Potomac Waterfront Flood Mitigation Study - City of Alexandria

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Technical Analyses Support climate change. The USACE provided a policy circular, titled Water Resource Policies and Authorities Incorporating Sea-Level Change Considerations in Civil Work Programs, dated July 1, 2009, that specifically states: “…engineering designs should consider alternatives that are developed and assessed for the entire range of possible future rates of sea-level change.” The USACE policy discusses the methodology to derive the sea-level rise at a specific location, which is based on an updated and modified National Research Council report from 1986. The methodology involves calculating the sea level rise based on location, year project is built, and expected age of project. The City of Alexandria is a highly urban area, which limits opportunities to provide structural flood mitigation measures. The floodwall proposed in this study provides 3 feet of freeboard above the 100-year flood elevation to meet FEMA’s levee certification requirements. Three feet of freeboard will accommodate the anticipated sea level fluctuations. However, if the City prefers to achieve and retain FEMA levee certification, additional freeboard should be considered. Consideration of sea level rise for the Jones Point berm can be accomplished in the same manner as the floodwall. Another large-scale flood control alternative proposed in this study is the elevated walkway. The elevated walkway height, at an elevation of 6.0 feet NGVD, was selected as the maximum practical height based on topographic information. The intent of the elevated walkway is to mitigate frequent flood events while preserving the look and feel of the waterfront; therefore, increasing the height of the elevated walkway to accommodate the sea level fluctuations will have a direct impact on the intent of the project. The proposed road height was also selected based on topographic constraints; therefore, consideration of sea level rise for this measure is not feasible. 28-JUL-10\\ 4-10
Economic Valuation (Benefit-Cost Analysis) SECTION FIVE: ECONOMIC VALUATION (BENEFIT-COST ANALYSIS) 5.1 DEFINING THE SCOPE OF ANALYSIS The economic valuation used in this study is a benefit-cost analysis (BCA). BCA is a technique to assess the relative desirability of competing alternatives in terms of economics. BCA is based on the economic notion of efficiency—allocating resources where they have the most added value to society. BCA does not incorporate the notion of equity, which relates to the fairness of allocation. The BCA determines the cost-effectiveness of flood mitigation alternatives by calculating the Benefit Cost Ratio (BCR). The BCR compares the net benefits of the project to the total project cost. This analysis helps select which flood mitigation measures to implement. To determine whether or not the alternative is cost-effective, benefits should outweigh the costs, resulting in the BCR equating to at least 1.0. A BCR is not a precise calculation; instead, it relies upon skilled conservative estimates of the parameters involved. The final result should be interpreted as having a wide error range. The net benefits are derived from the Net Present Value (NPV) of the project, thereby incorporating the value the project provides over time. The BCR was calculated for each of the structural mitigation measures. Section 6 summarizes the analyses for each mitigation measure and compares total expected benefits to the total expected costs and provides a resultant BCR for each measure within the appropriate area of study. As the BCR increases, the likelihood that the mitigation measure will be accepted increases. For this study, the benefits were defined as the flood damages mitigated by a specific structural mitigation alternative. In general, flood damages were divided into direct building damages, contents damages, and indirect losses. Direct building damages include any damage to the physical structure, cost of replacing utilities (e.g., electrical wiring, telephone), and restoring the structure to a pre-flood condition. Methodology for valuing residential properties differs from that of commercial properties. For example, content damages were any damages to personal or commercial property within a structure. For residential properties, contents include furniture, appliances, housewares, etc. For commercial properties, contents include any office equipment, retail stock, etc. Indirect losses were lost income (business losses) or costs incurred by a resident when they are unable to occupy their home (residential displacement). The project costs were derived from conceptual designs, specific contractor estimates in the case of certain floodproofing alternatives, property fair market values, and various administrative costs. The concept designs were developed using accepted engineering standards and codes, existing data, and engineering judgment. A unit measurement of the total materials and labor costs is calculated. Finally, unit costs values from national construction cost code guides were used to created total project costs. For acquisition, the fair market value was the main project cost. A more detailed discussion regarding the costs for each structural alternative is discussed later in this section. Results from the BCA are included as Appendix E. 28-JUL-10\\ 5-1
Page 2 and 3: F I N A L R E P O R T POTOMAC RIVER
Page 4 and 5: TABLE OF CONTENTS 6.1.2 Assumptions
Page 6 and 7: TABLE OF CONTENTS FIGURES Figure 1-
Page 8 and 9: ac acre A/E Architectural/Engineeri
Page 10 and 11: TES Transportation and Environmenta
Page 12 and 13: SECTION ONE: INTRODUCTION 1.1 POTOM
Page 14 and 15: Introduction Factory and Strand Str
