CALIFORNIA STATE UNIVERSITY, LONG BEACH
Geography 458/558: Hazards and Risk Assessment
Lab: Rates of Wildfire, Probabilities of Risk
If you look at this map of wildfire frequency, you will see it concentrates on western Los Angeles County and eastern Ventura County. Any colored area has experienced at least one wildfire. The areas left white may well have had fires, but these fires would be structural fires, not wildfires (even though some of them might have started as wildfires and some wildfires may have started as structural fires that got out of control).
In this lab, you'll use wildfires to become familiar with the conversion of incident frequencies into simple ways of estimating future risk: recurrence intervals and annual probabilities. As you work your way through the questions, please put your answers on the 1 page form provided here. When you download it, please rename it as YourlastnameFireLab.ods (just so I don't mix up your labs!). When you finish filling it out, please upload it to the Lab 1 BeachBoard Dropbox.
Looking more closely at that pattern, what can you say about the fire-prone landscape? What kind of terrain is it overwhelmingly in? Please put your answer at the bottom of the answer sheet provided. And don't forget to "autograph" it (make sure your name is on it)!
Zooming more closely in, you'll see there are five boxes outlined on the map, each marked with a number. On the spreadsheet, write down how many fires each little area has experienced in the 102 years that this map covers (1900 - 2001). You do this by matching the color dominating each box with the legend in the lower left. Put your answers in the Times Burned column.
You are now in a position to calculate the average recurrence interval (i) for wildfire in each area. This is the average time between fires. To do this, divide the number of records or N (102) for each area by the number of fires (or n):
i = N / n
You should be able to use Calc to do this by typing in "=C17/D17" in Cell E17. You can then drag the formula down from E17 to E21. However you do it, please put your answers in the Recurrence Interval in Years column (Column E) and round your answers to 1 decimal place of accuracy (which the spreadsheet should default to).
Once you have the recurrence interval (i), you can then figure out the probability (p) that any area will experience a wildfire in any given year. To do this, take the reciprocal of i. That is:
p = 1 / i
You can do this by typing "1/E17" in Cell F17. Please format the Probability of Recurrence column to 5 decimal places of accuracy (which the spreadsheet should default to, I hope). Drag that cell down from F17 to F21, as before.
Then, convert each probability into a percentage (most people visualize percentages more easily than probabilities) and put the result in the Recurrence Expressed as Percentage column (Column G). You just multiply the probability by 100 (move the decimal point two places to the right):
% = p ( 100 )
In spreadsheet-speak, that would involve typing "=F17*100" in Cell G17 and the answer should be shown at 2 decimal places of accuracy (the default). Drag that down from G17 to G21. And you're done.
What does all this mean?
So, each year, there's a chance that, if your house were in a particular location, it might get burned or damaged by a wildfire. That might seem kind of abstract. Let's evaluate this level of risk another way.
What if this risk were the risk of you dying in a medical procedure? Now, if you were terribly injured or ill, you might really have no better option and even a 10% chance of dying on the operating table doesn't look that bad if otherwise you had a 100% chance of dying without the medical procedure. But what if these were the probabilities of dying during purely elective surgery, perhaps cosmetic surgery? At which level of risk would you be comfortable judging the trade-offs between cosmetic surgery's benefits and its risks? What if your chance of living without the surgery was virtually 100%, but you had a 10% chance of dying during a liposuction or nose-job? That same 10% risk has a whole different meaning now.
Living in the fire-prone landscape for the sake of a great view is more like a nose-job tradeoff than an an open-heart surgery tradeoff! Keep that in mind if, one day, you get rich and famous and have the resources to live wherever you would like. Factor in fire risk when you consider buying a home in the hills: It may not be not such a bargain.
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Last revision: 08/07/20