Hydrodynamics and morphodynamics of shallow tidal channels and intertidal flats

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution, February, 1993In this thesis, mechanisms which control morphodynamics of shallow tidal embayments are investigated analytically. In the process of exploring these mechanisms (specifically asymmetries in bottom stress, τ), basis momentum and mass balances which govern flow in these systems are clarified. Temporal asymmetries in τ are investigated via a new perturbation scheme which quantifies nonlinear processes and combines geometric controls on asymmetry into a single non-dimensional parameter. Implications of spatial asymmetries in τ are investigated though stability criteria based on a uniform distribution of τ. Morphologic observations of both tidal channels and intertidal flats are consistent with a unifonn distribution of τ at equilibrium. Investigation of morphodynamic mechanisms leads to scalings of momentum and continuity which diverge from classical models. Scalings for prismatic channels with strong tidal asymmetries indicate friction often dominates acceleration in the momentum equation. The resulting "zero-inertia" balance gives a time-varing diffusion equation which requires along-channel amplitude to decay. Uniform τ justifies a new scaling of continuity for exponentially-shaped channels. In such channels, along-channel gradients in tidal velocity are small and are often dominated by gradients in cross-sectional area. The resulting first-order wave equation allows only constant amplitude, forward propagating waveforms which are independent of channel length.Funding was provided by the Offce of Naval Research through the American Society for Engineering Education and the National Science Foundation under grant OCE91-02429

Uniform Bottom Shear Stress and Equilibrium Hyposometry of Intertidal Flats

Hypsometry is the distribution of horizontal surface area with respect to elevation. Recent observations of tidal flat morphology have correlated convex hypsometry with large tide ranges, long‐term accretion and/or low wave activity. Concave hypsometry, in turn, has been correlated with small tide ranges, long‐term erosion and/or high wave activity. The present study demonstrates that this empirical variation in tidal flat hypsometry is consistent with a simple morphodynamic model which assumes tidal flats to be at equilibrium if maximum bottom shear stress (τ) is spatially uniform. Two general cases are considered: (i) dominance of τ by tidal currents, where τ is equal to maximum tidally‐generated shear stress (τT), and (ii) dominance by wind waves, where τ is equal to maximum wave‐generated shear stress (τW). Analytic solutions indicate that a tidal flat which slopes linearly away from a straight shoreline does not produce a uniform distribution of τT or τW. If the profile is adjusted until either τT or τW is uniform, then domination by tidal currents favors a convex hypsometry, and domination by wind waves favors a concave hypsometry. Equilibrium profiles are also derived for curved shorelines. Results indicate that an embayed shoreline significantly enhances convevity and a lobate shoreline significantly enhances concavity — so much so that the potential effect of shoreline curvature on equilibrium hypsometry is of the same order as the effect of domination by τT or τW.https://scholarworks.wm.edu/vimsbooks/1034/thumbnail.jp