Page 16 and 17: Refinement of Mitigation Measures S
Page 18 and 19: 8.8 ft Figure 2-2: Flood Levels Stu
Page 20 and 21: Refinement of Mitigation Measures T
Page 22 and 23: Refinement of Mitigation Measures s
Page 24 and 25: Overview of Flood Mitigation Measur
Page 26 and 27: 3.1.3 Floodproofing Overview of Flo
Page 28 and 29: FLOOD ELEVATION BASE FLOW Overview
Page 40 and 41: E LURAY AV E GLENDALE AV EARL ST N
Page 42 and 43: N ALFRED ST MUIRS CT S COLUMBUS ST
Page 44 and 45: Technical Analyses Support resource
Page 46 and 47: Table 4-1: Repetitive Loss Properti
Page 50 and 51: 5.2 CALCULATION OF PROJECT BENEFITS
Page 52 and 53: Economic Valuation (Benefit-Cost An
Page 54 and 55: Economic Valuation (Benefit-Cost An
Page 56 and 57: 6.1 FLOODWALL Structural Mitigation
Page 58 and 59: Structural Mitigation Measure Conce
Page 62 and 63: 6.1.3 Potential Impacts Structural
Page 64 and 65: VDEQ Section 401 Certification VDE
Page 66 and 67: 6.2.1 Description of Alternative St
Page 68 and 69: CLIENT PROJ RAMSAY AL SWIFTS AL S L
Page 72 and 73: 6.2.4 Permitting/Approval Requireme
Page 74 and 75: 6.2.5 BCA and Results Structural Mi
Page 78 and 79: 6.3.2 Assumptions Structural Mitiga
Page 82 and 83: CLIENT PROJ REVISION NO SCALE WALES
Page 86 and 87: Site Plan Approval Structural Mitig
Page 88 and 89: North Union Structural Mitigation M
Page 92 and 93: CLIENT PROJ REVISION NO SCALE S ROY
Page 94 and 95: King Street Residential Structural
Page 96 and 97: Waterfront Commercial Structural Mi
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Structural Mitigation Measure Conce
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Structural Mitigation Measure Conce
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7.2 KING STREET Elevated Walkway Re
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Figure 7-2: Elevated Walkway at Wat
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Recommendations The elevated walkwa
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King/Strand Street Intersection Roa
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Figure 7-6: Floodwall behind Torped
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7.7 REPETITIVE LOSS PROPERTIES Reco
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Recommendations (USBC) and Internat
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SECTION EIGHT: REFERENCES Access Fl
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References Huber Technology, 2009.
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Appendix A Initial Ranking of Poten
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Appendix B Ordinance and Sandbag In
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Exhibit 1-2. Sample ordinance from
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5. The necessity to the facility of
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Appendix C Non-Prioritized Measures
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Figure 1: Duckbill Check Valve A ty
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Relocate external electrical boxes
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Last, Hurricane Preparedness inform
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Potomac River Waterfront Flood Miti
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Figure A1-12 Exhibit 2: Field Visit
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Exhibit 2: Field Visit Summary - Ju
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Fig A11-1 Exhibit 2: Field Visit Su
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Figure A18-1 Exhibit 2: Field Visi
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Figure 21-3 Exhibit 2: Field Visit
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Figure 2: Screen shot from NOAA Atl
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Potomac River Waterfront Flood Miti
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38 40 38 4 6 40 34 28 14 30 30 40 4
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46 44 38 44 42 40 42 44 44 36 22 36
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48 44 38 36 38 18 46 42 38 42 26 22
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CLIENT PROJ REVISION NO SCALE City
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Table 4: Detailed cost estimate for
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Exhibit 3: Various Elevated Walkway
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Exhibit 4-2: Elevated Walkway - Kin
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Appendix H Berm
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Roadway Drainage Additional Data Ta
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Appendix J Property Acquisition
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Appraisals Based on data from Appra
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Exhibit 2. Total Other Costs for Pr
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Table 4: North Union Acquisition Co
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Appendix K Floodproofing
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Exhibit 2. Window Replacement Costs
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Elevate or relocate the electric pa
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Overview Potential Federal Funding
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Table 1. HMA Funding Options by Pro
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Hazard Mitigation Assistance The De
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program comparisons (continued) ELI
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program information GovDelivery Not
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