AN OVERVIEW OF COASTAL BLUFF STABILITY ALONG THE GREAT LAKES

Coastal bluffs of the Great Lakes are experiencing widespread hillslope failure in response to high rates of bluff toe erosion. Given the large amount of infrastructure as well as public and private property located near these bluffs and the ability for their eroded sediments to nourish beaches the stability of Great Lakes’ bluffs is of significant societal concern. Ultimately the impetus for bluff instability stems from elevated lake levels that have risen since 2013 leading to increased wave action eroding bluff toes, but a multitude of processes within the hillslopes regulate the rates and timing of bluff failures. Many of the processes that regulate the timing and magnitude of these cold coast bluffs (where air temperatures spend a portion of the year below freezing) differ from their warm coast counter parts.

Using remote sensing techniques, groundwater flow modeling, hillslope stability modeling, and novel in-situ instrumentation we have analyzed and assessed the mechanisms of bluff failure. We find that in many instances the maximum rate of bluff failure occurs in the early spring when temperatures begin to rise above freezing. The timing of this peak in bluff failure results from a transient rise in porewater pressure and the reduction in bluff sediment strength associated with thawing of interstitial ice. When both of these conditions are maximized in the early spring the bluff hillslopes are at their most unstable. Also, in general, we find that the hillslope instabilities along the bluffs are composed of many small failures and that failure fronts propagate up the bluff face at a rate of ~4 m/yr. Given that some of the taller bluffs are ~40 m high the crests of those bluffs (and the communities atop of them) still have not experienced retreat despite rapid erosion along lower segments of the hillslope. This delayed response at the bluff crest will likely be followed by a nearly equal delay in the return to stabilization of the crests once lake levels have lowered and toes have stabilized. The mechanisms and rates we have determined for bluff instability should provide private and government planners a baseline from which to plan.