A Field Study of How Wind Waves and Currents May Contribute to the Deterioration of Saltmarsh Fringe

Abstract

Deltaic landscapes, such as the Mississippi River Delta, are sites of extensive conversion of wetlands to open water, where increased fetch may contribute to erosion of marsh edges, increasing wetland loss. A field experiment conducted during a storm passage tested this process through the observations of wave orbital and current velocities in the fringe zone of a deteriorating saltmarsh in Terrebonne Bay, Louisiana. Incident waves seaward of the marsh edge and wave orbital and current velocities immediate landward of the marsh edge were measured. Through a dimensional analysis, it shows that the current and orbital velocities in the marsh fringe were controlled by the incident waves, inundation depth, submergence ratio, and vegetation density. Similarly, it is shown that the longshore currents in the inundated saltmarsh fringe depended on the local wave-induced momentum flux, vegetation submergence, and vegetation density in the fringe zone. The cross-shore current showed the presence of a return flow in the lower region of the velocity profile. A high correlation between the current direction and the local flow-wave energy ratio as well as the vegetation submergence and density is found, indicating the important role of surface waves in the fringe flow landward of an inundated wetland under storm conditions. The field observations shed light on the potential ecological consequences of increased wave activities in coastal saltmarsh wetlands owing to subsidence, sea level rise, limited sediment supply, increases in wind fetch, and storm intensity.

Wave Generation, Dissipation, and Disequilibrium in an Embayment With Complex Bathymetry

“…Correspondingly, the bottom stresses due to wave forcing are greater (up to 0.4 Pa) on the shallower flanks than in the channel (less than 0.1 Pa; Figure ). These regions of enhanced frictional dissipation correspond with wave‐induced bottom stresses that are more intensified locally than previously found in shallower and more spatially uniform estuaries (Karimpour et al, , ; Mariotti & Fagherazzi, ; Mariotti et al, ). For the northwest wind conditions, the simulated wave‐induced bottom stresses increase from around 0.1 Pa in the upwind region of the middle estuary to more than 0.3 Pa in the downwind region of the lower bay.…”

“…Wave energy dissipation is strongly tied to spatial gradients in bathymetry, particularly in the lower bay, as waves that are at equilibrium in deeper water propagated into shallower regions and are subject to greater bottom friction. These regions of enhanced frictional dissipation correspond with wave‐induced bottom stresses that are more intensified locally than previously found in shallower and more spatially uniform estuaries (Karimpour et al, , ; Mariotti et al, ; Mariotti & Fagherazzi, ). Although the energy dissipation due to bottom friction is small overall compared with dissipation by whitecapping, over steeply sloping topography bottom friction dissipation greatly exceeds the equilibrium balance.…”

Wind Wave Behavior in Fetch and Depth Limited Estuaries

“…A critical feature of such restoration projects is to understand how erosion may contribute to wetland loss in these shallow environments, where depth and fetch are perceived to limit wave generation. There is increased awareness that even in shallow estuaries dominated by wetland vegetation, the wave activity contributes to processes such as sediment re-suspension, mudflat erosion, turbidity alteration, marsh edge erosion and wetland losses78910.…”

“…Previous studies suggested that wind wave activities in wetland dominated estuaries are a potential factor in enhancing and accelerating wetland loss rates1011, which fundamentally can affect the physics and biology of estuaries. The conversion of wetlands to water caused by wind waves in shallow estuaries, leads to further increases in wind fetch, which consequently causes wave generation with higher energy.…”

“…As wetland loss occurs from complex interactions of sediment supply, subsidence and sea-level rise in estuaries with significant total area occupied by wetlands, the fetch limited wave functions become an important component of accelerated erosional force on wetland landscapes10. Fetch enlargements due to wetland loss lead to a wave generation with higher energy and consequently to a higher rate in wetland erosion.…”