Potomac Waterfront Flood Mitigation Study - City of Alexandria

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Overview of Flood Mitigation Measures SECTION THREE: OVERVIEW OF FLOOD MITIGATION MEASURES As described in Section 2, ten flood mitigation techniques were selected for further consideration. They include measures that have structural elements, such as flood barriers, as well as those that do not require structural changes, such as ordinance revisions and modification to the City’s sandbag program. It should be noted that the ranking analyses were performed without assessing specific applicability to the City. The flood mitigation measures and their applicability for use within the City of Alexandria are described in this section. 3.1 STRUCTURAL MITIGATION MEASURES 3.1.1 Structural Flood Barriers Flood barriers are man-made structures that are built to protect low-lying areas from the inundation of floodwaters. These barriers provide either permanent or temporary flood protection. Temporary flood barriers are described in Section 3.2.3. Permanent flood protection is a passive system, meaning it is always in place and requires no human interaction to activate during flood events. These measures include levees, floodwalls, and berms. Levees and berms are typically earthen structures that require significant land while floodwalls take up less space and are typically constructed of concrete or steel. Permanent flood protection is typically an expensive option, which requires ongoing maintenance for continued flood protection. Selection of the most appropriate flood barrier needs to take into account the frequency, typical depth, and duration of flooding. Next, the level of protection desired and the size of the area that needs protection need to be considered. Since the areas being protected by the systems are lowlying, all flood protection methods need to be extended to (i.e., tied-in to) high ground. Aesthetics is another important consideration in choosing an appropriate flood barrier. Levees, floodwalls and berms cause visual impacts and can be viewed as unattractive; they may also hinder access to waterways. Access to waterways through a flood barrier can be provided by using a floodgate, which is an opening in the flood barrier that is lowered or closed during flood events. Consideration of all of these factors will determine the best type of flood barrier for the project area. Once the flood barrier is selected, an important design component is interior drainage. During most rainfall storm events, the discharge from the interior areas can be conveyed by gravity through the existing stormdrain systems. Stormdrain systems are typically designed to convey the 10-year discharge. However, during periods of high elevation on the Potomac River, high water in the river prevents gravity flow through the stormdrain system, while flapgates prevent back flow. During this worst-case scenario (referred to as “coincident peaks”), the flood barrier system would need to convey the interior drainage for events at least up to the estimated 100-year flood discharge. Therefore, design concepts include pumping stations to pump the discharge into the Potomac River in the event the flapgates are sealed or blocked. Finally, if the flood protection barrier is to be recognized by FEMA as a flood protection device, the levee must meet the requirements contained in Section 65.10 of the National Flood Insurance 28-JUL-10\\ 3-1
Overview of Flood Mitigation Measures Program (NFIP) regulations. These requirements include at least 3 feet of freeboard above the Base (one-percent annual chance) Flood Elevation (BFE), an operation and maintenance plan. If these criteria are met, the areas on the landward side of the levee may be removed from the floodplain. Only permanent structural flood barriers are permitted to change the floodplain. Three potential permanent structural flood barrier solutions are evaluated in this report. They are: 100-year Floodwall along the Potomac River Waterfront Elevated Walkway in the Lower King Street Area Jones Point Berm Conceptual designs were prepared for each of these flood mitigation alternatives to evaluate the technical and cost feasibility. Further analysis is presented in Section 6. 3.1.2 Acquire Properties In recent decades, FEMA’s preferred flood mitigation alternative has increasingly been property acquisition because, in many cases, it is more cost-effective than large engineered solutions. Property acquisitions or flood buyouts are the process of purchasing flood-prone structures and demolishing them to eliminate future flood damage claims from those structures. Often these acquired properties become an amenity for the community through the creation of new open space that can be used to create parks or wildlife areas. It is also a permanent solution for mitigating those flood hazards. Various factors should be considered to determine whether or not property acquisition is a viable mitigation measure in the City. Because buyouts are a voluntary measure, a critical factor is the willingness of residents to participate in the program. To determine acquisition costs the following parameters were estimated: fair market value of each property, the number of properties likely to require a special survey, and project work schedule. Average costs were used for property appraisals, real-estate closings, structure demolition, debris disposal, and legal fees. Administrative costs are also expected to be incurred for report preparation, overtime, and incidental expenses. One disadvantage to the acquisition option is that it precludes the preservation of historic buildings. Potential political or socioeconomic implications involved with such a project need to be considered. Further, potential opposition from property owners reluctant or unwilling to support the acquisition must be considered. Acquisition within the study area is a technically feasible alternative, but it is not feasible for every property. Therefore, a more detailed assessment of the study areas is required. While acquisition may be technically feasible in some study areas, the cost effectiveness of this alternative is highly variable. The cost variability is dependent on characteristics such as real estate values and flood depths. Therefore, this alternative will be further analyzed in Section 6. 28-JUL-10\\ 3-2
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 26 and 27: 3.1.3 Floodproofing Overview of Flo
Page 28 and 29: FLOOD ELEVATION BASE FLOW Overview
Page 30 and 31: Overview of Flood Mitigation Measur
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
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6.2.5 BCA and Results Structural Mi
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Structural Mitigation Measure Conce
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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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