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A Tale of Two Cities (and two hurricanes)Part 1: Miami

Was Hurricane Andrew really a ‘worst-case scenario’ for Miami?

Thomas JusterDepartment of Geology, University of South Florida© 2011 University of South Florida Libraries. All rights reserved.

SSAChaz.TCJ.9(trad)

Core Quantitative IssuesFrequency, probability

Supporting Quantitative IssuesAverage

Core Geoscience IssuesHurricanes, risk

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Hurricanes and the southeastern US

Hurricanes are a fact of life in the southeastern US, as residents of Atlantic and Gulf of Mexico coastal communities know all too well. These tropical storms, which develop over the warm tropical waters of the southern Atlantic, Caribbean, and Gulf of Mexico, are most active during the months of August through October.

This map, from the National Oceanic and Atmospheric Administration (NOAA) shows the tracks of all major hurricanes (Cat-3 and above) in the southeastern US from 1888-2008 (only 20 years of data!).

Notice there are three places were the hurricane tracks converge: the southern Gulf coast, south Florida, and coastal North Carolina. In this unit you will learn about the effect of hurricanes on Miami.

Hurricanes are graded in intensity by the Saffir-Simpson intensity scale, from Cat-1 (sustained winds 74-95 mph) to Cat-5 (sustained winds > 155 mph). Notice the hurricane tracks on this figure are color-coded by their intensity.

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Review: estimating risk with the Risk Equation

Damage from Hurricane Andrew, courtesy of NOAA

The risk is a quantitative measure of the likely damage caused by a hazard, which for natural hazards takes into account the likelihood that the hazard will occur (called the ‘hazard), the number of people or structures that could potentially be damaged by the hazard should it occur (the ‘exposure’), and the susceptibility to damage of the people or structures in the way (the ‘vulnerability’). These four terms are related by the Risk Equation:

Risk = Hazard × Exposure × Vulnerability

Hazard is expressed as a probability, and thus is a dimensionless number between 0 and 1. Exposure is measured in terms of number of people or dollar value of property. Vulnerability is a dimensionless number that can be thought of as the conditional probability that death or damage will occur given the hazard. For example, if 3% of people are killed when a hazard occurs, then the vulnerability would be 0.03.

If you multiply these three numbers together you will see that risk, too, is expressed in terms of deaths or property value.

If you’re like most people, this is better understood with an example. Consider the property value risk to Pinellas County associated with a Cat-3 hurricane making landfall in Tampa Bay, causing storm surge on the order of 15-20 feet. There has been one Cat-3 hurricane in the last 167 years, so we can estimate the probability of the hazard as 1/167 = 0.006. According to Pinellas County’s web site, the total value of all property in the county is 64,800,000,000 ($64.8 billion dollars), which represents the exposure. Based on statistics from an analysis of the effect of a Cat-3 hurricane on Hillsborough County, I estimate the vulnerability factor to be 0.29. We now use the Risk Equation:

Risk = Hazard × Exposure × Vulnerability

Risk = 0.006 × $64,800,000,000 × 0.29 = $112,752,000

or about $110 million dollars rounding to two significant figures. Note that this doesn’t represent the actual expected damages from a Cat-3 hurricane—these would be much larger—but these damages ‘spread out’ over the 167 years that represents the recurrence interval for this storm.

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Example of using the Risk Equation

Figure from NOAA

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Miami and Andrew

Hurricane Andrew, the last major hurricane to make landfall near Miami, struck during early in the morning of August 24, 1992 with great fury. The storm had limped across the Atlantic, wavering between a weak hurricane and tropic storm until on August 22nd when it suddenly sprang to life again. Most models had the hurricane veering north and making landfall near Jupiter as a moderate Cat-2 hurricane, but Andrew surprised everyone by rapidly intensifying to a monster Cat-5 that took aim directly at Miami. Within 24 hours it ravaged the Bahamas, re-intensified as it crossed the Florida Straits, and slammed into Florida.

There is some debate about whether Andrew was a strong Cat-4 or a true Cat-5 upon landfall, but the results were devastating regardless: over $25 billion dollars in damage, 15 dead, and over 250,000 people temporarily or permanently without homes.

