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<br />0=2198 <br /> <br />-11 <br /> <br />The diversion will consist cf a compacted moisture-controlled <br />earthfill dike and an unlined vegetative channel approximately 2,000 <br />feet in length., It will divert the peak rate of runoff for a 100-year <br />frequency storm from a small (0.10 square-mile) unreservoired tributary <br />of Fishers Peak Arroyo into structure FPC-l at a maximum rate of 160 <br />cubic feet per second. <br /> <br />The stream channel improvement will consist of the following <br />items: (1) Excavation will be made to eliminate the rough spots and <br />grade inequalities in the existing floor of the closed conduit on <br />Fishers Peak Arroyo. This excavation will be filled with a reinforced <br />concrete floor slab of uniform thickness, which will give the conduit <br />a greatly improved coefficient of roughness throughout its entire length <br />and will provide a safe capacity for peak flows. (2) A reinforced <br />concrete and rock stilling basin will be constructed at the lower end <br />of the closed conduit to stabilize the flow within the conduit and to <br />control erosion downstream. <br /> <br />BENEFITS FROM WORKS OF IMPROVEMENT <br /> <br />Benefits were estimated from the reduction in damages with the project <br />installed. Flood damages were computed for all probable flood events on <br />Fishers Peak and Carbon Arroyos under present conditions and with the <br />project installed. With the project installed, flood events of less <br />magnitude than the project design flood (0.3 per cent chance event) will <br />cause no overbank flooding in Trinidad, and flood events of greater <br />magnitude than the project design flood will be materially reduced. <br /> <br />With the structures installed on Fishers Peak Arroyo, the one per <br />cent chance flood of 1,700 c.f.s. at Trinidad will be reduced to 950 <br />c.f.s., which is within the improved channel capacity and will leave a <br />margin of safety in the capacity of the improved conduit to prevent <br />build up of pressure flows. On Carbon Arroyo, with the structure <br />installed, the one per cent chance flood of 1,150 c.f.s. will be reduced <br />to 700 c.f.s., which is within existing channel capacity. The land <br />treatment measures to be applied will reduce land loss from gullying and <br />will reduce sediment production above structure sites by an estimated <br />10 per cent. <br /> <br />" <br /> <br />It is estimated that the average annual damages will be reduced <br />from $24,863 to $4,908 with the project in operation. The remaining <br />damages consist of minor damages to railroad bridges, road bridges, <br />and channels from floods less than channel capacities. There will also <br />be a small amount of damages from overbank flow occurring as a result <br />of runoff from the uncontrolled areas below the structures during floods <br />of greater magnitudes than the project design flood. At present there <br />are 14 blocks in the business district of Trinidad (see Figure 4) subject <br />to damages from floods on Fishers Peak and Carbon Arroyos. With the project <br />installed flood events of lesser magnitude than the project design flood <br />will cause no overbank flooding; however, there are four blocks (see <br />Figure 4) in the lower part of the buaness district which are subject <br />to damages from storm runoff originating within the City. Improved <br />storm water disposal facilities should be provided by the City of Trinidad <br />to prevent these damages. <br /> <br />An itemized account of the monetary damages and benefits expected <br />to accrue are shown in Table 7. <br /> <br />