Hazard Identification and Risk Assessment (HIRA)
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Northern Virginia Hazard Mitigation Plan Update Chapter 4: Regional Hazard Identification and Risk Assessment (HIRA) Requirement §201.6(c)(2): (The plan shall include) …a risk assessment that provides the factual basis for activities proposed in the strategy to reduce losses from identified hazards. Local risk assessments must provide sufficient information to enable the jurisdiction to identify and prioritize appropriate mitigation actions to reduce losses from identified hazards. The risk assessment shall include: (i) A description of the type, location, and extent of all natural hazards that can affect the jurisdiction. The plan shall include information on previous occurrences of hazard events and on the probability of future hazard events. (ii) A description of the jurisdiction’s vulnerability to the hazards described in paragraph (c)(2)(i) of this section. This description shall include an overall summary of each hazard and its impact on the community. All plans approved after October 1, 2008 must also address NFIP insured structures that have been repetitively damaged by floods. The plan should describe vulnerability in terms of: a. The types and numbers of existing and future buildings, infrastructure, and critical facilities located in the identified hazard areas; b. An estimate of the potential dollar losses to vulnerable structures identified in paragraph (c)(2)(ii)(A) of this section and a description of the methodology used to prepare the estimate; c. Providing a general description of land uses and development trends within the community so that mitigation options can be considered in future land use decisions. (iii) For multi-jurisdictional plans, the risk assessment must assess each jurisdiction’s risks where they vary from the risks facing the entire planning area. I. Introduction The 2016 update to the Northern Virginia Hazard Mitigation Plan includes the following participating jurisdictions: Counties Towns Arlington County Town of Clifton Fairfax County Town of Dumfries Loudoun County Town of Haymarket Prince William County Town of Herndon Town of Leesburg Town of Lovettsville Cities Town of Middleburg City of Alexandria Town of Purcellville City of Fairfax Town of Occoquan City of Falls Church Town of Quantico City of Manassas Town of Round Hill City of Manassas Park Town of Vienna 4-1 Northern Virginia Hazard Mitigation Plan Update Although some anecdotal information may be included regarding the towns located within these counties, these areas may not be fully included in this assessment due to the lack of data available. Where available, location-specific data is incorporated into the 2016 update. Where it was not available, it is assumed that adjacent county or municipal data includes or otherwise accounts for the town. For the purpose of simplicity, the study area will be referred to as the Northern Virginia planning area throughout the remainder of this chapter. Efforts to involve county, city, and town departments and community organizations that might have a role in the implementation of mitigation actions or policies included invitations to attend meetings and assist with the development process, e-mails of minutes and updates, and opportunities for input and comment on all draft deliverables. Additional information on how this chapter was developed is available in the Planning Process Chapter. The purpose of this section of the plan is to: 1) Identify the natural hazards that could affect the Northern Virginia planning area; 2) Assess the extent to which the area is vulnerable to the effects of these hazards; and 3) Prioritize the potential risks to the planning area. The first step, identifying hazards, assessed and ranked all the potential natural hazards in terms of probability of occurrence and potential impacts. It also identified those hazards with the highest likelihood of significantly impacting the community. This section was completed based on a detailed review of the planning area hazard history. The 2010 update evaluated and reviewed the 2006 ranking and it was determined by the steering committee to expand the ranking and better align it with the Commonwealth of Virginia’s methodologies. For the 2016 update, it was determined to continue the same methodology and hazards, with one minor change – rather than include extreme temperatures with other hazards, extreme temperatures is included in the 2016 update as an independent hazard. Prior to the beginning of work to update the HIRA, the planning committee determined that the 2016 plan update would focus on natural hazards, and that no man-made or technological hazards would be included in this update, even in a redacted appendix. The hazards determined to be of the highest risk were analyzed further to determine the magnitude of potential events, and to characterize the location, type, and extent of potential impacts. This included an assessment of what types of development are at risk, including critical facilities and community infrastructure. Finally, a prioritization of the risk to the planning area was compiled, to serve as an overall guide for the communities when planning development, implementing policy, and identifying potential mitigation measures. II. Data Availability and Limitations This study includes data collected from a variety of resources including local, state, and national datasets. Whenever possible and practical, data has been incorporated into GIS products to aid in analysis and to develop area-wide maps for depicting historical hazard events, hazard areas, and vulnerable infrastructure. Critical facility data has been collected from the FEMA loss-estimating 4-2 Northern Virginia Hazard Mitigation Plan Update module, Hazards U.S. (HAZUSMH), and has been supplemented, to the extent possible, by local data. The local data provided is summarized below in the Building Inventory & Local Critical Facility Data section. In accordance with FEMA mitigation planning guidance, the results of this study are based on the best available data. In most cases, detailed data regarding the structural characteristics of facilities does not exist in a usable format at the local level. Local Critical Facility and Building Data Building inventories were provided by the jurisdictions participating in this plan. In most cases, the building inventory captures only the location and estimated value of structures. Characteristics such as structure and construction type, (i.e., residential wood frame home) are not always recorded. This data was utilized to determine the risk to buildings based on the extent of known hazard areas that can be spatially defined through GIS technology. Hazards without known recurrence probabilities or mapped hazard extents are not deemed unique enough to make definitive risk and vulnerability assessments for potentially at-risk buildings or facilities that differentiate them from other areas of the region. The hazard-specific sections provide the analysis, if relevant, for the critical facilities, historic structures, and buildings at risk. Table 4.1 summarizes estimated building inventories per jurisdiction, estimated from both local inventories and HAZUSMH. Table 4.1. Local Building Inventory per Jurisdiction, from Local Inventories and HAZUSMH Estimated Number Jurisdiction Estimated Jurisdiction of Buildings per Number of Critical and HAZUSMH Historic Assets Arlington County 40,847 380 Fairfax County 328,867 448 Town of Clifton included 58 Town of Herndon included 37 Town of Vienna included 19 Loudoun County 99,182 176 Town of Leesburg included 171 Town of Lovettsville included 7 Town of Purcellville included 7 Town of Middleburg included 6 Town of Round Hill included 5 Prince William County 128,867 171 Town of Dumfries included NA Town of Haymarket included 8 Town of Occoquan included 11 Town of Quantico included NA City of Alexandria 41,158 21 City of Fairfax 7,986 16 City of Falls Church 4,602 9 City of Manassas 8,024 85 4-3 Northern Virginia Hazard Mitigation Plan Update Table 4.1. Local Building Inventory per Jurisdiction, from Local Inventories and HAZUSMH Estimated Number Jurisdiction Estimated Jurisdiction of Buildings per Number of Critical and HAZUSMH Historic Assets City of Manassas Park 4,152 19 Local historic asset, critical facility, and infrastructure data were provided in some form by most jurisdictions. However, a comprehensive inventory consistent across jurisdictions does not exist because there is no universally accepted definition of what constitutes critical facilities and infrastructure, nor is one associated with FEMA and DMA 2000 planning requirements. For purposes of this plan, critical facilities and infrastructure are identified as “those facilities or systems that are owned/operated/maintained by the jurisdiction whose incapacity or destruction would present an immediate threat to life, public health, and safety, or have a debilitating effect on the economic security of the region.” This includes the following facilities and systems based on their high relative importance for the delivery of vital services, the protection of special populations, and other important functions in the Northern Virginia region: . Emergency Operations Centers (EOCs); . Hospitals and medical care facilities; . Police stations; . Fire stations; . Schools (particularly those designated as shelters); . Hazardous material facilities; . Potable water facilities; . Wastewater facilities; . Energy facilities (electric, oil, and natural gas); and . Communication facilities. Because of their significance