Sedimentation and equilibrium in wave-formed bars: a review and case study

Abstract

A tentative classification of wave-formed bars is presented based on available evidence of both morphological and process factors. One specific group of bars (group VI), which has been described by numerous studies in the Great Lakes, the Mediterranean, and the Gulf of St. Lawrence, is examined in detail; models of bar formation and equilibrium, which may be relevant to this group, are reviewed and evaluated. These include (1) vortex action under plunging breakers; (2) interaction of incident waves with standing waves generated by reflection; (3) interaction of incident waves with standing edge waves; and (4) hydrodynamic instability of a uniformly long wave setup produced by incident waves.Results of field measurements of one set of crescentic group VI bars in Kouchibouguac Bay, New Brunswick, indicate that none of the theoretical models adequately explains the characteristics of these bars: (a) the stability of the bar form in an area of high longshore sediment transport; (b) sediment transport patterns, which reveal a continuous circulation of sediment through the bar; and (c) the occurrence of spilling rather than plunging breakers. A conceptual model of bar formation and equilibrium is proposed based upon: (a) landward sediment transport by shoaling waves, increasing towards the break point; (b) gradual reduction of transport rates associated with spilling breakers leading to a buildup of the bar; (c) maintenance of the trough through removal of sediments by longshore and rip current circulation; (d) seaward transport of sediment by rip currents; and (e) landward transport of sediments under reformed waves landward of the trough. The application of this model to other group VI bars, particularly straight bars, is discussed.