Ingeniero Civil

Appraisal of Chronic Hazard Alleviation Techniques

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2 FACTORS AFFECTING SHORELINE STABILITY
Factors affecting shoreline stability occur across a broad range of spatial and temporal scales (Figure 2.1). They involve a complex
combination of interactions between geologic, oceanographic, and, to a lesserextent, biologic processes.

2.1 LONG TERM, REGIONAL CONTROLS
Deposition and tectonic deformation that has occurred along the Cascadia subduction zone over the last sixty million years has
shaped the morphology of the Pacific Northwest coast. In this regard, a body of recent evidence indicates that a cycle of tectonic
activity occurs along the convergent plate boundary that fronts the PacificNorthwest coast. During one part of the tectonic cycle,
an extended period of gradual aseismic uplift of the coastal margin occurs in response to the accumulation of strain within the
subduction zone. Gradual variations in mean water level, and hence shoreline position, accompany this part of the tectonic cycle.
In contrast, the other part of the tectonic cycle is characterized by a major seismicevent which occurs as the strain that has
accumulated within the subduction zone is suddenly and dramatically released. Rapid variations in mean water level due to
subsidence of the margin are associated with this part of the tectonic cycle. Superimposed upon these tectonically-induced variations
in shoreline position are variations in global eustatic sea level due to the alternating growth andmelting of glaciers. These repeated
marine transgressions and regressions have also shaped regional coastal morphology.

One result of these processes operating over time scales of hundreds to thousands of years is that basalts deposited some 15 to 45
million years ago are today resistant headlands, prominent morphologic features along the Oregon coast. For the most part, the
prominence ofthese features is such that over time scales of tens to hundreds of years they restrict longshore transport and thereby
define discrete segments of shoreline. Viewing the Oregon coast in terms of headland-bounded segments of shoreline, or littoral
cells, is a concept that is useful for both scientific and management purposes (Figure 2.2).
Another result of these processes is that sands and mudsdeposited some 2 to 40 million years ago,

carried upward by tectonic

activity, and eroded by waves of ancient seas, today form a series of marine terraces that back much of the Oregon coast. Because
this uplift and erosion is differentially distributed along the coast, bluff-backed shorelines along some segments of the Oregon coast
stand in marked contrast to dune-backed shorelines foundelsewhere along the Oregon coast.

As much as 45% of the Oregon coast consists of dune-backed shorelines. The littoral cell that extends from Heceta Head on the
north to Cape Arago on the south, for example, contains the Coos Bay dune sheet. This approximately 150 mile long coastal dune
accumulation is the largest in the United States.
Other littoral cells characterized by dune-backedshorelines include the southern portion of the littoral cell that contains the
Columbia River and Clatsop Plains, the Rockaway Littoral Cell, and the Nestucca Littoral Cell, on the north Oregon coast, as well
as the littoral cells south of Cape Arago, south of Coquille Point, and in the vicinity of Gold Beach on the south Oregon coast.__________________________________________________________________________________
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Appraisal of Chronic Hazard Alleviation Techniques

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Bluff-backed shorelines also makeup large segments of the Oregon coast. Much of the central Oregon coast, from Cascade Head
on the north to Cape Perpetua on the south consists of bluff-backed shoreline for example. Similarly,...