PhD Dissertation Defense, Jana Haddad: Wave transformation in coastal marshes

Wave transformation in coastal marshes

Amid increasing frequency and intensity of storms, there is a growing need for resilient shoreline protection strategies, especially those that can also provide ecological and economic benefits to coastal communities. Coastal marshes, whether restored, protected, or constructed as part of a living shoreline provide habitats that support seafood economies, improve water quality, and contribute to marsh accretion by trapping suspended sediments. The salt marsh vegetation in these systems reduces erosion by attenuating wave energy. Though it is recognized that such strategies attenuate waves, there remains a need for better understanding the interactions between the canopy and waves, and better ways of representing these processes in wave models.

I have endeavored to address these needs using an energy-based wave model and a rich dataset of wave, vegetation, and elevation observations made at natural marshes in coastal North Carolina. In particular, this work addresses three overarching challenges: (1) Wave models applied towards prediction of wave transformation in salt marshes typically rely on empirically derived formulations of drag coefficient, but these formulations are highly variable even for similar marsh canopies and wave conditions. In answer to this challenge, the dataset and wave model were used to explore possible factors that may explain differences between past formulations. (2) There is also as yet no consensus on which dimensionless flow parameter best represents the underlying physics of wave dissipation by salt marshes.  A dimensionless flow parameter that incorporates plant motion was found to relate well to drag coefficients, and captured observed patterns between drag coefficient and canopy emergence. (3) While nonlinear waves are often present in nearshore regions, they are rarely accounted for in studies of wave dissipation by marshes, and the influence of nonlinear waves on drag coefficient estimates in salt marshes is unknown.  To address this, a nonlinear wave model was used to estimate drag coefficients, and factors controlling the influence of nonlinear waves on drag coefficient estimates were explored.