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.
STATE OF MINNESOTA
MINNESOTA DEPARTMENT OF TRANSPORTATION
ST. PAUL, MINNESOTA 55155
Stability of the channel of the Minnesota River near state bridge no. 93, Minnesota.
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