Click on the icon to open the embedded spreadsheet and immediately save itwith a unique name. This is what you’ll be submitting at the end of this activity. 2Cities-Miami

Miami and Andrew

Fortunately, Andrew was a tight, compact storm with hurricane force winds concentrated near the eye, and tropical storm force winds extending only 90 miles away.

Cat-1 winds (yellow)

Cat-2 winds (orange)

Cat-3 winds (pink)

Cat-4 winds (purple)

Cat-5 winds (blue))

Figure from NOAA

20 km

Q1. Was the hurricane’s intensity symmetrical about its eye? Why do you think this is true?

Q2. Use the scale bar measure the total distance across Andrew’s hurricane force wind field, then divide in half to estimate the average distance away from the eye that hurricane force winds extended. This is an estimate of the size of Andrew’s wind field.Go direct to End-of-module questions

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Miami and Andrew

Damage from Andrew was extensive. Maximum winds reached over 145 mph and storm surge reached 17 feet in places near the northern eyewall.

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Fortunately, Andrew didn’t actually make landfall in Miami!

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The eye of Andrew actually made landfall south of Miami, in the vicinity of Elliott Key. Although many millions of people lay in Andrew’s path, even more would have been affected had Andrew actually made landfall in downtown Miami.The pink circle shows the approximate area of hurricane force winds when Andrew came ashore. The blue circle shifts Andrew about 20 miles to the north. How much worse would it have been?

Q3. Which side of Andrew—north or south—received the brunt of the damage? Why? [HINT: think about how hurricanes rotate in the Northern Hemisphere]

Go direct to End-of-module questions

This a zip-code map, with the different colors corresponding to population density. The reds have the highest population density, the greens the least.

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Estimating the Exposure for an Andrew-like hurricane in Miami

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The spreadsheet you opened contains three tabs: “Answers”, “Zip codes-Andrew-actual” and “Zip codes-Andrew-north”. The second corresponds to the zip codes in the pink circle, and the third to the zip codes in the blue circle.

Here’s what the first 17 rows of “Zip codes-Andrew-actual” look like. For each zip code the spreadsheet lists the 2009 estimated population, number of homes or condos, and the average value of each dwelling

Click on the second tab to see the areas affected by Andrew in 1992.

We can use these data to calculate the total population and total property value that would experience hurricane-force winds from an Andrew-like storm in 2009. To compute the total population you will use the SUM function to total all the populations for each zip code zone. To compute the total value of property, we first need to compute the total value of property for each zip code zone by multiplying the number of dwellings by their average value, and then sum.

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Estimating the exposure for an Andrew-like hurricane

Q4. Calculate the total value of property in each zip code zone, and then sum up all the values to compute the total value of property that would experience hurricane force winds if a hurricane like Andrew made landfall exactly where it did in 1992. The first one is done as an example—make sure you can reproduce it!

Go direct to End-of-module Questions

Notice the formula for calculating the total value: =D2*E2

Remember, these values are in 2009 dollars. When Hurricane Andrew actually struck, the actual damage was less, because everything was worth less than it is now.

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Estimating the exposure for a landfall 20 miles north

What if an Andrew-like hurricane were to make landfall right in the heart of Miami?

Q5. Do the same thing to calculate the total value of property that would be exposed to hurricane-force winds if an Andrew-like storm made landfall 20 miles north of where the real Andrew landed in 1992. Once again, the first one is done as an example to check your work. Go direct to End-of-module Questions

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Estimating Risk for the two scenarios

To calculate the risk of an Andrew-like hurricane we need two more numbers: (1) the probability of the hazards; and (2) the vulnerability. In other words, we want to know: How often do Andrew-like storms occur in Miami? What would happen if one occurred?

Q6. Go to the Appendix to learn how to extract data from the NOAA Historical Hurricanes web site.

Then calculate the probability of a major hurricane (Cat 3 and up) coming within 25 nautical miles of Miami. This is the hazard term of the Risk Equation.

