BLUFF EROSION HAZARDS IN COASTAL MAINE

The Maine Geological Survey and University of Maine have mapped bluff-erosion hazards along 3,850 km of Maine’s 5,596 km of tidal shoreline. Three categories of bluff stability were mapped: highly unstable (near vertical bluffs devoid of vegetation), unstable (steep, partly vegetated bluffs), and stable (gently sloping, vegetated bluffs). A fourth category, no bluff (rock or < 1 m of unconsolidated sediment) was also mapped. Four bluff-toe environments were also mapped: vegetated, non-vegetated, bedrock, and armored (engineered).

Bluffs are absent along 47% of the coast largely because 32% of the coast is bedrock. Stable bluffs comprise 36% of the shoreline, with 17% fronted by salt marsh. Unstable bluffs make up 15% of the shore, with 6% in non-vegetated settings. Highly unstable bluffs make up only 2.3% of the coast, with most (1%) in non-vegetated areas. There is a coast-normal zonation of bluff stability, with outer, rocky areas and inner, vegetated estuarine regions generally more stable than central bay regions. Unstable areas are generally exposed to long fetch in elongate estuaries and possess relatively narrow intertidal zones. Unstable bluffs are concentrated along the shoreline northeast of Portland, and are less common in the beach-dominated area to the south.

Only a relatively small proportion of Maine’s coast is unstable and highly unstable, 581 km and 88 km, respectively, but many of these sites are composed of glacial-marine mud, which erodes relatively rapidly and occasionally experiences landslides. Many dangerous bluffs are developed for residences, especially in Casco Bay, and upland development is often associated with higher bluff instability. Significantly, 6.2%, or 240 km of the bluffed area is armored, presumably because of property at risk from bluff instability. Bluff erosion is also the primary source of mud to the coast, and sediment input to marine depositional environments is reduced by engineering along the shore.