A two-dimensional finite-element surface-water model was used to simulate the effects of the proposed State Highways 15 and 67 relocation on water-surface elevations and flow distributions for the 100-year flood on the Tchoutacabouffa River at D'Iberville, Mississippi. The Mississippi Department of Transportation plans to relocate State Highways 15 and 67 by removing a portion of the existing four-lane highway and constructing a four-lane facility upstream of the existing alignment. The proposed alignment is located on the northern floodplain and will tie into the existing highway about 1,000 feet north of the dual State Highways 15 and 67 bridges. The proposed alignment will intercept flows that cross the existing highway during large floods. Seven scenarios were simulated for the 100-year flood, including four proposed alternative configurations for drainage structures. The model grid was developed by using surveyed floodplain cross sections and channel bathymetry data obtained by using an Acoustic Doppler Current Profiler, in combination with a global positioning system. The model was calibrated and verified by using surveyed flood profiles through the study reach and flood discharge measurements obtained at the State Highways 15 and 67 crossing. Model parameters were adjusted so that the computed water-surface profiles agreed closely with the surveyed flood profiles. Computed water-surface differentials across the proposed alignment near the northern edge of the floodplain for the four alternatives proposed by the Mississippi Department of Transportation ranged from 1.4 to 2.6 feet. Much lower differentials were computed in the vicinity of the main-channel bridge. The computed water-surface elevation at McCully Drive, upstream of the proposed alignment, was 17.3 feet for existing conditions. Computed water-surface elevations at McCully Drive for the proposed alternatives ranged from 17.3 to 17.8 feet.
