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Bluff-top homes overlooking the Minnesota River valley in Eden Prairie are on solid footing, but slope and riverbank erosion must be stopped to ensure their safety in the future, a new watershed report has found.

Concern is focused on severe erosion along 1,200 feet of the river's north bank bordering Eden Prairie just west of Hwy. 169. About 540 feet of the bluff face above the river is failing too, according to the Lower Minnesota River Watershed District. It received a report from Wenck & Associates in December that diagnosed the erosion's cause and suggested solutions.

"The overall stability of the slope shows that the properties on the bluff are well within the acceptable minimum factor of safety," the report says.

The watershed district is scheduled to choose an erosion-control strategy designed to keep the homes safe next Wednesday. Eleven homes are in the area of concern.

Historical records and river photos indicate that at this location the Minnesota has cut 115 feet into its north bank since 1967. The line that once defined the north bank is now in the middle of the river. The water is eating into the bank at a rate of about 3 feet a year.

This erosion process is natural but may have accelerated in the past several years due to increased drainage or climate change, Wenck said. Without erosion controls, the meandering of the river will continue downstream, cutting into new sections of bank.

"We would like to have a permanent correction to this," said Terry Schwalbe, executive director of watershed district. "Part of the permanent solution is not pushing it down river."

Wenck recommended building about seven bumper-like rock vanes along the north bank of the river, at a cost of about $1 million, to stop erosion and rebuild the bank.

Rock vanes are strategically placed piles of rock that jut at an angle into the river. Those piles direct river flow away from the bank while encouraging sediment to drop out and reinforce the river's edge, said Joel Toso, Wenck project manager.

The rock vanes would move the river away from the bluff over time and bring out the bank to its previous location, Toso said. "Right now the bank is moving toward the bluff."

To stop storm water erosion of the bluff face, Wenck prescribed grading it, planting it with vegetation and conveying storm water down the side of the slope directly into the river with pipes or on a riprap channel.

Grading the slope to a stable angle would wipe out a walking trail on an old stagecoach route along the river.

Once the board selects an erosion-control strategy, engineering plans will be drawn up to determine a cost, Schwalbe said.

Laurie Blake • 612-673-1711