to be maintained, A jetty is normally a high structure to insure that 
sand will not be carried over it into the navigation channel. 
Possibly the most common shore protection structure is the groin. 
It is built usually perpendicular to the shoreline to trap littoral drift 
or to retard erosion of the shore (Figure 5). It is narrow in width 
(measured parallel to the shoreline) and its length may vary from less 
than one hundred to several hundred feet (extending from a point landward 
of the shoreline out into the water). It is constructed of timber, con- 
crete, steel or rubble. Groins may be classified as permeable or im- 
permeable, An impermeable groin is a solid or nearly solid structure, 
whereas a permeable groin has openings through it of sufficient size to 
permit passage of appreciable quantities of littoral drift. The top pro- 
file of groins generally parallels the slope of the beach out to about 1 
foot above MLW, and thence is horizontal to the outer end; An impermeable 
groin is normally designed with low profile to build a beach by impounding 
the drift to its top elevation, and then permitting the drift to pass over 
its top to nourish the downdrift beaches. A high groin is normally used 
only as a barrier at the ends of littoral zones, such as limiting groins 
of a bayhead beach or at a harbor entrance. 
Breakwaters may be either shore connected or detached (offshore). 
Both are used to modify the littoral processes as well as provide shelter 
for shipping. 
The shore-connected breakwater usually leaves the shore as a jetty 
but then changes alignment to roughly parallel to shore. It is generally 
built of heavy rubble, and designed to be stable under attack of storm 
waves extant in the area. Other construction materials are also used for 
breakwater construction, such as precast concrete shapes and, when sited 
on hard bottom, concrete, timber or steel caissons loaded with ballast. 
The length of the structure is dependent upon the required harbor area to 
be protected, Shore-connected breakwaters may be used singly or in pairs 
to totally enclose a harbor area. 
An offshore breakwater is normally set parallel to the shore. It is 
generally built with a trapezoidal section of heavy rubble similar to the 
jetty, and designed to be stable when attacked by the waves extant in the 
area. The length of the structure is approximately equivalent to the dis- 
tance the structure is sited from the shore, From an engineering point of 
view, the length is determined by the wave diffraction patterns around the 
ends of the structure. In other words, the length of the structure is so 
designed that a calm area of required size is created shoreward opposite 
the center of the structure. 
Undoubtedly the most widely used shore protection device at present, 
is the artificially constructed and nourished beach. It is included here 
20 
