Erosion and deposition of fine-grained sediments from the Bay of Fundy

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ABSTRACT

Simulations of the erosion, transport and deposition of fine-grained sediment, such as that of Greenberg & Amos and the Hydraulics Research Station, have illustrated a general lack of reliable field data. Consequently, some standard equations and constants used in modelling the sedimentation character of fine-grained cohesive sediment were evaluated based on data from two field studies and a flume experiment with undisturbed sediment from the Bay of Fundy.

Initial results showed that the resistance to erosion of intertidal fine-grained sediment is controlled largely by the degree of subaerial exposure and the consequent dehydration and compaction. The sediment shear strength was high (4 kPa), but generally decreased seawards across the intertidal zone. The resistance of intertidal mud to erosion can be 80 times greater than sub-tidal counterparts.

The rate of sediment erosion varied as a complex function of the applied bottom shear stress. At stresses immediately above the critical, the erosion rate decreased asymptotically with time. At higher excess stresses, the erosion rate was linear with respect to time. Thus sediment erosion cannot be represented by a single coefficient.

The Krone method of computing sedimentation rates of suspended material was shown, by comparisons with direct measurement, to overpredict by 29%. All variables used in his method were measured in the evaluation with the exception of the critical deposition stress (τd). The closest comparisons were obtained when τd was assigned a value of 0.1 N m−2 following Creutzberg & Postma.

The in situ still-water particle settling rate (Vo) was constant with respect to time (2.1 × 10−3 m s−1). However, the settling tube measures of settling rate, compared to in situ results, underpredicted particle settling by an order of magnitude (2.7 × 10−4 m s−1). The reason for this discrepancy is not apparent from our results.

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