Pennsylvania Sea Grant

Approximately 34 million people live within the North American Great Lakes Basin: ~32% of the Canadian population and ~8% of the US population. About 12 million of those people live on the Lake Erie coast of New York, Pennsylvania, Ohio, and Ontario. Unconsolidated Quaternary-age bluffs ranging in height from 1.5-55m dominate along 73km of the 123km Pennsylvania coast, and long-term records show that slow-continuous erosion is pervasive, and that rapid (but locally catastrophic) bluff failure is relatively infrequent. As a result, ~90% of the Lake Erie bluff coast in Pennsylvania is designated by the state as a Bluff Recession Hazard Area wherein regulations limit risky bluff-top development. A high degree of variability (space, time) in bluff-retreat rates exists because stratigraphy and geotechnical properties show variation due to materials, depositional geometries, post-depositional processes, hydrology, and anthropogenic influences. This makes it difficult to forecast the magnitude, frequency, and location of larger bluff-failure events and consequently makes pre-emptive mitigation efforts more challenging. Two methods are commonly used to establish coastal construction setbacks on Great Lakes bluff coasts: (i) the "AARRxT" method which uses a simple linear extrapolation of past bluff-change rates to estimate the future bluff position and the setback line for a building being constructed today; and (ii) the "AARRxT+" method which uses a similar approach but incorporates a slope stability factor and/or a relocation buffer. The limitation is that these deterministic methods assume that rates and magnitudes of processes driving change in the past will not change in the future, and they create the impression that bluff change is linear and more predictable than it is in reality. At the property and municipality scales, this makes hazard planning for continuous and catastrophic bluff failure particularly challenging.