FLOOD02245

• Designation Number: 522
• County: Grand
• Community: Grand Lake
• Basin: Colorado Mainstem
• Title: Floodplain Information Report - Grand Lake
• Date: 1/1/1986
• Designation Date: 7/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

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floodplain mapping needs for the community such as new delineations in areas where annexations
have taken place. The Town was receptive to the Board's proposed designation action, and sent an
official letter requesting that the CWCB designate and approve the floodplain information currently
available to the community. A meeting between Town officials and CWCB staff was not held prior
to the Board meeting at which the floodplain information was designated. A copy of the request
letter is included in the Appendix of this report.

ENGINEERING METHODS

Hvdroloeic Analvsis
Hydrologic information used for the FEMA floodplain delineations in Grand Lake is not available
or was not documented by FEMA. However, acceptable hydrologic information was developed by
the CWCB using a methodology developed by the CWCB for computing 100-year flood-flow
values.

The method involved the use of a publication entitled "Guidelines for Determining 100-year Flood
Flows for Approximate Floodplains in Colorado". The 100-year flow for Tonahutu Creek in Grand
Lake was estimated by utilizing the regional regression equation for the Mountains Region. The
Mountains Region equation is used for streams having drainage watershed areas above an elevation
of 7,500 feet. The CWCB regression equation is:

QlOO = 79.5(A)o.724 (For l<A<950)

The QlOO is the 100-year peak flow in cubic feet per second and A is the upstream (contributing)
drainage area in square miles. The regression equation is only valid for streams with drainage areas
between 1 and 950 square miles.

Table 1 presents 100-year flow estimates as computed using the CWCB regional regression
equation for Tonahutu Creek and North Inlet at Grand Lake.

100

Table 1
P kFl Eft t th T

fG dL k

-year ea ow sIma es or e own 0 ran a e
Drainage loo-year
Area Peak Flow (cfs)
Stream Name and Location (sq. mi.)
Tonahutu Creek at mouth 16.5 605
North Inlet at mouth 46.0 1,270

The CWCB regression equation has not been formally adopted by the Board and is subject to
revision at a later date. The equation is also not valid for Little Columbine Creek because it does
not consist of a natural, unregulated watershed area. There is a streamflow gaging station located

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