Friday, August 19, 2016

An update on the Elwha delta

The Elwha River delta and view to the west, photographed 23 July 2015

Its been a good while since I've posted about the goings-ons of the beach of the Elwha River delta, and there have been some exciting developments this summer.  In particular, the beaches on the east side of the delta really added some volume this spring and summer.  Lets take a look at a few profiles, referenced to the map below:

So lets start at the river mouth and move east.  Line 156 and Line 164 are right on the new part of the delta, and where there has been massive accretion in the ~4 years since late 2012.  That massive growth has largely stopped this year, and at Line 164 the profile data show fairly rapid erosion:

By the way, the way each of these profiles is configured is a top panel that shows a selection of profiles (in this case I chose one "winter" profile from February or January of 2016, and then a set of three from this summer (June, July and August).  The bottom panel on each figure is a time-series of the cross-shore position of the Mean High Water contour on the upward slope is accretion or seaward movement of the beach, and a downward slope represents erosion.  

Moving east things really changed this year.  Profiles on the east side of the new deposit near the river mouth showed nice even growth through this year after a period of very rapid accretion that occurred last summer.  Here is Line 174:

But it was to the east, at the very northern end of the delta, where things really heated up this summer.  Here is Line 190 and 198:

At these two sites a pattern of chronic erosion reversed in the summer of 2014, but it was something of an uneven and intermittent pattern of accretion until early this summer.  Moving around the northern tip of the delta, we were seeing slow but chronic erosion until early this spring.  Here is line 204 and 210:

One of the things I find interesting about this stretch of beach is the apparent movement of material from low on the beach towards the upper part of the profile.  Things east of here are a bit more complicated, largely because the shoreline had been highly modified by a long segment of old abandoned rip-rap that started over closer to Line 198, but connected to shore near Line 216.  Where it connected to shore it seemed to be acting as a groin, which I suspect is why profiles at Line 216, 222 and 229 really show very limited growth this summer.  However, as of last week a project led by Jamie Michel of the Coastal Watershed Institute removed nearly all of that old rip rap...and at Line 216 there was an immediate if subtle response:

 In this profile you can actually see the rip rap boulders that connected to shore.  Here is a photo of what this section of beach looked like prior to Jamie's shoreline restoration:

Line 216 cuts right through this picture above, up along the rip-rap near the line marking the annotation that starts, "Shore-connected...".  Here is what this same stretch of beach looked like on Thursday (albeit from a different perspective, looking alongshore towards the west...roughly from where the white arrow is in the photo above):

So if you look closely at the profiles for Line 216 you can see that between July and August two things happened...the rip rap came out (you can actually see that in the profile, by how smooth it is between about 1 and 3 meters), and the beach grew a bit there.  You might expect that this would take a while...after all the big machines used to remove the old boulders probably left some pretty substantial holes in the beach.  But Jamie reported that the area where they had pulled the boulders out was filled in, and actually grew a bit, just one day after those boulders were removed.  

Completing the trip around the delta, Lines 222 and 229 both show some late summer erosion, which is consistent with what I've seen in previous summers on the delta:

but with the "groin" out I will be very interested to see what the beach looks like next month...

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