Tuesday, April 16, 2013

Preliminary results from a Ledgewood Slide survey

The ProMark 200 RTK-DGPS system used for this survey leaning one one of the uplifted scarps along the edge of the slide. Those gravels on top of the scarp are the former high intertidal surface, uplifted 2-3 meters.

The Ledgewood Slide on Whidbey Island on March 27th was large enough to destroy at least one house and prompt the evacuation of more, and also to generate at least a blip on the national news scene. Bluffs are a predominant shore type in Washington State, and their failure - whether slow and creeping or large and catastrophic - is one of the many hazards that coastal residents of this state are subject to. Bluff erosion and failure is sort of a double-edged sword though - while a hazard to human infrastructure, the sediment supplied to beaches by bluff erosion is thought to be a building block for beaches and complex coastal habitats. As a result there can be tension between the desire to stabilize bluffs with armoring or other tools, and allow them to erode to support habitat restoration goals. In light of sea level rise projections, climate change projections related to increasing winter precipitation, and on-going construction of infrastructure on coastal bluffs it is likely that this tension will only amplify in the decades ahead.

Rebekah Sexton and Diana McCandless from WA DOE's CMAP program - about to get wet and collect more topography data than you can imagine.

For all of these reasons understanding how, when and why bluffs erode, and what sort of contribution they make to the coastal sediment budget are important research objectives in Washington. The Ledgewood slide, because of its scale and the deep-seated nature of the slide, generated significant interest amongst the small community of coastal geologists, geomorphologists, managers and others who think about these sorts of problems every day (see these two other blog posts on the slide, one by Dan McShane, and the other by the venerable Hugh Shipman, as well as the Washington DNR's Preliminary Report on the slide). As a result I carved aside a day (2 April 2013, six days after the slide) to join a few others from the WA DOE Coastal Monitoring and Analysis Program to investigate the slide. While the DOE staff (Rebekah Sexton and Diana McCandless) focused on collecting high resolution topography data on the beaches adjacent to the slide for post-processing, I decided to focus on collecting real-time topography data on the slide itself. I had two goals: 1) Add to the baseline data set that will allow us to track the evolution of the toe of the slide over time as it interacts with waves and tides, and 2) quantify the volume of sediment delivered to the coastal zone by the slide. Here I am providing some of the preliminary results and analysis from that short survey. To reiterate though, this is just a taster. Others are collecting reams of data and providing insight that should deliver an enhanced perspective on the slide.

The former log line from the high beach, uplifted to approximately 4 m above MHHW.

First, a bit on methods. All of these survey data were collected with a Peninsula College-owned Ashtech ProMark 200 RTK-DGPS system (PM200) on a survey pole (see photo above), receiving corrections through the Washington State Reference Network. Its always ideal to validate the RTK survey data against some other independent spatial data, preferably a monument with good published control. In this instance though there were no monuments adjacent to the site. As an alternative the PM200 survey data were compared to aerial LIDAR collected in 2001-2002 for the area around the slide, and available through the Puget Sound Lidar Consortium. Near the slide site 20-30 points were surveyed along a surface (the road leading down to the parking area north of the slide) that I though was likely there in 2001-2002, highly visible to the LIDAR sensor, and probably fairly stable over that 11 year intervening period. These 20-30 survey points were compared to the nearest individual LIDAR return and the measured elevations differenced to generate this distribution:

So this was good - an average 6 cm offset suggested that the PM200 and the LIDAR from 2001 were relatively close in the vertical...good enough for the purposes of this preliminary survey.

I focused on trying to collect cross-shore profiles from the water line as high as I could get on the slide, both adjacent to and on the slide, and then compared each to the 2001-2002 LIDAR data. Here is an example from a site in the middle of the slide (others are given at the end of this post):

As it turned out I was unable to survey high enough on the slide (due to vegetation) to get over the pre-slide surface (the LIDAR data). The slide toe was pushed 60-80 m seaward and so all of the areas I was surveying on the toe were over formerly sub-tidal or lower intertidal surfaces, and to make matters worse the 2001-2002 LIDAR in this area was collected during relatively high water - somewhere around 1.5 m above MLLW. Needing a pre-slide surface to calculate vertical changes against I turned to Dave Finlayson's Puget Sound DEM (also shown in the figure above). As is clear, though, this is a relatively coarse (10 m horizontal resolution) DEM and only roughly models the actual surface in the area of the slide.

Despite the coarse resolution of the Finlayson DEM it was the best available surface for most of the survey extent. Here are how all of the survey data compare to the DEM:

You can see from this map how I was unable to survey over the former beach surface along most of the slide. For comparison sake here is the comparison to the 2001 LIDAR data:

Since these LIDAR data were collected at relatively high water the only survey points that I collected that overlapped were on the upper beach adjacent to the slide.

Total volume contributed to the coastal zone? Well, kind of hard to get at with these data since I wasn't able to survey to the former MHHW contour and didn't have high resolution bathymetry to work with, not to mention how sparse my survey data were. However, based on the footprint of the slide now in the coastal zone (an estimated 21,260 m2 based on my preliminary survey), and assuming a mean difference between my survey data and the pre-slide surface (a loosely estimated 6 m based on eyeballing the profiles below), I estimate a contribution to the coastal zone (defined here as the area seaward of the former MHHW contour) of ~128,000 m3 of sediment. This is a VERY rough estimate (noting that a better estimate should come later from the higher resolution data collected by WA DOE), but if its close it represents a significant volumetric contribution to the intertidal zone. However, based on observations made of the slide it seemed to be composed of a large fraction of mud and silt which is unlikely to remain on the beach. And, of course, it remains to be seen if and how long it takes Puget Sound to redistribute this sediment.

Of course, one of the more interesting things about this slide was not necessarily its extent, or the volume it contributed to the coastal zone, but in how it slid, and the forms associated with it. I shot this video to try to capture some of the interesting features of the toe of the slide:

and here are the remaining profiles...all ten of them. If you are interested in my raw survey data email me at immiller@uw.edu and I can get them to you.

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