Phase A - Alternatives Analysis Report - Urban Drainage and Flood ...

PHASE A – ALTERNATIVES ANALYSIS REPORTMajor Drainageway Planning Update for Hidden Lake-Bates Lake6.5 Alternative SelectionAlternatives were evaluated on a reach by reach basis. The most significant problems in each reach wereidentified and alternatives that addressed these problems were evaluated to determine if they could eliminatethe problem in a cost effective manner. The alternative that solved the significant problems in each reachcost effectively was chosen as the recommended alternative.6.5.1 Reach H-1In Reach H-1 the most significant problems are inundation of structures west of Benton Ct., the overtoppingof the Grace Church access road, and the overtopping of Sheridan Blvd. In order to address all of theseproblems, composite Alternatives 5a and 5c were considered. As can be seen on Table 6-4, Alternative 5ahas a benefit-cost ratio of 2.51 and Alternative 5c has a benefit-cost ratio of 0.67. Because the benefit costratio of Alternative 5a is much higher than the benefit cost ratio of Alternative 5c and Alternative 5a alsoprovides water quality in the Arvada Center detention basin, this was the alternative that was selected for therecommended plan.6.5.1 Reach H-2In Reach H-2 the most significant problem is the water quality in Hidden Lake. Since there is very minimalflooding within this reach, it did not make sense to do a benefit-cost analysis for this reach. Alternative 4adoes help improve the water quality within Hidden Lake. In addition, this alternative was supported bycommunity members at the Public Meeting, the local sponsors, and also by the owner of Hidden Lake so itwas included in the recommended plan.plan event though it does not alleviate any flooding and it does not provide any water quality. Since there isalready a detention basin in Homestead Park that attenuates peak flows, no improvements were proposed inthe upper portion of Reach B-1. For Alternatives 5a and 5b, the flooding north of the Union PacificRailroad and the overtopping of the tracks was looked at in both Reach B-1 and B-2. A benefit-costanalysis was done for both Alternatives 5a and 5b. The benefit-cost ratios ended up being very closetogether with 1.77 for Alternative 5a and 1.49 for Alternative 5b. Alternative 5a was selected for therecommended plan because the proposed detention facility at the extension of Depew St. providedadditional water quality in Reach B-1 and the open channel north of the railroad tracks between SheridanBlvd. and Tennyson St. that was proposed in Alternative 5b would have impacted the proposed Gold Linelight rail station east of Sheridan Blvd.6.5.1 Reach B-2In Reach B-2 the most significant problems are the inundation of structures north of the Union PacificRailroad, and the overtopping of the railroad itself. Since the flooding in this reach was considered in thecombination alternatives for Reach B-1, and an alternative was selected that eliminated the flooding in thisreach without any actual improvements in the reach, Alternative 1, the “Do Nothing” alternative wasincluded in the recommended plan for this reach.6.5.1 Reach H-3In Reach H-3 the most significant problems are the water quality in Aloha Beach Lake, and Lake Sangracoalong with the overtopping of Lowell Blvd. at 66 th Ave. A benefit-cost analysis was done for Alternative3a, however the benefit-cost ratio turned out very low, 0.02, because Lowell Blvd is only overtopped instorm events exceeding the 50-year event and no structures are flooded due to the overtopping. However,due to the importance of keeping arterial roadways clear in major storm events to allow emergency vehiclesaccess and the relatively low cost of the improvements, this alternative was included in the recommendedplan. Alternative 4a was considered because it does help improve the water quality within Aloha BeachLake, and Lake Sangraco. In addition, this alternative was supported by community members at the PublicMeeting therefore it was included in the recommended plan.6.5.1 Reach H-4There were no significant problems identified with in Reach H-4 besides the water quality in Aloha BeachLake, and Lake Sangraco which is addressed with Alternative 4a in Reach H-3. Therefore Alternative 1, the“Do Nothing” alternative was included in the recommended plan.6.5.1 Reach B-1In Reach B-1 the most significant problems are the safety of the operation of the Bates Lake inlet and outletstructure, the inundation of structures west of Sheridan Boulevard, and the overtopping of the Union PacificRailroad. Since public safety is a major concern and it was documented by City of Arvada maintenancepersonnel that the grated manhole that acts as both an inlet and outlet for the detention basin shoots up intothe air when the pipes are surcharged in a major event, Alternative 2a was included in the recommended6-5