LIDAR MAPPING OF SHORT-TERM BLUFF RECESSION, LAKE ERIE, PENNSYLVANIA, USA: ROLES OF BLUFF GEOMORPHOLOGY AND GROUNDWATER FLUX

Coastal bluff retreat is a common problem along the world’s unconsolidated coastlines. On the North American Great Lakes coast of Pennsylvania, Quaternary clay-rich glacial till and sandy strandplain sequences overlie Devonian bedrock. Subaerial and lacustrine erosional processes cause permanent coastal land loss at spatially variable rates. This is of concern to environmental agencies because land-use planning should account for spatial and temporal variability in land-loss rates, and because bluff erosion contributes to temporary degradation in coastal water quality. The goal of this pilot study is to evaluate spatial variability in bluff retreat rates along a sector of Pennsylvania’s short Great Lakes coast. High resolution LiDAR data covering a one-decade time frame (1998-2007) permit bluff-crest mapping on two comparable data sets that capture change within a timeframe similar to planning intervals. Short-term recession analysis can be more useful, cost-effective, and accurate than long-term analyses that use lower-resolution field measurements, T-sheets, and historical aerial photography.

Bluffs along the 20 km coastal study site consist of up to 26 m of unlithified Quaternary sediments overlying a 1-4 m ledge of sub-horizontal Devonian shale and sandstone. Bluff slopes range from 20-90 degrees, beaches are narrow or absent, and the bluffs are seasonally shielded by ground-freeze and lake ice. DEMs, hillshades, and slope and contour maps were generated from bare-earth 1998 and 2007 LiDAR data, and checked against 2005 aerial ortho-photography. Maps were analyzed at a scale of 1:120 in ArcGIS and the bluff crest was identified primarily by the visual-break-in-slope method. Rates of bluff retreat derived using DSAS vary from unresolvable to as much as 2.2 m/yr, averaging less than 0.25 m/yr which is consistent with known long-term rates. In general, bluffs retreat relatively linearly where glacial till dominates the bluff stratigraphy, while along high-elevation strandplain-capped bluff sections, rotational earth slumps (<100 m diameter) are well developed. Retreat rates are highest at slump zones, and at 1^st - 2^nd order ravines (<300 m in length) maintained by focused groundwater discharge. Other factors influence bluff retreat spatially, but are inferred to be minor at the scale of this study.