SPATIAL VARIATION IN BLUFF RETREAT RATES ALONG TRANSGRESSIVE AND HIGHSTAND SYSTEMS TRACT DEPOSITS ON THE LAKE ERIE COAST OF PENNSYLVANIA

Coastal bluff retreat is common along the Lake Erie coast of NW Pennsylvania. Devonian bedrock at the shoreline is overlain by Quaternary clay-rich glacial tills, and by late Quaternary lacustrine strata of the transgressive systems tract and strandplain/beach ridge strata of the highstand systems tract. Lake-hydrodynamic processes and particularly subaerial processes (runoff, groundwater flow, bluff-top anthropogenic modifications to slope and vegetation) contribute to bluff instability which causes permanent loss of coastal land at spatially variable rates. This is of concern to planning agencies in this grape-growing region because land losses can be significant, and because bluff erosion contributes to degradation in coastal water quality (turbidity, aesthetics, light penetration, nutrient supply).

Ongoing LiDAR analysis of bluff change shows that recession is particularly problematic along the high-elevation, well-drained, sand- and gravel-rich beach-ridge complex between Twelvemile and Sixteenmile Creeks where the geology and lake microclimate favor intensive viticulture. Lower-elevation, poorly-drained lacustrine deposits up-coast and down-coast exhibit lower recession rates and different failure patterns. 1998 and 2007 LiDAR data indicate that, overall, bluff change rates range from -4.2m/yr to +0.98m/yr, averaging -0.27m/yr +/- 0.1m/yr. Excluding ~10% of the coast that shows no resolvable change or slight progradation, actively eroding areas have an average recession rate of -0.30m/yr +/- 0.1m/yr. Short term rates of recession can locally exceed 11 m/yr and are associated with high rates of groundwater discharge at the bluff face. On the beach-ridge complex, recession rates are about twice those of the lacustrine lowlands (-0.3 to -0.6m/yr versus -0.15m/yr).

The premise of the ongoing project is that groundwater flux (m³/m²/day) at the bluff face is variable seasonally and spatially, being greatest for the beach-ridge complex in the spring and fall. This leads to higher bluff retreat rates compared to elsewhere where 1^st thru’ 4^th order streams incise to bedrock and divert a component of groundwater flux away from the bluffs. The roles of bluff-top elevation, the presence/absence of beach, bluff-top slopes, and land use exhibit smaller correlations with bluff-change rates.