Modeling the Evolution of the Wax Lake Delta in Atchafalaya Bay, Louisiana

In this study, a process-based, depth-averaged Delft3D hydrodynamic and morphologic model of the Wax Lake Delta in Atchafalaya Bay, Louisiana was developed to simulate a five year period of delta development. The purpose of this modeling effort was to test the ability of process-based modeling tools to successfully simulate typical delta-building processes and the resulting morphologic and stratigraphic characteristics of the delta. Recent developments in conceptual modeling of mouth bar formation and full delta development have confirmed the capability of process-based models to simulate the processes necessary for delta growth and the resulting long-term, geologic scale morphologic and stratigraphic features. In this attempt to model the actual development of a prototype delta using similar techniques to those employed in the conceptual delta models, the applicability of physics-based modeling to delta evolution simulation will be further validated. Morphology qualitatively reproduced typical river-dominant delta growth through the establishment of new depositional lobes while maintaining approximate radial symmetry. More specific stratigraphic features were also reproduced. The successive stacking of coarsening upwards sequences observed in Wax Lake Delta mouth bar deposits was evident in the stratigraphy of modeled incipient jet deposits, a result of the varying discharge regime. Though incipient jet deposits developed in the model at the distal ends of distributary channels, the prograding bars did not aggrade sufficiently to induce flow bifurcation and the development of a mature mouth bar depositional lobe. The overall coarsening-upwards, though sand dominant stratigraphic sequence of typical friction-dominant river mouth deposits is reproduced. Significant mud-dominant prodelta deposition is observed basinward of the original delta front location. In the prototype Wax Lake Delta, deposition of fine sediments in this area is hindered by waves and wind-induced transport, so the modeled mud depositional bodies do not reflect prototype development. Distributary channels are significantly incised and narrowed over the course of the simulation. The narrowing proceeds from significant sand-dominant subaqueous levee deposition on the channel banks. Though distributary channels in the real Wax Lake Delta do frequently incise through the full deltaic sedimentary sequence with channel extension, the modeled incision is persistent throughout the simulation. Upstream accretion of established delta lobes through sand-deposition, similar to the observed primary process of subaerial development in the neighboring Atchafalaya Delta, was also present in the modeled development. The Wax Lake Delta is clearly river-dominant according to traditional classification schemes; however, the deposition of fine sediments is influenced by basin processes that resuspend and export significant quantities from Atchafalaya Bay. The processes contributing to the coarse sediment depositional features that dominate the Wax Lake Delta are qualitatively simulated under purely riverine forcings, but the fine sediment dynamics cannot be accurately simulated in the present, process-limited model. Recommendations for improving morphological simulation include model redevelopment with an alternative, total load transport formulation and the inclusion of limited marine forcings that inhibit fine-sediment deposition.CoMEM - Coastal and Marine Engineering and ManagementSection Hydraulic EngineeringCivil Engineering and Geoscience