Tuesday, July 31, 2012

Finding the sand from the Elwha River

I've posted recently about the sand that is accreting to the beach near the river mouth of the Elwha River. I and others are tracking this carefully to determine if and when the removal of two dams on the river acts to halt or reverses chronic erosion along the delta. Last week I also went out for the first in a series of dive sessions, with other divers from the USGS, the Lower Elwha Klallam Tribe and the EPA, to monitor a series of sub-tidal sites near the Elwha River mouth (the red dots in the photo below):

At each site we are collecting a suite of data to describe the invertebrate and kelp community, as well as the substrate. We started this project in 2008, and more information on the project and the biological community off-shore of the Elwha can be found here.

The new sand on the beach made me think that we might see some new sand at some of our sites, especially the shallow sites to the east of the river mouth. But I was struck by how similar everything looked (as reported by the Peninsula Daily News), from a grain-size standpoint, to our surveys from the previous year. So we went hunting for it...

We decided to enter the water at our monitoring site "D1" and swim a transect to the river mouth along a line that is surveyed at least once a year by the USGS for bathymetry:

Preliminary results from the most recent USGS bathymetry survey, in May 2012, suggest that there had been new sediment added to this transect between September 2011 and May 2012:

This is a series of profiles collected along the transect since 2005 (courtesy of Andrew Stevens, USGS), showing the transition of this particular slice of the earth from a beach to a river mouth (between about 2005 and 2008), and the subsequent accumulation of sediment in the shallow coastal zone (since 2008). Between September 2011 and May 2012 there was more sediment added...and I wanted to know what its composition was...sand? sand and gravel? mud?

The video that we collected gives it away. I shot video at various depths as we moved up the slope towards the river mouth. Each of these video segments is prefaced by showing the profile again, with a black arrow that corresponds to our position on the profile. Enjoy!

Monday, July 16, 2012

Recent Article in the PDN on the Elwha

I love my local paper. I am a loyal subscriber, and have no plans on ever changing that. In an age defined in part, we are told, by dying print media, I want to do everything I can to keep the Peninsula Daily News chugging along.

That being said, I have been known to criticize the PDN when it comes to reporting on the sorts of coastal scientific issues that I am most familiar with (see my blog post titled "Where Do They Get This Stuff From?" from last December).

But I'm overall pretty pleased with Arwyn Rice's article from yesterday, "Researchers Excited By Early Signs of Elwha Changes" and thought that she did a good job trying to capture what I was trying to say, representing the uncertainty in our science, and forming it into a decent summary.

That being said, lets move on to the errata. First, when Arwyn writes that "Once a sandy beach existed just east of the Elwha River mouth" the implication is that I said that. I didn't. In fact, there is virtually no evidence that this was the case. Really the only data that I've seen suggesting what the grain size on the beach was like prior to the dams comes from an 1908 Coast Survey chart, which marks the shoreline near the mouth as "Sand and Pebbles"...suggesting the possibility that it was sort of like what we see now.

Next, Rice quoted me as saying "In the past 15 to 20 years, erosion has taken 12 feet per year". If I did say that, I am sorry. What I should have said is "erosion has taken UP TO 12 feet per year on average". Erosion in places and over some time periods has exceeded that rate, and in others it is less. Erosion is like that.

Also, as I've written in the past, i am not yet willing to say that the removal of the Elwha dams will "rebuild Ediz Hook". To be fair this statement wasn't attributed to me in Arwyn's article, but it may be interpreted as such by association. I'm not ever sure that Ediz Hook needs "rebuilding" so much as a different perspective on how we manage it. But that is a complicated story, one that needs some study, and is definitely for a different time.

Not quite sure where this came from, "The amount of sediment to be deposited from the dam removal is the equivalent of a 100-year or 500-year event, he said." I think that one is mis-attributed since we didn't really talk about sediment quantities observed at the mouth, and I wouldn't have had much to say there anyway, since we don't yet have good data on that topic.

