Guy Gelfenbaum, Coastal Geomorphology

Guy Gelfenbaum, USGS

So what happens when the sediment gets to the coast or in our case, the Strait of Juan de Fuca – that’s our job to figure that out. Where will the sediment go? What process are important? How will habitat be modified?

Objectives include developing baseline conditions, impacts caused by the dams, develop models of expected response and track changes during restoration.

History of the shoreline changes – significant erosion of the east side of the river delta but not so much on the west side of the delta. Because of the dams in place, there has been a lack of sediment to flow out to the delta and naturally build the beaches.

Prior to construction of dams, there was a lot of sandy beaches, which are important to things like clams. Now, it’s very cobble-like and poor habitat.

Offshore, they have been mapping the bottom of the Strait. They have also been doing dive surveys to observe what animals and plants live on the bottom of the Strait. Because of lack of fine sediments, the habitat has adapted to a more coarse substrait. But we expect that to change as the sediment comes down.

Tracking the freshwater plume: Scientists are measuring the thickness of the plume, which is very thin at the surface, only 1-2 meters thick. The tides carry the plume back and forth, east and west, which aids dispersal of the fine sediments.

In order to understand the dispersal, they are tracking it during various phases of the tide. Often the plume is carried to the east side as a result of the complex flow pattern of the tides.

While tracking the plume movement, they also are mapping where they predict the sediment will end up over time. They are predicting it to mostly settled at the river mouth but also expect it to settle east of the river mouth.

With those predictions, they now want to know what’s really going on out there. So far, with one set of measurements, in May 2012, there is continual erosion on the shoreline, but accumlation is taking place at the river mouth and the eastern side. 94K cubic meters of sediment is what they’ve measured so far coming down the river.  In comparision, 2007 flooding resulted in 37,000 cubic yards of the sediment accumulation.

They are comparing model predictions to the actual measurements.

Beach sediment size is changing – in April 2012, coarse cobble was found on the beach. In May 2012, there is sand slowly starting to build up on the lower beaches.




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