A two-dimensional finite-element surface-water model was used to study the effects of proposed modifications to the U.S. Highway 231 corridor on water-surface elevations and flow distributions during flooding in the Tennessee River Basin south of Huntsville, Madison County, Alabama. Flooding was first simulated for the March 19, 1973, flood for the existing conditions in order to calibrate the model to measured data collected by the U.S. Geological Survey (USGS) and the Tennessee Valley Authority (TVA) during and after the flood. After model calibration, the effects of flooding were simulated for two scenarios---existing and proposed conditions----for the 100-year and 500-year recurrence intervals. The first scenario was to simulate the existing bridge and highway configuration for the U.S. Highway 231 crossing of the Tennessee River flood plain. The second scenario was to simulate the proposed modifications to this bridge and highway configuration. The simulation of floodflow for the Tennessee River flood of March 19, 1973, in the study reach compared closely to discharge measurement and flood profile data obtained during and after the flood. The flood of March 19, 1973, had an estimated peak discharge of 323,000 cubic feet per second and was estimated to be about 50-year flood event. Simulation of the 100-year floodflow for the Tennessee River for the existing conditions at U.S. Highway 231 indicates that of the peak flow, 92.1 percent (316,500 cubic feet per second) was conveyed by the main channel bridge, 4.0 percent (13,800 cubic feet per second) by the northernmost relief bridge, and 3.8 percent (13,200 cubic feet per second) by the southernmost relief bridge. The water-surface elevation predicted in the vicinity of the USGS gaging station was 576.91 feet. No overtopping of U.S. Highway 231 occurrec. For the 500-year flood, the simulation indicates that of the peak flow, 89.2 percent (359,000) cubic feet per second) was conveyed by the main channel bridge, 5.6 percent (22,600 cubic feet per second) by the northernmost relieft bridge, and 5.2 percent (20,900 cubic feet per second) by the southernmost relief bridge. The water-surface elevation predicted in the vicinity of the USGS gaging station was 580.91 feet. No overtopping of U.S. Highway 231 occurred; however, the girders of both relief bridges were partially submerged. Simulation of the 100-year floodflow for the Tennessee River for the proposed conditions indicates that of the peak flow, 93.2 percent (319,800 cubic feet per second) was conveyed by the proposed main channel bridge, 3.3 percent (11,400 cubic feet per second) by the proposed northernmost relief bridge, and 3.4 percent (11,800 cubic feet per second) by the proposed southernmost relief bridge. The water-surface elevation predicted in the vicinity of the USGS gaging station was 576.93 feet. No overtopping of U.S. Highway 231 occurred. For the 500-year flood, the simulation indicates that of the peak flow, 90.9 percent (365,400 cubic feet per second) was conveyed by the proposed main channel bridge, 4.3 percnet (17,300 cubic feet per second) by the proposed northernmost relief bridge, and 4.8 percent (19,400 cubic feet per second) by the proposed southernmost relief bridge. The water-surface elevation predicted in the vidinity of the USGS gaging station was 580.93 feet. No overtopping of U.S. Highway 231 occurred; however, the girders of both relief bridges were partially submerged.