Modeling hydrodynamics and sediment transport at a river-coastal confluence

With the Maumee River/Lake Erie confluence requiring continuous dredging of two permanent bed mounds to remain a viable harbor, the goal of this dissertation is to research the hydrodynamic patterns and sediment transport characteristics under different forcing conditions in the confluence, with the objective of seeking the origin and physics of the two mobile sediment humps in the navigation channel of the river. As an outcome, this dissertation should help provide guidance as to why, when, and how dredging should be done in the region.; An integrated three dimensional, hydrodynamic, sediment transport, wave current bottom boundary layer, and wave model is applied to simulate the hydrodynamics and sediment transport. A curvilinear planform grid with 208 by 79 cells in the horizontal direction and 12 sigma layers in the vertical direction is used to cover the whole lake and the 11 km long dredged channel of the Maumee River. After analysis of the existing Lake Erie literature, 18 hydrodynamic and 14 sediment transport application cases are constructed, based on different combinations of the forcing functions on the lake and river.; In addition to generating the Lake Erie circulation patterns often described in published literature by other researchers, the hydrodynamic modeling conducted in this dissertation also has obtained new insight on confluence physics. The thermal plume from the Maumee River mainly goes along the Maumee Bay coastlines. In the spring, storms create strong thermal stratification at the water surface in the Maumee Bay, whereas steady winds create strong stratification throughout the water column. In both steady wind and storm conditions, strong vertical thermal mixing exists along the dredged channel, which makes the thermocline deeper in the channel than its surroundings. Storm surges are more efficient in thermally mixing water in the Maumee Bay than steady winds. At the cessation of a storm, deeper mixing occurs for a short time (∼2 hours). It is also found that the depth-averaged velocities in the dredged channel in Lake Erie are opposite to the wind directions (SW or NE). This characteristic has significant impacts on the sediment transport from the Maumee River into the Western Basin.; The sediment transport simulations reveal that heavy sediment resuspension and transport occur in the northern and southern coastal waters in Lake Erie and in the neighborhood of the islands in the Western Basin.; It is found that strong longitudinal concentration differences approaching the nature of a front exist under low river inflow conditions. Strong storm surge/seiches accompanied by river reversal is more efficient in entraining river bottom sediments than steady winds and can cause an order of magnitude larger sediment concentration in the river water column. It is found that riverine fine sediment is more easily transported to the Maumee Bay in a weak seiche than in a strong, flow reversing seiche regime. The simulation also reveals that steady NE winds produce stronger vertical sediment stratification in the river than SW winds.; The analysis of the sediment simulation results shows that the hump closer to the Maumee Bay (hump 2) is mainly deposited and sustained by the river bottom sediment resuspension. The upstream hump (hump 1) is mainly deposited and maintained by the riverine sediment deposition. (Abstract shortened by UMI.)...