How much rainfall can MCD dams and levees handle?

By Kurt Rinehart, MCD Chief Engineer

With the heavy rains of recent hurricanes, especially Hurricane Harvey’s 50 inches, people are wondering how much precipitation can the MCD flood protection system handle?

The system is designed for the greatest reasonably expected storm but not the largest scientifically possible storm. In other words, the system is designed to handle more rain than the region has ever seen but not as much as meteorologists and scientists predict could occur in a worst-case scenario.

The 1913 Flood brought 9 to 11 inches of rain in three days across the entire 4,000-square-mile Great Miami River Watershed.

What the flood protection system is designed to handle

MCD’s integrated system of five dry dams, 55 miles of levee and acres of preserved floodplain is designed to withstand a storm the size of the 1913 flood plus another 40 percent. Eight to 11 inches of rain fell over the 4,000-square-mile watershed in three days in March of 1913. So the flood protection system is designed to handle about 14 inches of rainfall across the watershed over a three-day period.

The two largest high-water events since the 1913 flood were in 1959 and 2005. In 1959, 4 to 6 inches of rain fell between January 19 and 21. In January 2005, we saw 7 to 10 inches of rain in 14 days, plus another 1.5 inches of precipitation in snowmelt from a 15-inch snowstorm in December.

In both of those events, there was still plenty of capacity in the retarding basins behind the dams. In the 1959 event, floodwaters filled 32 percent of Germantown Dam’s retarding basin. That’s the most any dam has ever held.

Based on those numbers, there nothing to worry about, you might think. Which is true. Kind of.

A study commissioned by the Ohio Department of Natural Resources in 2013 predicts in an absolute worst-case scenario storm, this region would receive 16 inches of precipitation over the entire watershed in three days. The dams could hold the floodwaters but the levees likely would be overtopped.

Biggest storm possible

A study commissioned by the Ohio Department of Natural Resources in 2013 predicts in an absolute worst-case scenario storm, this region would receive 16 inches of precipitation over the entire watershed in three days. This is the most extreme scientifically possible event for our region.

If that were to happen, the dams could hold the floodwaters but the levees could be overtopped and flood the cities.

Maintenance and reinvestment are key

The cost-benefit ratio doesn’t allow for us to build a system large enough to handle a worst-case scenario. It doesn’t make financial sense to build for a storm that in all likelihood will never happen. But it is crucial that we continue to maintain our system to handle the smaller storms that could still flood our cities if we didn’t have a working flood protection system.

That’s why maintenance, reinvestment and preparation are key. Our dams and levees are nearly 100 years old. Fortunately, MCD has worked hard to maintain the structures over the last century.

More recently, MCD’s capital improvement project, called the Dam Safety Initiative, addressed potential seepage issues in the foundations and the crests of the dams. We also repaired and replaced concrete floodwalls and revetment. More repairs and investment, however, are needed in the coming years to ensure the dams and levees continue to protect our riverfront communities.

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