Text Size:   A+ A- A   •   Text Only
Site Image

Types of Coastal Hazards
Map of Juan de Fuca plateEarthquakes and resulting tsunamis occur over larger geographic areas and time frames  than chronic coastal hazards. Although not as frequent in occurrence, the damage caused by these catastrophic events is immediate and life threatening. Off the coast of Oregon, subduction zone earthquakes can be generated along the sloping boundary between the descending Juan de Fuca plate and the North American plate. This area - known as the Cascadia subduction zone - could produce an earthquake of magnitude of 8.0 to 9.0, or greater. Geologists have found evidence that earthquakes of this magnitude have occurred once every 300 to 600.
The last major earthquake of this magnitude occurred in late January of 1700 A.D. An earthquake of this size would cause enormous damage to the coast and large portions of Western Oregon. In many areas, especially on the coast, liquefaction and landslides could damage buildings and their foundations, destroy bridges and cause massive loss of life. A great subduction earthquake could last as long as four minutes.

Chronic hazards are those we can see clear evidence of along the shore – beach, dune, and bluff erosion, landslides, slumps, gradual weathering of sea cliffs, and flooding of low-lying lands during major storms. The damage they cause is usually gradual and cumulative. The regional, oceanic, and climatic environments that result in intense winter storms determine the severity of chronic hazards along the coast. The short term, chronic events resulting in coastal flooding, erosion, and landslides are categorized as wave attack, mass wasting, and human activities. These processes operate over relatively short time periods in limited geographic areas and affect shoreline stability. The importance of these factors vary from setting to setting.

Wave Attack
Image of wave-cut scarpAlong dune-backed shorelines, processes of wave attack, including wave overtopping [i.e. flooding] and undercutting [i.e. erosion] are the primary processes affecting shoreline stability.

Because winds and waves tend to arrive from the southwest during the winter and from the northwest during the summer, Oregon coast littoral cells generally exhibit a seasonal reversal in the direction of sand transport along the shoreline. Specifically, net transport tends to be offshore and to the north in winter and onshore and to the south during the summer. El Niño events have been shown to exaggerate the characteristic seasonal pattern of erosion and accretion. For example, significant short-term variation in shoreline extent and location has been associated with the 1982-83 and 1997-1998 El Niño events. The processes of wave attack significantly affect segments of the shoreline where rivers enter the ocean, known as inlets. They interact with ocean tides and river forces to control Image of riprap seaward of coastal houses patterns of inlet migration. Recent examples of the importance of inlet dynamics are the Bayshore Spit at Waldport and the Netarts Spit near Oceanside.
Land-use management practices and non-structural solutions to problems of erosion and flooding are preferred to structural solutions. However, shore protection structures are sometimes necessary. These water and erosion control structures, whether located in the waterways or on shorelands above ordinary high water marks, include jetties, bulkheads, seawalls, and similar structures, as well as fill. They are designed to minimize adverse impacts on water currents, erosion, and accretion patterns.

Sand Inundation
The El Niño winter of 1997-98 and the La Niña storms of 1998-99 have focused attention on the Image of sand buried house by Paul Komarthreats posed by beach and dune erosion along the Oregon coast. However, in association with, and following episodes of erosion, there are segments of the Oregon coast where the problem is too much sand. These areas tend to be located at the north ends of headland-bounded segments of shoreline consistent with littoral cell circulation and sedimentation. Growth in the height and width of the foredune in these areas has enhanced ocean flood/erosion protection, but rapid heavy sand accumulation has also resulted in the inundation of dwellings, restriction of ocean views, and loss of beach access. Further information can be found at NOAA El-Niño/La-Niña.

Mass Wasting
Along bluff-backed and slide-backed shorelines, processes of mass wasting are the primary controls on shoreline stability. Mass wasting refers generally to a broad range of gravity- Image of landslide on Hwy 101 driven rock, soil, or sediment mass movements. This includes weathering processes that result in gradual bluff recession, such as direct wind and rain impact.

Here, the term mass wasting refers to episodic slope movements also known as landslides. The distinction between bluff-backed and slide-backed shorelines represents differences in the scale of slope movement. Simple surface sloughing is the dominant process along bluff-backed shorelines. Complex deep-seated landsliding and slumping is the dominant process along slide-backed shorelines.
A number of factors affecting slope stability increase driving forces and/or reduce resisting forces. Material composition is a primary control on slope stability. Headlands, for example, while not immune to mass wasting, do not readily give way. In contrast, soft bluff-forming sandstones and mudstone are highly susceptible to slope movement. Prolonged winter rains saturate these porous bluff materials, Image of bluff slumping at The Capesboth loading the slope and lowering cohesive strength, to further decrease slope stability.

The geometry and structure of bluff materials also affects slope stability. They define lines of weakness and control surface as well as subsurface drainage. By removing sediment from the base of bluffs and by cutting into the bluffs themselves, processes of wave attack may also affect slope stability. The extent to which the beach fronting the bluff acts as a buffer is important in this regard.

Human Activities
Human activities affect the stability of all types of shoreline. Jetty construction and maintenance dredging are factors that affect shoreline stability for long time periods and large geographic areas. This is particularly true along dune-backed and inlet-affected shorelines. Cumulative effects of shoreline hardening and, specifically, the spread of non-native European Beachgrass have markedly affected shoreline stability along dune-backed shorelines of the Oregon coast.

Image of undercut house at Gleneden BeachExamples of human activities that affect shoreline stability over shorter time periods and smaller geographic areas include those associated with residential and commercial development, such as grading and excavation, surface and subsurface drainage alterations, vegetation removal, and vegetative as well as structural shoreline stabilization. With the exception of the latter two, these activities tend to be a particular concern along bluff-backed shorelines. Typically associated with heavy recreational use, pedestrian and vehicular traffic also affect shoreline stability over shorter time and smaller space scales. Because these activities may result in the loss of fragile vegetation cover, they are a particular concern along dune-backed shorelines. Along bluff-backed shorelines graffiti carving can be added to the list of human activities that affect shoreline stability and are associated with heavy recreational use.