FLOOD10235

• County: Fremont
• Community: Canon City
• Stream Name: Arkansas River & Tributaries
• Basin: Arkansas
• Title: Hazard Mitigation 404 Grand Application for FEMA - Includes 3 Maps
• Date: 2/1/1998
• Prepared For: FEMA
• Floodplain - Doc Type: Educational/Technical/Reference Information

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III. DESCRIPTION OF THE PROPOSED PROJECT

Project Location

The proposed project consists of seven small detention ponds to be located along the western
boundary of the City of Canon City, Colorado. Canon City is located approximately 35 miles
west of Pueblo on U.S. Hwy. 50. The west side of the city is bounded by two hogbacks which
run north-south. The higher and western-most of these two hogbacks rises approximately 350
feet above surrounding ground and its serrated ridge is traversed by the well known Skyline
Drive. The proposed detention ponds are to be constructed by placing low height dams across
openings in the lower, eastern hogback at the seven locations shown on Figure 11-2. Larger scale
delineations of the ponds and tributary drainage basins have been reproduced from the Hogback
Detention Master Plan and are included in this application as oversized attachments labeled
Sheets I, 2 and 3. Detention Pond Number I is the largest of the proposed basins and is an
existing structure, having been constructed by Canon City in the summer of 1995.

Project Description

Proiect Obiectives. The tributary drainage area for the proposed detention ponds consists of
134.4 acres of undeveloped land between two hogbacks which form the western boundary of the
City. Currently, stormwater flows exit through openings in the eastern-most hogback and
regularly overwhelm collection facilities in the historic downtown and adjacent residential areas.
Because of the steep, undeveloped nature of the tributary drainage basins, stormwater flows carry
substantial amounts of sediment and debris. This material ultimately deposits on City streets and
in City stormwater collection facilities necessitating regular maintenance. The proposed project
intends to alleviate these problems by constructing low height dams across seven openings in the
eastern hogback. During storm events, ponding behind these dams would remove the majority of
sediment and debris and reduce peak flow rates by approximately 50 percent.

Preliminary Engineering Design. Preliminary engineering design of the ponds is presented in the
Hogback Detention Master Plan by Graef, Anhalt, Schloemer & Associates, Inc. (February,
1995). The largest of the seven ponds proposed in the Master Plan was constructed in the
summer of 1995. For the remaining six ponds. volumes of water stored during the loo-year
storm would range from 0.4 to 1.0 acre-feet, maximum ponding depths would range from 3.3 to
5.2 feet, and the inundated area would range from 0.15 to 0.35 acres. Outlet works for each of
the basins consists of a 24-inch diameter corrugated metal pipe (CMP) through the dam structure
discharging to an improved ditch. The ditches would convey reduced stormwater flows from the
base of the east hogback to City collection facilities, a distance ranging from 250 to 1,100 feet.
Ditches would be trapezoidal in shape, generally 2 feet wide at the bottom and 3 feet deep with
2: I (horizontal:vertical) side slopes. At the pond outlet and along steeper reaches of each ditch,
rip rap lining would be required.

Scope of Work. Construction of the six proposed detention ponds will consist of the following
work.

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