FLOOD06085

• Designation Number: 570
• County: Douglas
• Community: Unincorporated Douglas County
• Basin: South Platte
• Title: FHAD - Cherry Creek Corridor - Reservoir To Scott Road
• Date: 5/1/2003
• Designation Date: 9/30/2003
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTIONFOUR

FUTURE ROODS

Flooding can occur with relatively little discharge in the event of channel blockage caused by debris
or ice. In these situations, water surface elevations rise until relief is achieved by flows over the
floodplain. Historical data, however, indicate that obstructions due to ice are not likely to occur in
the study reach.

4.3 HAZARDS OF LARGE FLOODS

The extent of damage caused by any flood depends on the topography of the area flooded, the depth
and duration of flooding, the velocity of flow, the rate of rise, the extent of development on the
floodplain, the amount of debris in the floodwater, and ice conditions. A 1 %-probability (100-year
frequency) flood on Cherry Creek in the study reach would result in the inundation of and
subsequent damage to primarily agricultural properties and associated developments as well as
public utilities and public roadways. A 0.2% probability (5oo-year frequency) flood, however,
would flood numerous residential and commercial buildings along both sides of the Corridor.
Significant residential flooding would occur at Cottonwood subdivision and Stroh Ranch
subdivision. Deep floodwater flowing at high velocity and carrying floating debris would create
conditions hazardous to persons and vehicles attempting to cross flooded areas. ,In general,
floodwater 3 or more feet deep and flowing at a velocity of 3 or more feet per second could easily
sweep an adult person off their feet, thus creating definite danger of injury or drowning. Rapidly
rising and swiftly flowing floodwater may trap persons in homes that are ultimately destroyed or in
vehicles that are ultimately submerged or floated. Isolation of areas by floodwaters could create
hazards in terms of medical, fire, or law enforcement emergencies.

4.3.1 Flooded Areas and Flood Damages.
Table 5 provides a summary of the flood characteristics from the HEC-RAS hydraulic evaluations
that were undertaken. The flood elevations at 219 reference cross sections for the 10-, 50-, 100-, and
5oo-year floods are presented in the table.

The Flood Hazard Area Delineation Profiles (FHAD Profiles), Sheet 1 through 18, show the water
surface elevations and water depths of the lO-year, 50-year, loo-year, and 500-year floods, relative
to the streambed. The water surface elevations of the 100-year and 5oo-year floods were used to
detennine flood limits on the cross-sections and topographic maps. The GIS-based computer
program HEC-GEORAS was used to delineate the floodplain on the digital topographic maps. The '
results are shown on the Flood Hazard Area Delineation Plan (plan), Sheet 1 through 16. An index
to the Flood Hazard Area Delineation Plans (FHAD Plans) is provided on Plate 2. Representative
cross sections, including those immediately upstream and downstream of modeled bridges, are
illustrated on Cross Section Profiles, Sheet 1 through 8. The cross sections show ground elevations
across the valley on both sides of the channel and the depths of overbank flooding. Reference
stream stations, coinciding with the locations of the cross sections, are shown on both the Plans and
the Profiles. Table 6 provides floodplain and floodway data at model cross sections, including
distance along the main channel centerline, elevations of the streambed, and elevations of the 10%,
2%, 1 %, and 0.2% probability (lO-year, 50-year, loo-year, and 5oo-year, respectively) floods.

The FHAD Plans show the area that would be inundated by the 1% probability (lOO-year) flood
event. The flood limits were located at each cross section and the intervening flood inundation
limits were drawn based upon detailed digital topographic mapping with fine tuning from
engineering judgment and field observation. It is, however, possible that more or less flooding could

be shown on the Flooded Areas given the uncertainty in selection of stream friction coefficient, which normally
varies seasonally. For a specific situation, where more detailed accuracy of flooded area is required, the flood
limits can be more accurately established by detennining the water surface elevation from the profile or reference
tables and then locating that elevation by a more detailed survey of the floodplain.

A reference line is shown on the Flood Hazard Delineation Plans for Cherry Creek. This line is in general
alignment with the current thalweg of the creek. Due to the alluvial nature of Cherry Creek, this thalweg is likely to
change over time.

The flood elevations shown on the profile and in Table 5 apply laterally from the channel over most of the
floodplain width. Road crossings and other topographic features can alter the lateral flood elevations. Depending
on whether these features divert or block flows, the flood elevation at the edge of the floodplain may be higher or
lower than at the channel.

Floodplain widths vary from one location to another throughout the study reach. The loo-year floodplain width
varies from a minimum near 520 feet to a maximum near 4,100 feet with an average of 1,500 feet. Floodplain
widths generally increase gradually from Scott Road to Cherry Creek Reservoir.

The major portion of the Cherry Creek loo-year floodplain in the study reach is in agricultural use. Few buildings
of any type exist on this floodplain.

According to the results of the hydraulic modeling, several existing developments and structures were identified to
encroach on the existing 1000year floodplain, including:

. The Town of Parker Fire Safety Training Center,
. Arapahoe Road,
. North Water Reclamation Facility
. Scott Road
. Cherry Creek Country Club

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The Town of Parker Fire Safety Training Center was located at a relatively low elevation by Cherry Creek. Under
the loo-year flow condition, the building would be under a depth of five to six feet of water. The administration
building at the Fire Safety Training Center would likely experience shallow flooding under the loo-year flow
condition.

Under the I % probability (100-year) flood, the Arapahoe Road would be under significant flooding with over 5 feet
of water at its lowest elevation. The total length of the road being flooded reaches 2700 ft.

The current modeling study indicates that the 100-year flood elevation at the North Water Reclamation Facility
immediately downstream of E-470 reaches the top of the berm that separates the site from Cherry Creek flooding.
With no freeboard, the site could be flooded in windy conditions or if the conveyance of the stream is reduced by
accumulation of debris. Failure of the berm, which does not meet FEMA levee standards, could also lead to
significant flooding of the site.

Under the 1% probability (loo-year) flood, the Scott Road Bridge would be overtopped slightly (< 0.5 ft) and the
road would be overtopped significantly with over 4-feet of water at the lowest spot.

Cherry Creek Country Club is located on the right floodplain of Cherry Creek just downstream of Arapahoe Road.
Several residential houses, mostly along East Caley A venue, are affected by the 1% probability (100-year) flood.
Flooding of these structures, however, is likely to be limited because the flood depth is less than 1 foot.

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