Wednesday, February 4, 2015

Changes in Bayocean Beaches Studied by DOGAMI

Jonathan Allan, a coastal geomorphologist with the Oregon Department of Geology and Mineral Industries (DOGAMI), is leading an Oregon Beach and Shoreline Mapping and Analysis Program that is mapping and analyzing changes in beach profiles along the Oregon coast, including seven sites on Bayocean. Their mission is to “provide important information concerning the temporal (time) and spatial (cross-shore) variability of the shape of a section of beach.” Bayocean is one of three sub-cells within the Rockaway littoral cell, which spans the shoreline between Cape Meares and Neahkahnie Mountain.

For ocean coastlines, “littoral” is used to describe the area from where waves splash on a beach, just above the high water mark, out to the continental shelf. As for "littoral cells", let's turn to the Scripps Institute of Oceanography :

All coasts are divided into natural compartments called littoral cells. Each cell contains a complete cycle of sedimentation including sources, transport paths, and sinks. The presence of sand on any particular beach depends on the transport of sand within the cell. When structures such as dams or harbors interfere with sand transport, downcoast beaches will erode. Therefore, the littoral cell and its budget of sediment are essential planning tools for regional and coastal management.

“Littoral drift” refers to migration of sand up and down a coastline, caused by wave action. Early scientists thought there was a littoral drift northward along the Oregon coast; but in Development and Erosion History of Bayocean Spit, Tillamook, Oregon, (1973) Oregon State University oceanographers Paul Komar and (his student) Thomas Terich showed that within the littoral cells along the Oregon Coast there was a near-zero net littoral drift.

Winter storms generally hit the Oregon coast from the southwest, moving the sand north, whereas summer winds and ocean waves generally approach from the northwest, moving sand back south. Bayocean was breached because the north jetty, especially after it was extended in the early 1930s, stopped the replenishment of beach sand needed to offset the winter removal. 

Professors Komar and Terich studied and wrote extensively about the geological processes that destroyed Bayocean, but it was another student of Komar, Jose Roman Lizarraga-Arciniega, who helped me understand some of their more complicated studies in his 1975 master's thesis, Shoreline Changes Due to Jetty Construction on the Oregon Coast. Building a jetty replicates the geological process of creating a small cape. The established, seasonally-neutral, littoral cell is cut in two, and two new, smaller ones must be formed on each side. If you look at any coastal capes along Oregon, you will see that sand beaches round off the edges on each side. That sand comes from the shores between the capes. Those natural processes engage very quickly to create the same effect on each side of a new jetty. 

In the summers following the construction of Tillamook's north jetty, the sand that accumulated north of it came from such a long expanse of coastline (up to Neahkahnie) that the loss of it was negligible in any one location. The distance from the jetty to Cape Meares, on other hand, was very short, so beach erosion noticeable. The other complication was the narrow, Tillamook Bay inlet, with it's powerful outflow on low tide. In the winter, all that sand that was moving up to smooth out the jetty line from the south, was being washed out to sea. Since the rounding of the jetty was every achieved, the beach of Bayocean kept eroding until the south jetty was built. The sand then had a place to nestle up against the south side of it during winter storms, and as it accumulated, rounding out the jetty, it applied back pressure slowing down beach erosion. Once balance was achieved, the erosion stopped. Unfortunately, it was too late for the town of Bayocean. 
Oregon coast beaches also shrink and grow on an annual cycle. Winter storms pull sand offshore and the milder waves of summer move it back. But the beach is never exactly the same. In The Pacific Northwest Coast (1992) Paul Komer explained why this is important for potential beach home buyers: “New retirees arrive from the Midwest in summer to settle into the comfort of a beach home fronted by a wide beach and gentle surf, only to see the sand disappear during the next winter and the waves lapping at their doors.” This is the dynamic DOGAMI is studying. Jonathan Allan has worked and co-authored publications with Paul Komer.  

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