100 YEARS OF COASTAL CHANGE FROM A DYNAMIC STRANDPLAIN SYSTEM AT PRESQUE ISLE, PA: SHOREFACE CHANGES REVEALED USING GIS ANALYSIS

Presque Isle is a large, late Holocene, beach-ridge strandplain located in a non-tidal, wave-dominated, Great Lakes setting. It is migrating eastward along the southern Lake Erie coast and is composed of at least nine mega-beach-ridge sets whose configuration reflects complex late Holocene variability in sediment supply and wave regime. The youngest beach-ridge set is over 1 km long and records rapid coastal changes that have occurred on the downdrift (east) end of the strandplain since the turn of the 20th Century. The entire strandplain is building into water depths of as much as 12 m and burying sand-veneered shale bedrock under as much as 8 m of progradational shoreface sands. As a result, the depositional record is that of a relatively thin strandplain depositional system prograding over a much thicker regressive shoreface system under conditions of normal regression.

Presque Isle has experienced significant anthropogenic modifications since the 1820s. Prior to the mid 1950s, hard stabilization was employed almost exclusively to counteract erosion on the updrift end of the strandplain. Since 1956, at least 7 x 10⁶ m³ of beach nourishment has been added to the system, and an array of 55 offshore breakwaters was constructed in 1992. The system is thus well suited to analyzing the response of a dynamic coastal system to anthropogenic forcing. Bathymetric surfaces were derived from 1:12,000-scale nautical charts in a GIS to characterize shoreface change at ~25-yr intervals since 1901. Preliminary data indicate that the majority of sediment eroded from the updrift and central portion of the strandplain's shoreface is transported in the nearshore sediment dispersal system to the downdrift end. Other than at the latter location, there is no evidence of significant offshore transfer and storage of littoral sediment between the 0 and -12 m LWD isobaths. Since the initiation of beach nourishment, the downdrift shoreface profile has developed a pronounced inflection point at -5 m LWD across which gradients decrease from 0.05 (“foreset” slopes) to 0.01 (“toeset” slopes). This, and the 25-yr spatio-temporal comparisons, indicate that a large fraction of natural sediment input and ongoing beach nourishment becomes sequestered on the upper part of the downdrift accretional shoreface at decadal timescales.