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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The hogback drainages tributary to the proposed detention ponds constitute approximately 13
percent of the North Ninth Street Drainage Basin as shown in Figure II-I. The runoff curve
number of 84 assumed in the Hogback Drainage Master Plan is similar to values which would be
appropriate for the residential and commercial areas downstream. The high curve number value
is due to the relatively impermeable shale of which the hogbacks are composed. Assuming that
the hogback curve number is approximately equal to the average curve number for the rest of the
North Ninth Street Basin, some general conclusions can be drawn regarding storm flows and the
relative impact of alternatives.

. The hogback drainages tributary to the proposed detention ponds account for
approximately 13 percent of the total runoff generated in the North Ninth Street basin.
. Adopting the Full Detention Alternative over the Partial Detention Alternative would
provide approximately an additional 6 percent reduction in peak stormwater flow in the
North Ninth Street Basin during a 24-hour, general type storm. Percent reductions for
short duration summer thunderstorm events should be somewhat greater than 6 percent.

Cost estimates for each of the structures proposed under the Full Detention Alternative are
provided in Table II-6. Note that costs for enlarging Basin I, which was constructed in 1995 to
Partial Detention Alternative design specifications, have been included. Land acquisition costs
and assumptions are the same as for the Partial Detention Alternative. Including a 25 percent
contingency, engineering, administration and land acquisition costs, the total estimated cost for
the Full Detention Alternative is $637,000.

Additional Alternatives Considered

In addition to these three alternatives which have been examined in detail, several other concepts
were analyzed for feasibility and subsequently discarded.

The possibility of creating a single large detention basin rather than a series of smaller basins was
considered. If a single basin were to be used, flows from each of the individual drainage areas
between the hogbacks would have to be collected and transported across topographic boundaries
to a detention site at the south end of the hogback area. There is a general downhill slope from
north to south in the hogback area, but the required crossing of topographic ridges would involve
a substantial amount of earthwork including cut depths exceeding 25 feet. It was judged that the
cost of such earthwork would be prohibitive and the concept was discarded.

Excavation of an interceptor channel along the base of the larger, western hogback was also
considered. Such a channel would intercept runoff generated on the steep east face of the
hogback. The intercepted area would constitute approximately 50 percent of the total hogback
drainage area. The channel would flow south toward a single large detention basin, accumulating
flows along the way. Stormwater flows generated on portions of the drainages located below the
interceptor channel would not be controlled or mitigated. Given the steep slope of the hogback
and the shale rock in which the channel would have to be excavated, the cost of this approach
would be prohibitive. Additionally, the visual impact of the channel would be undesirable.
Because of the high cost and negative aesthetic impacts, this concept was also discarded.

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