Wednesday, June 6, 2012

The Changing Elwha shoreline

While the removal of the Elwha dams is not yet complete, we are already getting a taste of the potential restoration benefit and scale of this project. Starting a few months ago we started to see sand on the beach in places where we never had before. Every few weeks I head down to the Elwha delta and measure a beach profile and collect grain size imagery at 3-4 set monitoring locations (the ones in red in the photo below): On 24 April 2012 I put together a little summary based on some observations of NEW sand, tentatively attributed to the dam removal, at one of three sites that I monitor every few weeks. This is from Line 164, just to the east of the river mouth, and you are looking at a profile collected on three different dates, plus some grain size photos and oblique photos of the beach: This sand accretion was relatively high up on the profile, around 0.5m (1.5 feet or so) above MLLW. While very cool (this was, after all, some of the first sand that I could attribute to the dam removal that I observed on the beach), the amount of sand, and its extent along the beach, were both pretty minor. Now, though, the lower tides of May and June have given us a better view of sand accretion lower down on the profile - and it turns out that this is where a lot of the action is. Right around the river mouth there is a substantial body of sand that I first observed at the beginning of May: (Sorry about the wind and bad audio in this vid). At the time it wasn't clear if this sand would persist, or if it was a temporary blip in a longer-term reconfiguration of the Elwha delta in response to the addition of new sediment from the dam removal. It now appears, though, that the various sand bodies are growing and seem to be set to stay. For example, this set of profiles from today, and 5 July 2011 (about a year ago), suggests there has been substantial profile accretion below about 0 MLLW (where the black line, representing the surface today, is well above the gray line, representing the surface in July 2011): and the grain size shift associated with that accretion is readily apparent in these "before and after" photos taken at an elevation of -0.15 m MLLW at line 164 on 5 July 2011 and today: I was also surprised to find that the fine sand depositing down low on the beach extends far to the east. At Line 204 the profile was covered with fine sand below a depth of about -0.25 m MLLW, as you can see in this series of photos taken at 0.25 m elevation increments starting at -1.00 m MLLW: While I don't have any grain size photos from earlier dates to compare to for this relatively low part of the profile (today's low tide was exceptionally low - I don't often get the opportunity to sample below MLLW), my own observations after 5 years of work out there suggests that this sand is NEW. While the upper beach itself looks about like it always has around most of the delta, at line 164 that small sand body that I originally reported on at the end of April (see above) seems to have grown and has now welded to the base of the foreshore. Here you can see a photo taken from low on the profile looking towards the shore, with a bar of sand that has attached to the upper part of the beach. Comparing profiles from today and two weeks ago (May 20), that profile growth is very clear...all sand. This observations is particularly interesting in light of one of my major motivations for doing this - this beach is seriously erosive, and I am interested in "if, how and when" the sediment associated with the dam removal may act to reverse the chronic erosion that causes all sorts of problems on the delta. This sand accreting to the foreshore is some of the first evidence that the upper part of the profile, which is the part that erodes so rapidly, might enjoy some sort of restoration benefit from the dam removal...very cool stuff. Thanks to colleagues at the US Geological Survey and the Lower Elwha Klallam Tribe for helping to collect and interpret some of the data you see here...

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