Potomac Waterfront Flood Mitigation Study - City of Alexandria

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SECTION ONE: INTRODUCTION 1.1 POTOMAC RIVER FLOOD MITIGATION STUDY OVERVIEW Introduction The Potomac River is a major flooding source within the City of Alexandria. Flooding from the Potomac River is a recurring threat that has significantly impacted residential homes, businesses, and infrastructure along the City of Alexandria’s waterfront. In response to the flooding issues, the City of Alexandria commissioned the Potomac River Waterfront Flood Mitigation Study in 2006 to identify and assess flooding problems and to develop, evaluate, and recommend solutions to reduce the threat of flood damages in the City along the Potomac River. The Potomac River Flood Mitigation Study applied a typical problem-solving process: 1. Identify the specific flooding problems 2. Determine the specific cause of the problems 3. Identify solutions 4. Evaluate solutions 5. Recommend the most effective solutions The Initial Flooding Assessment Report, prepared by URS Corporation and dated October 2007, addressed the first three steps: identify the flooding problems, determine the causes, and identify potential solutions. This report concentrates on the last two steps: evaluating solutions and recommending the most effective solutions. This report summarizes the detailed engineering assessments conducted as part of the feasibility evaluation of potential measures and recommends cost-effective solutions that consider historic/archaeological resources, business/tourism impacts, and environmental impacts. 1.2 BACKGROUND The City of Alexandria’s waterfront lies within the Potomac River watershed and frequently experiences flooding. Flooding severely disrupts businesses in the area and causes extensive damage to property. The City estimates that $32,000 is expended per flooding event for maintenance and public safety personnel and for material costs for sandbags and equipment. This cost does not include lost business revenue and water damage to businesses or residential properties. Flooding along the waterfront has resulted from heavy rains, snow melt, storm surges, strong winds, tropical storms, and hurricanes. Major floods within the City of Alexandria in recent history were recorded in 1972, 1983, 1996, and 2003. Two floods in 1996 significantly impacted Alexandria’s waterfront. The January 1996 flood was due to a heavy snowfall followed by a period of rain and warm temperatures. In September 1996, Hurricane Fran caused flooding along the Potomac River and evacuations of properties in Old Town Alexandria. In February 2003, record levels of snow followed by rain also caused flooding in Alexandria. The most significant recent flood event was due to tidal flooding occurring during Hurricane Isabel. Hurricane Isabel, which occurred in September 2003, made landfall on the North Carolina coast. Isabel weakened to a tropical storm in Virginia, but the storm’s 40- to 60-mile-per-hour 28-JUL-10\\ 1-1
Introduction sustained winds pushed a bulge of water up the Chesapeake Bay and the Potomac River. In Alexandria, the water level in Old Town reached 8.8 feet North American Vertical Datum of 1988 (NAVD88). Figure 1-1 is a representative photograph taken in the Lower King Street area soon after Hurricane Isabel passed through. Businesses and residential losses were extensive. 1.3 STUDY AREA Figure 1-1: Lower King Street Area after Hurricane Isabel, 2003 The study area for this project is defined as the area affected by flooding associated with the Potomac River. In general, the southern boundary is the Capital Beltway and the northern boundary is near the railroad tracks near Bashford Lane. For the purposes of this report, the study area was divided into four focus areas, which are shown in Figure 1-2: Jones Point: This focus area is named for the Jones Point Park that abuts the residential neighborhood. The houses are built of brick and many have basements. All of the houses in the flood prone areas are multi-family residential homes (e.g., townhouses) with the exception of 210 Lee Court and 211 Lee Court, which are single-family structures. The structures are all located in the National Register District. Approximately 17 of the structures in the Jones Point focus area are predicted to experience flooding for the 100-year event. King Street: This focus area is a mixed-use area (commercial and residential) near the Lower King Street. The boundary begins at the north at Fayette Alley, runs south down South Union Street, cuts through the neighborhood between Prince and Duke Streets and continues up to South Lee Street. Approximately 23 commercial and six residential structures in the King Street focus area are predicted to experience flooding for the 100-year event. Waterfront Commercial: The Waterfront Commercial focus area includes commercial structures fronting the Potomac River on the eastern boundary of the focus area. The Torpedo 28-JUL-10\\ 1-2
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 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 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
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6.1.3 Potential Impacts Structural
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VDEQ Section 401 Certification VDE
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6.2.1 Description of Alternative St
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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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