Shoreline Stability

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Shoreline Stability

Factors Affecting Shoreline Stability

Short-Term Trends

Long-Term Trends

References

Factors affecting shoreline stability operate across a broad range of spatial and temporal scales. For hazard assessment purposes, factors needing consideration will vary with location. For example, along segments of shoreline backed by a sandy beach and dune, the extent of short-term shoreline change due to episodic "wave attack" events is the primary factor affecting shoreline stability. Long term trends of shoreline change due to the sediment budget or relative sea level rise may also need to be considered.

Along segments of shoreline backed by a sandy beach and bluff, processes of "mass wasting" are the primary control on shoreline stability. These include weathering processes such as direct wind and rain impacts, that result in gradual bluff recession, as in well as episodic slope movement. A distinction between simple shallow sloughing and complex deep-seated landsliding/slumping is useful in this regard. The term landsliding is generally applied to translational mass movements, or motions that occur along a more or less planar surface. The term slumping is generally applied to rotational mass movements, or motions that occur about an axis. Most large mass movements possess both translational and rotational components of motion, however.

"Human activities" may affect the stability of all types of shoreline. At longer time and larger space scales these include jetty construction and maintenance dredging. Cumulative effects of shoreline hardening and the planting of European Beachgrass can also be considered in this context. Examples of human activities that affect shoreline stability over shorter time and smaller space scales include those associated with residential and commercial development, such as grading and excavation, surface and subsurface drainage alterations, vegetation removal, and vegetative and structural shoreline stabilization. Typically associated with heavy recreational use, pedestrian and vehicular traffic are other types of human activities that affect shoreline stability over shorter time and smaller space scales. 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.

Factors Affecting Shoreline Stability

Roll your mouse over the three icons on the left to see the processes that affect shoreline stability and how they relate to the space and time scales on the right.

What follows is a more detailed consideration of factors affecting the stability of dune-backed shorelines.

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Short-Term Events

Along dune-backed shorelines, processes of wave attack, namely "wave overtopping" and "undercutting", are the primary control on shoreline stability. In wave overtopping, it is the magnitude of an extreme event that is of particular interest. Tides, storm waves, barometric pressure effects, temperature effects, and baroclinic currents all affect mean water level. Superimposed upon these longer term variations in mean water level are short-term variations associated with the passage of waves and expressed at the shoreline as wave runup. Extreme water surface elevations achieved during storms result from the simultaneous occurrence of individual maxima within this range of forcing events.

In wave undercutting, it is not only the magnitude of the water surface elevation but its frequency of occurrence that is of interest. The regional wind/wave climate of the Oregon coast exhibits a marked seasonality. In summer, regional atmospheric circulation is dominated by the North Pacific High. This brings fair weather, north-north westerly winds, and low waves. In winter, regional atmospheric circulation is dominated by the Aleutian Low, a series of low pressure centers that pass over the North Pacific at intervals of several days to a week or two. These winter storms bring heavy rains, strong south to southwesterly winds, and high waves. Rip currents are an important element of nearshore circulation during these storms. By focusing wave attack, they accentuate erosion locally.

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 longshore as well as cross-shore transport. Specifically, net transport tends to be offshore and to the north in winter, onshore and to the south during the summer. Interannual events are superimposed on this seasonal pattern of erosion and accretion. They have been shown to have a significant effect on shoreline stability. (Komar et al., 1999) During both the 1982-83 and 1997-98 El Niño events, elevated mean water levels and strong northward-flowing currents eroded the southern ends of littoral cells and caused the northward migration of inlet mouths locally along the Oregon coast. During the 1998-99 La Niña, the passage of storm systems directly over the Oregon coast produced elevated wave energy levels and caused coastwide beach and dune erosion.

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Long-Term Trends

Along dune-backed shorelines, long-term trends of shoreline change attributable to factors such as relative sea level rise or the sediment budget also may need to be accounted for.

Relative Sea Level Rise

The Oregon coast is a convergent margin, where the oceanic Juan de Fuca Plate plunges below the continental North American Plate at the Cascadia Subduction Zone. Recent scientific evidence suggests that tectonic activity along the convergent margin is cyclic. 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 accompany 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 and melting of glaciers. What is particularly relevant in this regard is the net change in mean water level, or relative sea level rise.

The Sediment Budget

One result of such 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 of these features is such that over time scales of tens to hundreds of years they restrict longshore transport and thereby define discrete segments of shoreline, or littoral cells. The concepts of sand supply and the sediment budget are relevant in this regard. These concepts involve viewing a given segment of shoreline in terms of the positive or negative transfers of sediment that occur within it. The resultant balance of the sediment budget is determined by comparing the volume of sediment gained from "sources" (positive transfers) to the volume lost to "sinks" (negative transfers). A negative balance means that more sand is leaving than is arriving and, as a result, that segment of shoreline is eroding. Conversely, a positive balance means that more sand is arriving than is leaving so that the segment of shoreline is accreting. Along the Oregon coast, potential sources of sand include rivers, bluffs, dunes, and the inner shelf. Potential sinks include, bays, dunes, offshore, dredging sites, and mining sites.

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References

Shoreland Solutions. 1998. "Chronic Coastal Natural Hazards Model Overlay Zone." Unpublished report to the Oregon Department of Land Conservation and Development.
Komar, P.D., J. Allan, G. Diaz-Mendez, J.J. Marra and P. Ruggiero. In review. "El Niño and La Niña Processes and Impacts on the Pacific Northwest Coast."