Buried silt on the Elwha sea floor

This week I was, once more, part of an annual subtidal survey of the marine community around the Elwha River delta during which we visited our site called, 4SP1, just to the east of the Elwha River mouth, roughly here. This is a place that was buried under 2-3 feet of sand in 2013, as the dam removal pumped out tons of sediment into the Strait of Juan de Fuca.  The site IS still very sandy, here is a bit of a view from this year:

and it does still look very different than it did before dam removal started (check it out in 2011 here).  But this year the site was also obviously different from last in that much of the deep mantle of sand deposited during dam removal had been stripped off.  So that is interesting in and of itself...we've been curious about how the Elwha River delta would evolve in the long-term now that dam removal is long over, and this gives us some indication.  But what really struck me was what was UNDER the remaining thin layer of sand on the seafloor...a few inch thick layer of silt and mud, which you can see in this photo below, with my slate for scale:

And get a sense for the composition in this video.  So what is the big deal?  This is a high current site, where mud and silt generally doesn't stick around.  I think this layer of mud and silt tells us something about how rapid and how massive the delivery of sediment was during the dam removals.  I envision a pulse of very fine material blanketing the sea floor, presumably followed almost immediately (perhaps within a tidal cycle?) by a pulse of sand, both pulses massive enough to blanket the seafloor with many inches of material...

Cabins at risk of coastal bluff erosion removed at Kalaloch in Olympic National Park

 

Some of the cabins at Kalaloch are (or were) quite close to the edge of the coastal bluff. Photo from 6 July 2023 

Eroding coastal bluffs are hard to manage, especially when valuable assets are perched on their edges. Coastal bluffs are by definition tall, and their erosion can be rapid and forced by a variety of processes, so engineering solutions aren't always easy to devise, and are invariably expensive and difficult to maintain. At Kalaloch in Olympic National Park, two factors recently converged to create particularly dangerous problems.  First, rates of erosion of coastal bluffs in a small area just south of Kalaloch Creek accelerated, and second, the locations where those erosion rates accelerated also happened to be developed. Buildings, in this case valuable rentable cabins at Kalaloch Lodge, were put at increased risk and, as a consequence, in March 2024, five cabins were deemed unsafe and removed.

A similar perspective as in the photo above, with cabins removed.  The former cabin sites are marked with the brown burlap.  Photo from 10 April 2024

I was part of efforts that looked for every opportunity to try to understand that erosion, in the hopes that some cause of erosion might be identified that could illuminate a potential solution. As is often the case, though, the best we can do is use our monitoring data to characterize rates of erosion, and how those rates vary along the shoreline, as I did in this recent recorded talk.

    To me, one of the most interesting things that we've done at Kalaloch is to install five Chronolog stations, which encourage visitors to submit photos from a fixed vantage point, which can then be complied into a time-lapse video. Those stations, and the people that have contributed to them, have created a rich visual record of bluff erosion and the removal of at-risk buildings, and other poorly understood coastal processes (like the loss and recruitment of large wood along the beach). 

A Chronolog station at Kalaloch.