Flyboy said:
We have, what, maybe 250 years of semi-accurate data, less than a century of accurate data that can be used for modeling?
What "Once In A Hundred Years" Really Means
https://www.almanac.com/real-meaning-once-hundred-years
From time to time, when a major weather event occurs, I see it described as a “once in a hundred years” or even “once in a thousand years” event. But what does this weather phrase really mean?
Weather phrases such as “once in a hundred” years are commonly misunderstood. Usually, this refers to a particular storm that brings heavy rain or snow.
- How do we know that an event is this extreme?
- How can we have five “once in a hundred years” events in Texas in less than half a century?
The answer to the first question is that we have excellent precipitation records from about the past 30 years based on rain-gauge, Doppler radar, and satellite measurements. We have pretty good records from about the past 100 years, mostly with rain gauges. Before that, the records are much more speculative, with some data from weather instruments but most of the weather information on individual events coming from personal diaries, with climatological information coming mostly from indirect measurements of things like the thickness of tree rings.
The answer to the second question is more complicated. Instead of imagining a “once in a hundred years” event happening exactly once every 100 years, think about it as having a 1% chance of happening in any given year. And once such an event occurs, the chance of it occurring again in the following year is still 1%.
Very simply, the United States is large enough, with enough different locations and weather events, that an apparent anomaly like the Texas rainfall will occur over 1% of the country each year. This means that events of this nature will happen somewhere in the United States each year, despite the tiny chance of one occurring in any one location. And Texas is a large state with plenty of room for extreme events in different parts of the state.
Because we do not have good weather records going back for more than a century or two, these events may happen more often than we expect.
Perhaps there needs to be a better way of describing 'risk' to folks who are at risk of catastrophic flooding, many of whom in western North Carolina and eastern Tennessee were unprepared for what happened. Should folks have expected such flooding? Warnings were made at least 5 days in advance, but then the heavy rains could have gone more east toward Charlotte, or west more toward eastern Tennessee (e.g., Knoxville, Nashville, . . ) and up into the Ohio River valley. As it was, much of rain fell on the border of Tennessee and North Carolina in the mountains that fed the French Broad and Swannanoa Rivers and their tributaries (many mountain creeks). Mills River is a major tributary of the French Broad River, as is the Swannanona River, which meets the French Broad in Asheville, NE.
The
French Broad River is a
river in the U.S. states of
North Carolina and
Tennessee. It flows 218 miles (351 km) from near the town of
Rosman in
Transylvania County, North Carolina, into Tennessee, where its
confluence with the
Holston River at
Knoxville forms the beginning of the
Tennessee River. The river flows through the counties of Transylvania,
Buncombe,
Henderson, and
Madison in North Carolina, and
Cocke,
Jefferson,
Sevier, and
Knox in Tennessee. It drains large portions of the
Pisgah National Forest and the
Cherokee National Forest.
https://en.wikipedia.org/wiki/French_Broad_River
South of Rosman, the stream is joined by the Middle and East forks to form the French Broad River.
From there it flows northeast through the
Appalachian Mountains into
Henderson, and
Buncombe counties. In Buncombe County, the river flows through
Asheville where it receives the water of the
Swannanoa River. Downstream of Asheville, the river passes north through
Marshall and
Madison County. After passing through
Hot Springs in the
Bald Mountains, the river enters
Cocke County, Tennessee.
In Cocke County, the river passes through
Del Rio and receives the waters of both the
Pigeon and the
Nolichucky rivers northwest of
Newport.
The Nolichucky River also flooded, particularly in Erwin, Tennessee.
By 9am on the 16th, the river had risen to over eighteen feet and then an hour later, the gauge was washed away. The French Broad hit an estimated twenty-four feet while the Swannanoa River likewise crested at over twenty feet. The flood spread, foaming and writhing, to reach almost a mile-wide swath in Asheville.
Asheville, NC had a catastrophic flood in 1916, or 108 years before the current event.
https://climate.ncsu.edu/blog/2015/07/nc-extremes-flood-of-1916-wiped-out-railways-records/
https://www.ashevillehistory.org/july-16-1916-the-great-flood/
Western North Carolina is no stranger to flooding. “Freshets” as these floods were often called, were described by settlers as early as 1791. Larger ones also occurred in 1901, 1902, and again in 1910. Before the area’s industrialization, these innundations were not always entirely destructive. Native Americans and settlers learned to depend on these seasonal alluvial floods that deposited rich silt along river bottoms where they grew crops. Residents along the region’s rivers likely could not have anticipated just how different the flood of 1916 would be.
