An evaluation of the Bagnold (1956) theories for sediment transport in Northumberland Strait, Canada

Abstract

This study examines the seabed stability and sediment transport in the region of a proposed fixed link (bridge) across Northumberland Strait, Canada. The region is dominated by strong semi-diurnal and diurnal tides that have eroded, transported, and deposited material that modify the character of the local seabed morphology. The seabed is composed of rippled fine sand interspersed with a shelly gravel lag. The study provides an opportunity to examine the theories of R.A. Bagnold on sand transport that, by his own admission, were deficient in the prediction of the transport of fine and very fine sand. A further purpose was to determine the mechanisms of the sand transport (waves/currents or both). Representative sediment samples were collected and subjected to hydrodynamic forcing in a laboratory flume (Lab Carousel); relationships were derived between the applied stream power and sediment transport rates both as bedload and in suspension. In addition, a multi-parameter benthic lander (Ralph) was deployed at two sites in the nearshore regions of the fixed link for a total duration of 33 days. Flow velocity measurements derived from the lander were used to assess the transport potential using calibrations derived from the flume studies. The results showed that: (1) the total immersed bedload transport measured herein corresponded well to Bagnold’s dimensionless stream power function when normalized to flow depth and grain size; and (2) the tidal currents coupled with the mean flow in the region are sufficient in magnitude to erode and transport sand to the southeast where they are deposited to form a tidal delta. The sand transport in the shallow margins (depth < 10 m) took place mainly as bedload, though periodic suspension also took place. The nature of the predicted sand transport indicates that the seabed is in a continued state of evolution subject to the present-day hydrodynamic forcing.

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ORCID for Hachem Kassem: orcid.org/0000-0002-5936-6037

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