Improving Understanding of the Littoral Drift Direction in Texas

The work presented here is part of a wider project funded by the Texas General Land Office to inform the development of sediment budgets for each of their four coastal regions. A numerical model is developed to simulate sediment transport along the southern coast of Texas. The model computes the waves currents and sediment transport in the area using the fully coupled openTELEMAC modeling system (version 8p1r1). This model is calibrated against measured data on waves, currents, and water levels. In these comparisons with measurements, the model skills range from sufficient in very calm conditions to very good in more energetic conditions. The sediment transport model predicts sedimentation rates in navigation channels that match both observed channel infill and long-term dredging records well. Conventional wisdom is that residual sediment transport along the south Texas shores is directed northward and simple calculations of potential wave-driven transport (littoral drift) in the nearshore have confirmed that to be the case. However, when the combined influence of waves and currents is taken into account in the regional modeling, the sediment transport modes and the respective pathways in the area have been found to be more complex than reported by previous authors. In summer, residual sediment transport is still to the north, but in nonsummer conditions, strong winds cause significant southward directed currents. These wind-driven currents in the Gulf of Mexico change the residual transport direction from northward to southward. Averaged over the year, this leads to sediment transport pathways with a divergence point about 50 km north of Mansfield Pass, with southward alongshore sediment transport south of this point and northward transport north of it. Although the literature assumes a drift convergence near Kenedy County, the modeling presented here identifies this area as a drift divide.