HOW DOES THE SURF ZONE-SHOREFACE PROFILE ON A HIGHSTAND SYSTEMS TRACT STRANDPLAIN RESPOND TO HIGH-FREQUENCY (101-102 YR) TRANSGRESSIONS AND REGRESSIONS AT GEOLOGICALLY SHORT TIMESCALES?

Understanding coastal change at meso-timescales is a prerequisite for developing predictive coastal-response models and is a "holy grail" for coastal scientists and engineers. Presque Isle is a non-tidal Holocene strandplain system located on the Pennsylvania coast of Lake Erie and forms the eastern terminus of a 36 km long littoral sediment dispersal system. Its historical behavior at multi-decadal to centennial timescales provides insight into the complexities of net coastal response to a large set of geo-environmental variables. Principal among the latter are high-frequency transgressions and regressions (normal and forced) of up to 0.8 m in magnitude that occur at timescales of years to decades against a back-drop of longer term lacustrine transgression (8 mm/yr, 1901-1997).

We utilized 1:15K-scale bathymetric charts in a GIS to examine spatial variability in lake-bottom change along and across the nearshore profile (0-16 m water depths) between 1901 and 1997. Four multi-decadal sample intervals correspond to periods of moderate regression (-9 mm/yr), overall-constant lake level, moderate transgression (8 mm/yr), and major transgression (15 mm/yr). Several general rules describe the gross changes on this anthropogenically modified system. Moderate regression (1901-1923) is characterized by minor net erosion which is inferred to occur because sediment supply from updrift source environments (bluff face to shoreface) is reduced. Wave base sinks onto the Presque Isle substrate to allow profile cannibalization and enhanced littoral sediment bypassing as accommodation space is reduced. Overall-constant lake level (1949-1971) is characterized by major net accretion but the signature is strongly influenced by beach nourishment which raises sediment supply dramatically. Moderate transgression (1971-1997) is characterized by minor net accretion as sediment supply from updrift source environments increases. Shoreface cannibalization decreases because wave base lifts off the substrate, but this signature is also influenced by major beach nourishment and breakwater installation. Major transgression (1923-1949) is characterized by major net accretion because shoreface accommodation space increases (shoreface cannibalization decreases) and sediment supply from updrift and beach source environments increases.