Abstract:
During the period 1970-76, Siletz Spit on the mid-Oregon coast
has suffered foredune erosion. This erosion is associated with high
wave conditions along the coast, produced by intense storms in the
North Pacific.
During the winter of 1972-73 and during January through March
1976 the erosion was particularly severe. The pre-1975 erosion of the
spit has been documented in a previous study. One of the main purposes
of the present study is to document the 1976 erosion and to contrast
it with the earlier episodes. It differed principally in that a
small wash-over of the spit occurred in one area and beach drift logs
were thrown up onto the dunes. Neither occurred during previous erosion
episodes, even during the 25 December 1972 storm when wave
breakers exceeded a significant wave height of 7 meters. This is
because the 1972 storm occurred at a time of low water level, neap tide
conditions. In contrast, the 18 February 1976 major storm occurred
during spring tide conditions, causing the wash-over and log throwing,
even though the waves were smaller than in December 1972. The amount of dune retreat was somewhat less in 1976 than in 1972-73, partly
because of the shorter time period over which the 1976 erosion
occurred and partly because of the use of protective riprap.
The large waves causing the erosion at Siletz Spit are generated
by storms in the North Pacific. The storm systems for December 1972,
January 1973, and February 1976 were analyzed as to fetch distances,
wind speeds and directions, movements of the fetches, and other
factors important in the generation of waves. Waves hindcasted from
these data were compared with measurements of the waves obtained from
a seismometer system at the Marine Science Center in Newport, Oregon.
This comparison demonstrated that these distant fetches were responsible
for the waves causing erosion on Siletz Spit. Waves generated
locally by coastal winds appear to be of negligible importance in the
erosion. This is also shown by a comparison between the wave measurements
and data on the coastal winds at Newport during the times of
maximum erosion.
Tides were investigated for their role in contributing to coastal
erosion. High spring tides allow the storm waves to better reach the
dunes or sea cliffs and so permit greater erosion. Neap tides, with
their lower water levels, diminish the amount of erosion. Many of the
differences in the December 1972 and February 1976 erosion episodes
were due to the difference in tide levels.
Storm surges were determined for Yaquina Bay, Oregon, by subtracting
the predicted high-tide levels from the observed levels. It
was concluded that significant storm surges did not occur at the times
of maximum spit erosion and therefore did not contribute to the
erosion processes.
