The hydraulic effects of the proposed Texas Highway 43 crossing of the Sabine River near Tatum, Texas, were determined on the basis of results from a two-dimensional finite-element surface-water-flow model. In planning the replacement crossing by the Texas State Department of Highways and Public Transportation, approximations of apportionment of flow among the openings and velocities within the openings were of concern. The model was used to simulate flow in the river floodplain system for the proposed design, an alternate design, and for the natural condition. The proposed bridge design by the State Department of Highways and Public Transportation consisted of a 320-foot main channel opening, four left overflow bridges with widths of 120, 320, 320, and 280 feet, and one right overflow opening with a width of 440 feet. The alternate design consisted of a 950-foot main channel opening, three left overflow bridges with widths of 320, 320, and 280 feet, and one right overflow opening with a width of 200 feet. Preliminary one-dimensional computations were used as an aid in establishing the boundary conditions for the two-dimensional analysis.

The analysis evaluates the interaction of complex hydraulic characteristics of varied features found at many multiple-opening highway crossings. Valid applications of accepted standard one-dimensional techniques would not reveal some aspects of the overall hydraulics of the crossing.

The simulation of the proposed design shows, among other things, which openings are the most productive per foot of bridge length. The results of the two-dimensional simulation of the proposed design indicate some differences in the apportionment of flow among the openings when compared to the one-dimensional proposed crossing computations. The alternate design results in computed water-surface altitudes which were slightly lower than those of the proposed design. The alternate design would require less modification to the existing embankment. Velocities computed within the openings and at the bridge abutments, using the two-dimensional model, were within the design specifications of the State Department of Highways and Public Transportation.

The simulations of the proposed and alternate designs indicate a lateral component of the water-surface slope at the embankment. Redistribution of flow across the floodplain also is indicated in both simulations. Some of the differences in the response between the two designs are affected by geometric features of the floodplain other than the embankment-opening geometry.