Abstract
The objectives of this study are to better understand controls on bed erodibility in muddy estuaries, including the roles of both sediment properties and recent hydrodynamic history. An extensive data set of erodibility measurements, sediment properties, and hydrodynamic information was utilized to create statistical models to predict the erodibility of the sediment bed. This data set includes >160 eroded mass versus applied stress profiles collected over 15 years along the York River estuary, a system characterized by “depth-limited erosion,” such that the critical stress for erosion increases rapidly with depth into the bed. For this study, erodibility was quantified in two ways: the mass of sediment eroded at 0.2 Pa (a stress commonly produced by tides in the York), and the normalized shape of the eroded mass profile for stresses between 0 and 0.56 Pa. In models with eroded mass as the response variable, the explanatory variables with the strongest influence were (in descending order) tidal range squared averaged over the previous 8 days (a proxy for recent bottom stress), salinity or past river discharge, sediment organic content, recent water level anomalies, percent sand, percent clay, and bed layering. Results support the roles of 1) recent deposition and bed disturbance increasing erodibility and 2) cohesion/consolidation and erosion/winnowing of fines decreasing erodibility. The most important variable influencing the shape of the eroded mass profile was eroded mass at 0.2 Pa, such that more (vs. less) erodible cases exhibited straighter (vs. more strongly curved) profiles. Overall, hydrodynamic variables were the best predictors of eroded mass at 0.2 Pa, which, in turn, was the best predictor of profile shape. This suggests that calculations of past bed stress and the position of the salt intrusion can serve as useful empirical proxies for muddy bed consolidation state and resulting erodibility of the uppermost seabed in estuarine numerical models. Observed water content averaged over the top 1 cm was a poor predictor of erodibility, likely because typical tidal stresses suspend less than 1 mm of bed sediment. Future field sampling would benefit from higher resolution observations of water content within the bed’s top few millimeters.
Highlights
High sediment bed erodibility can lead to a number of ecological and societal complications within an estuarine or coastal system
principal component analysis (PCA) for Gust profiles was completed to reduce the shape profile containing seven eroded mass values and seven stress values, to one shape score that could be used as a response variable in multiple linear regression
Multiple linear regressions were applied to an extensive, 15-year data set from the York River estuary to determine and better understand which sediment and hydrodynamic properties are most important in controlling estuarine bed erodibility in terms of 1) the magnitude of eroded mass at a characteristic bed stress (0.2 Pa), termed “eroded mass” and 2) the normalized shape of the eroded mass profile between 0 and 0.56 Pa, termed “erosion shape.”
Summary
High sediment bed erodibility can lead to a number of ecological and societal complications within an estuarine or coastal system. Ecological implications relating to high bed erodibility include bed disturbance influencing benthic community structure (Schaffner et al, 2001) and increased suspended sediment concentrations decreasing photosynthesis (Cloern, 1987; Kruk et al, 2015). Sorption of harmful contaminants and excess nutrients to fine-particles can cause re-introductions of these pollutants during sediment erosion, which can lead to harmful bioaccumulation (Yujun et al, 2008) or increased nutrient loadings within a system (Moriarty et al, 2021). Aside from ecological complications, societal ramifications of high sediment bed erodibility relate to infilling of shipping channels (Brouwer et al, 2018) and potential burial or exposure of dangerous unexploded ordinance (Cooper and Cooke, 2018). A widely applied equation for predicting the rate of fine sediment mass eroded into suspension as a function of bed stress is the AriathuraiPartheniades equation (e.g., Mehta, 2014):
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