MANKATO — A study on the stability of the Rapidan Dam is complete, leaving the Blue Earth County Board with a decision on whether to pay for a $10 million fix or continue with what they hope will be relatively minor maintenance.

Put another way, do you plunk down $1,500 for a new transmission on your clunker or do you ride it out and hope to get some more miles out of it?

Or maybe you just scrap the car.

The $187,000 U.S. Army Corps of Engineers study estimated it would cost $29 million to remove the county-owned dam entirely.

The question boils down to risk.

“How much risk are you willing to accept and how much will you pay to avoid that risk?” said Craig Evans, a civil engineer with the Corps.

The Rapidan Dam as it stands now offers “far less security than most people would accept,” Evans said. The dam does not meet Corps standards.

But the Corps doesn’t own the dam and isn’t telling the county what they should do with it.

That will be up to the County Board, which received on Wednesday what Public Works Director Al Forsberg said was the best information to date and justifies the county’s one-fourth stake in the study’s cost.

The trouble with the Rapidan Dam, Evans said, is not in the visible structure towering above the Blue Earth River.

The problem is the concrete slabs and rock extending from the base of the dam downriver.

Over time, water rushing over the dam has landed with such force that it’s chipped away at the highly erodible sandstone foundation. The foundation varies by location; in some places it’s as porous as sand.

This sort of erosion is now believed to be the cause of a concrete gap discovered beneath the dam in 2007 that prompted the emergency closure of the nearby park.

Interestingly, the turbulence can cause erosion more than 100 feet downriver of the dam and threaten the dam itself, Evans said.

The solution would be to install a new basin at the foot of the dam that dissipates the energy of the crashing water in a way that isn’t harmful to the dam. That is estimated to cost at least $10 million with much of the cost coming from the difficulties in laying concrete in a river.

As it stands now, the basin downstream of the dam can absorb an estimated 28,500 cubic feet per second of water. By comparison, the 100-year flood is estimated to be 30,000 cfs and the largest flood recorded here is 43,100 cfs.

That flood, in 1965, took the dam’s hydroelectric generators out of commission. The county acquired the dam in 1970.

Of course, a longer-term answer would be to remove the dam entirely. The Corps’ perspective in its studies has always been to remove the dam and restore the river’s ecosystem.

There was some good news with the study. The most dangerous kind of erosion, which happens when water rushes underneath a dam, does not appear to be occurring. The massive amount of silt behind the dam — about 12 million cubic yards at last count — may actually be insulating the dam’s foundation from that sort of a leak.

The County Board didn’t make any formal decisions at its work session but did direct Forsberg to write a bid package to find a contractor that can respond quickly at the dam to do repairs in times of low water flow.