The beginning of this week was dedicated to grain size surveys on the beach at the Elwha delta. What that means is that I got up early each day and spent hours taking photos of the ground. To spice things up a bit I decided to try something that I've been thinking about for some time - creating a time lapse of the rising tide. I've got some kinks to work out (need a bigger SD card, for example, since mine filled within 6 hours taking a shot a minute) but the result is very cool. More importantly, it may help me to get at some things I've wondered about, like how wave height and breaking characteristics change as the tide rises over the low tide terrace. We've also talked about using a similar technique to monitor changing grain size and shoreline position. Probably more of this to come.
Today I head the opportunity to visit Discovery Bay with Dr. Brian Atwater from the USGS, Carrie Garrison-Lanery, a PhD student at the UW, Lee Whitford from the PT Marine Science Center, and a reporter from the Seattle Times. Carrie was investigating the site as a potential addition to her investigation of earthquake-related subsidence in Puget Sound. I went along to watch the proceedings and figure out if and how we can work with Carrie and Brian to do some education on the tsunami history in this area.
The sand layers that mark numerous tsunamis are clearly visible just inches below the surface of the marsh, and extend to about 8 feet depth, where we ran into what may be the bottom of the shallow bay from thousands of years ago. In total we counted maybe 8-10 sand layers, each perhaps marking an individual tsunami occurring sometime in the past 3000-4000 years.
We started by watching Brian dig a pit, about 3 feet deep, in which you could see two sand layers. Brian then used a corer to pull sediment from beneath the bottom of the pit, revealing further layers. The top three layers were also visible in the side cut of the creek channel draining the uplands. Very cool. Hopefully you can make out the lighter colored sand layers in the photos above...