Assessing Variations in Sediment Properties and Erodibility in Harrison Bay, Alaska, Using a Portable Free-Fall Penetrometer

Geomechanical data of Arctic nearshore and offshore seabed sediments remain sparse. Two field surveys were conducted in Harrison Bay, Alaska, in the summers of 2021 and 2022. These surveys involved portable free-fall penetrometer (PFFP) deployments, grab sampling, gravity coring, and compressed high-intensity radar pulse sonar for seabed investigation and bathymetric surveying. The goals of this study were to test geomechanical seabed surface sediments in situ using a PFFP, relate those properties to erodibility parameters from a jet erosion test (JET), and demonstrate the potential use of geomechanical seabed mapping to better inform numerical models of shelf morphological evolution in an Arctic environment. After deriving the firmness factor (FF) from the PFFP data, a classification scheme was developed with an FF = 450 m-1 threshold differentiating cohesive and noncohesive sediments at a threshold fines content of 30%. Strength properties were then calculated for cohesive or noncohesive sediments, respectively. For noncohesive sediments, the packing state in the form of relative density was related to the JET-derived detachment coefficient, kd, with the critical shear stress being determined via empirical relations. Three categories were assigned for cohesive sediments: an undrained shear strength separator of su = 2 kPa correlated well with groupings of kd obtained from JET performed on gravity core samples, and the third category for the least erosive sediments was developed for su values greater than 20 kPa. These categories helped explain variability in erodibility and sediment strength across the entire bay and a focused 13 km transect on the inner shelf. Recently deposited sediment from bluff erosion and ice-scoured seabed were both classified in the highest erodibility group, highlighting the usefulness of this classification system for studying morphodynamics in Arctic environments.