Laserfiche WebLink
<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 />-12- <br /> <br />necessary for the 100-year frequency. <br /> <br />reservoir outflow to the existing capacity, approximately <br />200 cfs,*or slightly less than the 2-year storm event, the <br />required storage volume for the 100-year event would be <br />approximately 1100 AF. For the two dam sites investigated, <br />this volume could not be accommodated. <br /> <br />Tabulated below are the present worth of costs and benefits, <br />the net benefit and the B/C ratio for the most economical <br />combination of approaches in this alternate. <br /> <br />P.W. Benefits P.W. Costs* Net Benefits B/C <br /> <br />To facilitate a more realistic ponding scheme, the maximum <br />release rate during a flood event was increased which would <br />require that downstream improvements be made in conjunction <br />with ponding. <br /> <br />Arvada $ 12,200,000 $ 1,400,000 $ 10,800,000 8.7 <br /> <br />Jefferson <br />County 6,000,000 1,600,000 4,400,000 3.8 <br /> <br />*Phase A Estimated Costs <br /> <br />The lower ponding site evaluated is located west of Oak Street <br />extended, on the south side of West 58th Avenue and east of <br />Tabor Street. This site is not well suited for a reservoir <br />of the size required as it is quite flat and would require an <br />extensive embankment in both height and length to generate the <br />needed volume. Site grading would have to be completed and <br />much more right-of-way acquired. <br /> <br />Alternate 3 - The implementation of stormwater detention along <br /> <br /> <br />with the recommendations of Alternate 2 were considered in <br /> <br /> <br />Alternate 3. Stormwater detention included the use of <br /> <br /> <br />major flood control reservoirs on the stream together with the <br /> <br />smaller detention facilities located in the developed areas <br /> <br /> <br />throughout the basin. The primary objective of detention <br /> <br /> <br />or ponding was to reduce the size and cost of needed downstream <br /> <br />improvements. <br /> <br />The area which would benefit most from ponding is the stream <br />reach through Arvada. Through this reach, potential flood <br />damage is the highest. Locating a ponding area immediately <br />upstream of town would serve to reduce the flooding conditions <br />through the town. The reservoir improvements considered were <br />based on eliminating downstream flooding. Since the design <br />hydrograph for the area is typically of long duration with a <br />large volume of runoff, reservoir ponding requirements are quite <br />substantial. <br /> <br />A second reservoir site was considered 3,200 feet west of Ward <br />Road. This site was more feasible in that it provided embankment <br />abutments at a natural valley constriction and also provided <br />adequate ponding volume. To maximize the efficiency of the <br />pond, a side channel spillway was considered. During a flood, <br />waters would not spill into the pond until such time that flood <br />stages were high enough. At that time, hydrograph peaks would <br />be near the pond and would provide an optimum routing situation. <br /> <br />Also, the capacity of exsiting downstream facilites would be <br />the basis for determining the reservoir outflow. Without <br />improving the conduit under the shopping center and limiting <br /> <br />A dam was also considered for the hogback, north of Table <br />Mountain. This site also is an optimum reservoir site, from <br />an engineering standpoint. However, analysis shows that a <br />reservoir at that location generates no benefit due to flood <br />peak reduction in the reaches downstream of the confluence with <br />the South Tributary. <br /> <br />*Phase A Estimate <br />