Model study of the Minnesota River near trunk highway no. 169 Bridge, Minnesota

Abstract

The Minnesota Trunk Highway No. 169 crosses the Minnesota River just north of the town of Le Sueur. The bridge is immediate1y downstream of the confluence of the Minnesota River and a small tributary, Le Sueur Creek, entering from the east. The bridge was completed in 1960. The approaches were designed so as to preclude overtopping. In fact, they were not overtopped during the largest flood on record, which occurred in 1965. During severe floods, the entire floodplain carries a considerable discharge. The severe constriction at the bridge was partially alleviated by excavating the western bank and immediately adjacent floodplain to provide a floodway. The bridge spans the original channel (east side), Where the piers are deep, and the original f1oodway (west side), where the piers are shallow. During the episodic flood of 1965, the severe constriction of overbank flow at the bridge caused scour that endangered the western approach. The recession of the flood waters revealed a large bar at the mouth of the tributary. This bar had occupied the old channel and forced the river to the west, against the shallow floodway piers. At one point several of these piers were subjected to scour below their seals. Although various corrective measures were taken, the problem became progressively worse. In 1981, contingency measures were implemented, including filling of a portion of the west side with riprap and excavating a portion of the east side to provide a pilot channel. In addition, the model study described herein was commenced, in order to obtain guidance as regards more permanent corrective measures. The model study was conducted using Froude similarity with a horizontal scale of 1:200 and a vertical scale of 1:40. Crushed walnut shells were used to model the bed sediment. The model study indicated that scour resulting from the severe constriction at the bridge becomes an increasingly difficult problem as the fraction of discharge that is overbank increases. During severe floods, sediment from Le Sueur Creek cannot enter the channel due to slack water at the mouth of the tributary. Sediment collects in Le Sueur Creek, and enters the channel as a slug as flood flows recede. Additional deposition can occur when the Minnesota River is low, but the tributary is high. It was found necessary to distinguish between corrective measures operable at below- and above-bankfull stages. At above-bankfull stages, a T-spur dike on the western floodplain adjacent to the channel can pull floodplain flow away from the western approach and deflect it to the east, is near the mouth of the tributary. At lower flows, two in-channel permeable dikes can encourage deposition on the west side, and deflect the flow to the east side. The height of the permeable dike must be well below bankfull stage in order that appropriate conveyance be provided for higher flows. These, and certain other maintenance-oriented corrective measures, are proposed in order to return the channel to its original position and control the growth of the bar at the mouth of the tributary. At flows above about 60,000 cfs, severe scour is induced at the bridge due to the intense constriction of floodplain and channel flow. Increased conveyance would reduce the potential for severe scour. If this cannot be provided, extensive riprapping of pier footings is recommended.