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<br />flows exceeding the 1300 cfs capacity of the Fountain Creek channel into the <br />lliversion tunnel. This water would then be released into a stilling basin <br />in order to dissipate energy before reentering the Fountain Creek channel. <br />Capacity of the diversion tunnel would be 7800 cfs, which is equivalent to a <br />fifty year flow. Total costs for this solution are estimated at over <br />$17,000,000. A larger diversion tunnel capable of carrying a higher <br />frequency flood would naturally have a higher cost associated with it. Any <br />further consideration of this alternative would require a detailed <br />floodplain analysis to determine the impact of this sol ution on residents <br />along Black Canyon. <br /> <br />b. Large Detention Structure. The valley through which Fountain Creek <br />flows just above Manitou Springs is very steep and narrow, making it <br />difficult to obtain much storage for any possible detention structure. Any <br />structure over 30 to 40 feet high would require the relocation of U.S. <br />Highway 24. This would also be difficult due to the narrow valley and the <br />large road cuts that would be necessary. For this reason, pOSSible <br />locations of detention structures were investigated farther upstream. One <br />possible site is about one mile upstream of the community of Crystola. <br />Contributing drainage area above this location is 4.2 square miles. The <br />maximum possible height of any structure without affecting U.S. Highway 24 <br />at this location is approximately forty feet. This would result in a storage <br />(lapacity of 250 acre-feet, less than a ten year volume. There is also a <br />llecond poSSible site about 3,000 feet below Crystola with a contributing <br />drainage area of 5.2 square miles. Thi s location would allow a maximum <br />height of 80 feet before affecting U.S. Highway 24. Capaci ty would be 500 <br />acre-feet which is about the 25 year volume. Even if both detention dams <br />oould be built, they would not provide adequate protection for Manitou <br />Springs. No other adequate sites were found for a large detention structure <br />1~hich did not require maj or relocations. <br /> <br />A Corps of Engineers report enti tIed "Arkansas River Above John Martin <br />Dam Survey Report" wri tten in 1968 had previously examined the feasibility <br />of constructing a dam on Fountain Creek two miles west of Manitou Springs. <br />This report also found that the costs associated wi th relocating U.S. <br />Highway 24 made such a project too costly. <br /> <br />c. Channel ization Project. Channelizing Fountain Creek through Mani tou <br />Springs was considered. Construction costs would be high because a <br />(loncrete-lined, high veloci ty channel would be required to handle the design <br />flow of 40,000 cfs (SPF). Channel projects allowing for protection from the <br />~56 year flow (10,000 cfs) and the 17 year flow (5000 cfs) were also <br />studied. Problems with these alternatives are mainly associated with <br />obtaining rights-of-way through the heavily developed downtown area of <br />Manitou Springs. Alterations to buildings and relocating bridges were other <br />areas of concern. The estimated cost of the SPF project was $32,000,000 <br />(1967 cost) while the 56 year and 17 year projects have an estimated cost of <br />$10,500,000 and $9,000,000 (October 1984 cost), respectively. <br /> <br />d. Small Dams. Small dams were examined as possible methods of <br />controlling the flood flows from Ruxton and Sutherland Creeks as well as <br />l.illiams and Waldo Canons. The investigation showed that because of the <br />steep slopes of the terrain the number of dams needed would be extremely <br />,high in order to store the necessary volume of water (100 year flow). This <br />would cause the construction costs to be high as well. <br /> <br />-26- <br />