And finally, and really digging into the weeds, I can see why Arwyn wrote, "Once Lake Mills is removed, the river's sediment load will increase dramatically, he said." since we talked about how a larger fraction of the stored sediment in the Elwha system is still stuck in Lake Mills. But the term "load" is some circle suggests that the amount of sediment that the river is carrying at one point in time, versus the long-term delivery to where I care about it - the mouth and coastal zone. The instantaneous load may or may not increase when Glines comes out(it will probably depend more on river flow than anything), but certainly the amount of sediment available for transport down the river will increase.

Tuesday, July 3, 2012

Boat-based LiDAR to measure bluff erosion

The shoreline of the Strait of Juan de Fuca, like much of Puget Sound and even some of the outer coast of Washington, is a strip of tall and very actively eroding bluffs. These bluffs, built primarily of loose sediment deposited by retreating glaciers >15,000 years ago, are the subject of considerable interest in the management of marine resources in our area. Despite being fairly easily eroded, their edges seem to be preferred places to build homes. Great views, indeed, but over even relatively shore time periods - 10 or 20 years - these bluffs can move shoreward due to erosion many 10's of feet. In most cases regionally and around the country, their have been two inevitable outcomes: 1) Either homes are lost to the sea, or 2) the bluff is "armored" in some way to reduce or stop erosion. This engineering solution, while attractive at first glance, carries a hidden cost - the sediment eroded from the bluffs deposits on the beach at the base of the bluff, where it can contribute (via processes that we are only just starting to understand) to building habitat. In some cases, the connection between bluff erosion and habitat-building processes seems stunningly clear; Dungeness Spit, widely considered a regional gem in terms of coastal habitat, is probably maintained almost purely by the erosion of soft bluffs to the west.
The WA DOE research vessel R/V George Davison (in the far field, right) scans the bluffs immediately east of the Elwha River delta

Despite the apparent importance of these bluffs to shallow marine habitats, our understanding of how they erode, and how much they erode, is surprisingly naive. Most of the work that has been done to quantify rates of erosion around Puget Sound and in the Strait of Juan de Fuca was/is done by drawing lines on aerial photos taken at various times over the last century or so. This was state-of-the-are even 20 years ago, but there are a lot of uncertainties when making measurements off of photos - how good is the photo? Does vegetation block the view of the bluff edge? Does the line actually represent the bluff face, or is the bluff face overhung? Also, it is very difficult to quantify the volume of sediment contributed to the beach from aerial photos.

A ground control marker on the bluff west of Port Angeles

As a result, new techniques are warranted. Over the last 10 years or so there has been an jump forward in remote sensing technologies, including Light Detection and Ranging (LiDAR), which uses a laser fired from a scanner to very rapidly estimate the distance to a surface some distance away. Associated with a high-precision GPS unit, and a device called an inertial motion unit, it is now possible to very quickly collect hundreds of thousands of discrete measurements of a surface from a moving vehicle. This has been done from planes for a good two decades or more, but planes aren't the best platform to measure bluffs from, since the bluff face is oriented up and down relative to the plane. Last week, for the first time, bluffs in the Strait of Juan de Fuca were scanned from a LiDAR unit mounted on a boat. Using a boat, large distances of bluff can be scanned fairly quickly (miles of bluff per day).

Diana McCandless, WA DOE, surveys a ground control point on the beach east of the Elwha River delta

These bluff erosion measurements are led by the WA DOE Coastal Monitoring and Analysis Program, with support from WA Sea Grant and WA DNR. My role in this project was to help with local logistics and help set-up ground control points - large targets visible on the boat that can be used to assess how well the LiDAR scanner is measuring the surface. Our goal is to measure bluff erosion as a volume (i.e. so many cubic feet of sediment were eroded from the bluff over x number of years), and so the ground control points allow us to add error bars to that estimate. The first survey was a huge success - a least logistically. We had phenomenal support from private and public land-owners all over the county. And this bluff measurement project isn't even the whole story...it is just a part of a larger project coordinated by the Coastal Watershed Institute, with involvement by Clallam County and Earth Economics, as well as a whole suite of supporters.