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 by 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