Mitigating Shore Erosion Along Sheltered Coasts (2007) / Chapter Skim
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2 Understanding Erosion on Sheltered Shores
2 Understanding Erosion on Sheltered Shores
Pages 25-43
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From page 25... ...
Because rising sea level exposes portions of the shoreline to actions of waves and current, sea-level rise can exacerbate erosion. The principal tool for understanding erosion is the law of conservation of sediment mass, which requires information on the sediment (grain size, composition, sediment type)
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From page 26... ...
Con FIGURE 2-1 Two examples of nomenclature for classifying sediment types; this classification is for mixed clastic sediments formed from varying contributions from sand, silt, and clay fractions: (A) symmetrical conceptual scheme and (B)
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From page 27... ...
Large waves can mobilize and transport larger grains than can small waves, thus the ability of wind-driven waves to entrain or transport sediment of various sizes may change through time as winds change in intensity or direction. Similarly, physical features that focus wave energy can alter how waves generated by a more or less uniform wind velocity impact various portions of shoreline.
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From page 28... ...
Shorelines that respond to wind wave energy span a range from mudflats2 to rocky shores. These shorelines respond to incident wave energy through adjustments in planform and profile.
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Although more complicated, the behavior of these sediments in oscillatory (wave) flow is somewhat analogous in that deposition, transport, and erosion of a specific sediment type requires increasing flow velocities.
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From page 30... ...
. For sheltered coasts, episodic wave energy and the common occurrence of subthreshold wave orbital velocities can cause stranding of storm deposits offshore.
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From page 31... ...
Thus, the size of the body of water adjacent to a shoreline, particularly the distance in the direction of strong winds, has a strong influence on the potential severity of damage due to the worst-case storm waves. Because sheltered coasts are fetch limited, the impact of wave height and energy on erosion depends on the wave energy generated during storm events as constrained by the fetch in the direction of the storm.
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From page 32... ...
Depending on the climate of a given region and the nature and attitude of the exposed bedrock, these outcrops may form various types of resistant rock headland, including steep cliffs or shallow ramps along the water's edge. Because sheltered coasts, by definition, lie along water bodies with limited fetch, the current and wave energy that characterize these water bodies generally is insufficient to rapidly erode fully indurate bedrock, thus such settings are not explored further in this chapter in the remainder of the report.
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From page 33... ...
Sand mining from beaches is an example of an anthropogenic sink, while beach nourishment is clearly an anthropogenic source. Littoral Cells The principle of conservation of mass embodied in Equation 2-1 is a simple, yet very powerful tool for understanding coastal erosion and for defining littoral cells.
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From page 34... ...
The Role of Changes in Sea Level The previous discussion only deals with half of the problem, land loss due to the removal of sediments from the underlying shore profile. Landward shoreline movement will also occur for the case of a fixed shore profile on which there is a long-term rise in sea level relative to the land.
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From page 35... ...
Figure 2-4 illustrates the regional variation in sea-level trends around the United States over the past century. The nature of the nearshore system response to rising sea level for both roll-over and hold-the-line strategies is an ever-increasing stress on the system followed by failure and roll-back.
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From page 36... ...
MITIGATING SHORE EROSION ALONG SHELTERED COASTS FIGURE 2-4 Relative sea-level trends, in centimeters, for selected U.S. cities from 1900 to 2000.
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From page 37... ...
are found in estuaries, lagoons, or sounds where the shore exposure is fetch limited. Rising sea levels inundate old fluvial systems and create geomorphic units that are generally shorter (measured parallel to the shoreline)
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From page 38... ...
. 2-5 reaches or littoral cells are often much longer, even size reduction in hundreds of kilometers (or arrows have been darkened and heavier line weight miles)
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From page 39... ...
39 UNDERSTANDING EROSION ON SHELTERED SHORES D Deposition C B A Drift Cell Erosion FIGURE 2-6 Shoreline of a littoral (or drift) cell that illustrates the transition 2-6 from the zone of erosion (A)
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0 MITIGATING SHORE EROSION ALONG SHELTERED COASTS FIGURE 2-7 Three levels of bank erosion: Stable, Intermediate, and Unstable. SOURCE: Hardaway et al., 1992.
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From page 41... ...
1 00 Yr. M HW MHW MLW MLW FIGURE 2-8 Six typical shoreline profiles found on sheltered coasts.
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From page 42... ...
Since property owners only address erosion on their shorelines, it is important that the contractor, the consultant, and local permitting authority all understand the potential cumulative impacts. A shoreline management plan that considers the impacts of continued modification of the shoreline can prevent the unintentional domino effect of armoring which may eliminate the natural shoreline along the reach.
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From page 43... ...
? The effectiveness of these four options in addressing erosion will depend on the context of the littoral system being evaluated.
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