff Channels in
Colorado" (2CCR 408-1).

On July 30-31, 1997, Logan County and the City of Sterling (City) experienced a major flood
event in the Pawnee Creek Basin. On the mainstem of Pawnee Creek, the flood frequency of the
July 30-31, 1997 event was in excess of a 100-year flood. Due to the magnitude of the event,
flood overflows entered the City of Sterling. Previous engineering studies projected that these
flood overflows would occur. In fact, an NRCS report entitled "Flood Plain Management Study,
Pawnee Creek in Logan County, Colorado dated April 1992" stated and acknowledged that
during a 100-year flood event (or greater), floodwaters would enter the City. In addition to the
findings of the 1992 engineering analyses, it has been documented that flood overflows during
the June 1935 and June 1965 flood events entered the City.

DISCUSSION

The flood path for the July 30-31, 1997 flood event did not track the 100-year floodplain outline
or the path of the 1935 or 1965 flood events. The 1997 Pawnee flood overflows tracked and
inundated portions of the mapped 500-year floodplain through the City. The flood event
demonstrated a flood outline and flood hydrology/hydraulic characteristics which were not