Tuesday, May 21, 2013

The Changing Elwha Shoreline (continued again)...

Two things made me think its time for another update regarding shoreline change at the Elwha River delta. First I've finally had a few moments to do a bit of work with the profile data I've been collecting approximately twice per month (at four sites around the delta) since I started with Sea Grant in March 2011. Notably I've been wanting to add an analysis specifically looking at movement of the Mean Higher High Water contour as a proxy for the position of the shoreline. As the restoration continues our conceptual models suggest that erosion, especially to the east of the river mouth, should slow down and perhaps reverse as sediment is supplied to the shoreline. Its been very clear since about January that the beach at Line 164 (see map in attached below) was growing dramatically. This site is close enough to the river mouth, though, that it is possible that this accretion is less about alongshore transport processes and more about that site really becoming part of the expanding river mouth.

30 April 2013 aerial image of the Elwha River delta courtesy of Andy Ritchie, Olympic National Park. The four transect lines that I measure topography and grain size on every two weeks are shown in red.

As a result I've been particularly intrigued by patterns of shoreline change at Line 190, which is further to the east and at least a few hundred meters away from the large pile of sediment that deposited in front of the river mouth in December-January this winter. Even with an enhanced sediment supply to the coastal zone, which really kicked off in April of 2012, however, my data from late last year and early this year showed continued erosion at this site. Lately, however, (and this is the second thing that made me feel like its time for an update) I've started to "feel" like that section of the shoreline has stabilized a bit relative to how its been eroding in the recent past. I think that my profile data back me up a bit on that. I see the possibility in these data that rates of erosion at Line 190 (the green line in the lower right hand corner of the figure below) have slowed since about the middle of 2012, and that data from the first half of this year even suggests a possible pattern of accretion developing there.

Profiles from May 2011, 2012 and 2013 from four sites on the Elwha River delta (reference to map at lower left). Relative position of the MHHW contour (positive is accretion, negative is erosion) over time is shown at lower right.

The interesting thing here is that if there is a pattern of beach growth emerging at this site it is happening in a different way than the accretion at Line 164. At Line 164 its all about sand...huge amounts of sand. At Line 190 the entire intertidal zone is seemingly as coarse as its been for a good while. The photo below, for example, shows both an oblique of Line 190 (looking landward from about MLLW) from 2012 and 2013, as well as a grain size image from just about the mean sea level contour on the beach. So what is going on? Not sure, really, but it is possible to imagine that there is alongshore transport of sand, and perhaps gravel, from areas closer to the river mouth that are adding volume to the beach at Line 190...even without an obvious decrease in grain size on the surface of the beach.

Oblique images taken looking landward from about Mean Lower Low Water (top) and grain size images taken at about 1.40 m above MLLW on the beach (bottom) from May 2012 and 2013.

But the title of this post really says it all...this data collection effort will continue in the hopes that we can puzzle out how this dam removal will also act as a beach nourishment...and if it will reverse decades of erosion. This is an important question, since communities all over the nation and the world are increasingly grappling with questions about how to respond to projections of shoreline erosion due to climate change. Changes to the way that society manages sediment delivery to the coastal zone, via rivers primarily, is one possible way that some of that projected erosion of shorelines might be addressed in the future.

No comments: