By Mike Ekberg, manager of water resources monitoring and analysis
MCD flood protection dams are storing water more often than at any other time since the dams were completed almost 100 years ago. That’s because the Miami Valley’s climate is getting wetter. Can the flood protection dams handle more rain?
A rising 30-year average precipitation
Average annual precipitation in the Miami Valley for the 30-year period of 1951 to 1980 was about 37 inches a year. Average precipitation for the last 30 years (1991 – 2020) has climbed to almost 42 inches a year—a nearly 14 percent Increase.
Average precipitation increased for every month except August. Average precipitation for the months of January, April, May, June, July, September, October, November, and December increased by more than 10 percent. October showed the largest increase in average precipitation—more than 30 percent.
With increased precipitation comes increased runoff and higher river flows. When river flows become high enough to be a flood threat, our flood protection dams go into action and begin to store water. When any one or more of our dams begin to store water, we call that a “storage event.” Storage events at each of the dams are recorded separately. So if all five dams are in storage at the same time, it is counted as five storage events. The storage event ends at each dam when that dam is no longer holding back any water.
Waters stored behind dams more frequently
MCD tracks the number of storage events that occur each year. The following chart shows the number of storage events that have occurred during each full decade since the dams were completed in 1922. You can see how the number of storage events has climbed throughout the decades of the 1990s, 2000s, and 2010s. Prior to the 1990s, no single decade had more than 200 storage events. The number of storage events in the last three decades all exceeded 200, and storage events for the decade of the 2010s exceeded 300.
More frequent large storage events
Increasing frequency of storage events at MCD dams is one thing. What about the size of those storage events?
The answer seems to fall in line with simple odds. All other things being equal, the more storage events there are, the greater the chance of having really large events.
We rank storage events based upon the total storage volume of water held behind our five dams. The following chart shows the number of events by decade that rank in the top 100 largest storage events. You can see that 32 of the top 100 largest events took place in the last 20 years. The decades of the 2000s and 2010s each had 16 storage events that ranked in the top 100. Only the 1950s comes close to this number. That decade produced the largest storage event since the MCD flood protection system was built—the January 1959 event.
MCD flood protection system is resilient
The climate of the Miami Valley is changing and getting wetter. Flood risk is increasing. Fortunately, the MCD flood protection system is well designed to respond to these changing conditions in the Miami Valley.
The system was designed to withstand a very large event—the 1913 flood event plus 40 percent more runoff (which is the equivalent of 11-14 inches of rain over three days). The January 1959 event was the largest storage event since the completion of the flood protection dams. The 44.8 billion gallons of floodwater the dams held back used only 16 percent of the five dams’ total storage capacity. That means 84 percent of the capacity has never been used but is there if we need it.
The MCD flood protection system is resilient—based on design, capacity and performance. Resiliency is a good thing to have in a rapidly changing world.
Learn more about the flood protection system.