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

SECTION 2.0
STUDY AREA DESCRIPTION

2.5

2.1

Topography and Physiography

The Dakota Hogback runs through the Willow and Little Willow Creek Watersheds creating a
significant natural hydrologic and topographic boundary. The slope of the terrain and
tributaries within the headwater areas of these watersheds in the foothills of the Front Range
becomes very steep, having grades greater than 10%, The general slope of the terrain in the
middle and lower reaches of the watersheds in the Eastern High Plains province ranges from
2% to 6%.

2.2

Soils

Hydrologic soil types within the watershed study area are shown in Figures 2.IE and 2.1 W.
Hydrologic soil types were identified based on the U.S. Department of Agriculture, Soil
Conservation Service's Soil Survey of Castle Rock Area. Colorado, dated November 1974
(Reference 3).

2.6

Hydrologic soil Types B, C and D are predominant in the watershed. Type B soils have
moderate infiitration rates, whereas Types C and D soils have lower infiltration potential.

2.3

Reservoirs and Detention Basins

There are four small reservoirs within the watersheds. The Platte Canyon Reservoir is located
in the lower reach of the Little Willow Creek Watershed immediately upstream of the South
Piatte River. It is primarily used to control irrigation flows along the High Line Canal which
traverses through the lower portion of the watershed. The Aurora Rampart Reservoir is
located in the middle of the Little Willow Creek Watershed and serves as a raw water storage
facility for the City of Aurora. A regional on-stream detention/water quality pond was
constructed as a part of the Roxborough Village Park Subdivision and is located along Willow
Creek about I mile upstream of the Platte Canyon Reservoir. There is a low dam located on
private property along Willow Creek about three quarters of the way up the watershed, which
is privately owned.

2.4

Flood History

No documentation of flooding in the study area is known to exist.

Vegetation

Vegetation in the watershed consists primarily of native grasses typical of High Plains areas,
with a portion of the prairie grassland being used as pasture. About one-half of the study area
(primarily the northern one-half) is currently in agricultural use as cropland. Intermittent
stands of willow trees grow along the creek beds in the lower reaches, becoming increasingly
more dense in the upper reaches.

Natural riparian vegetation is evident along many of the major streams in the study area and
consists primarily of cottonwood, willow, and herbaceous wetland vegetation. In general, the
natural riparian vegetation is confined to the alluvial channel corridor and varies in width
from about 8 to 10 feet in the middle reaches of the watershed. The lower reach of Willow
Creek supports a well developed, diverse, and lush riparian corridor. The existing wetland
and riparian areas within the watersheds have been deiineated and are described in a report
entitled "Willow Creek Watershed and Riparian Areas. Douglas County. Colorado"
completed by ERO Resources Corporation, July 1996 (Reference 4).

Watershed Imperviousness

Areas of imperviousness, representative of future development conditions in the study area,
are shown in Figures 2.2E and 2.2W (located in the Appendix). Projected development for
the basins is based on the Douglas County Master Plan, and discussions with the Douglas
County Planning Department and the UD&FCD. A large portion of the study area consists of
undeveloped grassland and agricultural land. Small areas of existing development have
occurred, primarily in the Little Willow Creek Watershed, The Roxborough Village
Subdivision located along Little Willow Creek below the hogback and the residential areas
behind the hogback have been developed according to the Douglas County Master Pian,
adopted by the Douglas County Planning Commission in January 1992 (Reference 5). The
homes in the Roxborough Village area have typically been built at a density of 6 per acre,
while the development of homes behind the hogback have historically been on one-half acre
lots. In accordance with Volume I of the District's Urban Storm Drainage Criteria Manual
(USDCM) (Reference 6), the imperviousness used in the CUHPF/PC model where
development has occurred in these areas is 45% and 30%, respectively.

Existing development within the Willow Creek and Little Willow Creek Watersheds is
described by the County as separated urbanized areas. According to the Douglas County
Planning Department, future development in the separated urbanized areas can be expected to
occur at a density equal to developments in the past. Therefore, the imperviousness used in
the CUHPF/PC model for the future development conditions within the separated urbanized
area is 30% west of the hogback and 45% east of the hogback. The Foothills Water
Treatment Plant is situated in the Willow Creek Watershed. This facility is assumed to have
an imperviousness of 10%. Outside of the separated urbanized boundaries, there are a few
pockets of land currently zoned general industrial or business. For future development
conditions, these areas are considered to have an 85% imperviousness. The majority of the
area outside of the separate urbanized boundaries is currently zoned agricultural. Because of

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