Paul McWilliams
Ocean/ESS 230
Field Trip #4 Washington Beaches
Evidence of tsunamis can be found near the shores of Willapa Bay in southwest
Washington State. Exposed strata feature compelling evidence of prehistoric earthquakes
and the massive tsunamis that charged ashore as a result. The subduction zone of the
Washington coast is an area of frequent and powerful earthquake activity. Evidence of
such tsunamis exist in the form of soil and vegetation that were quickly buried and
preserved beneath sandy layers of sediment that were deposited within the minutes and
hours that followed earthquakes. Further evidence of such earthquakes can be seen
where the land had subsided because of an earthquake and as a result the soil and
vegetation were buried beneath muddy tidal flats.
Beaches exist in dynamic equilibrium. Rather than being stationary, sediment that makes
up beaches is in transport and thus sediment that is moved away must be replenished if
the beach is to remain stable. Littoral Transport, or longshore drift, is the primary
mechanism, a result of wave refraction, which transports sediment along coastlines.
Where a break in the shoreline occurs because of flooded river valleys or other natural
features, sediment continues to be moved by littoral drift and is deposited in offshore
barrier spits. The barriers are normally prevented from entirely cutting off the ocean
from the lagoon. The erosive force of flood and ebb tides, that service the lagoon,
maintain an inlet to the lagoon behind the barrier. As sediments are moved and deposited
at the end of the spit on the up-current side of the inlet, the inlet itself serves as a barrier
to the transport of sediment so that the down current side of the inlet is no longer
replenished with new sediment supply. Erosion of the beach occurs because littoral
transport continues to operate in that area although the sediment supply from up current is
cut off, and therefore sediment is carried away without being replenished. Thus, as the
up-current spit grows across the inlet the down-current side of the inlet is eroded away
and the inlet migrates. At some point the barrier may began to reach the end of the
natural feature that created the lagoon, when this happens a new inlet may form farther
up-current and the inlets migration begins over again.
Jetties, such as those at the mouth of Gray’s Harbor, are manmade engineering structures
constructed to stabilize inlets in place. These large structures, on the order of kilometers
in length are built of large immobile materials, such as boulders, on the edges of the inlet
perpendicular to the shore and parallel to the channel. They work by cutting of the
supply of sediment by littoral drift to the inlet. Often they are carefully engineered to
restrict the width of an inlet and focus the erosive energy of the tidal interchange in order
to maintain the channel. Jetties, however, come with a price. Increased sediment
deposition occurs on the up-current side of the jetty system resulting in large, and often
problematic accumulations. On the down-current end of the system the supply of
sediment has been cut off and even more problematic erosional trends remove large
portions of the shoreline.
The ocean beaches of the Washington coast from Cape Disappointment, at the mouth of
the Columbia River, to Point Grenville are heavily dependent on sandy sediment supplied
by the Columbia River. Although sand generally makes up only ten percent of a river’s
sediment load, the Columbia River moves a lot of sediment—and ten percent of a lot is a
still a lot. Sandy sediments delivered to the mouth of the Columbia are moved northward
by the prevailing littoral drift. As the sandy sediment continues to migrate northward,
beaches must be continually replenished if they are to remain stable. In the past few
decades, erosion has been observed along this part of the Washington coast. It is thought
that because much of the sediment traditionally supplied by the Columbia River is now
trapped behind Dams.
Unlike their neighbors to the south, the beaches north of point Grenville are independent
of the Columbia River Littoral system. Instead they are fed by the erosion of cliffs that
exist because of the proximity of the Olympic foothills to the coast. The size of the
material on the cliff-fed beaches is much larger than that of the sandy river-fed beaches.
As a result, the foreshore of the headland beaches near Kalaloch is steeper because the
erosion force of backwash is lessened by the fact that the larger grained beach is more
porous and allows more water to drain through to the water table instead of washing back
over the beach.
Much like the Washington shore within the Columbia River Littoral system, Ediz Hook,
near Port Angeles, and Dungeness Spit near Sequim are fed and maintained by sediment
supplied by rivers and transported by littoral drift. Ediz Hook in particular depends
largely on the supply of sediment from the El’wha River. Because the El’wha river has
been dammed, and much of the sand sediment is trapped in the reservoir above the dam
the supply to Ediz Hook is greatly reduced. Because littoral transport in the Strait of Juan
De Fuca continues to move sediment erosion is occurring at Ediz Hook and engineering
measures have been required to artificially strengthen this natural feature. Dungeness
spit is similarly vulnerable to the reduction in supplied sediment, although the impact
may be less pronounced because much of its sediment supply comes from cliff erosion
that occurs along the strait to the west of spit. The most obvious solution to this problem,
besides continual maintenance of artificial reinforcement to these natural spits, seems to
be a removal of the dams on the El’wha River. Doing so would restore the supply of
sediment to the spits, but this fix does not come without costs. These costs are not fully
understood, but it is known that the sediment currently collected by the dam would be
eroded and released and a relatively sudden slug of natural organic pollutants would be
quickly released into the river and the Straits of Juan De Fuca. This relatively sudden
change in the nature of the river’s sediment load will have great impacts on life in the
river and upon sea life in the straight. Oysters and Salmon are among the creatures that
will be most affected. While the pollutants that would be released by a removal of the
El’wha dams are mostly natural, s similar removal of dams on the Snake and Columbia
rivers will be likely to contain heavy industrial contaminants, agricultural byproducts
such as pesticides, and even radioisotopes from nuclear power plants. Such releases have
great potential to wreak ecological havoc and must be considered with great care.