Monday, November 16, 2015

Tillamook Coastline Studied

The Oregon Department of Geology and Mineral Industries (DOGAMI) recently published Coastal Flood Hazard Study, Tillamook County, Oregon to "develop a digital flood insurance rate map (DFIRM) and flood insurance study (FIS) report for Tillamook County, Oregon." Much of Oregon's coastline is discussed within its 274 pages, but specific focus is placed on the four littoral cells (coastline sections between capes) from Neahkahnie Mountain to Cascade Head. Bayocean is located within the Rockaway littoral cell.
The report reviews ancient geological processes that created Oregon's coast, then uses previous research to provide context for the most recent data gathered about earthquakes, tsunamis, tides, erosion, wave runup, overtopping, and floods. Wonderful color graphics - photographs (including aerials), charts, diagrams and maps - help explain the detailed analysis. Much of it is still beyond my understanding, but I didn't see anything that conflicts with Pre-historic Geomorphology of Bayocean Peninsula and Changes in Bayocean Beaches Studied by DOGAMI; most likely because the study's lead author, Jonathan Allan, was kind enough to give me feedback while writing those earlier posts.

The jetty built on the north side of Tillamook Bay's inlet, primarily the extension in 1931, is blamed for Bayocean's eventual destruction. The report provides specific distances and rates of erosion post starting on page 36. After the south jetty was finished in 1979, the beaches rebuilt. These two maps depict shoreline changes across the last century. Other diagrams depicting these changes in different ways are at Bayocean Shoreline Changes Over Time and Oregon Coastal Atlas.


Shoreline changes at north end of Bayocean Spit (page 37)
Shoreline changes at south end of Bayocean Spit (page 39)
Those who attended Perry Reeder's presentation at the Tillamook County Library on October 24th heard and saw the evidence he has gathered over the last decade showing beach expansion (accretion) and dune growth (aggradation) parallel to the Bayocean town site. He noted that the area close to Cape Meares seemed to have stayed the same. Allan and his colleagues agree with him, using precise measurements taken from several gauging stations along the Bayocean shoreline. This figure and text is from page 66:


Figure 3-1 depicts the changes that have taken place over the past 15 years. In the far south, the beach is backed by an extensive gravel beach that provides considerable protection from erosion to the backshore properties. As a result, this section of the beach is essentially stable, oscillating between minor bouts of erosion and accretion. With progress north along the spit, it is apparent that the dunes have fully recovered from the late 1990s winter storms (Figure 3-12) and are now actively aggrading along the length of the spit. Accretion rates are highest along the north end of the spit (reaching around +1m/yr [3.3 ft/yr]) and lowest in the south.

The report listed 128' as the height of the highest dune measured on Bayocean. This concerned me because I'd reported hiking to 152' back in January. So I contacted Allan. He clarified that they measured the dune closest to each transect, not older ones farther from the beach.  His Lidar map showed the highest point on Bayocean to be approximately 153'. Close enough. 

This older comparison, based on U.S. Coast and Geodetic Survey records, shows changes in the shoreline of Tillamook Bay between 1867 and 1971. It's in a 1972 Oregon State University study. Though not as detailed or colorful, it shows how much has changed over the last 150 years. 

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