Structurally, a groin must resist wave action, currents, the impact of floating debris, and earth
pressures created by the difference in sand levels on the two sides. As with other structures, a groin must
also resist the scour created by waves breaking on the structure and by currents adjacent to it.
Problems with both downdrift erosion and flanking can be avoided by filling the groin
compartments after construction. This provides material to immediately nourish the downdrift beaches
while increasing the size of the protective fillet (Figure 10). Although some downdrift erosion may still
occur, pre-filling should limit its extent compared to allowing the groin compartments to naturally
accrete.
APPENDIX D contains specific information on possible groin configurations.
BEACH FILLS
Beach fills are quantities of sand placed on the shoreline by mechanical means, such as dredging
and pumping from offshore deposits or overland hauling and dumping by trucks. The resulting beach
provides some protection to the area behind it and also serves as a valuable recreational resource.
The beach fill functions as an eroding buffer zone. As large waves strike it, sand is carried
offshore and deposited in a bar. As the bar grows, it causes incoming waves to break farther offshore.
The useful life of such a beach, which depends on how quickly it erodes, can be completely eliminated
in a short period of time by a rapid succession of severe storms. The owner must expect, therefore,
to
periodically add more fill as erosion continues. Beach fills generally have a relatively low initial cost
but a periodic maintenance cost of adding new fill (periodic nourishment).
The rate at which new fill must be added depends on the relative coarseness of the fill material in
relation to the native beach material. Ideally, fill and native beach materials should be perfectly matched,
but this is virtually impossible. Generally speaking, if fill material is coarser than the native material, the
fill erodes more slowly and if it is finer, it erodes more quickly.
Figure 11 illustrates the important design factors in constructing a beach fill. The berm elevation
should decrease the likelihood of overtopping by storm waves. The berm width should provide,enough
material for several years of erosion before refilling. The portion of the fill that is very fine material is
lost almost immediately after placement, requiring an initial supply of material somewhat over
expectations. Overfill by 50 percent the first time and then adjust this ratio as experience dictates. For
instance, if 100 cubic yards of sand is needed for the required berm width, you should initially place
about 150 cubic yards to compensate for the high initial losses of fine material.
The final factor is the beach slope, which should parallel the existing profile and slope on the
theory that the existing beach is in equilibrium with the wave forces and that the new beach eventually
assumes a similar shape. Equipment can shape the beach fill cross section as it is placed, or the fill can
be reshaped by waves. The final equilibrium slope depends on the relative coarseness of the fill material
because coarser sand results in a steeper beach slope.
If fill is placed over a short length of shoreline, it creates a projection that is subjected to increased
wave action. Therefore, it is generally preferable to make the transition to the existing shoreline over a
longer distance, which may require cooperation from other landowners. If this is impractical, protective
structures, such as groins, may be required to retain the fill.
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