FLOOD07934 - Laserfiche WebLink

Home Browse Search

City of Fort Collins December 4, 1980 Page 3 benefit/cost ratio must be greater than one for the plan to be economically viable on the basis of flood control. The larger the ratio, the more efficiently funds are being utilized for flood control purposes. Examination of the floodplains and benefit/cost data while keeping in mind the goals of the study leads to the selection of one alternative plan for each reach as the overall most bene- ficial. A review of the flood plain conditions reveals that flood damage in both the Mail Creek and McClellands study reaches is localized, due primarily to specific structures or channel segments which are inadequate for the design flows. Several reaches are located in undeveloped areas and design flows are contained within the existing natural channel resulting in minimal flood damages. Where existing natural channels are adequate, it is because of the basin topography and the fact that upstream development has been orderly and considerate of storm drainage; detention is widespread. Because of the nature of the flood hazards and potential damages in Basins I & K, it is not surprising the the alternatives which have the highest benefit/cost ratios are those which address specific problem areas. The selected structural improvements in Reaches lB, lC, and McClellands, which consist of improvements to culverts and street crossings are the most effective. Similarly, in Reaches 2, 3, 4 and 5, those selected structural alternatives have the highest benefit/cost ratios because of the localized nature of the flood damages. Because of the City's policy requiring detention with development and because detention enhances the B/C ratios in downstream reaches by reducing construction costs, detention alternatives were considered separately and in combination with local structural improvements. Starting with a total of 14 detention ponds in Mail Creek and 3 in McClellands, the number and size of the ponds were systematically reduced until estimated construction costs became'reasonablein comparison with potential benefits. We used the SWM~1 program to model the down- stream effects of the proposed detention ponds. After modeling several arrangements and discharges, three detention ponds in Mail Creek and none in McClellands appeared cost-effective. It is important to note that the cost effectiveness of the detention ponds used strictly for flood control is not that good. In consider- ing implementation of detention alternatives, therefore, multiple use facilities should be envisioned. By using the sites for parks and recreation opportunities, water quality control points, and other activities, the ROW acquisition and construction costs attributable to flood control would be reduced, raising the B/C ratios.