SECTION 4: RISK ASSESSMENT
wildfire hazards.
NCEM’s IRISK database incorporates county building footprint and parcel data. Footprints with an area less than 500 square feet were excluded from the analysis. To determine if a building is in a hazard area, the building footprints were intersected with each of the mapped hazard areas. If a building intersects two or more hazard areas (such as the 1-percent-annual-chance flood zone and the 0.2-percent-annual-chance flood zone), it is counted as being in the hazard area of highest risk. Parcel data provided building value and year built. Building value was used to determine the value of buildings at risk. Year built was used to determine if the building was constructed prior to or after the community had joined the NFIP and had an effective FIRM and building codes enforced. Census blocks and Summary File 1 from the 2010 Census were used to determine population at risk. This included the total population, as well as the vulnerable elderly and children age groups. To determine population at risk, the census blocks were intersected with the hazard area. To better determine the actual number of people at risk, the intersecting area of the census block was calculated and divided by the total area of the census block to determine a ratio of area at risk. This ratio was applied to the population of the census block. For example, a census block has a population of 400 people. Five percent of the census block intersects the 1%-annual-chance flood hazard area. The ratio estimates that 20 people are then at risk within the 1%-annual-chance flood hazard area (5% of the total population for that census block). Certain assumptions are inherent in any risk assessment. For the Wake County Multi-Jurisdictional HMP, three primary assumptions were discussed by the HMPC from the beginning of the risk assessment process: (1) that the best readily available data would be used, (2) that the hazard data selected for use is reasonably accurate for mitigation planning purposes, and (3) that the risk assessment will be regional in nature with local, municipal-level data provided where appropriate and practical. Key methodologies and assumptions for specific hazard analyses are described in their respective profiles. PRIORITY RISK INDEX The conclusions drawn from the hazard profiling and vulnerability assessment process can be used to prioritize all potential hazards to the Wake County planning area. The Priority Risk Index (PRI) was applied for this purpose because it provides a standardized numerical value so that hazards can be compared against one another (the higher the PRI value, the greater the hazard risk). PRI values are obtained by assigning varying degrees of risk to five categories for each hazard (probability, impact, spatial extent, warning time, and duration). Each degree of risk was assigned a value (1 to 4) and a weighting factor as summarized in Table 4.5. The results of the PRI scoring are provided in each hazard profile and Section 4.6 Conclusions on Hazard Risk.
Wake County, NC Multi-Jurisdictional Hazard Mitigation Plan
WSP June 2024 Page 70
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