An earthen embankment of the Willow River Dam failed in 2016 during a large flow event resulting from rainfall in excess of seven inches in a 24-hour period. The Minnesota DNR, in consultation with the community of Willow River, elected to restore the upstream reservoir level by constructing a rock arch rapids to span the 13-foot elevation change between the reservoir and the downstream river. The rock arch rapids would improve site safety, provide upstream fish passage, and allow for small boat passage when conditions permit.
Barr performed detailed design and modeling for the proposed rock arch rapids, including development of a two-dimensional hydraulic model to evaluate flow velocity and shear stress under a variety of flow conditions. The hydraulic modeling allowed us to size the rapids appropriately to maintain stability during critical flow conditions. Because the rock rapids maintain upstream lake levels that existed previously, the rapids are classified as a dam by the DNR.
From an environmental perspective, the new “dam” represents a major improvement to the river ecosystem, while maintaining the upstream reservoir level. The dam consists of a series of rock rapids with large boulders that break up the flow and create ideal fish habitat. The rapids were designed to allow upstream migration of warm-water fish species while safely accommodating much larger flood flows than the old dam.
Senior Water Resources Engineer
Vice President Senior Water Resources Engineer
Water Resources Engineer