The Stone Arch Bridge, a historic 19th-century railroad bridge in downtown Minneapolis, spans the Mississippi River at Saint Anthony Falls. It now serves pedestrians and cyclists. In 2024, the Minnesota Department of Transportation initiated repairs to five of the bridge's piers near Mill Ruins Park, but strong river currents posed a safety risk for divers working on the project. Contractor J.F. Brennan Co., Inc. (Brennan) partnered with Barr to perform hydraulic modeling and structural analysis for a custom temporary flow deflector system designed to slow water around the piers during underwater repairs.
Barr built a two-dimensional computer model of a 1,550-foot stretch of the river around the bridge. The model accounted for water splitting between the Xcel Energy powerhouse and spillway as well as the influence of the downstream dam. Using existing riverbed and surface data, we tested conditions up to 18,000 cubic feet per second, the maximum flow rate considered safe for diving.
Five flow deflector options were tested—one for each pier. The modeling showed that the deflectors could successfully slow water in underwater work zones to less than four feet per second, which would allow divers to operate safely. Barr also calculated the forces acting on each deflector. The deflector designed for Pier 10, facing the most intense flows, was estimated to withstand up to 50,000 pounds of pressure. These results informed the final frame and anchor designs.
By combining hydraulic and structural expertise to inform deflector design, Barr helped Brennan ensure diver safety, minimize construction delays, and support the rehabilitation of one of Minneapolis’s most iconic landmarks.
