in-ground pool, even when filled, weighs less than the soil it replaces and 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 bluff materials. As described earlier, waves sort this material, moving clays
and silts offshore while leaving sands and gravels for the beach. During storms, however, waves can
reach the bluff itself and erode or undercut the toe. Depending on the bluff soil characteristics, 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.
Low Erodible Plains and Sand Beaches. By far, the most common shoreform 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
inaccessible 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.
During periods of either increased water levels or wave heights, the sand above the low water level
is eroded, carried offshore, and deposited in a bar. Eventually, enough sand collects to effectively
decrease the depths and cause the storm waves to break farther offshore. This reduces wave action on the
beach, and helps re-establish equilibrium. At open coast sites, the process eventually reverses, and long-
period swells again return the sand to the beach after storms. At sheltered sites where no exposure to
oceanic swells exists, the recovery does not occur, and storm caused erosion becomes permanent.
13
Previous Page
