LANDSLIDE RISK ASSESMENT ON THE COAST OF MAINE

Landslides on the Maine coast occur frequently in areas underlain by Presumpscot Fm. glaciomarine sediments. They are usually progressive rotational failures (slumps) in areas of thick glaciomarine sediment exposed at coastal bluffs. They range in size from a few hundred m² in many locations to greater than 1.4 hectares (3.5 acres), such as the Rockland harbor slumps of 1973 and 1996. There appears to be a rough inverse correlation between slump size and frequency of occurrence (yearly to several decades). Slumps and landslides can cause property damage in the hundreds of thousands of dollars, and have the potential to damage important public infrastructure such as roads, sewer lines, power and water supplies. Widespread bluffs of Presumpscot Fm. within many estuaries and bays of the southern and central Maine coast undergo a cycle of weakening at incipient cracks, slumping, stabilization by salt marshes on the slumped sediment platform, and then renewed steepening through bluff erosion. Similar slumps are also found on river banks in the interior of the state. We use ground-penetrating radar (GPR) and sequential airphoto GIS mapping techniques to characterize historic and ongoing slumps, and to define areas of potential future failures. GPR allows imaging of disturbed sediments and displacement by incipient or active failure surfaces. More detailed sedimentology at fresh and older exposures details spacing of cracks, secondary evidence of groundwater flow, such as iron and manganese oxide stains, and relationship of sediment texture and stratigraphy to slump failures. This study is planned to allow us to better inform the public of the risks associated with unstable coastal bluffs and identify specific slump hazard potentials.