Thursday, January 19, 2012
The eastern shore of southern Whidbey Island forms a well-defined drift cell with the dominant southerly storms and waves transporting sediment northward. The cell originates at Possession Head at the south end of the island and terminates at Sandy Point, near Langley. The abundant sandy sediment has led to the development of long spits parallel to base of the high bluffs. One of these lies just north of Possession Head (Possession County Park lies at its southern end). Another is Columbia Beach, the long low strip of land lined with waterfront homes that you see from ferry as you approach the Clinton terminal.
North of the ferry dock, the shoreline bends slightly westward and another barrier beach emerges, forming Randall Point, Brighton Beach, and Camper's Row along Hastings Road (aerial view). The original spit must have fizzled out along here somewhere, but ingenuity and some earth-moving equipment (perhaps just a big hose) filled the narrow strip of low beach berm and wetlands to create a wide enough platform on which to build cabins.
It makes for a neat neighborhood, but a geologically hazardous one, as homes are tucked into the base of steep slopes that periodically fail during heavy rain storms. Elliott, my guide today, recalls living in one that got hit by a slide several decades ago. Some owners have built walls to block or divert possible mud and debris. In addition, a couple of steep stream gullies also empty onto this beach, their lowermost reaches in pipes and channels that would be challenged by a large debris flow or a flash flood.
I have no trouble seeing the appeal of these places, but the geologist in me can't help but shake my head. I doubt I'd sleep comfortably during rainy spells.
For more on the history of this community and a wonderful sense of what draws people to these places, look for Frances Wood's Down to Camp: A History of Summer Folk on Whidbey Island.
This time of year, the highest tides arrive right around sunrise. A couple of days after Christmas I headed out early to Golden Gardens to see the effect of the month's highest water levels. The predicted high tide was 12.7' above MLLW (Mean Lower Low Water or a "0" tide). According to the Seattle tide gauge, the actual water level may have come in a couple inches higher. These photos were taken 15-30 minutes after the peak tide.
This is not an unusually high tide. A 13' tide (these numbers hold for central Puget Sound, but would need to be adjusted for other areas) occurs an average of 7-8 times a year, although the annual variability is huge. Some years never see a 13' tide at all. And in El Nino years, we may get 20 or 30! (Alki Beach: January 2010).
The number of high tides at progressively higher levels falls off quickly. In Seattle, Mean Higher High Water (the average of the highest high for every day of the year) is 11.35' - and we can expect a couple hundred every year. As noted, 13' tides average less than 8 times a year. And 14' tides take extraordinary conditions: either a strong El Nino, like January 2010, or a big storm with a strong surge (February, 2006). Seattle's highest high tide was 14.5' and occurred on January 27, 1983 (another El Nino year).
In these photos at Golden Gardens, we see that the still water level is probably a foot or so below the sandy berm, but that the runup of the waves is just enough to send water over the berm crest. Berm elevations vary and can depend on wave exposure and the abundance of gravel, among other factors, but 2' above MHHW is often a good place to start.
More on Golden Gardens:
Golden Gardens: July, 2011
I'm trying to catch up with some posts started in December.
The original shoreline here consisted of a low bluff, a narrow beach, and the endless tidal flats of Padilla Bay. Development involved building a rectangular fill jutting out into the bay, using sediment dredged out from under the eelgrass on the flats. No, they couldn't get permits to do that today! The aerial image (see link below) shows the artificial geometry of this piece of the world.
The north and south ends are marked by riprap walls, which are extended out into the bay to act as terminal groins for the artificial beach that lies on the western shore of the feature. Two more intermediate rock groins apparently help control the beach's shape - although I'm not sure how much they add.
The original beach was built of imported sand and gravel - the material in the tide flats is too fine - and was renourished as the beach gradually eroded back into the artificial fill. There is no sediment in the corner south of the fill, but a very nice little pocket beach has formed on the north side. Some of this sediment may have come from the eroding bluffs to the north - while some may have leaked around the northern groin from the nourished beach.