Monday, December 31, 2012
The west side of San Francisco slopes down to the ocean in rolling sand dunes, now stabilized by the endless residential blocks of the Richmond and Sunset neighborhoods. The sandy beach is all that remains of the original system, cut off from the city by the Great Highway and the impressive recurved seawall built to keep the Great Highway where it was built.
This is a complicated beach that has experienced long cycles of erosion at one end or the other for the past century. Climate and wind patterns shift sediment back and forth as they do on most beaches, but there are also interesting dynamics related to the availability of sand from offshore deposits at the mouth of the bay (USGS Field Trip Report, ASBPA 2005). The beach is doing well at the north end but there have been serious erosion problems at the south end down near Sloat Avenue for many years and there have been recent efforts to nourish the beach at that area.
The O'Shaugnessy Seawall at the north end is often held up as an example of a successful seawall - although I suspect what it really shows is that seawalls work best, and have the least impact, where the beach in front of them is stable or building. It is a substantially built structure which means it may continue to protect the Great Highway even if the beach were to go away or sea level were to rise. For the time being, it provides a nice promenade with easy access to the wonderful beach.
Saturday, December 29, 2012
The southerly hook of Point Reyes forms Drakes Bay (after Sir Francis). Drakes Beach lies along its northern shore and is an elegant curve of high bluffs, broken by a few small barriers and a couple of larger spits.
I believe the bluffs are Miocene siltstones and sandstones of the Purisma Formation. There is a wonderful planed-off platform at the base of the cliffs. Wave-cut platforms - shore platforms, more generally - have generated plenty of research and speculation. I suspect this one reflects some combination of modern waves, historic uplift?, and lithologic variation - but that's mainly a guess. The sun was setting and we had to get back to the city for dinner - so much for more exploration.
Point Reyes is separated geographically from the rest of the Northern California coast by Bolinas Lagoon, the Olema Valley, and Tomales Bay. It is separated geologically by the San Andreas Fault, which bisects all three of these features. That means Point Reyes is on the Pacific Plate and heading north relative to most of northern California, which is still attached to North America. It's geolgoic affiliation is really with the coast much farther south - and ultimately with rocks that belong in Southern California. The headland of Point Reyes is Cretaceous granite, much of the peninsula is Miocene sediments, and much of the west side is mantled in both ancient and modern sand dunes.
The beach on the ocean side consists of a long strand of coarse sand extending north from the headland. It's backed by dunes and low bluffs cut into much older dunes. The aerial shows the dunes climbing southeastward up the long western slope, which says something about the prevailing winds.
The dunes are largely stabilized by non-native European beach grass and Ice Plant. Dunes are like meandering rivers and receding coastal bluffs - they are supposed to erode. And if they can't - due to levees or seawalls or invasive vegetation, the landscape and the ecology begins to change. The ice plant was beautiful in its winter colors -- even if there was far too much of it!
Monday, December 17, 2012
In November 2011, I came up here during a windstorm and wondered what it would look like if the tide were two feet higher. This morning it was three feet higher, although I missed the peak by 30 minutes or so.
The road signs pretty much tell the story. I've included a picture from a nicer day (several years ago) for comparison!
And here's a video clip -- excuse the poor quality, but it was hard to stand up in the wind.
This morning was already going to be a very high tide, but an incoming storm turned it into something special. Below are the barometric pressure and tides in Seattle for the last two days. As pressure dropped yesterday, the water levels rose above their predicted levels (green curve is difference). This added "surge" increased to about 2' last night, then fell off a little by morning as the system arrived. The predicted 12.9' tide arrived at about 14.5' (relative to MLLW in Seattle), roughly matching the previous record set in January, 1983 (during an El Nino).
I spent the morning on the west side of Whidbey Island. I missed the peak of the tide, but got some great shots of storm waves at West Beach (see subsequent post) and numerous other shots on the way down the island. The winds were out of the southwest on the northern end of the island but from the northwest as I moved down the island. Ultimately, it was if the wind was barreling eastward down the Strait of Juan de Fuca, then splitting when it reached the west shore of Whidbey near Point Partridge. In an earlier post (Keystone: October 2012) I mentioned that while winds are often out of the south in the Admiralty Inlet area, the most significant waves in terms of sediment movement might still be out of the northwest. I think today was a good example.
|Bowman Bay, Deception Pass State Park|
Gravel Beach: March 2012
|Failing bulkhead south of Libbey Road|
Gravel Beach: February 2009
|Removing logs off road at Ebey's Landing|
Gravel Beach: February 2008
|Another failing bulkhead, this one at Ledgewood Beach|
(for another perspective, see cover image of 2010 USGS Report)
Gravel Beach: April 2006
|Lagoon at intersection of Shoreview and South Woodard in Freeland|
|Lagoon at Freeland Park (you may note that Freeland has several|
historic back-barrier lagoons that periodically reassert themselves)
Gravel Beach: October 2006
Wednesday, November 28, 2012
This large multi-unit residential project was proposed several years ago, but was delayed by the economy. Now things are underway again, with one of the first steps being the innovative log-crib erosion control structure.
I'm not quite sure what to make of this project - I can't think of anything like it on the Sound. It's an improvement over the seawalls farther east - in large part because it doesn't project as far onto the beach - but it will be interesting to see how it does over time.
For some more history on this area:
Skyline: April 2006
And to see what Flounder Bay has become, check out the view from above:
The best use for a big deep-seated landslide on Puget Sound is probably a forest. The next best use may be a relatively low-intensity public park. The northern day-use area at Camano Island State Park sits on a mid-slope bench, the contents of which began their jerky movement to the Sound hundreds or thousands of years ago. The uneven surface of the picnic area and the offsets in the parking lot suggest nothing has changed.