From 2016 -
100 years after the Flood of 1916, the City of Asheville is ready for the next one
https://www.ashevillenc.gov/news/10...-city-of-asheville-is-ready-for-the-next-one/
Um - they were not ready this time, not even close.
One hundred years after the Flood of 1916, Asheville collectively wonders, “Can it happen again?”
The answer is yes. And no.
Modern meteorology now gives advance notice of major rain events. “Today FEMA has mapping systems that can predict how much rainfall we are going to get, and which areas are going to be flooded,” said Kelley Klope of the Asheville Fire Department. “The City can help notify people ahead of time through its
Everbridge emergency notification system (called Citizens Alert), announcements to local media and messages on our social media channels.”
The answer is yes, and yes, of course! Emergency Alerts work fine until power and cell service go out.
Still, the force of nature continues to impact Asheville with recurring floods. “About every 20 years we have a major flooding event,” Coates noted.
In 2004, virtually the same scenario walloped Asheville when Tropical Storms Ivan and Frances converged upon Western North Carolina, producing the wettest September ever recorded. Again, Ivan came up from the Gulf, followed by Frances for another double tropical storm wallop. Frances alone dropped 23 inches of rain on some parts of WNC. When the storms hit, Asheville’s North Fork Reservoir was full. For safety reasons, some of the water had to be released.
Portions of Biltmore Village flooded 4 feet deep when Tropical Storms Ivan and Frances converged on WNC in September 2004.
Unfortunately, 11 people died in Western North Carolina during the 2004 floods. One hundred forty homes were destroyed, another 16,234 damaged. There was $7 million dollars in damage in a seven-county area. It served as a sobering reminder of what can happen, even today.
In the 1916 Asheville Flood, Eighty (80) people died.
FOX News - The death total in Asheville is at least 72 people and now we're learning one family lost 11 loved ones to landslide during Hurricane Helene.
https://www.foxcarolina.com/2024/10/10/hurricane-helene-kills-11-people-one-asheville-family/
Asheville means the metropolitan area and Buncombe County. The 11 family members who perished lived in Fairview, up stream of Asheville. A landslide occurred
Fox News - Helene disaster exceeds devastation caused by the ‘Great Flood of 1916’ in North Carolina
At least 190 deaths are reported in western North Carolina, which are attributed to Helene, and many are still missing.
https://www.foxweather.com/extreme-weather/helene-flooding-extremen-asheville
Repeat: Perhaps there needs to be a better way of describing 'risk' to folks who are at risk of catastrophic flooding, many of whom in western North Carolina and eastern Tennessee were unprepared for what happened.
Anyone living along the Gulf Coast (from Brownsville, TX to Key West, FL) and along the Atlantic Coast (from Key West to New Brunswick and Nova Scotia, Canada) is at risk. Time will tell if the risk has increased.
One area in which I lived started seeing increased flooding during the mid-1970s. My family had lived there for 4 years when we experienced a first flooding event, in which the water level got to the curb (street flooded). The next year, the water level was halfway across the front lawn. The next year (3rd flood) the water covered the front lawn to the front of the house. In the fourth year, water entered the house, and usually, several times afterward, if we got a prolonged rain event (not even a hurricane).
In a neighboring area, there were three major flooding events with Hurricane Harvey being the 3rd. A friend's sister was told to raise the house by 6 feet or move. She chose to move. The neighborhood where my parents had lived was flooded by 2 to 3 feet; my parents had sold their property/house and the new owner demolished my parents' house and built a large house, which was common in the area. Despite flood mitigation efforts, those neighborhoods still flood.
Hurricane Harvey is cited in the Almanac article:
For example, in 2017, Hurricane Harvey brought exceptional rain amounts to the Houston, Texas, area—including 60.5 inches in the city of Nederland— while 30 inches or more fell on an area near the Texas coast the size of the state of Maryland. The 60.5 inches in Nederland was a record for a single storm in the continental United States that created an unprecedented “once in a thousand years” flood event. No rain event of that magnitude has ever happened in the history of the United States.
My parents' old house was probably in the region that got about 30 inches of rain. Even with half of that, the area houses would flood.
