The effectiveness of vegetation is also limited by characteristics of the site. For example, the
effectiveness of marsh planting depends on the elevation and tidal regime which determine the degree,
duration, and timing of plant submergence; slope of the site; exposure to wave action; soil type; salinity
regime; and oxygen-aeration times. Plants which are specially adapted for higher beach elevations must
tolerate rapid sand accumulation, flooding, salt spray, abrasion by wind-borne sand particles, wind and
water erosion, wide temperature fluctuations, drought, and low nutrient levels. Appropriate species also
vary with geographical location, climate, and distance from the water.
APPENDIX E contains specific information on applicable plant species
INFILTRATION AND DRAINAGE CONTROLS
Infiltration and drainage controls are often needed for stability along high bluff shorelines.
Although many factors lead to slope stability problems, groundwater is one of the most important. The
majority of slope failures and landslides occur during or after periods of heavy rainfall or increased
groundwater elevations. Infiltration controls prevent water from entering the ground, while drainage
controls remove water already present in the soil or on the surface.
Since water entering surface cracks can lead to further instability, the cracks should be filled
with compacted, impermeable soil (preferably clay) as they develop. Surface runoff should also be
diverted from critical areas of the bluff by either drainage ditches or swales.
The treatment of subsurface drainage problems is complex.
Where such problems exist, a
geotechnical engineer should be consulted.
SLOPE FLATTENING
A bluff slope may be flattened to enhance its stability when adequate room exists at the top and it
does not interfere with the desired land use. Freshly excavated slopes should be planted to prevent
erosion due to surface runoff. It may also be necessary to build a revetment or bulkhead at the toe of the
slope to protect against wave action.
PERCHED BEACHES
Perched beaches (Figure 12), which combine a low breakwater or sill and a beach fill, are beaches
elevated, or perched, above the normal level. The perched beach provides a broad buffer zone against
wave action while offering a potentially excellent recreational site. The sill (which is structurally like a
low fixed breakwater) must be impermeable to the passage of retained beach sand. This is
accomplished through the use of, for instance, a filter cloth behind and beneath the structure. The cloth
prevents fill from escaping through voids in the sill and also may be used to stabilize the sill against
settlement. While a small-sized stone core within a rock sill could also be used in place of the filter
cloth, the limited height of such sills generally precludes use of multi-layered structures of this kind.
Figure 12 also shows a splash apron to prevent scour and erosion of the beach fill from overtopping
waves.
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