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ALTERNATIVE EVALUATION V. I I I I off-channel detention located off the main stream and used only when flow above a specified design amount is exceeded. Two sites were identified which offered possibilities for construction of new detention storage. One site is located in the upper portion of Denver West upstream ofI-70, and the other site is located in Jefferson County Openspace downstream of West 6th Avenue and adjacent to Mount Vernon Road. Development Alternative development consists of identifying possible improvement options and determining their feasibility. Options which are determined to be feasible will be combined into alternative plans. The best alternate plans are evaluated on the basis of benefits versus implementation costs I Acquire Prope : This option involves identification of residential or commercial structures in the floodplain whose removal would provide a significant reduction in total damages. This alternative was determined to be potentially feasible in reach L-5. Relocate Channel: This alternative requires the relocation or realignment of the channel'S original flow path. This option typically could be used to direct flood flows from a developed area into an undeveloped area, thereby reducing flood damages. No areas where this alternative appeared applicable were identified in Upper Lena Gulch and therefore no additional evaluation of this alternative was performed. Several options were evaluated in each reach during an initial screening analysis. Table 7 shows a matrix of the alternates considered in each reach. The screening analysis consisted primarily of a qualitative assessment of the feasibility of each improvement option. Factors such as the estimated damage reduction, the existing land use adjacent to the drainageway, property owner concerns, and past improvements made to the drainageway were considered, The following is a brief description of the alternatives shown in the Alternative Screening Matrix in Table 7 I I I Flood Warning System: This option generally involves the installation of precipitation and/or streamflow gaging equipment and development of a mechanism whereby residents along a drainageway can be alerted in the event that flood flows are imminent. Portions of Lena Gulch already have a flood warning system in operation. Although not specifically targeted for evaluation, the possibility of expanding the existing flood warning system should be factored into floodplain management decisions the Maintain Existing Condition: This alternative involves leaving the drainageway in its present condition. This is status quo in every reach since it involves no improvement costs. This is also the option against which all other improvements are measured. I section to provide several reaches involves enlarging an existing natural channel identified as a potentially feasible solution in This alternative This alternative was Enlarged Natural Channel adequate conveyance capacity. I Flood ement: This alternative requires proper regulation of development in the basin to insure that no practices occur which increase runoff or cause additional damage. Proper floodplain management should be considered as an integral part of all alternative improvement plans. This alternative provides additional capacity in channel sections by construction of hard- concrete. This alternative is not considered feasible for future improvements to Enlarged Lined Channel lined sides and bottom such as Upper Lena Gulch I structures in natural channels to stabilize the is usually necessary in steep channel sections This alternative was considered potentially to other improvement options. Erosion Control: This alternative involves the installation of control channel and reduce erosion damages. The application of this option where flow velocities are expected to be high such as in Apex Gulch. feasible in several locations within the study area due to its low cost relative Enlarged Existing Storm Sewer: This alternative consists of replacement of an existing storm sewer system, or adding additional pipes to the existing system, to provide more capacity. Because nearly all of Upper Lena Gulch consists of an open channel system, this was not considered applicable and was not studied further. I Construct New Storm Sewer: This alternative consists of constructing a storm sewer system in an area where there is not an existing system. This alternative was not considered feasible and was not studied further, I Limited Structural Improvements: Generally, limited structural improvements involve minor modifications to existing facilities which yield a reduction in damages from flooding. Examples might be constructing small berms or performing minor grading at certain structures to prevent them from being damaged, Locations in the Upper Lena Gulch drainageway were identified where limited structural improvements were potentially feasible alternatives. Therefore, additional evaluations of this alternative were made Improve Street Crossings: This alternative involves enlarging or making improvements to a bridge or culvert increase the structure capacity. Several structures throughout the study area have been identified which have inadequate conveyance capacity. This alternative was determined potentially feasible for many of those structures and required additional evaluation. to I I Following an evaluation of the alternative screening matrix by the project sponsors, several specific alternatives in each reach were identified for further study. Each feasible conveyance alternative was evaluated at various design discharges Enlarge E Detention Stor3J!:e: This alternative provides additional detention storage to reduce peak flood flows by enlarging an existing detention pond. Either raising the height of the embankment or providing additional storage volume through excavation could be included in this alternative. The possibility that some additional storage volume could be provided at Denver West by raising the pond embankments was identified as a possible option. However, after comparison of the inflow hydrograph volume to the existing pond storage volumes, this alternative was considered not feasible and was not studied further I Benefit-Cost Analysis The more detailed evaluation of the various alternatives identified during Alternative Development was performed using the benefit-cost approach. This approach enables the benefits associated with various alternatives to be weighed against the implementation costs. If a favorable benefit-cost ratio is achievcd. the alternative can be 25 and construction of new stream or sites rcctly on the main identification of potential dctcntion locatcd d involves include possible on-channel New Detention Storage: This alternative storage facilities. This would Construct detention I I I