FLOOD01081

• Designation Number: 286
• County: Douglas
• Community: Unincorporated Douglas County
• Title: Flood Hazard Area Delineation - Spring Gulch
• Date: 12/1/1986
• Floodplain - Doc Type: Floodplain Report/Masterplan

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In order to determine the flood limits for such a flow, it was necessary to

evaluate the site hydrologic conditions. Through the various methods

described later in this report, peak flood rates were calculated at key

locations in the drainage basin. Hydraulic calculations were then

performed to determine the water surface profile and flooding limits

corresponding to these flow rates. Also, floodway limits were evaluated

and backwater flood plain limits and profiles were determined for tentative

culvert/embankment type road crossings.

II. DRAINAGE BASIN CHARACTERISTICS

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A. GENERAL

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The Spring Gulch drainage basin consists of approximately 2.058 square

miles of undeveloped land in the northwest corner of Douglas County,

Colorado. Currently, the Gulch carries runoff following precipitation, but

it is normally dry. Further study is needed to determine if development

will cause more frequent runoff due to uybanization characteristics.

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B. COORDINATION

The basin originates at Section 22, Township 6 South, Range 68 West (see

Vicinity Map, Basin Boundary Map and Sheet Index (sheet 1 of 12 in

Appendix), and extends approximately 3.4 miles in a northwesterly direction

to its eventual outfall point at U.S. Highway 85. Development in this

basin will consist of single and multifamily residences with school sites

and nonurban use areas I and is expected to increase runoff rates over

existing condition flows.

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Appropriate methodologies for perform lng the necessary analyses were
established through consultations with the Urban Drainage and Flood Control
District.

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C. METHODOLOGY

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The basic mapping is at 1 inch 40 feet scale with one,foot contours, and

subsequently reduced to 1 inch 200 feet scale for use in this report.

Land use data was obtained from the approved Development Plan for Highlands

Ranch. The Spring Gulch drainage basin was divided into 12 sub,basin

segments (see Basin Boundary Map), and peak discharges at the downstream

end of each of these sub-basins were determined using the Colorado Urban

Hydrograph Procedure (CUHP) in conjunction with the Storm Water Management

Model (SWMM). Water surface profiles and flood plain limits, as well as

floodway limits due to encroachments, were determined using the HEC,2 Water

Surface Profiles computer program. The reader should refer to the

following sections for greater detail on the hydrologic and hydraulic

analyses used and to the tables, figures and drawings included in the

Appendix of this report for the results of these analyses.

The drainage basin is contained wholly within Douglas County. Various
utility easements including those for the City of Aurora and the Denver
Water Board cross this drainage basin. Spring Gulch basin is contained
wholly within Urban Drainage and Flood Control District jurisdiction. This
report addresses only that portion of the drainage basin flood limits which
lie upstream of the Corps of Engineers embankment upstream of U.S. Highway
85.

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B. SPRING GULCH

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Spring Gulch basin consists of one major watercourse (reach). The extent

and relationship of this reach to the basin can best be seen on the

previously referenced Basin Boundary Map in the Appendix. The channels

delineated on this map vary considerably along their lengths in terms of

channel cross section, side slopes, stream bed (thalweg) slopes, roughness

and channel material. Detailed mapping in the Appendix (Sheets 1-6) has

been prepared for the hydraulic analyses of this major reach.

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