Vessel Wake Impact Forces on Marsh Scarps

Abstract

Priestas, A.M.; Styles, R., and Bain, R., 2023. Vessel wake impact forces on marsh scarps. Journal of Coastal Research, 39(2), 207–220. Charlotte (North Carolina), ISSN 0749-0208. A common morphological feature of high-energy coastal plain salt marshes is an erosive scarp separating the marsh platform from the abutting tidal flat. A noted deficiency in the understanding of scarp erosion is a lack of direct wave impact measurements to better constrain the physical processes driving material loss. To measure wave impact forces, load cells were embedded into a scarp face along the Intracoastal Waterway in northern Florida, which is frequented by shallow-draft vessels. The largest impact forces occurred when the mean water surface elevation was just below the midpoint of the scarp and decayed as the mean depth either increased or decreased. Early wave breaking around low tide reduced the direct impact on the scarp, whereas wave reflection around high tide reduced wave breaking and the associated maximum impact force. The sensitivity of impact force to tidal stage suggests that wave energetics (breaking vs. reflection) and scarp morphology (ramp vs. near-vertical surface) combine to produce a force pattern that favors maximum impact around midtide. Regression analysis indicated that the impact pressures were positively correlated with wave power and dynamic pressure, which are primary metrics used to model shoreline erosion. This study argues that the critical threshold for erosion may be refined by evaluating the relationship between impact pressures and marsh resistance, since wave exposure alone does not necessarily initiate erosion.