Help enlisted to diagnose erosion along riverbank

Eden Prairie has asked the U.S. Army Corps of Engineers to diagnose a severe riverbank erosion problem on the city's southern limits along the Minnesota River -- one that could eventually threaten homes along the bluff above if not corrected, officials say.

Since 1937, the river has cut 300 feet into a point on its north bank in Eden Prairie, forming a sharp bend in the river and even washing away a chunk of old Riverview Road, a historic gravel road now used for hiking along the riverbanks. The erosion is occurring about a mile and a half west of Hwy. 169 at the base of a tall bluff lined by about a dozen homes overlooking the river valley.

The homes along the west end of the current Riverview Road are 500 feet up the bluff from the river and not in imminent danger, the city says. But the river erosion -- combined with bluff landslides caused by storm-water runoff, seeping groundwater and river flooding -- all add up to a concern about the long-term stability of the bluff, officials say.

"The long-term issue is how stable is that slope and is that endangering the houses and everything above it?" said Dale Thompson, supervisor of the municipal storm water unit of the Minnesota Pollution Control Agency (PCA).

"You are fighting the forces of nature here," Thompson said. "Man has meddled and moved in where Mother Nature wasn't very stable, and now we are starting to see some of the costs associated with trying to develop in an area like that."

The city wants answers from the Corps of Engineers concerning how urgent the problem is, how much of a threat it is to the homes, what's causing it, how to fix it and what it would cost, Eden Prairie Public Works Director Gene Dietz said. He expects Corps representatives to visit today.

Repairing the old road and shoring up that erosion would cost more than $400,000, but three more similar points of erosion are beginning as well, Dietz said. "When we have upward of half a mile of riverbank subject to the same forces, we think we need some serious expertise about how to manage the river."

The erosion "looks like a big washout, like a slide, a big severe erosion problem in the bend of the river," Lower Minnesota River Watershed District Administrator Terry Schwalbe said. The district wants to stop it, he said, because "there is public property and private property that could be lost if the erosion continues."

The watershed district has joined in the request for help from the Corps, but also plans its own study of the area that will look at how much storm water and seeping groundwater from the bluff might be contributing to the riverbank erosion, Schwalbe said.

The district wants to be sure that any corrective measures address all issues, because more river erosion could pose a future danger for the houses on top of the bluff, he said.

The PCA is also active in the area. It has brought enforcement action against Eden Prairie to force the city to build a new settlement pond to clean storm-water runoff from the top of the bluff before it goes into the river.

The city's original pond, built in 1987 in the river's flood plain, is just east of where the major erosion has occurred. Now the pond is missing a wall on the river side and allows storm water to flow directly into the river.

But the PCA contends that high storm-water levels weakened the pond wall and that it collapsed at least in part because the city did not manage the pond properly.

"We are requiring them to come up with plans to correct the situation," Thompson said.

Because of movement of the river, Dietz said there is no space to build a new pond. The PCA wants the city to find another spot. The storm water is coming from the top of the hill, Thompson said. "They could capture it and redirect it."

The PCA is taking enforcement action because the city has not volunteered to find and buy another site, Thompson said. The PCA may give the city a proposed schedule this week for replacing it, he added.

"What the Corps does will not change what has to be done from the municipal storm-water side," Thompson said. "They [the city] are supposed to be managing their storm water to reduce the loading of pollutants to the river to the maximum extent practical."