The Influence of the Shore Slopes Ratio on the Nature of a Transgressing Shore

Roger N. Dubois

Abstract


In this study a cross-shore profile extends from the foredune crest to the shoreface toe and is characterized by the shore slopes ratio (SR). SR is obtained by dividing the slope tangent of a cross-shore profile by the slope tangent of a shoreface base segment that is abandoned during times of transgression. SR predicts the type of response that an equilibrium shore profile will take as it transgresses in the face of a relative sea-level rise. When SR is greater than unity, the landward trend of the abandoned shoreface base slope intercepts the shoreline below sea level, and a net loss of sediments occurs from a cross-shore profile during times of transgression. Sediments lost from profiles are deposited in landward and seaward compartments normal to the shoreline and at the shoreline termini. When SR is at unity, the trend of the abandoned shoreface base slope intercepts the shoreline at the crest of the foredune, and there is no net sediment loss or gain at a shore profile during transgression; the volume of sediment eroded from a beach is equal to that deposited on a shore bottom. This volumetric sediment balance is recognized as Bruun' rule. Finally, when SR is less than unity, the trend of the abandoned shoreface base slope intercepts the shoreline above the foredune and during times of transgressions a net gain of sediment derived from terrestrial sources is required to preserve the cross-shore profile symmetry. Of the three shore conditions, field evidence reveals that SR is frequently greater than unity. Thus, care must be taken before applying Bruun's rule at a field site; field data must first show that SR is at unity.


Keywords


Barrier island; beach erosion; Bruun's rule; nearshore; ramp ; sea-level rise; sediment budget; shoreface; shorerise

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