ST. LOUIS, Mo. – Flash Flooding hit our area extremely hard on July 26th and 28th. Rainfall rates over two inches per hour caused historic flash flooding. That kind of flooding happens about once in 150 years in the St. Louis area.
“This rainfall event was the c one the St. Louis metropolitan area has seen since records began in 1874. Roughly 25% of our normal yearly rainfall fell in about 12 hours, and the highest 6-hour total (7.68″) surpassed the normal amount of rain for the months of July and August combined. What is most impressive is how quickly the all-time record for 24-hour rainfall (7.02″ in 1915) was surpassed. It took only 6 hours since midnight CST (all climate records are kept in standard time) to surpass the old record,” states the National Weather Service.
The high number of impervious surfaces like roofs and concrete contributed to the rapid run-off and fast rises on streams.
You can not really compare this past summer’s event to the Flood of ’93. But, we can try to visualize the amount of rain that fell in the area.
One inch of rain over one square mile equals 17.4 million gallons of water weighing 143 million pounds (about 72,000 tons), or the weight of a train with 40 boxcars. The heaviest rain was 10″ and it fell in a band that ran from Wentzville to St. Peters. It was around three miles wide by 14 miles long, or 42 square miles.
Now multiply 42 square miles times the 10 inches of rain per square mile times 17.4 million gallons. That is approximately the weight of 420 trains with 40 boxcars each.