Evolution of Critical Shear Stress in the Seabed of an Urbanized Estuary and Natural Estuary after the Passage of Hurricane Ian

Sediment transport and mixing in estuaries impact a number of ecosystem services, including the flux of nutrients and the mediation of turbidity of the water column, which in turn affects the health of seagrasses and other benthic primary producers. A key factor to predicting the direction and strength of sediment transport is the critical shear stress required to erode sediment from the bed. But the erodibility of fine sediments is poorly constrained because of the complicated interactions between grainsize, consolidation, and biological factors. This study assessed the evolution of critical shear stress and erodibility of the seabed in southwest Florida, USA after the intense disturbance of Hurricane Ian. We also compared how the evolution of the bed differed in a location that has had extensive development with a nearby but undeveloped bay with no anthropogenic development. Erodibility and critical shear stress were measured with Gust-type erosional chambers. Profiles of ⁷ Be and Xradiographs were used to determine the extent of new sediment deposition and bioturbation. Hydrodynamics were measured with an acoustic doppler current profiler. Hurricane Ian initially eroded the seabed down to a consolidated layer with high critical shear stress (1-1.5 Pa) and low erodibility at both sites. In the subsequent months, new sediments were deposited and rapid bioadvection of the top 6 cm ensued. The shear stress was reduced (~0.25 Pa) and erodibility increased by the end of the study. Recovery was more rapid in the undeveloped site because the hydrodynamics were more energetic. Both sites returned to stability within one year of the passage of the storm.