FLOOD01923

• Designation Number: 462
• County: Douglas
• Community: Unincorporated Douglas County
• Stream Name: Willow, Little Willow, East Willow Creeks
• Title: Flood Hazard Area Delineation - Willow Creek, Little Willow Creek, East Willow Creek, Douglas County, CO
• Date: 12/1/1996
• Designation Date: 7/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Mr. Ben Urbonas, P.E.
Page 2
December 30, 1996

GREENHORNE & O'MARA, INC, 3131 SOUTH VAUGHN WAY . SUITE 42B.AURORA, COLORADO.B0014.PHONE[phone]

December 30, 1996

Subject:

Flood Hazard Area Delineation (FHAD), Wiilow, Little Wiilow and
East Willow Creeks - UD&FCD Agreement No. 95-06.10

This FHAD report is intended to provide a tool which will enable the sponsoring agencies to manage
development activities adjacent to the defined floodplains through established planning and
development approval processes. The attached drawings show the lOa-year floodplain, 10- and 100-
year water surface profiles, representative channel cross sections, and major roadway crossings.
Tabular information for each cross-section is given for stationing, thalweg elevation, 10-, 50-, and
lOa-year discharges, 10-, 50-, and laO-year water surface elevations, laO-year floodplain and
floodway topwidths, floodway water surface elevations, and mean velocities in the floodplain and
floodways. The floodways have been based upon a total increase of not more than 0.5 feet and 1.0
feet in the computed energy grade line. It should further be noted that the channels in the study area
are erodible and may undergo changes in shape, width, and direction as a result of flood events.
Therefore, flood hazards may exist outside the floodplain limits indicated in this report.

Mr. Ben Urbonas, P.E.
Chief, Master Planning and South Platte River Programs
Urban Drainage and Flood Control District
2480 West 26th Avenue, Suite-B
Denver, CO 80211

Dear Ben:

We have enjoyed working with the project sponsors and your staff throughout the preparation of this
FHAD report.

Greenhorne & O'Mara is pleased to submit the attached report entitled Fiood Hazard Area
Delinealion - Willow Creek, Little Willow Creek, and East Willow Creek, dated December 1996.

Sincerely,

We would like to acknowledge the help and support in the preparation of this report that was
furnished by your staff and the staff of Douglas County. In particular, Mr. John Doerfer and Mr.
Biil DeGroot from the District's staff and Mr. Brad Robinstein from Douglas County have been
instrumental in providing input and timely reviews.

GREENHORNE & O'MARA, INC.

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Project Manager

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Matthew J. Ursetta, P.E.
Project Engineer

This FHAD report provides floodplain information for three drainageways located in the
transitional zone between the mountain foothills and the plains in unincorporated western
Douglas County, Colorado. The three drainageways and the approximate reach lengths included
in this study are:

. Willow Creek - 6.8 miles
. East Willow Creek - 2.55 miles
. Little Willow Creek - 3.54 miles

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Wil10w Creek is a southern tributary to the South Platte River, upstream of the Chatfield
Reservoir. East Willow and Little Willow are tributary to Willow Creek with confluence's near
the Willow Creek/South Platte River confluence. Delineated portions of the creeks are generally
the northern halves of the overall drainage basins. At the time of this study, significant
development has occurred only along portions of Little Willow Creek, with the remainder of the
drainage basins being primarily open grazing fields.