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

PHASE A – ALTERNATIVES ANALYSIS REPORTMajor Drainageway Planning Update for Hidden Lake-Bates Lake7 RECOMMENDED PLAN7.1 Plan DescriptionThe recommended plan for the Hidden Lake and Bates Lake watersheds is a combination of selectalternatives from each of the reaches. The improvement alternatives included in this plan include; themaintenance of existing drainageway facilities, regional detention facilities, major underground conduits,roadway culvert improvements, water quality ponds, removing directly connected impervious surfaces, andincorporating water quality into proposed detention facilities. The recommended plan is presented in Table7-1 and on Figures 7-1 through 7-6.In the existing condition there are two street crossing locations that are overtopped during multiple stormevents along the main stem flow path of the Hidden Lake; Sheridan Boulevard at 69 th Avenue and 66 thAvenue at Lowell Boulevard. In the existing condition there is one street crossing that is overtopped duringmultiple storm events along the main stem flow path of the Bates Lake watershed; Tennyson Street at theUnion Pacific Railroad crossing. The recommended plan eliminates the overtopping at these three locationsfor all storm events up to and including a 100-year storm event.Multiple structures were identified as being inundated in multiple storm events within the study area in theexisting condition. The structures identified as being inundated in the hydraulic analysis for this study alongHidden Lake Reach H-1 will be removed from the 100-year floodplain with the improvements proposed inthe recommended plan. The structures identified as being inundated in the hydraulic analysis for this studyalong Hidden Lake Reach H-2 will not be removed from the 100-year floodplain since they are immediatelyadjacent to Hidden Lake and no flood reduction improvements are proposed to Hidden Lake as a part of thisstudy. There are no structures identified as being inundated along Hidden Lake Reach H-3. There are onlytwo structures identified as being inundated along Hidden Lake Reach H-4. These structures are along theeast bank of Lake Sangraco the will not be removed from the 100-year floodplain since no improvementsare proposed to Lake Sangraco as a part of this study. The structures identified as being inundated in thehydraulic analysis for this study along Bates Lake Reach B-1 will not be removed from the 100-yearfloodplain since no improvements are proposed to reduce flooding in that reach as a part of this study. Thestructures identified as being inundated in the hydraulic analysis for this study along Bates Lake Reach B-2will be removed from the 100-year floodplain with the improvements proposed in the recommended plan.7.2 Operations and MaintenanceOperation and maintenance costs have been included in the alternative cost estimates. Since the studycovers such a large area, only existing facilities immediately adjacent to proposed alternatives are beingconsidered for the “Do Nothing” alternative operations and maintenance costs.7.3 Recommended Plan CostsA summary of costs associated with the recommended plan, the existing flooding damages, the floodingdamages after the recommended plan is implemented, and benefit-cost ratios are presented in Table 7-1.The costs have been broken into sub-categories including Structural Improvements; Traffic Control; UtilityRelocations; Property Acquisition; Engineering, CM, and Contingencies; and Operation and Maintenance.The overall cost of the recommended plan is $3,525,006.7-1