structures constructed of rubble stone or concrete shapes. When a wave 
impinges on the structure, its energy is absorbed in the interstices of 
the surface units, resulting in a greatly reduced run-up and backwash. 
The downward eroding effect of vertical seawalls and bulkheads is greatly 
reduced by use of this type of structure. 
It is to be emphasized that the above-mentioned structures are used 
only to establish a delimiting line between land and sea, and they do not 
modify the littoral regimen either to preserve or build a beach. Conse- 
quently material transported in the littoral zone is not obstructed, and 
So moves past the seawall, bulkhead, or revetment. 
Jetties are used at harbor entrances to maintain the channel through 
the littoral zone (Figure 10). In addition to providing shelter to the 
channel from incident waves they also provide protection by preventing 
sand carried alongshore as littoral drift from entering the channel. In 
order to serve this latter function they must be impermeable and high, 
Since they are high they present a complete obstruction to the- littoral 
drift, and thus deprive the downdrift shore of sand supply. This un- 
desirable feature has long been recognized, but only in the last decade 
has it been possible to develop a satisfactory means of transporting the 
impounded material downdrift to re-establish the natural processes. At 
the present time systems have been devised whereby material is moved across 
harbor entrances by a fixed pumping plant mounted on the updrift jetty. 
This system, while helpful, has not been shown to be entirely satisfactory, 
Another type now being used will be discussed in subsequent paragraphs. 
A groin is the most important shore structure for the enlargement or 
stabilization of beaches (Figure 11). It is almost always built perpen- 
dicular to the shore to impound the available littoral drift or to reduce 
the rate of loss of sand from the area by littoral forces. This structure 
has been developed in a variety of forms. It may be long or short; high or 
low; and permeable or impermeable. The length of an impermeable groin is 
usually determined by measuring the distance from the inner edge of the 
berm to the 6-foot depth, on the usual assumption that most of the material 
movement along the beach takes place shoreward of the 6-foot depth. The 
height of the groin depends on its function. If the structure is designed 
to impound all available drift except that which moves around its outer end, 
it is built high. It is pointed out, however, that complete obstruction 
of the drift creates serious erosion downdrift and may cause flanking of 
the structure on the inner end of the downdrift side. 
If a groin is required to hold or build a beach at its original pro- 
file, a low groin is used, with its top coinciding with the existing or 
desired beach profile. The advantage of this type of groin is that when 
full, it permits material to pass over its top and nourish downdrift 
beaches, thus eliminating shore starvation and possible flanking at the 
inner end. 
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