Willow River Dam - Innovative Conversion to Compound Rock Arch Rapids

In 2016, floodwater overtopped a state-owned dam on the Willow River in East Central Minnesota, breaching the dam’s right earthen embankment. The dam failure drained an 86-acre reservoir, created a bypass channel around the dam, and routed significant quantities of sediment downstream.
The Minnesota Department of Natural Resources (DNR) developed a concept design to replace the dam with a series of irregular compound rock arches and hired Barr Engineering Company Co. (Barr) in the spring of 2019 to take the project through final design and construction. The varying shape of the weirs required the development of a high-resolution, 2-dimensional HEC-RAS model to evaluate the stability of the design under various flow conditions. Collaboration between Barr and DNR engineers over a year led to a design constructed in the winter of 2020–2021.
The rapids have achieved the project goals of reducing maintenance, increasing safety, facilitating aquatic organism passage, and re-establishing runout elevation of the reservoir.