Tuesday, May 9, 2017

A Very low Angle Landslide and a Low Angle Tree

I had a revisit to a shoreline bluff that I did some work on 15 years ago. At the time, a relatively recent slide had taken place that left a pile of debris on the upper beach. The slide had also pushed over a tree that was growing near the base of the bluff. Seventeen years after the slide the tree is still hanging on and in fact, two of its limbs are now growing upward as leads. Rather impressive that a tree could maintain this angle.

This shore reach is fairly stable. Despite being fairly exposed to open fetch, waves do not reach the toe of the bluff very often and the beach is well supplied with sediment from eroding bluffs to the west. When waves do reach the bluff toe, the erosion is slow due to the hard compact nature of these preglacial deposits.

The slide that pushed the tree over was a but unusual in that it was not from the steep bluff face collapsing, but was instead a flow of loose material that had "waterfalled" down the cliff from a very low angle slide on the upland above the bluff.

I found my notes on the slide "Failure slope is impacted by perched water being directed into a more sandy ice contact deposit that is not consolidated. Failure does not appear to be the result of clearing. Several old growth trees impacted. Failure slope is less than 10 degrees. Failure extends 250 feet back from the top edge of the bluff."

This slide taught me a few lessons and made me very alert for similar slides.


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