SEDIMENT TRANSPORT TRENDS ALONG MARSH-BAY BOUNDARIES: IMPLICATIONS FOR MARSH SHORELINE RESTORATION

Between October and April, Louisiana experiences frequent cold-fronts, increasing erosion rates along marsh shorelines. Passing through the northern Gulf of Mexico, these cold-front systems cause large variations in wind speed and direction, inducing changes in sub-tidal water levels and wave heights. Waves and rapid water level fluctuations generated by these storms act as the main driver for erosion releasing unknown volumes of sediment and influence local sediment fluxes. Sediment released can be deposited proximal to the marsh edge and/or on the marsh platform, or dispersed to adjacent water bodies, directly or indirectly through resuspension and reworking of bay bottom sediment. Engineered structures have been placed in some areas to combat erosion. However, the long-term effects of these structures on sediment transport and delivery to the marsh platform are unknown and could have unfavorable consequences on marsh health. We investigate various stabilization structures along marsh shorelines comparing them to adjacent natural counterparts using field measurements, previously collected data and modelling. Through deployments of hydro acoustic sensor arrays along a cross shore transect from bay-to-marsh platform, coupled with short term sedimentation (e.g. sediment tiles) and longer-term marsh accretion methods (e.g^{. 210}Pb and ¹³⁷Cs) we compare and correlate sediment transport and water fluxes in restored shorelines and compare them to proximal control sites that received no restoration. Our results show that water and sediment flux to the marsh platform is a function of storm intensity, and directly proportional to sub-tidal water level excursion and local wave climate. Waves play a primary role in contributing to sediment transport onto the marsh platform when marsh inundation is low, and play a lesser role when inundation increases, at which point tidal and storm induced currents dominate transport processes. Moreover, water and sediment exchange is proportional to the restoration method used and exhibits a non-linear response as storm energy increases. Ultimately, our study will contribute to a spatio-temporal framework that can be used to inform shoreline protection and mitigation efforts in coastal Louisiana, with transferable science to marshes in the Gulf coast and the rest of the world.