FREQUENCY OF EFFECTIVE WAVE ACTIVITY AND THE RECESSION OF COASTAL BLUFFS - CALVERT CLIFFS, MARYLAND

The Calvert Cliffs, Chesapeake Bay, Maryland, USA, erode by direct wav e undercutting or by freeze/thaw erosion accompanied by wave removal o f slope debris. Directly undercut slopes recede more rapidly, with lon g-term rates exceeding 1.0 m/yr; freeze/thaw slopes recede at rates ap proaching 0.5 m/yr. The frequency of wave height and water level at th e shoreline is estimated for eleven sites based on a 37-year wind reco rd, estimates of storm surge, offshore wave geometry, nearshore wave t ransformation, and breaking wave type. Locations experiencing the larg est slope recession are not uniformly those with the largest cumulativ e wave energy; the resistance to erosion of the slope toe must also be accounted for. An index of relative wave strength is defined as the r atio of wave pressure T and the cohesive strength S of the slope mater ial. For the Calvert Cliffs, a minimum relative wave strength for init iating erosion of intact material is 0.05 < T/S < 0.1. A cumulative du ration of greater than or equal to 50 hours per year for T/S greater t han or equal to 0.1 distinguishes undercut and nonundercut slopes and recession rates greater or lesser than 0.5 m/yr. The relative wave str ength index may be used to identify sites at risk of increased erosion . At one site with a small historical erosion rate, the loss of a prot ective beach and associated decrease in toe elevation caused a positiv e shift in the frequency of large TIS. Direct wave undercutting and in creased slope recession may be anticipated at this site, as indicated by the development of an undercut notch during the course of the study .