Pop quiz: In the last 100 years in Harris County, Texas, were you more likely to get flooded in the first or second half of the year? And the answer is…second half. But surprisingly, it’s a close tie. Looking at the data, also revealed that a major flood happened every 2.5 years on average.

How Numbers Were Compiled

Harris County Flood Control District keeps a list of major floods. It actually goes back further than 1920. However, the pre-1920 records don’t reliably record the month of the flood, so I limited the sample to 100 years for the purpose of this quiz.

HCFCD shows 38 events through 2016. For my count, I added Hurricane Harvey (2017) and Tropical Storm Imelda (2019). Both happened since the last HCFCD chart update. And both produced major flooding in Harris County. That brought the total to 40 events.

22 of 40 Events Happened in Second Half of Year

Of the 40 major floods in 100 years, 18 happened from January through June; 22 from July through December.

That means you’re almost as likely to get flooded in the spring as you are by a tropical event in the summer or fall.

23 of 40 Events Happened During Hurricane Season

However, if you phrased the question as, “How many major floods happened during hurricane season?” you would get a slightly different answer. Seventeen of the 40 did not and 23 did.

That’s because:

• June falls in BOTH spring and hurricane season. Note that two floods, Audrey in 1957 and Allison in 2001, both occurred in June.
• In 1935 a major flood occurred during December, which is outside of hurricane season.

Major Flood Intervals Average 2.5 Years

The other major, mind-bending, slap-you-in-the-head statistic that comes out of this quiz concerns frequency. Forty events in 100 years represents a 40% chance of a major flood happening in any given year.

The average interval of major flood events: 2.5 years. The shortest interval: one month in 1929, 1989 and 2016. The longest interval: eight years between 1961 and 1969.

If those statistics don’t make you a believer in flood insurance, I don’t know what will. It should also make you a believer in flood control and drainage districts if you live in a southeast Texas county, such as Montgomery, that doesn’t have one.

For those whose screen is too small to read the data above, here’s a printable PDF.

If you don’t have a printer at home, here’s the breakdown:

First-Half-of-Year Floods: 1929 (April and May), 1930, 1955, 1957, 1960, 1969, 1972, 1973, 1983, 1989, 1992, 2001, 2006, 2009, 2015, 2016 (2 in April and May)

Second-Half-of-Year Floods: 1932, 1935, 1940, 1943 (July and October), 1945, 1959, 1961, 1979, 1983 (August and September), 1984, 1994, 1998 (September, October, November), 2008, 2012, 2014, 2017, 2019

Posted by Bob Rehak on 4/17/2020 based on HCFCD data

962 Days since Hurricane Harvey