A Comparison between Summer 2016 Flash Flood Observations and Rainfall ARIs Across the North Central United States

Due to the threat flash floods pose to life and property, an attempt is being made to utilize the average recurrence intervals (ARI’s) of significant rainfall events as a threshold for flash flood warnings. To aid in such efforts, the relationship between ARI’s and flash flood observations was explored. It is hypothesized that the majority of flash flood observations fall within a specific recurrence interval or greater. To test this hypothesis, a region of the continental Unites States was chosen in which a total of 20 flash-flood-producing rainfall events were studied. The events occurred during the summer months of 2016 and were chosen based upon the number of flash flood reports produced, and the value of resulting property damage. Overall, among the 20 events studied, 527 flash flood observations were reported, 3 fatalities occurred, and 55.54 million dollars worth of property damage ensued. It was found that the majority of observed flash flooding in the north central region of the United States does not occur within a specific ARI threshold. But, as one may expect, an ARI of 1 year was found to encompass the most flash flood observations, near 44% of the entire flash flood reports. However, for events of a greater magnitude, such as those with a maximum ARI of 50 or 100 years, over 50% of flash flood reports were found to occur within the first year ARI or greater. In addition, it was found that rainfall events with ARI’s of greater magnitude, such 25, 50, and 100 year events, are responsible for producing a larger number of flash flood reports, a larger number of “major” flash flood reports, and greater property damage. Several challenges arise when considering flash flood reports. Report density is dependent on many different factors including population, time of day, and antecedent conditions. In addition, what is considered to be a “flash flood” is relative, and there exists no consistent method of issuing flash flood reports. Such inconsistencies allow for error and/or bias when drawing conclusions based upon flash flood reports. Therefore, consistency in reporting flash flood observations needs to be considered if flash flood prediction is to improve.