PHASE A – ALTERNATIVES ANALYSIS REPORTMajor <strong>Drainage</strong>way Planning Update for Hidden Lake-Bates LakeAlternative PlanBates Lake WatershedTable 6-2: Reach B-1 Combination of <strong>Alternatives</strong> Cost EstimatesStructuralImprovementsTrafficControlUtilityRelocationsPropertyAcquisitionREACH B-1: Watershed Headwaters to Railroad Tracks at Sheridan Boulevard5a5bCombination of<strong>Alternatives</strong> 2b, 4c,3a & 3b.Combination of<strong>Alternatives</strong> 3a &3b from Reach B-1<strong>and</strong> 3a & 3b fromReach B-2.Engineering,CM, &ContingenciesOperation &MaintenanceAlternativeTotal$782,896 $63,752 $71,021 $108,000 $274,014 $29,000 $1,328,683$1,088,108 $108,811 $108,811 $38,475 $380,838 $32,000 $1,757,0436.3 <strong>Flood</strong> Damage EvaluationAs mentioned in Section 4.4, flood flows overtop the channel banks in some of the reaches <strong>and</strong> the existingpipes <strong>and</strong> street cross sections do not have enough capacity to convey runoff is some of the lager stormevents. In order to determine how much damage would be done in each storm event, a depth of floodinganalysis was done along both the Hidden Lake mainstem <strong>and</strong> the Bates Lake mainstem. As part of thisanalysis the depths of flooding for each of the structures along the mainstems was determined. After theflood depths at each of the structures were determined, a percent damage of the structure was determinedfrom the <strong>Flood</strong> Insurance Administration (FIR) Depth-Building Damage Data Table. A portion of the tableis included in Table 6-3. The structure values for each of the inundated structures were acquired from theAdams <strong>and</strong> Jefferson County Assessors offices. A listing of the inundated building in each of the stormevents studied can be seen in Appendix F on Tables F-1 through F-6. The buildings are identified by theirparcel ID <strong>and</strong> their address. It is identified in the tables what type of buildings are affected, the actual valueof the structure, the source of the value estimate, the depth of flooding at the structure, the percent damage,<strong>and</strong> the actual value assumed for the damages in each storm event. The FEMA st<strong>and</strong>ard default value of30% for the contents of structures was used in the benefit cost analysis.Table 6-3 : Building Damage Percent1 or 2 Story Structure with<strong>Flood</strong> DepthBasement(ft) (%)-2 4-1 80 111 152 203 234 28is overtopped at 66 th Avenue. Along the Bates Lake mainstem, Tennyson Street is overtopped just north ofthe Union Pacific Railroad crossing. First it was determined in which storm events each of the roads wouldbe overtopped. Next it was determined for how long of a time the road would be inundated <strong>and</strong> how long itwould take travelers to detour around the flooded intersection. Last the average daily traffic on each of theaffected roadways was obtained from the local sponsors. All of these values were input into FEMA’sLimited Data Module: Benefit-Cost <strong>Analysis</strong> of <strong>Flood</strong> Mitigation Projects to determine a monetary value forthe loss of function at each of the affected roadways. All of the assumptions made <strong>and</strong> information used forthe loss of function evaluation can be seen in Tables F-7 through F-9 in Appendix F.6.4 Benefit-Cost <strong>Analysis</strong>The benefit-cost analysis is a tool used by policy makers to help decide if a project, or a series of projects, isa reasonable expenditure of public funds. The concept of this analysis is that if the benefits exceed the costs,it makes sense to build the project(s).The result of a benefit-cost analysis is typically the benefit-cost (B-C) ratio. The B-C ratio is the sum of allthe benefits divided by the sum of all the costs. B-C ratios greater than one immediately indicate the projectis worthy of investment since the tangible benefits are greater than the costs. Projects with a B-C ratio lessthan 1.0 may be viable to meet certain community “level-of-service” expectations during rainfall events.The benefits <strong>and</strong> costs are measured in dollars, <strong>and</strong> while the costs of alternatives can generally bedetermined with reasonable accuracy, the benefits are not always as simple to quantify in terms of dollars.Therefore, a listing of benefits not included in the B-C ratio can have equal or even greater importance topolicy makers as the computed ratio.Although many items are considered in computing project benefits, the