COASTAL BLUFF MANAGEMENT: EVALUATING SHORT AND LONG-TERM RISK FROM RARE EVENTS

Coastal bluff erosion is often episodic, and is associated with storms or landslides. Thus, predicting short- or long-term rates of bluff retreat is a management challenge when it is driven by relatively rare, and often catastrophic, events. In 2005, Acadia National Park initiated a study of bluff erosion as a result of a perceived threat to 3 cultural resource and recreational sites: Fernald Point (FP), Frazier Point (FRAP), and Thompson Island (TI). FP and FRAP have low bluffs and contain archeological sites severely impacted in a 1978 northeaster. TI is a popular picnic site with an unsuccessful attempt (rip-rap) at bluff-erosion control. In an effort to access annual erosion rates, the three sites were photographed seasonally, and surveyed in August 2005 and 2006. Aerial photographs spanning a 50-year period were examined to evaluate erosion rates and mechanisms. Over 50 years, bluff retreat up to 4 m occurred at parts of FP and TI, but was all associated with the interval that included the 1978 storm. FRAP appears to have experienced surface overwash and sediment movement during this interval. Direct surveys and seasonal photographs detected no bluff erosion at any site in 2005-06, with many cobbles remaining unmoved on the bluffs through the interval. Large-scale retreat at these sites, thus, occurred mainly during very rare storm events, and suggests that annual monitoring is a preferred management strategy to movement of the cultural resources at the present time. Minor erosion associated with visitors, though visible at all sites, is best controlled through social engineering, not with armor.