Assessing the Risk of Natural Disasters

Assessing the Risk of Natural Disasters
Modified from ‘Hazard and Risk’, by Lisa Gilbert (Williams College), Josh Galster (Montclair State University), Joan Ramage (Lehigh University)

The assessment of risk is complex. Each day, all of us face a variety of risks. Some, such as being involved in a car accident, we almost take for granted. Others, such as becoming seriously ill, are so ‘scary’ that most of us don’t want to think about them on a daily basis.

Risk from natural hazard is something that most people infrequently think about, unless they or a loved one previously has encountered one. Nevertheless, we all should be prepared for the unlikely occurrence of an earthquake, tornado, forest fire, hurricane, etc. Many people keep emergency kits at home in case of indefinite power outages due to storms. We all (hopefully!) are able to afford health insurance to mitigate the cost of going to the doctor or being hospitalized. We are required by law to have automobile insurance to cover the costs of accidents and loss. Anyone who holds a mortgage on their home is required to pay property insurance.

This simple exercise is designed to help you understand the assessment of natural hazard risk from both qualitative and quantitative perspectives. The objective is to help you to better understand that daily qualitative assessment of risk is often based upon culture and personal experiences, and also how government agencies and insurance companies can use our personal data to assess quantitative risk.


Part 1:  Consider the following cartoon:




1) What is a hazard?


Any danger that cause harm or destroy human life, property and the environment.


2) What are the hazards in the cartoon? Which are natural hazards, and which are non-natural hazards?


The hazard in the cartoon is a volcano eruption that may affect the aircraft and crash it. A natural hazard in any phenomenon that occurs naturally such as landslide, volcanic eruption, hurricane and earthquake among others. Non-natural hazards are phenomena that occur as a result of human activity such as poisonous gas leak from a factory near a residential area.

3) How could the parachutist reduce the number of hazards?


He should avoid parachuting from the aircraft and they should fly away to avoid coming into contact with volcanic ash

Part 2:  Read the New York Times article That Daily Shower Can Be a Killer, attached to the end of this assignment.


4) How do you qualitatively define ‘risk’?



5) What is the natural hazard you could potentially encounter from which you perceive the greatest personal risk?



6) Thinking of the example of New Guineans not sleeping under dead trees, what is at least one thing we do as a society to reduce risk to the natural hazard you listed above?


7) Propose a way to calculate your personal risk from the natural disaster identified in question 2.




Part 3: It is often useful to quantify risk. Insurance companies, for example, calculate risk before deciding to issue insurance policies and what premiums to charge. It can get quite complicated, of course. Your earthquake hazard risk is a lot less in Louisiana than in southern California, but obviously your hurricane hazard risk is much higher.

A very simple example of how we can calculate risk from natural disasters is illustrated by the following formula:


Risk = Likelihood x Cost


This equation is fundamental to assessing risk. The terms are defined as follows:

Likelihood: For our purposes, likelihood refers to the percent chance of a hazard happening in a specific place over a certain amount of time. Likelihood is an estimate, based on past events.

Example 1: California has had 8 major (magnitude > 7) earthquakes in the last 100 years. That is about 12 years between major earthquakes, on average (also called the return period). So, the likelihood of a major earthquake somewhere in the state is 1/12 X 100 = 8.33% each year.


Example 2: The return period of landslides on Smith Lane is 5 years. The likelihood of a landslide on Smith Lane this year is 1/5 x 100 = 20%.

Cost: Cost refers to the impact of a particular hazard. Cost is both a dollar amount in terms of damages to property and infrastructure, but can also be more difficult to quantify in terms of time/productivity lost, or injuries and deaths.

Risk: Simply stated, risk is likelihood times cost (in dollars). If the cost is very small or the frequency is very low, the risk will likely be low. Insurance companies use risk to calculate the portion of property insurance related to natural hazards.


8) Calculate risk from hurricanes to a beach house in the following three areas: coastal west Florida, coastal North Carolina, and coastal Maine. Use the figure below to calculate the average frequency of hurricanes in each area. Assuming that the value of the beach house and personal household belongings to be $2,000,000 what is the risk to each house being hit by a hurricane in this year?


  1. a) What is the average return period for hurricanes in each area?
    Coastal west Florida:

Coastal North Carolina:
Coastal Maine:


  1. b) Using the above answers, calculate the likelihood of a hurricane in each area.
    Coastal west Florida:

Coastal North Carolina:
Coastal Maine:


  1. c) Using the answers to question 2, calculate the risk to the $2,000,000 beach home in each area. (Your answers will be in $.)

Coastal west Florida:
Coastal North Carolina:
Coastal Maine:




9) Based upon the above calculations, in what area beach house insurance policies the most expensive?




