Potomac Waterfront Flood Mitigation Study - City of Alexandria

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Refinement of Mitigation Measures Type of Measure Solution Description S6. Improve flapgate operation at outflow points. S7. Add sump pumps. Underground storage S8. Create areas for underground storage. Detention structures S9. Provide detention/retention structures. Construct an offshore groin S10. Construct an offshore groin. Prevention Emergency Services Public Education and Awareness Sewer backflow preventers Enhancement of floodplain ordinances Minimizing electrical and gas outages after a flood ID system Flood warning system PR1. Add backflow preventers in homes to prevent stormwater (and sewer) backups. PR2. Improve the City’s floodplain and zoning ordinances. ES1. Isolate service so that only the buildings affected by flooding would have service turned off. ES2. Improve/enhance existing business identification system for returning to impacted area. ES3. Implement system to provide text messages, announcements, and/or phone messages regarding the status of the flooding. Emergency response ES4. Improve the City’s emergency response. Temporary structures ES5. Provide sandbags or other flood deterrents for residents and businesses. Cleanup program ES6. Provide improved cleanup program. Media involvement and outreach Transportation plans Insurance outreach 2.3 REFINEMENT OF MITIGATION OPTIONS EA1. Provide education to area media outlets about what is causing the flooding, how to avoid flooded areas, and what in Alexandria would remain open and accessible. EA2. Create maps, provide signs, and help erect barriers (that would be only as large as needed) to show visitors and residents how to navigate the streets and show what businesses and parking areas remain open. EA3. Inform business and residents about reimbursement for damages other than just exterior building damages. As part of this study, the mitigation alternatives in Table 2-2 were ranked using a numerical evaluation criteria to select flood mitigation measures for further consideration. The criteria, along with the respective weighting, are shown in Appendix A. City staff reviewed and approved the scoring matrix criteria. The criteria that were considered in the ranking are listed below, with the heaviest ranked criteria listed first: Reduction of Flooding Extent Cost to Property Owners 28-JUL-10\\ 2-5
Loss of Business Revenue Aesthetics Constructability City Liability Effect on Potomac River Viewshed Private Property Acquisition State/Federal Funding Repetitive Loss Property Mitigation Cost of Flood Insurance Property Ownership General Environmental Impacts (wetlands, forested areas) Loss of Recreational Use Historic/Archaeological Resources Regulatory Requirements Refinement of Mitigation Measures Each criterion was given a weighting factor, based on the goals of the overall flood mitigation study. A heavier weight was given to options that would prevent more frequent floods. Measures that protect against frequent floods typically provide the greatest cost benefit, because those floods occur more often and result in extensive damages. Second, mitigation measures that provide protection to a large number of structures were also heavily weighted. Since the City’s budget is a concern, capital costs were also weighted heavily as directed by City staff. Therefore, project capital cost, the criteria that directly relate to reducing nuisance flooding and the number of structures protected were given the highest weights, a value of 10. The next level were those criteria that related to extreme or intermediate flood protection, aesthetics, reducing damages, reducing actual flooding extents, loss of business revenue and impacts to the Potomac River viewshed (loss of views along waterfront). These were ranked slightly lower, a weight value of 5, as they are important criteria, but were not considered drivers in this screening process. A slightly lower weight value of 3 was given to constructability, city liability, maintenance costs, private property acquisition, state and/or federal funding availability, and protection of repetitive loss structures (discussed in Section 4.1.5). Lastly, impacts to flood insurance costs, property ownership, environmental impacts, loss of recreational use, and regulatory requirements were weighted the lowest, at a value of 1. Each flood mitigation alternative was then given a score for each criterion and the total weighted rankings were summed to provide an overall score. The ranking system was developed so that positive impacts were given a score of 10 and negative or no impacts within that criterion were given a score of 1. In conjunction with the City, a series of sensitivity analyses were performed with slight variations in the weight factors for certain criteria and for each alternative’s ranking within a 28-JUL-10\\ 2-6
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 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 48 and 49: Technical Analyses Support climate
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 60 and 61: 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
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Structural Mitigation Measure Conce
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6.2.4 Permitting/Approval Requireme
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6.2.5 BCA and Results Structural Mi
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6.3.2 Assumptions Structural Mitiga
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CLIENT PROJ REVISION NO SCALE WALES
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Site Plan Approval Structural Mitig
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North Union Structural Mitigation M
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CLIENT PROJ REVISION NO SCALE S ROY
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King Street Residential Structural
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Waterfront Commercial Structural Mi
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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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Potomac Waterfront Flood Mitigation Study - City of Alexandria

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