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| Looking at Ediz Hook from the west, September 2013 |
When I present about Elwha I frequently get asked about if, and how much,
Elwha River sediment has made it to
Ediz Hook. I've also addressed this question in a
previous blog arguing at the time (summer of 2016) that I didn't see evidence for any Elwha influence on Ediz Hook. This year, though, I started answering that question with a bit more confidence - I think it is likely that Elwha sediment has made it to Ediz Hook in quantities adequate to lead to measurable changes on the beach. I'm going to lay out some preliminary evidence for an influence on the beach of Ediz Hook in this blog.
First off, what is the shoreline response that I'm looking for? I've framed the expected response to dam removal on the shoreline along the Elwha littoral cell as the "X-hypothesis". Essentially I am measuring two things - the location of the beach profile, and the grain size of the beach. When the dams came down we expected to see something like this:
in which a previously coarse eroding beach becomes finer and starts to move seaward. We definitely saw this pattern on the Elwha delta, and in fact just
published a paper focused in particular on the beach profile position part of the story (the red line in the conceptual model above).
We also expect this response to move alongshore, driven by alongshore transport processes. So, essentially, the profile response and grain size response should sort of propagate alongshore with time, something like:
One of the most interesting insights from that paper is that we took a crack at
estimating the RATE that the dam removal response moved alongshore, at least on the Elwha River delta nearest to the river mouth...and came up with an estimate of 1 meter per day. If we extrapolated that rate to Ediz Hook, which is 8.5 km from the Elwha River mouth, we wouldn't expect a response there for quite some time...around 2033. The evidence I'm going to lay out below suggests the possibility that Ediz Hook is seeing a response now. Other evidence suggests faster response rates than our paper came up with as well. The Coastal Watershed Institute, for example,
published this account of beach accretion east of the delta in 2014...suggesting early slugs of sediment propagating along that shoreline that may have influenced Ediz Hook in some way.
So what is the evidence on Ediz Hook? I'm going to show summer annual beach profiles dating back to 2012, and oblique photos of the beach from three locations dating back to 2014, the first being the transect that I survey
at the very base of Ediz Hook. So here, the evidence that I see is in the beach profiles:
So what I think I see here is a fairly stable beach between 2012 and 2016, but then a period of accretion between 2016 and 2018. Its not huge...the beach moved seaward by a handful of meters...but it is out of character for this beach at least based on the limited data we have. The grain size story at this site isn't quite as compelling. Here is an oblique from 2014:
and 2016:
and then 2017, in which if you focus on some of the rip rap material in the far field you can really see the beach accretion
and 2018 in which we seem to see a finer beach face:
and finally this year (2019), in which overall we continue to see a relatively finer shoreline:
So lets turn our attention a bit further east, and further out on Ediz Hook. This
site sits just past the mill, and just about a kilometer from the site above. The story once you get to the mill and beyond is complicated, and the evidence isn't strong. But it may be there. Here are the profile data:
Erosion from 2012 to 2016, then a big bump seaward by the summer of 2017 associated with a cobble nourishment project (more on that below), erosion again between 2017 and 2019, but then, critically, a movement seaward between 2018 and 2019. Lets look at the oblique photos, starting in 2014:
then 2016:
2017...this is the cobble nourishment:
2018...amazing how quickly that cobble is eroded from the site:
and 2019:
The story at this site is complicated by
cobble nourishment, placed very 5 years or so under contract with the Army Corps of Engineers. Ediz Hook was nourished in this way, and at this site, in 2011 and again in 2017. However, one of the things that seems clear to me looking at these photos and profiles though is that the nourish material erodes very rapidly after it is placed, and the beach probably continues to erode until the next nourishment. The summer of 2017 photo above, for example, is a very visual example of what this beach looks like just before nourishment - coarse and heavily eroded. So the evidence that I see here for an Elwha influence is that between 2018 and 2019 the beach didn't erode...it grew, apparently (based on the photos), because of an influx of sand and gravel. Scant, I know...but something.
The final site is
right around the middle of Ediz Hook. Here are the profile data:
Its hard to see in this profile view, and the MHHW time-series plot isn't really working here, but you can see the influence of the 2011 nourishment in this profile, and then erosion through 2018 (interesting that there is no obvious influence of the 2017 nourishment in this profile, though there might be in the grain size)...but then a little accretion between 2018 and 2019. Here are the photos, starting with 2014:
then 2016:
2017:
2018:
and 2019:
So here the summer of 2017 stands out for how coarse the beach substrate was, perhaps reflecting alongshore transport of cobble that was placed near the mill in early 2017. That nourishment, though, based on the profile data, didn't really add too much volume to the beach. However, between 2018 and 2019, as at the last location, we see an increase in beach volume driven by sand and gravel (based on the photos).
So is there an Elwha influence on Ediz Hook? I think so. I think the profile and grain size data support it, and while its conceivable that there is another source of sediment at play here (bluff erosion for example), I don't think that the
rate of erosion on the Elwha bluffs accounts for what we see in these profiles. As always, looking forward to next year's survey data.
