Sunday, September 28, 2008
A 20th-century industrial midden north of Gig Harbor. Every piece of concrete on this site has a history - some came from railroad bridges in Tacoma, some from piers in Ferndale and Anacortes, some from old ferry terminals. Now the concrete lego blocks are holding together the edges of a large fill built at the base of the bluffs in the early 1950s, along a landslide-prone shoreline now blistering under the influence of large subdivisions creeping north from Tacoma.
A series of groins made from sandstone blocks and construction rubble trap or divert the southerly drift, creating several small pocket beaches. Although most of the storms come out of the south, the biggest fetch is straight down Colvos Passage from the north and that's where the biggest waves come from and what controls net drift. The bluffs immediately south of this site are eroding rapidly, the expected downdrift result of this heavily modified shoreline. Efforts underway to clean up the site stem the chemical seepage, but do relatively little to restore the beach or the shoreline itself.
Thursday, September 25, 2008
High bluffs of sand and gravel above a beach with great views of Mount Rainer and the Olympics (well, not this day). The shoreline of Marine View Park is relatively natural - if you discount the stair tower and its rock base, a scattering of cut-timber driftwood, and an infestation of English Ivy. No armoring. There's a house in a ravine just to the north, but it's well back from the beach. There's lots of drift wood, some large madrone trees, a small stream mouth, and the normal mix of sand, gravel, and large boulders. A sand bar was extending northward (downdrift) from the little stream delta - apparently as waves tried to work the sandy stream deposits back onto the beach.
Farther north, the base of the 250' angle-of-repose slope is marked by a 1950's era concrete seawall. The beach doesn't look much different, except there is no drift wood, and the lower beach is coarser grained - probably a function of changing geology, erosion patterns, and shoreline orientation as the beach curves around the point. But one can't help but wonder how different this shoreline would be had the bluff continued to erode for the past 50 years. That's an awful lot of gravel that would have contributed to beaches farther north.
The view down the beach from the Marine View Park stair tower shows very nicely where the Mean Higher High Water line typically lies on Puget Sound beaches - the previous high tide, which left an obvious waterline, was within a couple of inches of MHHW. It shows why the old practice of allowing bulkheads to be built out to MHHW was so destructive. This picture should be paired with the picture taken, and posted, last year (Normandy Park) - when the tide was even higher.
Wednesday, September 24, 2008
The northern tip of the Kitsap Peninsula has two great coastal marshes. One is a large salt marsh behind a recurved spit just west of Foulweather Bluff, but unfortunately it's private, or at least access to it is. The other is the brackish marsh at the Preserve - the Nature Conservancy's signature holding on Puget Sound.
I call this a closed marsh or lagoon, meaning that most of the time the barrier is continuous and there is no surface connection to the Sound. Occasionally a big storm may wash over the berm and if the water behind wants to get out badly enough, it may develop a shallow channel for a short period of time. But it always closes up. This one is large for a closed marsh, since as the potential tidal prism increases, the ability to maintain a channel becomes easier.
Closed marshes occur behind barriers and generally lack significant stream input (otherwise the stream would maintain a channel). In theory, it's easier to maintain a closed marsh behind a coarse gravel berm than behind a sandy one, since drainage can occur more readily through the permeable berm.
Closed marshes haven't gotten as much attention in recent years as tidal wetlands, since they are usually small, and without an open inlet, they have limited value as refuge or forage for passing fish. But they're still significant coastal features. Many may have originated as open systems. And maybe with sea level rise, some will become open systems again! Generally, they seem more common as you move north in Puget Sound, for which there are several potential explanations. More wave exposure leads to greater sediment transport and the North Sound has more barriers behind which estuaries and lagoons can form. Smaller tidal range in the north may reduce the hydraulic viability (smaller tidal prism) of these systems on the brink of being open or closed. And because precipitation in less as you go north, their are fewer streams intersecting the coastline and a larger proportion of small barriers occur along shorelines with no significant upland drainage.
This feature occurs in a low glacial trough - the till dives below beach level on each side creating a shallow bowl. Sea level eventually flooded the trough, but there was sufficient sediment to form a barrier across the mouth. This is a pretty sandy beach, although the high berm is gravel. It is swash-aligned - almost a pocket beach, held in place by the resistant promontory of till at the west end. For years, there was a beautiful madrone desperately hanging out over the bank, but in the mid-1990s, it finally gave up.
It's unfortunate that many investigations of Puget Sound's most interesting landslides have been done by consultants working for the people who want to build houses on the landslides! Which sometimes results in really great landslides being interpreted as old logging operations or marine terraces.
The slide plane dips below the beach in this area and emerges mid-beach as a band of uplifted, distorted clays. The toe of the slide is marked by an arcuate ring of large boulders (I'm not sure I understand just why this occurs). In the 1970s and again in the late 1990s this portion of the slide reactivated -- both times making it a little easier to question ambitious schemes to develop the upland portions of the slide.