Low Erodible Plains and Sand Beaches.

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For instance, rainwater is naturally absorbed and seeps down to lower levels. Soils, such as coarse sand,

which allow rapid and free passage of water are permeable. On the other hand, impermeable soils, such

as clay, do not allow the free flow of water except through cracks or other openings. In the figure, the

large tree's roots penetrate the clay layer and provide a path for seepage to the sand layer beneath.

Likewise, as the clay fails, cracks form at the surface, providing a path for seepage to penetrate the soil,

further weaken it, and accelerate the failure process. Water can also enter the clay along the existing

circular failure surface, leading to further movement.

Once seepage penetrates the clay and reaches the permeable sand layer, it passes freely to the

lower clay layer, where it flows along the clay's surface and exits the bluff face. This seepage can

increase the risk of a slope failure. In addition, surface flow can erode the bluff face, causing gullies and

deposits of eroded material on the beach area below. The seepage exiting the bluff at the clay layer can

also cause surface erosion.

The added weight of buildings and other structures can increase soil stresses and contribute to

slope failure. Structures located near the top edge of the bluff have the greatest impact. Any leakage or

splashing from a swimming pool could produce additional seepage. An in-ground pool, even when filled,

weighs less than the soil it replaces and, therefore, would not adversely affect stability, provided no

leakage exists and splashing is minimized.

The other major cause of bluff shoreline problems is wave action at the toe. Figure 5 shows a

beach formed from fallen materials. As described earlier, waves sort this material, moving clays and silts

offshore while leaving sands and gravels for the beach. However during severe wave activity, waves can

reach the bluff itself and erode or undercut the toe. Only a short time may be needed under such

conditions for the entire bluff face to fail.

The slope of the offshore bottom is important to wave action on a bluff. If the offshore slopes are

steep, deep water is closer to shore, more severe wave activity is possible, and maintenance of a

protective beach is more difficult. Flat offshore slopes, on the other hand, result in shallower water near

the shoreline, which inhibits heavy wave action at the bluff and provides for potentially better protective

beaches.

To summarize, in almost all cases, except where toe protection or runoff controls are needed, slope

stability problems on high bluffs are beyond the range of property owner-implemented solutions.

Analysis and treatment of such problems should be entrusted to a registered professional geotechnical

engineer.

Low Erodible Plains and Sand Beaches. By far the most common shoreforms throughout the

United States, beaches and erodible plains are composed of loose sediments ranging from silts to gravels

that slope gently up and away from the water's edge. Because they seldom reach more than five to ten

feet above stillwater level, such shorelines are susceptible to flooding as well as erosion. Erosion

problems are caused by wave action although wind can be important in some cases.

Figure 6 depicts an idealized beach profile. Waves approach from offshore, finally breaking and

surging up the foreshore. At the crest, the profile flattens considerably, forming a broad berm

unaccessible to normal wave activity. The beach berm is often backed by a low scarp formed by storm

waves, a second berm, and eventually a bluff or dune.