Wednesday, May 29, 2013

Time Lapse of the Ledgewood Slide

A few months back I wrote about a survey of the Ledgewood Slide, a large deep-seated "rotational" slide on the west coast of Whidbey Island. During that survey I left a camera behind, which faithfully collected an image of the toe of the slide every hour during daylight hours. My interest was in determining how quickly the toe of that slide would erode, as well as the processes that are most responsible (high water versus waves, for example). And while survey data would be most useful, the time lapse is a great tool when you can't be there all the time measuring.

I was able to quickly revisit the site over the Memorial Day weekend, and while I didn't have time or tide for a survey I was able to recover the photos. Check it out:

Note that this time lapse was updated on 28 June 2013 to include photos collected through 26 June 2013

There are a few marked episodes of erosion of the toe, occurring over the night on April 4-5, on April 10, and the night of April 28-29. All of these periods fall during spring tides (see water level observations from Port Townsend, below), with high astronomical tidal water level, suggesting that water level plays a critical role. This seems obvious, except to note that the last two days captured by this session, May 24-26, also feature very high tides but no obvious erosion events.

It therefore seems likely that wind, and specifically wind-generated local wind waves, played a role in those erosion events, and the wind data from Port Townsend seems to support this, with some notable spikes around April 4th, 10th and 29th.

Regardless, the bluff toe had changed dramatically. Notably, as the toe of the slide erodes it exposes more of the sand layer that was exposed as a relatively thin (~1-2 m) layer during my survey on 2 April. This material is likely a significant contributor to the budget of adjacent beaches...and it would be cool to get out and do a survey to look at profile and grain size change...just need to find a time to get out.

Here are a few photos I shot on May 26...

Steep and high sand scarps prevail across much of the toe of the slide at this point, which contrasts with the early toe, which was composed of a lot of uplifted clays.

A view from the top of the scarp looking down to the beach...a distance of ~6-7 meters

Another view of the scarp at the toe of the slide on 26 May

The beach to the north of the slide...appears sandier than before

A view of the beach to the south of the slide, taken on 26 May 2013 from the revetment. Below is a photo taken on 2 April 2013 taken from a point looking towards the slide from about MLLW. The photo above was shot from about where the stairs are in view right. Sandier beach?