August 9, 2023
On July 9, 2023, newscasters reported that 1-in-1000 year floods were occurring in the Northeastern United States.
Heavy rainfalls had fallen: 6.96 inches of rainfall in three hours at the West Point Military Academy, 5.35 inches in Reading, Pennsylvania, and 10 inches in the Hudson Valley of New York.
Such predicted floods were reported based on the chance—i.e., the statistical probability—of the flood magnitude.
A 1-in-1000-year flood means there is a .1 percent (one-tenth of one percent) chance of a flood of a specific size in any one year.
In the 1960s, the United States Geological Survey (USGS) developed an annual exceedance probability (AEP) to measure floods. For these statistical analyses, scientists use both the height of the water level and the flow rate in rivers to create complex statistical models to report the magnitude of a flood.
USGS has some 7,500 stream gages on rivers nationwide. The stream gages measure the height of the water and flow rate (streamflow in cubic feet per second). The agency uses the data to explain the magnitude of floods.
A “1-in-1,000-year flood” does not mean such a flood will occur at 1000-year intervals.
Think of each flood as an individual event.
From the statistical viewpoint, the occurrence of a flood in one year does not influence if a flood will occur the next year. That means a series of 1-in-1000-year floods could occur two, three, or more years in a row, or there could be no 1-in-1000 flood for 2,000 or more years to come. The chance of an remains the same.
Many factors influence the magnitude of floods—prior rainfall amounts; soil types in the watershed (the area that drains water into a river); soil water absorb capabilities; number, intensity, duration, and frequency of thunderstorms; snowfalls and accumulations; snow-melt rates; topographical features; area geology; human-made features; and other factors.
Floods can happen at any time.
For example, when my wife and I moved to Harrisburg, Pennsylvania, in January 1971, I noticed that Harrisburg buildings and houses were built close to the Susquehanna River. The river appeared about a mile wide and flowed from New York State on the north to Maryland and the Chesapeake Bay on the south. Its watershed (area from which rain runoff flows) covers about half of Pennsylvania.
The magnitude of the Susquehanna River basin and the chance of flooding concerned me.
So, I asked Ted, a new colleague in Harrisburg, “ Does it flood here?”
He responded, “No, it hasn’t flooded here since the 1930s–don’t worry about it.”
Why was I concerned?
I have vivid memories of the 1951 Kansas River flood in Kansas City, Kansas.
I was a grade-schooler, and my parents, brother, and I lived in a suburb south and west of Kansas City, Kansas. Our house sat on a hilltop well above the floodplain.
When the Kansas (Kaw) River flooded, my dad took my brother and me down to the 55th Street bridge to see the flood. The flood had not reached the south end of the 55th Street bridge, but it had reached the north end of the bridge. The bridge was not in danger of collapsing.
We parked two blocks south of the 55th Street bridge and walked over the bridge to where the flood had reached its very north end.
There, we watched the muddy brown water that stretched from bluff-to-bluff, flooding the Kansas River valley. Houses, cars, buildings, massive logs, debris, and trash floated by as we looked to the north. The flood also contaminated the public water system that provided our drinking water. We made trips to a public well for our drinking water and disinfected water with a disinfectant to kill any germs it might have contained. It was weeks before bridges were repaired or opened before we could see our grandparents in Kansas City on the north side of the Kansas River.
My concern about the Susquehanna flooding proved right.
A year and a half after arriving in Harrisburg, June 1972, we experienced the Susquehanna River flooding. Hurricane Agnes settled in the Susquehanna River Basin. It rained heavily for days, and the Susquehanna River flooded downtown Harrisburg and surrounding communities.
–Don Z