Field-based numerical model investigation of wave propagation across marshes in the Chesapeake Bay under storm conditions

Abstract

Saltmarshes have the potential to protect coastal communities from the impact of waves caused by severe storms. While recent developments in numerical models are enhancing the simulation of wave-vegetation interactions, the estimation of the drag coefficient (Cd), used to account for the reduction of wave energy by vegetation, is still a source of uncertainty. In this study, we use a unique combination of field measurements (wave parameters, topo-bathymetric survey and vegetation characteristics) and numerical modelling (XBeach) to evaluate the significance of Cd for calculating wave attenuation by vegetation, and ultimately, improve the representation of the wave propagation across these ecosystems. Wave height propagation simulated by five empirical Cd formulations, including Jadhav, 2012, Anderson and Smith, 2014, Garzon, 2018 (based on Reynolds [Re] and Keulegan-Carpenter [KC] numbers) and Ozeren et al., 2014, was evaluated by comparing against wave heights collected at different locations of two saltmarshes of the Chesapeake Bay during two different storms. This analysis revealed that: 1) Garzon, 2018 (based on Re number) and Anderson and Smith, 2014 formulations provided the most reliable results (relative bias lower than 20%), especially at the first 100 m across the vegetation field; 2) Results provided by Garzon, 2018) (based on KC) formulation yielded relative bias lower than 30%, and it mainly overestimated wave heights; 3) Jadhav, 2012 simulations clearly underestimated wave heights, showing relative bias up to −60%; and 4) Ozeren et al., 2014 (currently implemented in the model) simulations highly overestimated wave heights over the marsh field and displayed a large scatter. Finally, the validated formulation (Garzon, 2018; based on Re number) and vegetation characteristic measurements collected during the fall and winter were employed to explore the seasonal coastal protection variability of these saltmarshes. Our simulations indicated that under similar hydrodynamic conditions, marshes offered between 15% and 30% less protection against waves in the winter than in fall. However, while saltmarshes would provide additional coastal protection from hurricanes in comparison to nor'easters (cool-season extratropical cyclones), we demonstrated that the marshes still offer more protection than non-vegetated fields in both seasons in these locations.