Stability of the channel of the Minnesota River near state bridge no. 93, Minnesota

Abstract

The Minnesota State Highway No. 93 bridge crosses the Minnesota River near the town of Le Sueur. The bridge is situated at the apex of a bend the outside of which impinges against the eastern river valley wall. Near Le Sueur, the Minnesota River is a meandering stream with actively migrating bends. The valley walls has helped stabilize the channel in the immediate vicinity of the bri~ge, where the channel has moved little in the course of a century. Just upstream of the bridge is a reach consisting of several short bends, nowhere impinging against the valley walls, that are migrating downstream and outward at a relatively rapid rate near 9 ft/year (3 m/ year). The western approach to the bridge was riprapped in order to thwart the downstream progression of a bend. As a result, the bend ravelled up against the riprap and cut itself off. The channel now impinges against the riprap at a ninety degree angle, and then flows along the base of the riprap to the bridge opening. A large scour hole exists at the point of impingement, where the approach is in danger of being washed out. In the future, successive bends can be expected to migrate into the riprap, until the channel breaches the western approach and abandons the bridge. The Minnesota Department of Transportation is presently considering replacement of the bridge deck. The actively migrating bends in the reach in question preclude relocating the bridge away from its present stable location near the valley wall. A short channel relocation just upstream of the bridge can improve alignment. It can also mitigate the possibility of a natural cutoff causing the bridge to be abandoned.