A Conceptual Fairweather-Storm Model of Beach Nearshore Profile Evolution at Duck, North Carolina, U.S.A.
Keywords:Slope, coastal erosion, sediment budget, shoreface.
Limited long-term (decadal) beach-nearshore profile observations suggest that during fairweather conditions, the beach-nearshore slope gradually steepens while the shoreline remains relatively stable. The steepening process is terminated by an extreme storm event during which sand is carried offshore. The shoreline migrates landward and the beach-nearshore slope flattens. When the interval between storms is large, the profile approaches a maximum steepness and its susceptibility to erosion and shoreline recession is maximized. To test this conceptual fairweather/storm model, storm events were related to shoreline position, sediment volume and slope changes obtained from the high precision profile data at Duck, North Carolina. Beach-nearshore profiles have been collected for 10 1/2, years at approximately biweekly intervals.
Four major groups of storms occurred during the observation period. They all caused an abrupt increase in the volume of the upper shoreface; in two cases there was a net volume gain to the overall profile. During intervening fairweather conditions, there was a steady onshore transport of sand from the upper shoreface while the total volume remained constant. Slope changes, determined by linear regression of the upper shoreface, support the conceptual fairweather/storm model. Inclusion of the more landward element of the profile was difficult due to the presence of bars. The shoreline at Duck was insensitive to these offshore changes due to the coarse grain size at the shoreline. However, other sites composed of finer-grained sediments might be expected to more sensitive to such changes. These results show that the fairweather/storm model may be a useful conceptual tool to examine medium - to long-term (years to many decades) beach-nearshore profile behavior.