In two recent blog posts, we marked September as National Preparedness Month by exploring the concepts of social vulnerability and community resilience in the face of natural disasters. We’ll next look at the potential for loss from such events, specifically as measured by the Federal Emergency Management Agency (FEMA) in its National Risk Index. FEMA expresses risk of potential loss in terms of dollar amounts, using a concept called Expected Annual Loss, or EAL.
EAL is composed of three components that are multiplied together. First, we have exposure: the value of buildings, people[1], and agriculture that may be impacted by a natural hazard. Second, we have annualized frequency: the expected frequency or probability of an occurrence per year, based on historical data. Third, we have the historic loss ratio: the estimated percentage of the value of the people, buildings, and agriculture that would be destroyed by an event, again based on historical data. Thus, the value of the assets at risk, times the percent expected damage, times the likelihood of an event in a year gives us the Expected Annual Loss.
FEMA calculates EAL for losses to buildings, people, and agriculture for eighteen different types of natural hazards[2], though some are only applicable to parts of the country.[3] Figure 1 shows the total EAL by county for Georgia; categories represent natural breaks based on the distribution of values across the United States.[4]
Figure 1: Expected Annual Loss From Natural Disasters by County
Data Source: Federal Emergency Management Agency National Risk Index Data
As seen in this map, most of Georgia falls in the very low or relatively low categories. Exceptions are counties along Georgia’s coast due to higher probabilities of hurricanes and coastal flooding, and counties in Metro Atlanta, which have higher population density and more valuable buildings. With the exception of Chatham County,[5] no county in Georgia rates above “relatively moderate” in terms of EAL. But that doesn’t mean that we should be complacent: our 11-county region has an Expected Annual Loss of $313.9 million from natural hazards.
What natural hazards impose the greatest expected losses to our region? Table 1 shows EAL by hazard for the ARC region.
Table 1: Expected Annual Loss by Type of Hazard for the 11-County Atlanta Region
| Tornado | $146,598,008 |
| Earthquake | $43,701,539 |
| Hail | $42,035,534 |
| Riverline flooding | $25,104,494 |
| Hurricane | $21,902,839 |
| Lightning | $18,074,638 |
| Winter weather | $5,609,070 |
| Strong wind | $5,349,110 |
| Ice storm | $2,483,804 |
| Landslide | $1,428,762 |
| Wildfire | $621,352 |
| Cold Wave | $453,048 |
| Heat Wave | $305,739 |
| Drought | $264,500 |
| Total | $313,932,439 |
Data Source: Federal Emergency Management Agency National Risk Index Data
Per FEMA’s methodology and calculations, this table shows that tornadoes pose the greatest Expected Annual Loss to our region, followed by earthquakes, hail, and riverline flooding. It’s important to keep in mind the interplay between likelihood and frequency on one hand, and the damage that would be caused by an event on the other. Hail and riverline flooding events are fairly common, with multiple events each year in every county. But each event causes much less damage than would a tornado or earthquake. FEMA estimates only a 0.050% (1 in 2,000) chance of an earthquake hitting Fulton County in a given year. Given that our buildings are not designed to withstand a quake, however, a single event would be devastating. And though a tornado would cause less damage than an earthquake, tornadoes are much more common in our region. For example, there is about a 3 in 10 chance a tornado will touch down in Fulton County over the course of a year. And while “heat wave” is very low on the list of expected annual losses. It is worth noting that heat waves cause the highest number of fatalities per year in the United States, according to the National Weather Service’s Weather-Related Fatality and Injury Statistics program.
Figure 2 provides a closer look at the EAL for tornadoes in our region, by zooming to the Census tract level. Categories again represent natural breaks based on the distribution of values across the United States.
Figure 2: Exepected Annual Loss from Tornadoes in the Atlanta Region by Census Tract
Data Source: Federal Emergency Management Agency National Risk Index Data
As this map shows, the highest EAL for tornadoes is outside the urban core. It is highest in the both the northwest corner and Canton within Cherokee County; southwestern Forsyth County; the northeastern corner of Clayton County; east Douglas County; and Cobb County in and north of Marietta.
Figure 3 focuses on EAL for riverline flooding, again at the Census tract level. Categories again represent natural breaks based on the distribution of values across the United States.
Figure 3: Expected Annual Loss from Riverline Flooding in the Atlanta Region by Census Tract
Data Source: Federal Emergency Management Agency National Risk Index Data
Not surprisingly, this map shows higher annual losses along the Chattahoochee, near the Flint River by the airport, as well as in northwestern parts of Atlanta (NPUs A and C) where Peachtree Creek and Nancy Creek often flood. Pluvial flooding, including what we have recently seen in some downtown areas when heavy rainfall events overwhelm drainage systems appear not to be picked up by this measure.
The next (and final) post in this series will examine how FEMA bring together all three concepts, social vulnerability, community resilience, and Expected Annual Loss in a single index score and see what that tells us about our region.
Want to investigate further? With data available for this many hazards at both the county and Census tract levels, we can only scratch the surface in a blog post. Fortunately, FEMA makes these data available in a convenient mapping tool: https://hazards.fema.gov/nri/map.
Notes:
[1] This only measures the economic losses (i.e., wages) when people are harmed; obviously, there is no adequate way to put a price on a human life.
[2] The eighteen hazards are as follows: avalanche, coastal flooding, cold wave, drought, earthquake, hail, heat wave, hurricane, ice storm, landslide, lightning, riverine flooding, strong wind, tornado, tsunami, volcanic activity, wildfire, and winter weather.
[3] For example, there are no active volcanoes in Georgia. Coastal flooding only poses a risk in coastal counties; FEMA also does not consider avalanches or tsunamis as posing a risk in Georgia.
[4] FEMA divides counties and tracts into five risk categories ranging from very low to very high. Rather than breaking the groups at quintiles, however, it utilizes a machine learning technique called k-means clustering to determine natural break points in the distribution of scores. That means that the break points will be different for each type of EAL at each level of geography. For overall EAL at the county level, the resulting percentile breaks are as follows: very low runs through the 51.75th percentile; relatively low through the 95.2nd percentile; relatively high through the 99.5th percentile, and very high covers the remainder. The legend omits the very high category because no Georgia county falls in this group.
[5] Chatham County, where the city of Savannah is located, stands out with an EAL in the 98th percentile among all counties nationwide, with a total EAL of $271.6 million.