An in situ erosion rate for a fine‐grained marine sediment

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Abstract
Near‐bottom current and particulate concentration data and a theoretical model of sediment resuspension together have led to inferences about the erosional response of a fine‐grained sediment to bottom stress. Tidal boundary layer observations, made 5 m from the bottom in the main basin of Puget Sound, Washington, show that currents with speeds exceeding 40 cm/s cause an increase in concentration as much as sixfold from a background level of 1 mg/1. When the erosion rate, E = α|τ|η, is given a power law dependence on bottom stress τ, in accord with laboratory fine‐sediment entrainment measurements, the field data and theoretical model together suggest that η = 4.0 and α = 1.7 × 10−6 g/cm2/s. Uncertainty in these values stemming from a variety of factors is discussed. The analysis also points to sediment resuspension of material with relatively large settling velocity (ws = 0.1 cm/s) taking place against a background of much finer particles and a depth of sediment reworking of approximately 0.3 mm under typical strong flow conditions. Comparison of this in situ erosion rate with previous laboratory rate estimates shows it to be comparable to rates measured for some freshwater sediments.

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