10) Would this influence your decision as to where to purchase a beach house? Why or why not?  What other factors might you consider in making that decision?





Part 5: Since 1974, Murfreesboro has experienced 4 significant tornado outbreaks resulting in significant property damage to multiple homes.

11) What is the return period for significant tornado outbreaks in Murfreesboro?


12) What is the likelihood for a significant tornado outbreak in Murfreesboro in any given year?






13) Assume the median value of property and belongings in Murfreesboro to be $250,000. Calculate in dollars the average risk from a significant tornado outbreak to homes in Murfreesboro.


Average risk:




14) What other natural hazards might contribute to the overall risk to property in Murfreesboro?


That Daily Shower Can Be a Killer



The other morning, I escaped unscathed from a dangerous situation. No, an armed robber didn’t break into my house, nor did I find myself face to face with a mountain lion during my bird walk. What I survived was my daily shower.


You see, falls are a common cause of death in older people like me. (I’m 75.) Among my wife’s and my circle of close friends over the age of 70, one became crippled for life, one broke a shoulder and one broke a leg in falls on the sidewalk. One fell down the stairs, and another may not survive a recent fall.


“Really!” you may object. “What’s my risk of falling in the shower? One in a thousand?” My answer: Perhaps, but that’s not nearly good enough.


Life expectancy for a healthy American man of my age is about 90. (That’s not to be confused with American male life expectancy at birth, only about 78.) If I’m to achieve my statistical quota of 15 more years of life, that means about 15 times 365, or 5,475, more showers. But if I were so careless that my risk of slipping in the shower each time were as high as 1 in 1,000, I’d die or become crippled about five times before reaching my life expectancy. I have to reduce my risk of shower accidents to much, much less than 1 in 5,475.


This calculation illustrates the biggest single lesson that I’ve learned from 50 years of field work on the island of New Guinea: the importance of being attentive to hazards that carry a low risk each time but are encountered frequently.


I first became aware of the New Guineans’ attitude toward risk on a trip into a forest when I proposed pitching our tents under a tall and beautiful tree. To my surprise, my New Guinea friends absolutely refused. They explained that the tree was dead and might fall on us.


Yes, I had to agree, it was indeed dead. But I objected that it was so solid that it would be standing for many years. The New Guineans were unswayed, opting instead to sleep in the open without a tent.


I thought that their fears were greatly exaggerated, verging on paranoia. In the following years, though, I came to realize that every night that I camped in a New Guinea forest, I heard a tree falling. And when I did a frequency/risk calculation, I understood their point of view.


Consider: If you’re a New Guinean living in the forest, and if you adopt the bad habit of sleeping under dead trees whose odds of falling on you that particular night are only 1 in 1,000, you’ll be dead within a few years. In fact, my wife was nearly killed by a falling tree last year, and I’ve survived numerous nearly fatal situations in New Guinea.


I now think of New Guineans’ hypervigilant attitude toward repeated low risks as “constructive paranoia”: a seeming paranoia that actually makes good sense. Now that I’ve adopted that attitude, it exasperates many of my American and European friends. But three of them who practice constructive paranoia themselves — a pilot of small planes, a river-raft guide and a London bobby who patrols the streets unarmed — learned the attitude, as I did, by witnessing the deaths of careless people.


Traditional New Guineans have to think clearly about dangers because they have no doctors, police officers or 911 dispatchers to bail them out. In contrast, Americans’ thinking about dangers is confused. We obsess about the wrong things, and we fail to watch for real dangers.


Studies have compared Americans’ perceived ranking of dangers with the rankings of real dangers, measured either by actual accident figures or by estimated numbers of averted accidents. It turns out that we exaggerate the risks of events that are beyond our control, that cause many deaths at once or that kill in spectacular ways — crazy gunmen, terrorists, plane crashes, nuclear radiation, genetically modified crops. At the same time, we underestimate the risks of events that we can control (“That would never happen to me — I’m careful”) and of events that kill just one person in a mundane way.


Having learned both from those studies and from my New Guinea friends, I’ve become as constructively paranoid about showers, stepladders, staircases and wet or uneven sidewalks as my New Guinea friends are about dead trees. As I drive, I remain alert to my own possible mistakes (especially at night), and to what incautious other drivers might do.


My hypervigilance doesn’t paralyze me or limit my life: I don’t skip my daily shower, I keep driving, and I keep going back to New Guinea. I enjoy all those dangerous things. But I try to think constantly like a New Guinean, and to keep the risks of accidents far below 1 in 1,000 each time.


Jared Diamond, a professor of geography at the University of California, Los Angeles, is the author of the new book “The World Until Yesterday: What Can We Learn From Traditional Societies?”


A version of this article appears in print on January 29, 2013, on Page D1 of the New York edition with the headline: That Daily Shower Can Be a Killer.

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