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

Table 4.3 Hydraulic Summary of Crossings Along Little Willow Creek

Crossing Location Peak Discharge Overtopping Depth
(cfs) (feet)
10-Year 100- Y ear 10-Year 100-Year
Waterton Road 35+25
2 - 36" CMP's 805 1881 N/A 1.7
Village Circle East 70+10 754 1844 N/A N/A
2- 6' X 8' RCB's
Rampart Range Road 117+40 726 2034 N/A 1.0
2-ll'I"X 6'10" CMPA's
Foothills Treatment Plant 159+20 604 1840
Road, 78" CMP 0.3 1.5
Roxborough Park Road 179+90 604 1840 1.2 2.3
58" X 36" CMPA
Access Road to Fire Station 182+80 604 1840
48" CMP 1.0 2.2

Willow Creek crosses the High Line Canal and four roadways along the study reach. Table 4.4
presents a hydraulic summary of the crossings.

Table 4.4 Hydraulic Summary of Crossings Along Willow Creek

There is an existing detention pond upstream of Village Circle East that reduces the 1 OO-year peak
discharge from 2293 cfs to 1844 cfs.

Crossing Location Peak Discharge Overtopping Depth
(cfs) (feet)
10- Year 100- Year 10- Year 100- Year
High Line Canal 49 + 50 1577 5071 N/A 3.2
2 - 84" CMP's
Rampart Range Road 56+50 1577 5071 N/A 1.6
Triple RCB (9'-12'-9' X 8')
Roxborough Park Road 161 [phone] 1.3 2.6
72" X 44" CMPA
Private Dirt Road 217+20 1197 4164 3.0 3.9
42" CMP
Private Dirt Road 346 + 00 724 2419 1.1 2.3
42" CMP

East Willow Creek

Willow Creek

The Willow Creek 100-year floodplain is generally narrow and confined to the channel along about
half of the study reach, with top widths ranging from 50 feet to 120 feet. In the areas where flooding
is confined to the channel, the depth of flow ranges from 8 feet to 13 feet while the velocity ranges
from 9.2 feet per second to 15.6 feet per second.

East Willow Creek drains into the High Line Canal and then overtops the canal embankment and
drains a short distance (800 feet) to its confluence with Willow Creek. Currently, there is not a
drainage structure under the High Line Canal. The 100-year floodplain along East Willow Creek
is confined to the channel banks along most of the upper reaches, but becomes wider in the lower
reaches due to the lack of a defined channel.

In the remaining areas, the floodplain is wider and not confined to the channel due to ponding behind
existing roadway crossings or insufficient channel capacity. The depth of flow in these areas varies
from 2 feet where the channel is shallow and flooding is widespread to 13 feet upstream of roadway
crossing where ponding occurs. The channel velocity in these areas ranges from 4.1 feet per second
to II feet per second. In the unconfined areas, the 100-year floodplain ranges in width from 150 feet
to 500 feet.

In the lower reaches the floodplain ranges in width from 150 feet to 520 feet. The depth of flow
varies from 2 feet where the channel is not well defined to 11 feet upstream of the High Line
Canal. Velocities in the lower reaches range from 2.4 feet per second to 6.2 feet per second.

In the upper reaches the floodplain ranges 40 feet to 115 feet in width and generally flows 4 feet to
5 feet deep. Velocities in the upper reaches range from 5.7 feet per second to 10.1 feet per second.

There are currently only a few structures in the 100-year floodplain adjacent to Willow Creek
between stations 203+00 and 210+00.

There are two embankments along Willow Creek located on private property near stations 256+00
and 315+00. Since both of the embankments are located on private property it was assumed that they
do not have any flood control benefits and that the available stormwater ponding volume upstream of
the embankments was full at the beginning of any storm. Based on this assumption the 100-year
storm will overtop the embankments at station 256+00 and 315+00 by depths of about 3.0 feet and
2.5 feet, respectively.

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