CGRG Bibliography of Canadian Geomorphology
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Author : Duffy, G.P.; Hughes-Clarke, J.E.; and Parrott, R.
Date : 2005.
Title : Bedforms and associated sand transport on a Banner Bank, Saint John, NB.
Publication : Joint Meeting of the Geological Association of Canada, the Mineralogical Association of Canada, the Canadian Society of Petroleum Geologists and the Canadian Society of Soil Sciences. May 15-18, 2005. Studley Campus of Dalhousie University, Halifax, Nova Scotia.
Issue :
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Abstract
Monthly repetitive multibeam surveying of the submarine sand dunes of Mispec Bay sand bank has revealed that there is a complex pattern of sediment transport across and around the sand bank. Mispec Bay bank is the westernmost of a pair of banner banks associated with the headland Cape Spencer. ADCP current observations made over the sand bank close to spring tides in 2002 and 2003 show that a peak depth averaged flooding current of 80 cm/s toward the headland remained reasonably uniform from offshore to inshore of the bank. In contrast, the disturbing effect of the ebb tidal eddy caused peak ebbing currents flowing away from the headland to range from 110 cm/s offshore to 20 cm/s inshore of the sand bank. On the flood tide, bottom shear stress over the sand bank is strong enough to induce movement of the bed causing migration of the bedforms, whilst on the ebb tide, competent currents flowing around the headland suddenly decelerate in the eddy causing accretion of sediment onto the sand bank. A new processing algorithm has allowed migration vectors to be extracted from the repeated surveys of the shifting asymmetric bedforms, which in turn allow estimation of tidally-averaged bedload transport rate. In this way, the bedload sediment transport rate over the sand bank is estimated to be a reasonably uniform 30 kg/m/tide (2.6E-7 m3/m/s). The banner bank abruptly comes to an end at its tip close to Cape Spencer where bedforms are seen to be washed away by increasing bed shear stress, and increasing sediment transport rate, as they move into the bedload parting zone around the headland. In addition to remobilized sediment deposited in the ebb tidal eddy, migration vectors and spatial analysis of grain size, sorting and skewness have revealed that sand is also conveyed from the west by currents flowing towards the headland. Sediment may be continuously exchanged between the pair of banner banks on alternate tides indicating that these banks should be considered as a sedimentary system rather than as being independent bodies. Repetitive multibeam surveying in tandem with ADCP profiling has alluded to the existence of this important bank-maintenance process.
Bibliography of Canadian Geomorphology