The Ohio Department of Transportation doesn’t plan on engineering any changes to local highways following unprecedented flash flooding that closed the interstates on May 21.

The decision follows an ODOT review of the flooding that reached at least 4 feet in some spots and required the rescue of motorists trapped by the waters.

So much rain fell in some locations that the storm was the equivalent of a 1,000-year flood, ODOT said in a report released Thursday.

ODOT District 7 Deputy Director Randy Chevalley said flooding of more than one lane at a time on Ohio highways is highly unusual, including during tornadoes.

“This was a doozy,” Chevalley said.

The highway agency based its analysis on records kept by the Miami Conservancy District and other weather trackers. Total rainfall in the area of flooding was approximately 4.5 inches in roughly a two-hour time period.

The interstates are designed for a 50-year flood, the standard since the interstates were built, ODOT said. “We build highways to take a beating,” Chevalley said. “We lucked out and had minimal damage in this region.”

“According to the National Oceanic and Atmospheric Administration’s point precipitation frequency estimates this equates to what is commonly referred to as at least a 1,000-year flood,” ODOT said. “Another way to say it would be the probability of that amount of rain in that duration has the probability of happening, 0.1 percent chance in a year’s time.”

Said ODOT: “Specific to the interstates, we have determined they held up well and are in good shape. You cannot plan nor design for such a rare and extreme event and we have no plans to make any changes to the highway system at this time.”

Flooding closed Interstate 70 near the Bellefontaine Street overpass in Huber Heights and I-75 near the Eldean Road overpass north of Troy and the Peters Road overpass south of Troy,

The flooding did no lasting damage to highway drainage systems, ODOT said.

On I-70, Drylick Run creek channeled flood waters. A concrete stream channel beneath the highway built in 2006 remains in good condition and is functioning as designed, ODOT said. On I-75 north of Troy/Eldean Road, drainage pipes also remain in good shape. The same is true for a concrete drainage structure south of Troy/Peters Road.

In adding up the damage that required repair, ODOT found erosion at a location just south of the 25A exit between Piqua and Troy. The erosion was at two 16-foot pipes that were built in 1957.

“This required extensive work to replace and compact the material that washed away and re-constructed the guardrail,” ODOT said. The cost for this repair was $13,500. Additional inspections and patrols to examine potential damage cost $7,000.

Storm was in ‘exact place’ for highway shutdowns

WHIO-TV Channel 7 Chief Meteorologist Jamie Simpson said a 100-year or 1,000-year flood is a statistical possibility.

“The 1,000-year flood indeed means in any given year there is a 0.1 percent chance of it happening, likewise a 100-year flood means a 1 percent chance of an occurrence in a given year,” Simpson said. “Can a 100-year flood happen two years in a row? Yes, and chances are the same every year based on history. Since we only have good records for the past 100 to 150 years, it is possible that 100-year floods or any other 100-year occurrences of any weather phenomena may be recalculated over time. What is believed to be a 100-year flood may end up being only a 50-year flood is it happens more frequently with time.”

Simpson also said it is important to note that this “1,000-year flood” just happened to be in the exact place for these highway shutdowns to happen.

“If the heaviest rain shifts in any direction by 20 miles we aren’t having this discussion,” he said. “But perhaps we are having a different discussion depending on where it was located. Bottom line is that that much rain in a short period of time is very uncommon, but not impossible here, as we saw that day.”