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 3,0
BASELINE HYDROLOGY ANALYSIS

3.5

3.1

Major Watersheds

The study area was divided into three major watersheds: Willow Creek, Little Willow Creek,
and East Willow Creek, which is a right bank tributary to Willow Creek. These three
watersheds are combined into one model.

3.2 Subwatersheds

Each of the three major watersheds in the study area were divided into subwatersheds as
shown in Figures 3.IE and 3.1W. The subwatersheds were limited to a maximum of 130
acres where potential development could occur, but were permitted to be a maximum of 320
acres where no future development was possible due to zoning and topographic limitations.
The subwatersheds were numbered consecutively from north to south along the major
drainageways. The numbering system generally follows a given stream to its end, and
resumes where the next branch begins.

3.3

Streams

Streams (major drainageways) draining 130 acres or more of subwatershed area were defined
as shown in Figures 3.2E and 3.2W. To provide an orderly system of managing stream
information and hydrologic results, streams (above each confluence point) were designated
with an identification number. The stream numbers were grouped such that East Willow
Creek has streams numbers between 100 and 200, Willow Creek has numbers 200 to 300, and
Little Willow Creek has numbers above 300, Streams were numbered from downstream to
upstream (north to south) in such a way that the conveyance element had the same last two
digits as the upstream design point; e.g, the stream designated 121 was downstream of design
point 21. Design points (direct routing elements) were coded into the hydrologic model
between each pair of conveyance elements. The design points were designated according the
highest subwatershed ID number directly tributary to the design point.

Willow Creek and Little Willow Creek have been identified with numbers on UD&FCD
Drainage Basin Description Maps (UD&FCD, 1990). Willow Creek corresponds to UD&FCD
stream number 6900, and Little Willow Creek corresponds to UD&FCD stream number 7000.

3.4 Hydrologic Models

The UD&FCD's Colorado Urban Hydrograph Procedure, version F (CUHPF/PC) (UD&FCD,
1995) model was used to develop runoff hydro graphs for each subwatershed. The runoff
hydro graphs were routed through the existing stream network using UD&FCD's Storm Water
Management Model (UDSWM386) (UD&FCD, 1985b). The UDSWM386 model does not
have a channel infiltration component; however, channel infiltration, even though it occurs, is
not expected to significantly affect peak discharges during extreme flood events.

Rainfall

3.5.1 Criteria: The design rainfall events modeled for the study area were developed using
criteria from the Douglas County Drainage Criteria Manual (Douglas County, 1986) and the
Urban Storm Drainage Criteria Manual (USDCM) (UD&FCD, 1969).

3.5.2 Rainfall Depths: The one-hour rainfall depths used in the CUHPF/PC model are
presented in Table 3.1.

Table 3.1

Return One-hour
Period Rainfall (in.)
10-year 1.66
50-year 2.26
100-year 2.60
500-year 3.20

These one-hour rainfalls are distributed over a two-hour design storm internally by the
CUHPF IPC model.

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