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.

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