Friday, February 14, 2014

An(other) update from Elwha

I recently had an opportunity to compile some thousands of individual photos taken from a site above the mouth of the Elwha River into a single timelapse - this collapses over a year of observations into two minutes. It is hard to watch - this is raw with all photos taken at night, during bad weather and during all phases of the tide, included. Despite that, though, it provides a nice view of the changes that have taken place at the river mouth over the past year. A WWU student is currently working on a cleaned up version of this perspective, which should be available in a few months.

Interesting patterns on the beach after the last survey a few days back. Notably, the most interesting changes are happening on the beach to the WEST of the river mouth (Line 132 above), which continues to accrete due to the transport of sand on to the beach face. The sand is also moving higher on to the beach there, and now reaches up to over 4 m above MLLW:

Grain Size photo from 12 February 2014 at 4.25 m elevation at Line 132.

even just a few months ago that elevation looked like this:

Grain size photo from 15 December 2013 at 4.25 m elevation at Line 132.

To the east of the river mouth at Line 164 the areas that I can access are no longer really beach - the river channel now cuts across that site such that I can no longer reach the outermost bars. You can see the profile truncated in the profile from 12 February - that is a river channel. You can also sort of make out the migration of the river channel back to the east that happened in December in the timelapse above, though you sort of have to look hard.

Finally, on the eastern part of the floodplain not much appears to be happening on the beach. There appears to be some sand moving around (there are pockets of it everywhere, especially down low on the terrace at a variety of locations), but not enough of it has moved high enough on to the beach face to really start inflating those profiles (though it does appear that erosion has slowed down at both Line 190 and 204. We are POSSIBLY seeing some finer grain size material moving in on the beach at Line 190:

Grain Size photo taken at 1.50 m elevation on the beach at Line 190 on 12 February 2014.

Though its worth noting that the main channel of the river now appears to be position just to the east of this site (approximately between Line 164 and 190) and is pointing east in the alongshore direction. In fact, there are new exposures of the former coarse low tide terrace in places that had been covered with new sediment for the past months, probably because river flow has excavated this new material and moved it further alongshore. The river itself may be transporting some of this sand up on to the beach. Additionally, this beach has seen pulses of fine material before, especially this time of year:

Grain size photo taken at 1.50 m elevation on the beach at Line 190 on 5 February 2013

Grain size photo taken at 1.50 m elevation on the beach at Line 190 on 14 February 2012

No comments: