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 the Oregon Beach and Shoreline Mapping and Analysis Program which 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 the migration of sand up and down a coastline, caused by wave action. Because of the sand that accumulated north of the north jetty at Barview, the Corps of Engineers assumed that there was a southward littoral drift along the Oregon coast; but in in1973 Thomas Terich, a doctoral candidate in the Oceanography School at Oregon State University showed that there was the littoral drift in the Rockaway cell was net near zero. He and his advisor, Professor Paul Komar, made Terich's findings public the same year in  Development and Erosion History of Bayocean Spit, Tillamook, OregonThey pointed out that if sand were continually moving south, the beach at Barview would have continued growing until it reached the end of the jetty, which is obviously not the case. And the south side of Cape Meares is made up of rock and gravel rather than sand. They also broke a longheld belief that jetty construction had no effect in cases of net-zero littoral drift. 
Komar's next student, master's candidate Jose Roman Lizarraga-Arciniega, confirmed that net-zero drift applied to all of the littoral cells along the Oregon Coast. Their 1975 co-authored  Shoreline Changes Due to Jetty Construction on the Oregon Coast helped me understand that building a jetty replicates the geological process of creating a small cape. The established, seasonally-reversing, littoral cell is cut in two, and two smaller ones cells are formed on each side. If you look at any cape along the Oregon Coast, 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 sand removed in the winter was more noticeable, and it never returned because bay waters flowing out the inlet washed it all out to sea twice a day. As soon as construction of the south jetty began, in 1969, sand began filling the "embayment" created by it and continue to do so until a new shoreline ran parallel to predominant winter wave crests. Only then did Bayocean erosion come to an end. 
Oregon coast beaches naturally fluctuate 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) Professor 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 that Jonathan Allan is studying for DOGAMI. He has co-authored several publications with Paul Komer 

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