Potomac Waterfront Flood Mitigation Study - City of Alexandria

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Overview of Flood Mitigation Measures In addition to these items directly recommended by the CRS program, it is recommended that the City take several other steps to enhance their existing regulations. Permitting and Inspection – In addition to reviewing permits, it is recommended that the City increase the frequency of inspecting new construction to ensure that the work is being conducted according to the provisions of the floodplain ordinance. The ordinance can also be amended to give the floodplain administrator the right to issue a stop work order or revoke building permits if the inspections show that a violation has taken place. Sample code from the City of Charlotte in Mecklenburg County, NC is shown in Appendix B, Exhibit 1. This activity would ensure that buildings are constructed in compliance with the building permits and prevent unauthorized work, such as converting basements into living spaces, from occurring, thereby reducing flood damages. Costs for the City include additional staffing to support more frequent inspections. Accessory Structures – It is recommended that additional regulations regarding accessory structures such as sheds and garages be added to strengthen the existing ordinances. For example, regulations could prohibit structures from being constructed within the floodplain. The sample code from Charlotte in Mecklenburg County, NC is shown in Appendix B, Exhibit 1. This activity would prevent accessory structures from being constructed in the floodplain, which would lower costs from damages. Costs for the City would be associated with changing the zoning ordinances and educating permitting officials on the change. Variances – A review of approved City variances indicates that no variances related to floodplain protection were granted within the last 3 years. However, it is recommended that the City consider strengthening the language to ensure that floodplain variances are discouraged. The sample code from Roseville, CA is shown in Appendix B, Exhibit 1. Reducing the number of floodplain variances would potentially lower costs from damages by further minimizing construction within the floodplain areas. There may be an increased cost for homeowners and business owners who will not be granted variances. For the City, the only costs would be associated with changing the zoning ordinances and educating permitting officials on the change. Regulatory Consistency During this study, a discrepancy between the City of Alexandria’s floodplain ordinance and building code was discovered. The specific discrepancy is that under the floodplain ordinance (Section 6-307), the FFE of new or substantially improved structures must be at or above the BFE. However, the City’s Building Code (Section 8-1-2) is more restrictive in that there must be a minimum freeboard requirement with regards to the FFE. The City of Alexandria building code states that the City has adopted the Virginia Uniform Statewide Building Code (USBC). Therefore, the USBC is the guiding, legal document when the City’s code does not contain construction specifications. Within the USBC, the flood-resistant construction provisions of the International Building Code (IBC) are specifically adopted. Therefore, the City has effectively adopted the IBC with regards to flood resistant construction. 28-JUL-10\\ 3-11
Overview of Flood Mitigation Measures The IBC (Section 1612.4) states that “…the design and construction of buildings and structures located in flood hazard areas, including flood hazard areas subject to high velocity wave action, shall be in accordance with ASCE 24.” This means that all design and construction of structures located in flood hazard areas are governed by the specifications within American Society of Civil Engineers (ASCE) Standard 24. ASCE Standard 24 is titled “Flood Resistant Design and Construction,” and is the guiding document for construction within the floodplain. ASCE 24 states that with the exception of Class I structures, which are limited to agricultural, temporary, and minor storage, all new and substantially improved structures must be designed and built with a FFE at the BFE plus 1 foot or more. Therefore, all construction within the Zone AE floodplain of the City of Alexandria is required to meet the more restrictive ASCE 24 design, rather than the NFIP design. FEMA is aware of this inconsistency; currently a guidance document is being prepared that advises communities on how to deal with this within their floodplain ordinances. However, it is recommended, at a minimum, that the City require conformance to the required building codes, thereby requiring 1 foot of freeboard to the FFE. Therefore, the City, by reference, requires 1 foot of freeboard. It is recommended that the City request that FEMA consider awarding CRS points for this element. 3.2.2 Elevate Internal Supplies and Goods Elevation of supplies, products, or goods above the flooding depth is a type of wet floodproofing that can be readily implemented and can protect structure contents from flood damage. This measure would require businesses and residents to realign their internal work and storage areas, which may affect the function of the internal spaces. Although this solution is applicable for buildings that are flooded by an extreme flood, this mitigation solution focuses on buildings affected by nuisance flooding because it is believed that business operators and residents that experience frequent flooding would be willing to restructure their internal spaces. For this mitigation solution, elevating supplies and utilities to a height of approximately 2.5 feet, which is a standard table or desk height, was considered. Supplies could also be stored in shelving units or overhead suspension systems that are above that height. Another important component of this solution is outreach and education to residents and business owners who could benefit by internal elevation. Approximately 23 structures are located within the area of nuisance flooding. Using either the FFE or the minimum topographic contour, 13 of these structures have an FFE at or above the 4foot contour and are not expected to experience nuisance flooding. An additional two buildings receive too much flooding for elevation of internal supplies to be feasible. The final eight buildings have flooding depths less than 2.5 feet and would be candidates for this mitigation measure. All of these buildings are commercial properties within the Waterfront Commercial focus area and are listed in Table 3-1. 28-JUL-10\\ 3-12
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 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
Page 78 and 79: 6.3.2 Assumptions Structural Mitiga
Page 82 and 83: CLIENT PROJ REVISION NO SCALE WALES
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Structural Mitigation Measure Conce
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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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