Step 2a: Map risk consideration areas for hazards.

The difference between risk and vulnerability is an important distinction in this step. Risk consideration areas identify geographically (on maps) those areas most likely to be affected by a given hazard. The people and resources located within the risk consideration areas are considered to be at risk from hazards and may or may not be vulnerable to hazard impacts. The vulnerability of the people and resources within the risk consideration areas is a function of their individual susceptibility to the hazard impacts. For example, in one neighborhood of 50 homes there are 10 structures located within the floodplain (risk consideration area). These 10 structures would be considered potentially at risk to flooding and would be the targets for vulnerability assessment. Seven of the structures are elevated above the 100-year flood elevation and the remaining three structures are not elevated. The three non-elevated structures would be considered vulnerable to flooding. In this example, the risk consideration area (floodplain) helps narrow the target of the detailed vulnerability assessment from 50 structures to 10 structures.

To effectively narrow the focus of your vulnerability assessment, you must first identify the risk consideration areas for your hazards. The more risk data that are available, the more opportunity there is to focus vulnerability assessment activities in your highest-risk areas. It is possible, however, to develop some prioritization capacity using limited publicly available data and to improve upon it over time using more accurate local data sources. For each hazard being addressed, you should research the available data concerning the location of high-risk areas. Internet Data Resources includes information on some potential data sources.

If you have hazards with limited areas of risk (i.e., coastal erosion is limited to coastal interfaces) you will want to limit your vulnerability assessment to only those areas. Similarly, if you have hazards with varying degrees of risk throughout your community (i.e., flooding can occur almost anywhere but floodplains are particularly high risk) you will want to target vulnerability assessment in your highest-risk areas. It is appropriate to refer to these designations as risk consideration areas since they are locations you consider at risk to hazard impacts based on your best available information sources. Obviously, the better the risk data available, the more accurate your assessment will be.

In New Hanover County, a risk consideration area was established for each hazard. Some of the risk consideration areas are descriptive and relatively effective in targeting vulnerability assessment activities, while others are default designations, lacking useful risk data. Below is a description of the risk consideration areas:

Hurricane Storm Surge

Risk consideration areas were mapped using output from the National Oceanic and Atmospheric Administration (NOAA) Hurricane Storm Surge Inundation Model. These areas represent locations that might expect to be impacted by storm surge events.

Figure 3.2

Flood

Figure 3.3

Coastal Erosion

Risk consideration areas were confined to the barrier islands and mapped using distance from the first line of stable vegetation as a baseline.

Figure 3.4

Wind

Risk consideration areas were mapped using the barrier islands as a boundary for high-wind potential.

Figure 3.5

Wildfire

Risk consideration areas were mapped by identifying the amount of forested land available as potential fuel for the hazard.

Figure 3.6

Earthquake

The same general level of risk exists throughout the county.

Figure 3.7

Tornado

The same general level of risk exists throughout the county.

Figure 3.8

Step 2b: Assign scores within risk consideration areas, where possible.

Within your risk consideration areas there could be additional boundaries representing varying degrees of risk. These varying degrees of risk should be represented in your risk consideration areas both graphically (additional boundaries on the maps) and through some type of relative scoring system (higher scores for higher risk areas). For example, hurricane storm surge maps are generally created for five different category storms. Category 1 storms are generally associated with the least severe winds and storm surge while Category 5 storms are considered most severe. Generally, those areas subject to storm surge in the lower category storms are also projected for inundation in all of the higher categories. When developing a relative priority scoring system for storm surge inundation, Category 1 storm surge areas would therefore have the highest risk of being flooded since they are at risk of inundation in all storm events.

The table below shows the relative priority scoring system developed for the risk consideration areas in New Hanover County. The general concept is that locations with no consideration for risk will have a score of 0 and each incremental increase in risk adds 1 point.

In this example, only two of the hazards have any locations with a risk consideration score of 0 (hurricane storm surge and coastal erosion). In both cases the maximum extent of the hazard risk does not realistically include the entire county but is limited to proximity to coastal waters.

The minimum risk score for each of the remaining hazards is 1 since there is some potential for each of these hazards to occur anywhere throughout the county. Due to a lack of detailed geographic risk information on tornadoes and earthquakes, the maximum risk score of 1 is evenly distributed throughout the county.

Hurricane Storm Surge

Figure 3.10

Surge zones are generally delineated according to hurricane categories on the Saffir-Simpson scale (Categories 1 through 5). Locations that are subject to inundation from the lowest category storm event are considered at highest risk, as they will likely be inundated during stronger events as well. Therefore, Category 1 and 2 hurricane storm surge inundation areas are given a high-risk consideration score of 4. Category 3 inundation areas are given a score of 3, and Category 4 and 5 inundation areas are assigned a score of 2. Because of the difficulty in making clear boundaries, a 0.25-mile buffer was established around the surge inundation zones and given a risk consideration score of 1. All other areas of the county receive a 0, as they are not likely to be impacted by hurricane storm surge.

Flood

Figure 3.11

The risk consideration area scores for flood hazards were determined by using

Federal Emergency Management Agency's (FEMA) Flood Insurance Rate Maps (FIRM). The FIRMs are developed from the output of hydrologic models, identifying areas with high potential for flooding. The risk consideration area score of 5 is given to the Velocity Zone (V-Zone) where coastal flooding and wave action risks are highest. The second highest score of 4 is applied to the 100-year floodplain and a score of 3 is given to the 500-year floodplain. Areas located outside the floodplain but appearing on flood-prone soils are rated with a score of 2. All other areas of the county receive a score of 1.

Coastal Erosion

Figure 3.12

Risk consideration scores for erosion were determined based on distance from the first line of stable vegetation along the shore. This vegetation line was delineated using New Hanover County's aerial photography as a base. Those areas inland of the vegetation line to 210 feet are assigned a score of 3. Areas between 210 and 420 feet are assigned a score of 2 and the remaining areas on the barrier islands receive a rating of 1. The mainland portion of the county is assigned a 0, as there is an insignificant level of erosion risk in those areas.

Wind

Figure 3.13

The risk consideration area scores for wind hazards are determined by proximity to the coast. The barrier islands receive a high score of 2 while the remainder of the county receives a score of 1.

Wildfire

Figure 3.14

Risk consideration area scores for wildfires are determined by the concentration of the primary fuel source, pine trees. The area in the northern part of the county contains dense pine cover and receives a score of 3. The central portion of the county, where pine is interspersed with development, receives a score of 2. The barrier islands, which are least densely forested with pine, are assigned a score of 1.

Earthquake

The risk consideration area score is 1 throughout the county.

Tornado

The risk consideration area score is 1 throughout the county.

Summary Scores

Figure 3.15

Using a GIS, the seven risk consideration areas were combined and the scores were added together to create summary scores for every location in the county. These summary scores were used to develop a summary risk area map. The summary scores also provide the foundation for ranking high-risk areas in the remainder of the analyses.

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