Go direct to End-of-module Questions

Q7. Using a vulnerability factor of 0.50, use the Risk Equation calculate the risk associated with an Andrew-like storm that follows the exact path as Andrew in 1992, and one that makes landfall 20 miles farther north. Use the exposure values you calculated on Slides 10 and 11.

The vulnerability can only be estimated, but is probably higher than the 0.29 factor used for a Cat-3 storm in Tampa Bay. Based on estimates of damage and property value from Andrew in 1992, a reasonable value might be 0.50. This means that the average structure would sustain damage equal to half of its value if it were exposed to an Andrew-like storm.

Remember, the Risk Equation says:

Risk = Hazard × Exposure × Vulnerability

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End-of-Module Assignment

Answer all questions in the spaces provided in the embedded spreadsheet (Slide 5), which you should then save with a different name (e.g., “YourName-miami.xls”).

1.Answer questions Q1-Q7 on Slides 6, 8, 10, 11, and 12.

2.Calculate the total area of the zip codes affected by an Andrew-like storm shown on Slide 8 for both the original trajectory (second tab) and the more northerly one (third tab). These areas should be about the same—are they?

3.Compare the two exposures you calculated in Slides 10 and 11:

• Calculate the average dwelling value (in 2009 dollars) for both areas;

• Calculate the total population for both areas;

• Which area has the larger exposure? Based on the two calculations above, discuss why it has the larger exposure.

4.Using data extracted from the NOAA Historical Hurricane Tracks web site, calculate the recurrence interval for hurricanes making landfall within 25 nautical miles of the center of four cities: Jacksonville, Miami, Tampa, and Pensacola. Then do the same thing for major hurricanes (Cat-3 and above). Fill in these values in the table provided. [HINT: the probability of a hurricane making landfall near Tampa is done as an example in the Appendix; you need only convert the probability to a recurrence interval.] To obtain the number of major hurricanes making landfall in a given area you will need to specify only major hurricanes under “Category”.

5.Can you draw any conclusions about the recurrence intervals you obtained for question 4? Which cities are more prone to hurricanes? Considering the paths of typical hurricanes (e.g., see Slide 2), why do you think these cities are more prone than the others?

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Appendix: Hurricane data from the NOAA Historical Hurricanes site

Click here to open the web site.

NOAA maintains a web site that stores information on all known hurricanes, from 1842-2008. This is what it looks like.

You will use this web site to extract information you can use to calculate the frequency or recurrence interval of hurricanes for certain locations.

This Appendix will show how to calculate the probability that any hurricane makes landfall near Tampa, Florida

How many years are included in the range 2008-2010? (Count them!). Notice this is not the same as the difference between 2010-2008 (2010-2008 = 2). Where does the extra year come from?

Based on this inference, how many years of hurricane data are on the NOAA web site (from 1842-2008)? [Answer: 167!]

Appendix: Hurricane data from the NOAA Historical Hurricanes site

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First, enter “Tampa, FL” for the location . . .

You will need to select the city of Tampa from the list of ‘Tampa’ locations that pops up.

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Wow, that’s a lot of tropical storms! The first way to widdle the list down is to restrict the location to a smaller radius from the center of Tampa. Change the search area to 25 nautical miles.

Appendix: Hurricane data from the NOAA Historical Hurricanes site

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By default, the program retrieves all storms, from subtropical storms all the way to cat-5 hurricanes. We’re only interested in hurricanes, so click on “Add” to bring up the list of intensities.

Here’s what the list looks like.

Now hold down the <CTRL> key and click on each of the five hurricane categories, H1-H5, to select all hurricanes. Save.

Appendix: Hurricane data from the NOAA Historical Hurricanes site

Back to Slide 12

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This is the final result. The map shows the paths of each of the hurricanes that met our criteria—within 25 miles of Tampa, category H1-H5, from 1842-2008 (we didn’t change the default time, which is all years). Notice that the paths are color-coded showing how the strength changed.

On the left is the list of hurricanes that met the criteria. There were six.

Using the frequency approach to probability, we can calculate the probability of any hurricane making landfall near Tampa as 6/167 = 0.03593.

Appendix: Hurricane data from the NOAA Historical Hurricanes site