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