FLOOD10480

• County: Fremont
• Community: Canon City
• Stream Name: Arkansas River
• Basin: Arkansas
• Title: Canon City Flood Control and Drainage Analysis
• Date: 9/1/1996
• Prepared For: Canon City
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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to protect property from IOO-year flow that would overtop the railroad embankment.

3) The construction of a concrete floodwall structure in lieu of an earth levee along the southern banle '

These measures would require significant land purchases, relocation of Riverside Drive and levee
improvements on the north bank of the river to keep property north of the railroad out of the 100-year
floodplain.

A meeting was held at the City's offices on August 12, 1996 at which WRC presented the IOO-year flood
results. It was agreed during the meeting that construction of improvements to contain the 100-year flow was
not economical. WRC was asked to determine the flow that could be carried in the river without overtopping
the existing levee before the diversion weir was installed and to evaluate alternatives that would provide the
same level of protection to the southern bank as there existed before the diversion weir was installed. WRC
determined the magnitude of this flood event to be 11,350 cubic feet per second (cfs) corresponding to a return
period of approximately eighteen years.

Another case was evaluated to determine the flow rate conveyed by the existing channel if the existing levee
were to fail. This case included the effect of the diversion weir. WRC estimates that the existing channel
will convey approximately 7,000 cfs without the existing levee (corresponding to a return period of
approximately 9 years).

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Alternatives were considered that would convey the design flow, but that would allow for overtopping during
higher flows. These alternatives included levee protection by lining the entire face of the southern bank in
the area of concern with roller compacted concrete (RCC) or soil cement. Another alternative considered was
to riprap line the channel bank and construct a RCC or soil cement levee with the base lift flush with
Riverside Drive. According to experts in the cement industry, RCC and soil cement are comparable materials
for this particular application. Thus the recommendation of the use of soil cement over RCC was governed
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by cost. Modifications to the weir itself were not considered.

A typical cross section of the proposed channel improvements for the 11,350 cfs flood is shown in Exhibit
D. The dimensions of the rip rap and granular base were taken from Urban Drainage and Flood Control
District Design Manuals (Reference 7). The existing channel slopes are known to be steeper than the
recommended side slopes of 2 (horizontal) to 1 (vertical). Thus, some soil excavation would be required.

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