Waterfront structures may be broadly divided into three types as fpllows: (1) harbor-shelter structures, (2) stable-shoreline structures, and (3) wharfage structures.

Harbor-shelter structures are offshore structures that are designed to create a sheltered harbor. Various types of these structures are discussed below. A breakwater is an offshore barrier, erected to break the action of the waves and thereby maintain an area of calm water inside the breakwater. A jetty is a similar structure, except that its main purpose is to direct the current or tidal flow along the line of a selected channel.

The simplest type of breakwater or jetty is the rubble-mound (also called rock-mound) type shown in figure 1-13. The width of its cap may vary from 15 to

70 feet. The width of its base depends on the width of the cap, height of the structure, and the slopes of the inner and outer faces. For a deepwater site or from with an extra-high tide range, a rubble-mound breakwater may be topped with a concrete cap structure, such as shown in figure 1-14. A structure of this type is called a composite breakwater or jetty. In figure 1-14, the cap structure is made of a series of precast concrete

Figure 1-16.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.