EVALUATING THE SIGNIFICANCE OF SHORELINE INDICATORS COMMONLY USED TO ASSESS SHORELINE CHANGE

Coastal scientists, managers and planners commonly measure changes of the shoreline location to describe and predict the evolution of coastal areas. While various shoreline indicators, such as the High-Tide Line (HTL), Wet-Dry Line, and Mean-High-Water Contour (MHWC), are commonly used as “shoreline” positions, defining the ephemeral land-sea interface in an unambiguous manner is necessary as it has strong implications related to short and long-term erosion, storm response, coastal planning, management, and tourism. This study was designed to evaluate the relative significance of changes in the HTL and MHWC, with beach volume as indicators of shoreline change. A 2.4 km section of beach in Myrtle Beach, South Carolina served as the study site, in which nearshore video imagery and an extensive database of beach profiles exist. Seven high-resolution real-time kinematic GPS surveys were collected from December 2002 until September 2003. A specific survey design combined with rigorous spatial interpolation algorithms were used to create detailed digital elevation models (DEM’s) from which changes in the HTL, MHWC, and sub-aerial beach volumes were quantified. Results illustrate a unique longshore variability pattern in the MHWC and beach morphology during ordinary conditions, small coastal storms and the recent Hurricane Isabel. For the surveys considered to represent ‘normal conditions’, a strong relationship between the average change in the MHWC and volume change was interpreted from a significant correlation coefficient r of 0.995. In contrast, a weak anti-correlation r of -0.820 between changes in the HTL and volume change was revealed. Another correlation analysis including pre- and post- Hurricane Isabel surveys was attempted to evaluate these relationships under the influence of a large coastal storm. While the MHWC was still significantly correlated (0.881) to volume change, it appeared to be less correlated than during normal conditions. During storm conditions, higher-than-normal water levels affect the position of the HTL, disrupting the correlation with beach volume changes. The strong correlation between the MHWC and beach volume changes, which appeared to hold during both normal and storm conditions, suggests that the MHWC is a spatially robust indicator of sub-aerial beach volume.