Fry Virginia A.
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ThesisTidal velocity asymmetries and bedload transport in shallow embayments(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1987-08) Fry, Virginia A.Tidally forced circulation can cause a net near-bed transport of sediment when the tidal velocity is asymmetric about a zero mean (flood or ebb dominant) and the transport rate is nonlinearly related to velocity. The relationship between elevation and velocity is elucidated here to enable one to determine from tide gauge data and sediment transport relations whether tidal asymmetry may cause net sediment transport. Tidal elevation and tidal velocity are related through the equations of motion of the fluid. If the estuary is shallow, the change in cross-sectional area of the channel with the tide is significant with respect to total area: the equations become nonlinear and an exact solution does not exist. A relationship between elevation and velocity in a nonlinear system is derived through the continuity equation and shown to be significantly different than the linear relation. Finite difference numerical solutions of the one dimensional, shallow water nonlinear equations are compared to the continuity relation and are in good agreement. The relationship between elevation asymmetry and ratio of flood-to-ebb bedload transport is calculated for both the linear relation between elevation and velocity and the nonlinear relation. Results show that the ratio of flood·to-ebb bedload transport as calculated from the nonlinear relation between elevation and velocity is similar to the flood-to-ebb ratio calculated from the linear relation.
Technical ReportKings Bay, Cumberland Sound, Georgia part II : numerical modeling(Woods Hole Oceanographic Institution, 1987-03) Aubrey, David G. ; Fry, Virginia A. ; Lynch, Daniel R.As a complement to field measurements of waves, surface tides, currents, and sediment transport, numerical modeling of King's Bay/ Cumberland Sound was initiated. Diagnostic numerical models 1 (both 1- and 2-dimensional) were applied to determine their applicability to estuaries of the same scale as King's Bay. One-dimensional models showed the estuarine system to be ebb-dominant, in accord with observations. This model did not reveal any extreme system sensitivity to changes in channel geometry on the scale expected from maintenance dredging. The two-dimensional model (a finite element model having a moving boundary formulation) was run to examine its applicability for diagnostic modeling of these systems. Preliminary results indicate the method is promising, but some model developments are indicated. Suggested model developments include: semi-implicit algorithm to reduce run-time: mass-conserving boundary conditions at tidal boundaries: implementation of a two-level momentum equation: algorithm development to extend the deforming element concept for smaller estuarine space scales: and formulation of a comprehensive interactive graphical package to facilitate model formulation, boundary and domain gridding, and presentation of results. This latter graphical task is essential for successful application of these numerical models. Results from these studies suggest that diagnostic models of shallow estuaries will be a valuable tool to be used in conjunction with more expensive "predictive" models, to understand circulation and transport processes under natural and impacted conditions.