Potomac Waterfront Flood Mitigation Study - City of Alexandria

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FLOOD ELEVATION BASE FLOW Overview of Flood Mitigation Measures Figure 3-4: LAG Elevated to Protect Building from Flooding A raised patio is a patio that would be rebuilt at a new, higher elevation to prevent the flow of floodwaters into the structure. Other localized flood barriers can be designed to accomplish the same effect if a patio is not an option, including localized yard berms and small privacy walls that are designed to be floodwalls. The raised patio or other localized flood barrier must be designed by a registered architect, engineer, or other certified professional who is responsible for ensuring that the design prevents flooding. Two main differences need to be considered when evaluating dry floodproofing for residential structures verses for commercial structures. The first difference is that dry floodproofing a residential structure does not remove it from the FEMA floodplain and, thus, does not alleviate the requirement for flood insurance. However, dry floodproofing can be used to remove commercial structures from the FEMA floodplain. In addition, for a floodproofed structure to be removed from the FEMA floodplain, it is important that any construction that is below the BFE meet the FEMA criteria for flood damage resistance. For more information, refer to FEMA Technical Bulletin 2 (August 2008). Most wall materials, including brick, will leak unless constructed or modified using special waterproofing techniques. Care should be taken when applying a sealant to the outside of a brick wall. Waterproofing compounds can deteriorate or fail if exposed to floodwater for extended periods of time. In addition, sealants may also be subject to damage, particularly in areas that experience high velocity floodwaters, or waters containing debris or ice. Floodproof membranes or coatings can also affect the aesthetic quality of a building. Clear coatings, such as epoxies and polyurethanes, are generally not as effective as cement or asphalt based coating. Therefore, the aesthetic appeal of a brick wall is lost with the use of higher quality sealants. One way to solve this problem is to add an additional layer of brick to the structure with the sealant located between it and the original brick surface. However, this is not considered an appropriate technique for historic structures. Floodproofing the walls of a structure by applying a membrane or coating to the surface could be considered for almost all of the floodprone buildings in the study area. However, without careful care and maintenance, these sealants may still leak. In addition, it is difficult to use this floodproofing method without compromising the building aesthetics. Modifications would require review and approval by the City’s Board of Architectural Review, since the focus area is within the designated historic district. Therefore, floodproof membranes or coatings have not been considered as an option for this study. 28-JUL-10\\ 3-5
Overview of Flood Mitigation Measures Structure openings can be floodproofed using permanent or removable shields or valves. Such openings primarily include doors, windows, and air vents. Floodgates are widely available for floodproofing both garage doors and pedestrian doors. However, most of these are active systems; requiring installation after a flood warning has been issued. Special floodproof doors are also available that look and function the same as a regular door (Figure 3-5). Although they are more expensive than a floodgate, these doors have the advantage of being a passive floodproofing measure. Figure 3-5: Floodproof Door It is generally less costly to floodproof windows and air vents. One option for windows is to remove them and replace with brick. Another possibility is to seal the window shut with waterproof caulking, which allows the homeowner to retain the aesthetic benefit of the window. A third option is a shield on the outside of the window. These are usually made from Plexiglas, aluminum, or plywood and can be screwed in place or slid into predesigned framed slots. Air vents can only be floodproofed through active systems. Two options include a slide-in-place shield or a watertight adhesive material. As previously described, dry floodproofing offers many options. The following dry floodproofing options are considered technically feasible in locations within the study area. Floodgates Floodproof openings Raised patios Internal elevation of contents is another type of dry floodproofing described in Section 3.2.2. 28-JUL-10\\ 3-6
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 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 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 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
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6.3.2 Assumptions Structural Mitiga
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Structural Mitigation Measure Conce
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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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