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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The Highlands Ranch Development Plan delineates that a significant amount

of adjacent land use will remain as nonurban use; therefore, for hydrologic

analysis, the hydraulic length and slope of the maj or basin channel is

assumed not to change between existing and future development conditions.

It should be noted that the presence of a number of existing check dams

within the Spring Gulch drainage channel have been considered in

determining the maximum water surface profile and 100'year flooding limits,

since this would represent the worst possible flooding condition.

Numerous thunderstorms during spring and summer months, while generally

limited in area, produce occasional periods of intense rainfall, and most

of the maj or floods in this area have resulted from intense thunderstorm

activity. The thunderstorms are generdlly the result of convective

activity occurring during the afternoon or early evening. These storms are

generally of short duration and moder8te intensity. The most severe

thunderstorms are associated with frontal activity coupled with orographic

effects. No specific flood history is available for Spring Gulch.

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The Spring Gulch basin lies near the southwestern edge of the Denver basin,

an area extending from Colorado Springs north to Greeley, and from the

foothills eastward, approximately 60 miles, onto the plains. The Denver

and Dawson Formations underlie the Spring Gulch drainage basin which is

characterized by sand, gravel, silt and clay ranging in depths up to 60

feet, with slopes up to 15 percent.

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 urbanization characteristics.

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III. HYDROLOGIC ANALYSIS

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The maj ori ty of the Spring Gulch basin topography is generally rolling

rangeland. Vegetation consists of three maj or plant communities:

grassland, shrubland, and minimal riparian woodland. The predominant

vegetation type is grassland. Shrubs and wooded areas are located mainly

along the larger drainage channel with low, growing shrubs sometimes

overgrowing the channel bottom.

The latest UDFCD methodology was used to find and route the respective

sub,basin hydrographs. The Colorado Urban Hydrograph Procedure (CUHP) was

utilized to develop hydrographs for the respective sub,basins. These

hydrographs were then routed utilizing the Storm Water Management Model

(SWIM) program.

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The CUHP Model was used to develop flow rates for the 100'year developed

sub,basin (see Exhibit 3 & 4 ' in the Master Plan of Drainage Report). The

Storm Water Management Model was then used to route the developed

sub,basins. Applications of both the CUHP and SWMM computer models are

described in this section, along with the assumptions used for the Spring

Gulch drainage basins.

C. CLIMATOLOGY AND FLOOD HISTORY

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The climate over the Spring Gulch drainage basin can be classified as

semi,arid. The average annual precipitation in the Spring Gulch drainage

basin is 17 to 19 inches, 75 percent of which occurs during the months of

April through September. The period of heaviest precipitation is during

these spring and summer months. Snow melt is not usually a contributing

factor.

A. DESCRIPTION AND APPLICATION OF CUHP

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The CUHP computer model is described in the Urban Drainage and Flood

Control District (UDFCD) drainage manual (Reference A) and in the user

manual for the CUHP computer program (Reference B). The unit hydrographs

utilized in the CUHP method are determined from empirical relationships

between parameters of the unit hydrograph and physical characteristics of

the watershed. These relationships are based on field data from developing

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