Sediment and Fluid Mud Modeling of Atchafalaya Pro-Delta Channel

The three-dimensional, curvilinear, hydrostatic hydrodynamics model CH3D-Z was modified to include single-grain, cohesive sediment suspended transport and bed transport of fluid mud on a slope. CH3D-Z is a z-plane finite difference model that in this case has 25 layers each 60-cm thick, except the surface layer that is thicker to include all water level variations. Both settled mud and fluid mud processes are included in the new model CH3DZ-FM. A seven-layered bed structure simulates the formation of a surface layer through hindered-settling consolidation of newly-deposited material and, if necessary, tracks the descent of an erosive surface into the bed. Depending on the sediment surface density, cohesive sediment transport is treated either as settled mud (particle erosion and floc deposition) or as fluid mud (fluid entrainment and settling). Wind wave resuspension is an important process to suspended sediment, and winds are used by hydrodynamic and sediment transport sub-models. A model module computes down-slope gravity forces, and density-dependent yield stresses through the fluid mud layer and moves material accordingly. The Atchafalaya River flow averages 9,340 m³/sec and carries about 98 million tonnes of sediment annually forming two deltas in the shallow, micro-tidal Atchafalaya Bay. A 6.1-m-deep by 122-m-wide navigation channel transects the bay and extends some 28 km offshore unconfined by jetties. Fine, cohesive material forms a peak shoal height near the extent of salinity intrusion. Because of funding limitations, annual maintenance dredging of the bar channel by the Corps of Engineers has been limited to about 7.6 million m³, even though project depths are not always maintained. Other operational measures have been tried yet project depths have not been maintained for more than a couple of months each year. A model mesh extending 97 km long-shore and about 64 km cross-shore was developed. The model has about 3,000 surface and 25,000 total cells. Year-long simulations are performed for the period of 2001–2002 when extensive field data were collected. Structural alternatives tested in the model have included: jetties, artifical reefs, sediment traps, channel destratification measures, and a sloped channel reach with a sump. Deeper channel dimensions have also been tested.