The head scarp is a high steep, forested slope that rises above the parking lot. Big drain pipes convey water from the land above (part of which is the park entrance road, but other parts of which are private homes) down to the bench, where hopefully it then gets captured and sent to the beach. Water is, after all, the grease on which these slides move, and the simplest way to slow their movement is to keep the water out. Of course, even that may be far from simple. The bench often slopes landward, making natural drainage tough, and slide movement can break pipes or otherwise disrupt drainage. And often the water is deeper groundwater, which doesn't lend itself to easy solutions.
These big slides often have portions that are more susceptible to movement than others - often, but not always, due to past or present drainage problems. The most active portion of this slide is the outside lane of the road down the bluff -- if it expands, access to the day-use area may become a lot more difficult. Or a lot more expensive.
But again, better a park. All too often, developers covet these benches for rows of million-dollar homes, and the developer is long gone by the time the foundations start cracking and the septic tank slides past the patio.
Other Big Landslides:
Perkins Lane, Seattle
Ledgewood Beach, Whidbey Island
Camano Head, Camano Island
Discovery Park, Seattle
Kopachuck State Park, Gig Harbor
Turnagain Heights, Anchorage
Termination Point, Hood Canal
Wednesday, October 31, 2012
I'll try to keep this post relatively short. The recent work has only just been completed and I'm sure I'll be back often to check on how the restored beach performs. Also, I don't know all the construction details, so there are elements I can't really comment on very intelligently (I suspect some readers will point out that this has rarely stopped me before).
Here's a post from several years ago to compare with:
Cornet Bay: 2008
The old creosote timber bulkhead is gone and much of the fill has been removed. The beach and backshore slope has been regraded and erosion control measures have been put in place. I suspect this is to protect the site until planting and landscaping can be completed (possibly late this winter, but maybe they're planning to do something sooner?).
Based on the tides during my visit, I believe the straw wattles are located 6-12" above MHHW. This means that the final beach should extend up into the area that is currently covered in bark, so it's a little hard to appreciate the scale of the beach restoration in these photos.
This will be a valuable site to watch and I hope to be back up a few times over the next few months. There are a whole suite of neat beach projects in the vicinity - too bad that my informal monitoring is sort of an extracurricular activity!
I suspect this short stretch of beach just east of Keystone Harbor wins the award for most rapid, sustained, long term erosion rate on Puget Sound. But there's a catch!
We don't know a whole lot about erosion rates on Puget Sound, but we know that one foot per year is unusually high, at least over any significant period of time, and such rates are probably limited to the most exposed sites along the Strait or the toes of big, recently reactivated, deep-seated landslides.
Where erosion rates are unusually high, we usually find out that it is an area that was historically filled. You pile dirt out into the ocean and, of course, it goes away quickly.
This particular gravel bank erodes 5-10 feet per year, every year, and has done so for five decades. Every five years or so, the Corps dredges gravel from the mouth of Keystone Harbor (so the ferry doesn't run aground) and places it on the adjacent beach - a classic, albeit fairly small, bypass operation. The pile of material forms a steep, rapidly eroding bluff that advances rapidly on the parking lot for five years, at which time the Corps comes back, dredges again, and the whole process repeats.
The simple version of the sediment transport story is that drift is from north to south, so that gravel eroded from the bluffs farther north on the island gets moved south around Admiralty Head and then east past Keystone to accumulate on Keystone Spit (the large barrier that fronts Crockett Lake). This is why the bypass notion works. On the other hand, storms here are still largely form the south, so why isn't the drift the other direction? This issue arises elsewhere on the west side of Whidbey, too. My preferred explanation, but one that is purely speculative, is that longer period waves from the north, down Admiralty Inlet, are more effective at moving sediment than the choppier waves that come more frequently from the south.
Friday, October 19, 2012
This is another site where an old bulkhead - or perhaps more accurately, several old bulkheads and a lot of rubble - have recently been removed. A complex partnership of many different people came together to make this project happen.
This is a relatively low energy shoreline - wave action fades quickly entering the bay and the last real beach ends just short of this site. That doesn't mean that the banks aren't subject to erosion, but the rates are low and the site is developed in such a way that minor erosion is not a problem. Over time, some of the bank will crumble and a few trees will fall, but the resulting shoreline will be richer and more natural. Ironically, it looks like much of the past erosion is of fill material that was placed over the natural bank - which is so often the case.
This is a concept that could apply to many, many sites in Puget Sound and maybe some more examples like this one will help show folks that unarmored shorelines can provide as many benefits as those that are buried in rock and concrete.
The old concrete bulkhead at Anna Smith Park had stood for decades - well, most of it had stood for decades. Some segments hadn't made it so long. But its original purpose was at best a fuzzy memory and it offered an opportunity to recover a natural shoreline. The wall, along with some of the rubble behind it, was pulled out a few weeks ago and the remaining back fill is rapidly redistributing itself on the beach.
Kitsap Sun (30 August 2012)
The sites that lend themselves to removing seawalls are probably a lot harder to find than the sites that lend themselves to building seawalls - although there is some thinking these days that maybe some sort of balance can be achieved. I suspect that even if we can, we will find that the types of locations where we can take them out are significantly different than the types of locations where we can't stop them from going in! Which makes simplistic thinking about the net gain or loss of armored shorelines ... simplistic.