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 4.0
HYDRAULIC ANALYSIS

4,3 General Overview Of Flood Impacts

Little Willow Creek

4.1 Methodology

Water surface profiles were computed for each of the streams being studied using the U.S. Corps of
Engineers' HEC-2 Water Surface profiles program (Version 4.6.2). Channel cross section data was
taken from topographic mapping developed for the project by Benchmark Mapping. The mapping
was provided as digital, three-dimensional mapping at 2-foot contours.

Little Willow Creek drains under Waterton Road directly into the High Line Canal and then
overtops the Canal and discharges into the Platte Canyon Reservoir. Peak discharges draining into
and out of the Platte Canyon Reservoir are presented in Table 4.2. The information presented in
Table 4.2 assumes that the reservoir is full to the invert elevation of the emergency spillway
(elevation 5536.1) at the start of all storm events.

Culvert crossings were not modeled using the HEC-2 software. Due to the majority of the culvert
crossings being arched or elliptical in shape, and the inability of the program to model these shapes, it
was decided that the appropriate nomographs should be used to determine the water and energy grade
lines at culvert crossings. The resulting water surface elevation was entered in the HEC-2 model at
the upstream face of the culvert crossing.

Table 4,2 Hydraulic Information for the Platte Canyon Reservoir

Geometric measurements and invert elevations for culvert crossings were provided by Nolte and
Associates, and supplemented where necessary by additional field measurements. Photographs were
taken at each crossing and a sketch of each was prepared showing the appropriate dimensions,
Additional photographs were taken to document the existing channel and overbank conditions for
determining roughness coefficients for use in the hydraulic analysis. This material is on file at the
UD&FCD as backup information to the project in the Technical Addendum. The Manning's "n"
values utilized in the hydraulic analysis ranged from 0.035 to 0.080 in the channel and 0.030 to 0.110
for the overbank areas.

Storm Event Inflow (cfs) Outflow (cfs) Maximum Water Surface Elevation
10- Year 805 276 5537.4
50-Year 1517 624 5538.7
100- Year 1881 874 5539.2
500- Year 2203 1246 5540.2

The 100-year floodplain along Little Willow Creek is confined to the channel through about one-
half of the study reach. In the remaining areas, the floodplain extends beyond the channel banks
and flooding is widespread. The unconfined flooding is generally caused by inadequate roadway
crossings and channel capacity.

Water surface profiles were computed for the 10- and 100-year events for each stream under study.
In addition, flood ways were calculated for all reaches where the 100-year floodplain was not confined
within the channel banks. The floodways were based on a total increase of not more than 0.5 feet and
I. 0 feet in the energy grade line. The data is summarized in tabular form and referenced to the
channel stationing in Table 4.1 (located in Appendix A).

In the areas where the floodplain is confined to the channel, top widths range from 40 feet to 100
feet while the depth of flow varies from 4 feet to 9 feet. Velocities in these areas range between
6.5 feet per second and 12.3 feet per second.

4.2 Results

In the areas where the floodplain extends beyond the channel banks, top widths range from 150
feet to 500 feet while the depth of flow varies from 2 feet where the channel is shallow and
flooding is widespread to 10 feet upstream of roadway crossing where ponding occurs. Velocities
in these reaches range from 3.1 feet per second to 8.6 feet per second.

The results of the hydraulic analysis are shown on drawings and in tables in this report. Drawings
include plan view maps and profiles, each indicating flood elevations and cross section locations. A
total of 27 sheets are provided indicating the 100-year floodplain. An index of map sheets is
provided. Sheets I to 8 depict the Little Willow Creek floodplain, Sheets 9 to 22 show the floodplain
on Willow Creek, and Sheets 22 to 27 show the East Willow Creek floodplain.

The only structure within the future 100-year floodplain is the fire station located on the southeast
corner of Rampart Range Road and Roxborough Park Road. This flooding is most likely caused
by the inadequate crossings at Roxborough Park Road and the access road to the fire station. Little
Willow Creek crosses six roadways within the study reach. Table 4.3 presents a hydraulic
summary of the roadway crossings.

Detailed floodplain and floodway data are provided in Table 4.1. At each cross section used for the
hydraulic analysis, the resulting information for the water surface elevations, velocities, and widths of
the floodplain and floodways are given.

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