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Last modified
1/26/2010 10:12:30 AM
Creation date
10/5/2006 5:01:15 AM
Metadata
Fields
Template:
Floodplain Documents
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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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />, I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />are presented in Table II-2. Updated cost estimates for each of the proposed basins are presented <br />in Table II-3. The land acquisition cost of $54,810 presented in the table assumes that three <br />parcels of land totaling 78.3 acres will be purchased from their respective owners at a cost of <br />$700 per acre. Land acquisition costs are based on purchase prices for similar pieces of property <br />recently sold in the area. It has also been assumed that the State of Colorado will grant Canon <br />City an easement at no cost on the property where Detention Basin 9 is planned. Including a 25 <br />percent contingency, engineering, administration and land acquisition costs, the total estimated <br />cost for the Partial Detention Alternative is $319,000. <br /> <br />Full Detention Alternative <br /> <br />This alternative utilizes the same seven detention ponds as the Partial Detention Alternative, but <br />the ponds have been enlarged approximately 300 percent to provide storage for the full volume <br />of the 24-hour, lOO-year storm hydrograph. The additional volume would be provided through a <br />combination of excavation and increased dam height. The proposed height of each of the dam <br />structures across hogback openings would be increased to 10 feet at the weir crest. By limiting <br />the dams to this height, all would be non-jurisdictional structures and not subject to State review <br />or inspection. Small diameter outlets would reduce peak flows from the hogback drainage basins <br />to a maximum of 3 cfs. Outlet works would consist of 12-inch diameter CMP's with 6-inch <br />diameter orifice plates restricting the entrances. The lower peak discharge rates associated with <br />the larger ponds would substantially reduce the required dimensions for outlet channels. A small <br />trapezoidal ditch, I foot wide at the bottom, I foot deep and with 2: I (horizontal:vertical) side <br />slopes should be adequate. Preliminary design parameters for each of the six proposed basins <br />and their outlet works are provided in Table Il-4. <br /> <br />Drain times for the detention ponds would be significantly longer under the Full Detention <br />Alternative because of the larger volume stored and smaller diameter outlet. Table II-5 shows <br />computed drain times for each of the ponds under the Full Detention Alternative. Drain times for <br />the Partial Detention Alternative would be 20 minutes or less for all storm scenarios presented in <br />Table II-5. Long drain times for the ponds could be problematic if they resulted in street flooding <br />which was sustained for an extended period. This is not expected to be a concern under the Full <br />Detention Alternative because the small discharge rates maintained after the storm (less than 3 <br />cfs) would easily be accommodated by the City's stormwater drainage system. Long drain times <br />can also be problematic from the standpoint of accommodating back-to-back storms. In the case <br />of the Full Detention Alternative, drain times for the lOO-year storm (both the general and <br />thunderstorm type event) are less than 24-hours for six out of the seven proposed basins. Basin <br />I, the largest of the proposed basins, drains in 1.6 days following the lOO-year, 24-hour storm <br />and in 1.3 days after the lOO-year, 3-hour storm. Based on this data, back-to-back storms are <br />thought not to be a significant concern. <br /> <br />The longer detention times provided by the Full Detention Alternative should assure that <br />essentially all of the incoming sediment load will be trapped by the ponds. For the purpose of <br />benefit/cost analysis, it will be assumed that 100 percent of the sediment load will be trapped. <br /> <br />1I-3 <br />
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