Sheet Piling. Sheet pile sills are similar to bulkheads. Timber sheet piling will generally require
filter cloth backing on the shoreward face to prevent loss of the retained sand backfill through joints in
the structure. This is not generally a problem with steel or aluminum sheet piling. Sheet pile sills also
form an abrupt step to deeper water, which would definitely be hazardous to bathers, particularly
children.
The same precautions regarding adequate ground penetration and toe protection for a bulkhead
also apply to a sheet pile sill.
Concrete Boxes. Precast, open concrete boxes (for use in drainage structures) can be placed side
by side and filled with sand to form a sill (Figure 57). During placement, the gaps between adjacent
boxes must be minimized to prevent excessive wave transmission through the structure and to help retain
the perched beach. Filter cloth backing is required and toe protection
should be provided on the offshore side.
STRUCTURES AND FILLS
In addition to perched beaches, fills can also be incorporated in groin systems and with
breakwaters. In fact, auxiliary fills are almost mandatory in most cases, because if such structures fill by
natural accretion, serious erosion problems almost surely occur downdrift.
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