Monday, July 1, 2013

Coastal Sediment Distribution from the Ledgewood Slide

Much of the toe of the Ledgewood slide has eroded back dramatically, consistent with time lapse photography from the site. The scarp is now quite steep and tall (3-5m throughout most of the slide - the survey pole in the photo is 2m high). In this photos, at the very base of the scarp, you can make out the darker uplifted clays that were so prominent on the toe immediately after the slide.

I've posted a few times now about the large Ledgewood slide on Whidbey Island, which happened early this year. I've had the opportunity to visit the slide three times now to both do some very small-scale surveying of the shoreline, as well as collect time lapse photos of the toe of the slide as it erodes. My overall goal is to generally contribute to the efforts to understand the slide as a whole, but specifically look at the coastal consequences of large bluff failures like this one. In particular I wanted to try to document where, how and how much sediment from the slide recruited to adjacent beaches.

My approach was to do repeat surveys on a small number of transects both on and adjacent to the slide toe. The first set was collected just a few days after the slide...and I finally made it out last week with a small group of Island County residents to repeat those surveys. In general, we found erosion of the sub-aerial portion of the slide - the toe material that was in or above the intertidal, and some indication of deposition of some sediment below MLLW (though this is scanty since we couldn't really work below about MLLW). We also found deposition on the intertidal beach both north and south of the slide. Here are some example profiles from on the bluff toe itself:

Beach profiles from three sources: 2001 aerial LIDAR (courtesy of the Puget Sound Lidar Consortium), which I use here as a suggestion of the pre-slide beach position, a 2 April 2013 profile (immediately post-slide), and the data we collected on 26 June 2013. On these transects in the middle of the slide the scarp became very steep and tall (>3 m; see photo above), and I wasn't able to collect data on the scarp itself, or on the upper surface due to both safety concerns and vegetation on the upper surface that would have blocked the GPS signal.

At either end of the slide I was able to survey to the same surface I surveyed on 2 April...more or less "connecting the dots"

The profile from the other end of the slide

Profiles from the beach adjacent to the slide suggest some deposition of that eroded material on the intertidal beach, but probably not too far north. Here are the profiles from two transects north of the slide:

This transect was the furthest north one that I collected on 2 April 2013, and there is limited, if any, deposition at this site 139 meters north of the toe of the slide.

By contrast the transect immediately north of the slide shows uniform deposition across the entire profile...about 1 m vertically.

The grain size data I've collected isn't ready for prime time yet, but you can get a sense for the grain size transition on the beach by looking at these photos, suggesting that the material accreted to the intertidal beach is a mix of lots of sand, and a good bit of gravel. These photos are on the profile line above, immediately north of the slide. Check out the first one, collected 2 April 2013:

and compare it to this one from 26 June, taken at approximately the same elevation on the beach (just above MLLW) and more or less the same perspective (note the shoreline protection in the far field to orient yourself.

Based on the very limited number of profiles that I measured, it does appear that more sediment from the slide has migrated south

Here are the profile data from the transect immediately south of the slide. Here too there is a fairly uniform deposition of 1 m of sediment across the intertidal profile. If you watch closely, this deposition, as well as some of the dynamics associated with the transport of sediment on to the beach, is visible in the time lapse video.

To the south, as we moved away from the toe of the slide, we were still seeing measurable accretion on the profile. This is the furthest south repeat profile that I have, but based on grain size variation we estimated that sediment from the toe of the slide had deposited on the intertidal beach at least 375 m south of the slide.

Back of the envelope estimates of sediment volumes, based on these very limited data, suggest on the order of 18,000 m3 eroded from the toe of the slide (that is probably a low estimate)...and ~2000 m3 deposited on the intertidal beach to the north, and maybe 6000 m3 deposited on the beach to the south. Again, VERY loose estimates...and based on the observations of a variety of people, if I had to guess, I would say that the remainder (>10,000 m3) has moved into the sub-tidal zone (below MLLW) or was finer material (mud, silt and clay) that was stripped off of the toe and moved away from the site by currents.

A special thank you to Paul Morris, Jan Wright, Mark Kennedy and Jan Kennedy for their help with this re-survey! Also, John Erlick for hosting my time lapse camera...

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