Letter writer: Understanding a 100-year flood

Graphic by Lori Deaton

Your article on “Remembering the Great Flood of 1916: A Q&A with Filmmaker David Weintraub,” [June 15, Xpress] was very interesting and certainly timely in this new era of weather extremes.

One common error needs to be corrected, however. The filmmaker, David Weintraub, in making a point about land use and flooding, says: “This is where we live — a 100-year flood happens every 20 years here. Since the last one was 2004, you might say we’re due.” I understand the point he is making, but he doesn’t seem to understand the concept of the 100-year flood.

It is not that a flood of a certain magnitude will happen every 100 years, but rather that it has a 1 percent chance of occurring any year. You could experience a 100-year flood in consecutive years. That would be bad luck, but not unprecedented. So stay out of the flood plain!

— Paul Kelman
Asheville

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3 thoughts on “Letter writer: Understanding a 100-year flood

  1. boatrocker

    Thank you for explaining floods via science.

    From the title of the LTE, I was expecting the ignorant theocratic right ala
    ‘Blame the Negro in the White House Coming For your Guns and White Women’ on the fringe right or the
    ‘Holocaust like oppression of people who do not wear shoes in public and please buy a crystal on your way out’ on the fringe left.

    Science rules. Middle finger to any faith/spiritual based system of decision making.

  2. Good point in the letter to the editor: It is not that a flood of a certain magnitude will happen every 100 years, but rather that it has a 1 percent chance of occurring any year.

  3. David Weintraub

    In response to Paul Kelman’s letter commenting on the interview that was recently done with me about my film on the 1916 flood. Kelman expressed concern that I didn’t understand what a 100 year flood was but I can assure him that I do. While in theory, the 100 year flood is the one percent chance of a flood of a certain magnitude occurring, the facts tell a far different story. In the past 120 years, 100 year floods have occurred in Western North Carolina in 1896, 1916, 1928, 1940, 1977, 1996 and 2004. A rate five times greater than the one percent chance. According to a recent study published by Rick Wooten, the state geologist among other researchers, storms in the Southern Appalachians capable of causing thousands of landslides have occurred here every twenty-five years. Additionally, if you add in major and catastrophic storms in this region which have resulted in hundreds of landslides over the period between 1916-2013, the report states that their average frequency is 7 years. (Frequency and Magnitude of Selected Historical Landslide Events in the Southern Appalachian Highlands of North Carolina and Virginia (Wooten, et al 2015)

    I may not be a math major, but given this history, I think its clear that the percentage chance of storms capable of causing massive numbers of landslides in WNC is probably closer to 10%. If that is the case, wouldn’t we want building codes in this region to be more reflective of the real peril that these storms cause. And its not the flood plain I’m concerned about as much as the steep slopes construction where most of the damage here is caused. Much of that danger is not limited to the McMansion crowd whose homes have a beautiful view on the sides of mountains, but also to the folks living at the bottom of mountains where the debris flows will hit hardest.

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