STABLE-SHORELINE STRUCTURES

Figure 1-6.—Various types of seawalls. boxes called caissons, each of which is floated over its place of location, and then sunk into position. A monolithic (single-piece) concrete cap is then cast along the tops of the caissons. Sometimes, breakwaters and jetties are built entirely of caissons, as shown in figure 1-15. A groin is a structure similar to a breakwater or jetty, but it has a third purpose. A groin is used in a situation where a shoreline is subject to alongshore erosion, caused by wave or current action parallel or oblique to the shoreline. The groin is run out from the shoreline (usually there is a succession of groins at intervals) to check the alongshore wave action or deflect it away from the shore. A mole is a breakwater that is paved on the top for use as a wharfage structure. To serve this purpose, it must have a vertical face on the inner side, or harborside. A jetty may be similarly constructed and used, but it is still called a jetty. STABLE-SHORELINE STRUCTURES These structures are constructed parallel with the shoreline to protect it from erosion or other wave damage. A seawall is a vertical or sloping wall that offers protection to a section of the shoreline against erosion and slippage caused by tide and wave action. A seawall is usually a self-sufficient type of structure, such as a gravity-type retaining wall. Seawalls are classified according to the types of construction. A seawall may be made of riprap or solid concrete. Several types of seawall structures are shown in figure 1-16. A bulkhead has the same general purpose as a seawall; namely, to establish and maintain a stable shoreline. However, while a seawall is self-contained, relatively thick, and is supported by its own weight, the bulkhead is a relatively thin wall. Bulkheads are classified according to types of construction, such as the following: 1-9