Tuesday, March 20, 2012
Rosario Beach lies just north of Bowman Bay. It's a smaller pocket beach, the southern portion of which lies in the State Park. I've posted from here before (September 2009), but the purpose of this entry is mainly to provide a counterpoint to the previous post from Bowman Bay.
The Walla Walla University marine research station lies immediately north of the park. As at Bowman, they have had recent erosion problems, but the setting and the history are a little different. At the far end there is an eroding bank and there are also facilities vulnerable to storm damage. The closer end is a low lawn, perhaps fill across the historical backshore. High tide storms likely throw both water and logs up and over the berm and back toward the buildings -- erosion in itself may not have been a problem, but overtopping certainly was.
The solution that emerged is an interesting example of the kind of hybrid structure we are increasingly seeing on the Sound. There's some large rock, a row of logs, and an artificial berm or dune. I suspect the latter is actually the most useful element of this project, since it adds elevation and will likely reduce overtopping. I'm a little more skeptical about the purpose or the utility of the rock and the logs (they provide a sense of security, if nothing else), but regardless, the result is a naturalistic approach that provides benefits during storms while maintaining the basic character of the beach. It will look great when the dune grass grows in and the natural logs begin to obscure boulders.
The landslide at Cama Beach, combined with the realities of work and home, meant that that this post and the next have been stuck in my drafts folder for over two weeks. These two posts are from two pocket beaches on Fidalgo Island, just northwest of Deception Pass. Although both beaches tell many stories, these entries focus on recent efforts to deal with erosion control.
At the north end of Bowman Bay, the beach lies at the base of a gradual slope, but at the south end, it forms a barrier with a wetland behind it. The wetland used to be larger, but historic development led to filling of the central portion of this area. It appears that this fill was placed out across the backshore, which maximized upland real estate (for the state fish hatchery that used to occupy the site), but created a shoreline subject to erosion where a stable beach may have existed before.
For decades, the resulting bank had eroded, despite efforts to armor it with riprap and old concrete. This erosion was a very real problem, but one likely attributable to the placement of the historic fill. If there were homes or a railroad or a nuclear power plant built on edge, perhaps the only viable option would be to further armor the bank. But here, what is threatened is a narrow strip of a large poorly drained lawn. Not to trivialize the task, but pulling back the waterward edge of the fill might simply eliminate the erosion problem while creating a natural and much more enjoyable beach.
This recent effort to address the erosion focused on improving the ragged armor that was previously on the site. It's an engineering fix focused on the integrity of the rock structure, not the underlying cause of the erosion nor, unfortunately, the recreational potential of the beach. Technically, the solution may be fine. It was probably less expensive and bureaucratically simpler than other options. But it's also a missed opportunity (but not a lost one).
The north end of the beach, up near the old CCC shelters and the campground, offers a wonderful template for the eventual future of this central segment.
Just to show that blogging naturalists can visit the same site, yet find completely different things to marvel at, you might check out Wild Fidalgo: The Robins of Bowman Bay (also posted today). Check out Dave's other blog, Fidalgo Island Crossings, too.
Sunday, March 18, 2012
I went back to Cama Beach Saturday morning to see how the landslide was evolving. It's hard to tell how much new material is being added from the headwall, although small chunks kept falling during my visit. The debris in the gully has continued moving on down to the beach, with new trees and large blocks of till perched on top.
The landslide itself is interesting enough - but I think I am most curious about how the beach responds to the slide. The debris cone at the base projects out onto the beach, acting like a groin, and has resulted in the accumulation of a small wedge of beach up against the southern side of the landslide debris. Some of this new material may be beach sediment from the south, while some is derived directly from the eroding toe of the slide. Most wave action here comes from the south. While I was on site today, a gentle northerly breeze gradually gave rise to stronger southerly waves over a period of several hours.
Fine grained sediment is being winnowed from the toe of the slide during high tides and moved way in suspension - there is turbid water along the water's edge and extending in a plume offshore. What is less clear is how the coarser fraction - sand and gravel - is moving. We'll watch this, qualitatively at least, over the next weeks and months to see if it leads any noticeable change in beach profiles or sediment texture in the vicinity.
The headwall of the slide remains scary, with the loud thump of falling blocks of till reminding me to stay back from the edge. Too much is still moving and there are some significant overhangs.
I've posted more photos of the landslide at:
Photos: March 7th
Photos: March 17th
It was raining when I left Seattle shortly after 7:00. It was slushing in Marysville. But it stopped by the time I got to Cama. And then the sun gradually worked its way out from behind the clouds. I posted some less geologic pictures at hshipman, for those less interested in landslides and dirt and more interested in new places to enjoy coffee and scones while overlooking the beach.
Wednesday, March 07, 2012
Along the high bluffs north of Cama Beach, there is a deep box canyon cut back into the till. The upper end was pretty spooky when I first visited it in the 1990s, with vertical till cliffs on three sides, but a small window in the till at the base of the head wall offered a glimpse of the underlying fluvial sands. At the beach at the base of the ravine, there was a block of hardened debris that suggested some sort of big slide in the past.
There was a slide in the canyon last year - you can actually see the toe on the beach in the 2011 Google image linked at the bottom of this post. And then during the past couple of weeks, more material began sliding out of the canyon and onto the beach.
I estimated close to 1000 cubic yards of debris on the beach and an awful lot still in the ravine. Water flowing across the edges of the debris cone were depositing soft fans of sand, but the bulk of the slide was remarkably firm and dry and appeared to have arrived in an avalanche of fairly dry material. Most of the surface of the slide consisted of sandy material, but there were also large angular blocks (some 1-2 meters across) of till.
The source of the slide was massive failures in the high cliffs at the head of the canyon. Water was cascading from the forested surface (perhaps 140-150' above the beach?) down the face of the till - as I recall, the canyon coincides with a low point in the upland topography and may be drainage related. I couldn't get close enough to see if there was seepage from within the fluvial sediments themselves. Given the most recent failure last night, despite the lack of rain, and the current character of the steep cliffs at the head of the slope, I suspect additional large failures are likely.
The geology along these bluffs consists of till draped down over the seaward edge of the older flat-lying fluvial beds. In places the till comes all the way to to the beach, yet at the head of the slide, the till appears to be just 10-15' thick on top of the very thick sandy unit. You can sort of see this if you blow up the photo looking up the slope.
This is a fairly quick account - I'm sure I'll check back on this one -- and maybe the story will become a little clearer.
ADDENDUM: Here's a link to a cool video taken last week (not by me) - before the big stuff came down.
Landslide @ Cama