Friday, October 11, 2013

The Elwha season heats up again...

A juvenile Dungeness Crab utilizing the new sand accreting to the lower profile at Line 204...the first real indication from our surveys that this material is being used as habitat...

Fall is an exciting time for Pacific Northwest rivers, since this is the time period when we start to see opportunities for really high flows. For the Elwha River in particular many of us are pretty addicted to watching the flow of the river since, in general, the big morphologic changes in the river and coastal zone tend to correspond to these flow events. And this year it came early, on September 28th, with the flow gauge at MacDonald Bridge recording what was likely the largest flow since dam removal started:

Based on previous work it seems likely that this flow delivered a substantial volume of new sediment to the coastal zone, but much of it is probably below the water's surface. I started getting questions almost immediately, though, regarding whether we were seeing any changes on the beach due to this river flow. The answer...possibly? In the time lapse video below, for example, you can see some large wood moving on to the river mouth during the high flow period (28-30 September or so). Its not totally clear to me from the time lapse that the morphology of the river mouth changed substantially during that flow, though...

Beach surveys on 4 October and 7 October do show some interesting changes to the beach...but whether these are linked directly to the Sept 28th high flow isn't clear. In the figure below I present data from the four transects that I monitor once or twice a month, collecting a profile and grain size photos at each site. Across the top are the profiles from my last three surveys at each site, and I've circle some of the interesting developments at each site. At lower right in the figure is a time-series of the position of the beach on the if the line is pointing downward that suggests erosion, whereas a move into the positive scale on the y-axis suggests beach growth.

Interestingly enough, the beach at line 132 has seen some of the most obvious change over the last few months of summer, at least in terms of changes high up on the beach face (#1 in the figure above). In this case I've circled accretion of a storm berm over the last month, composed of sand. This part of the profile at this site is typically very coarse - this grain size photo from 3.50 m elevation at Line 132 from 6 August 2013 photo is representative:

but can be compared to the photo from the same profile and elevation collected on 4 October 2013:

At Line 164 I've circled an area of new accretion (#2 in the figure above). This is a place where the high flow in September may have made a difference, since the river channel now runs along and truncates that profile. So that accretion that I've circled appears to be, effectively, a bank of the river. During our survey on 7 October we were unable to cross the channel at this point, and could not extend the profile.

At Line 190 the big story is what appears to be a very slight inflation of the profile on the high intertidal beach. If you track Line 190 in the time series of beach position at lower right you can see that this beach has eroded dramatically over the last two to see even a hint of profile accretion is something. Interestingly enough, though, the grain size photos suggest that the beach at this elevation is still very coarse:

so its not totally clear what is going on here.

Finally, the news from my most eastward profile is continued accretion of sand in the low intertidal (#4 in the figure above). This is all medium sand that appears to be transporting on-shore from a deposit in shallow water just off-shore. This brings me, though, to the most exciting observation from my shoreline survey week...the first observed utilization of sand on the lower profile as habitat. We found a Dungeness Crab snuggled up in the sand waiting out the low tide at about the 0.5 m elevation on Line 204 (see photo at top of this post). This is a site that, one year ago would have looked like this:

and probably wouldn't have been viewed as attractive by Dungeness Crab. Now, though, it appears to fit the bill.

Thank you to Karsten Turrey for assisting with the 7 October 2013 beach survey...

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