this region, the dominant plants are forbs such as the Sea Fig (Carpobrotus edulis and C. aequilaterus). These
are effective for sand stabilization but are not good dune builders.
Great Lakes Region. Dune development is mostly confined to the Michigan and Indiana shores of Lake
Michigan; however, the discussion, which follows, is applicable to all the shores of the Great Lakes. American
beachgrass is the dominant species. Native species, especially prairie sandreed, will often invade naturally.
Once the dunes have been stabilized, volunteer or planted species of upland vegetation can be established.
Species of grasses suggested would include reed canarygrass, big bluestem, little bluestem, and switchgrass, all
native to the area. These grasses may be planted from early May to the middle of June at a rate of about 0.5
pounds of seed per 1000 square feet. All require full sun and may be mowed occasionally. Reed canarygrass is
especially useful in wet spots.
Various ground covers may also be planted. The species which may be utilized are best suggested by local
agricultural experts. The same holds true for shrubs and trees.
An additional problem, which landowners in the Great Lakes region have, is the stabilization of bluffs.
Often, structural corrections are required in concert with vegetation. Once the structural stabilization is
accomplished, vegetative cover will aid in preventing erosion, reducing seepage, and slowing runoff.
The type of vegetation, which can be established on bluff slopes, is dependent upon the slope angle.
Slopes steeper than 1 on 1 generally preclude successful vegetation; slopes flatter than 1 on 3 can be planted as a
lawn and maintained in the usual manner. Slopes between 1 on 3 and 1 on 1 can be planted with grasses which
will not be mowed, ground covers, trees and shrubs, or combinations of these three. As mentioned before, local
expertise (e.g., agricultural extension agents) can aid the landowner in selecting suitable species, and in
describing the most practical methods of establishment and maintenance.
PERCHED BEACHES
Perched beaches are constructed by placing sand fill behind a low breakwater or sill. Sills can be
constructed of virtually any material described earlier for fixed breakwaters. Beach material should be chosen in
accordance with guidelines previously given for beach fills. Proper filtering should be provided beneath and
behind the sill to prevent settlement and loss of retained fill. In some cases, navigation markers may be required.
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.
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