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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Partial Detention Alternative (Hogback Detention Master Plan)

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The Partial Detention Alternative is the flood mitigation scheme as proposed in the Hogback
Detention Master Plan by Graef, Anhalt, ScWoemer & Associates, Inc. (February, 1995). The
plan calls for seven small detention ponds to be formed by constructing low height dams across
openings in the lower, eastern hogback. Locations for the seven detention ponds proposed in the
Master Plan are shown on Figure II-2. Larger scale delineations of the ponds and tributary
drainage basins have been reproduced from the Master Plan and are included in this application
as oversized attachments labeled Sheets 1,2 and 3.

According to hydrologic modeling results presented in the Master Plan, the proposed detention
ponds would provide an approximate 50 percent reduction in peak storm flows generated by the
25-, 50- and 1 DO-year, 24-hr storms. Modeling results for shorter duration, thunderstorm type
events are not presented in the Master Plan. Percentage reductions in peak flows for
thunderstorm events should exceed reductions for general storms because of the lower volumes
associated with shorter duration events.

Estimated sediment trap efficiencies for the proposed detention ponds are not presented in the
Master Plan. Based on observations by the City, silt sized particles make up a significant portion
of the sediment load carried by runoff from the hogback drainage basins. Because of the fine
nature of these particles and relatively short detention times (20 minutes or less) provided by the
ponds it is thought that a portion of the incoming sediment load will pass through the pond
outlets and continue to be deposited in the City. For purposes of benefit/cost analysis of
alternatives in this report, it will be assumed that the Partial Detention Alternative ponds will trap
75 percent of the incoming sediment load and that clean-up costs are proportional to sediment
load.

Volumes of water stored in the seven proposed detention ponds during the 24-hour, lDO-year
storm would range from 0.4 to 2.0 acre-feet. Maximum ponding depths would range from 3.3 to
5.2 feet and inundated area would range from 0.15 to 0.35 acres.

Outlet works for each of the ponds would consist of an 18- or 24-inch diameter corrugated metal
pipe (CMP) through the dam 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. 2 feet wide at
the bottom and 3 feet deep with 2: 1 (horizontal:vertical) side slopes. At the pond outlet and
along steeper reaches of each ditch, rip rap lining would be required.

Details of preliminary design parameters for each of the proposed detention basins are presented
in Table II-I. Note that Detention Basin 1 (see Figure II-2), the largest facility proposed in the
Master Plan, was built in 1995 and is an existing structure. It has been included in Table II-I for
the sake of completeness, but is excluded from the cost estimate for the alternative.

Cost estimates presented in the Master Plan have been updated using bid data from the
construction of Detention Basin 1. Unit costs utilized in developing the updated cost estimates

II-2