largest item is the reduction ofdamages to structures <strong>and</strong> content. And typically, the greatest damage reduction occurs when flooding isexperienced during frequent storm events such as the 2, 5-, <strong>and</strong> 10-year storm events. Increased damages doresult from larger storms; however their frequency of occurrence is significantly lower when compared tothe more frequent flooding events.A benefit-cost analysis was done for the alternatives or combination of alternatives that help to reduceflooding along the Hidden Lake <strong>and</strong> Bates Lake mainstems. There were seven benefit-cost analysis run, thealternatives that were analyzed are Reach H-1 Alternative 5a, Reach H-1 Alternative 5b, Reach H-1Alternative 5c, Reach H-1 Alternative 5d, Reach H-3 Alternative 3a, Reach B-1 Alternative 5a, <strong>and</strong> ReachB-1 Alternative 5b. The print outs from FEMA’s Limited Data Module: Benefit-Cost <strong>Analysis</strong> of <strong>Flood</strong>Mitigation Projects are included in Appendix F for each of alternatives analyzed. A summary of thealternatives costs, pre <strong>and</strong> post project damages, <strong>and</strong> the benefit-cost ratio for the recommended plan <strong>and</strong> therest of the alternatives which a benefit-cost analysis was run can be seen in Table 6-4.A loss of function was also taken into consideration when doing the flood damage evaluation. The loss offunction is an important factor to consider when doing flood damage evaluations because if a major road isflooded in a storm event, emergency response vehicles will be delayed. There are two road crossings alongthe Hidden Lake mainstem <strong>and</strong> one along the Bates Lake mainstem that are overtopped during storm events.Along the Hidden Lake mainstem, Sheridan Boulevard is overtopped at 69 th Avenue <strong>and</strong> Lowell Boulevard6-2
PHASE A – ALTERNATIVES ANALYSISMajor <strong>Drainage</strong>way Planning Update for Hidden Lake-Bates LakeStructuralImprovementsTrafficControlAlternative PlanREACH H-1: Watershed Headwaters to Sheridan Boulevard5a Construct detention basin withwater quality in open space atthe Arvada Center. Upsize boxculvert under Sheridan Blvd.Raise the Grace Churchaccess road <strong>and</strong> install a boxculvert.UtilityRelocationsTable 6-4: Alternative Plans Costs <strong>and</strong> Benefit RatiosPropertyAcquisitionEngineering, CM,& ContingenciesOperation &MaintenanceAlternativeTotal CostsTotalAnnualizedDamages *Total AnnualizedDamages AfterMitigationBenefit-Cost Ratio$965,596 $89,975 $59,983 $15,585 $337,959 $47,000 $1,516,098 $384,670 $0 2.515b Convert the pond inFaversham Park into aregional detention basin.Upsize box culvert underSheridan Blvd. Raise theGrace Church access road <strong>and</strong>install a box culvert.$746,915 $94,420 $62,946 $15,585 $261,420 $24,000 $1,205,286 $384,670 $377,934 0.065cConstruct additionalconveyance betweenWadsworth Blvd. <strong>and</strong> SheridanBlvd. Upsize box culvert under$4,705,677 $303,307 $470,568 $15,585 $1,646,987 $57,000 $7,199,124 $384,670 $0 0.67Sheridan Blvd. Raise theGrace Church access road <strong>and</strong>install a box culvert.5d Upsize box culvert underSheridan Blvd. Raise theGrace Church access road <strong>and</strong>$680,232 $102,035 $68,023 $15,585 $238,081 $17,000 $1,120,956 $384,670 $377,934 0.07install a box culvert.REACH H-2: Hidden Lake4a Construct wetl<strong>and</strong> east ofSheridan Blvd, south of 69th $200,272 $0 $0 $0 $70,095 $10,000 $280,367 0.00Ave.REACH H-3: Between Hidden Lake & Lake Sangraco along Lowell Blvd.3a Upsize the elliptical pipesunder 66th Ave.$127,583 $6,379 $12,758 $0 $44,654 $3,000 $194,374 $334 $0 0.024aRemove the low flow concretechannel <strong>and</strong> replace with soil$107,460 $5,373 $0 $0 $37,611 $4,000 $154,443 0.00riprap <strong>and</strong> check structures.REACH H-4: Lake Sangraco to Clear Creek1a Do nothing $0 $0 $0 $0 $0 $0 $0 0.00NotesImproves water quality,eliminates structural damage inthe 100-year event, <strong>and</strong>eliminates overtopping ofSheridan Boulevard in the 100-year event.Eliminates overtopping ofSheridan Boulevard in the 100-year event.Eliminates structural damage inthe 100-year event, <strong>and</strong>eliminates overtopping ofSheridan Boulevard in the 100-year event.Eliminates overtopping ofSheridan Boulevard in the 100-year event.Improves water quality.Eliminates flooding of LowellBoulevard in the 100-year event.Improves water quality.6-3
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