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Tales of the Coast
Corps and the Coast

Structures

Often a navigation project requires one or more engineered structures to accomplish its objectives. Structures can serve a variety of purposes, such as dissipating wave energy and focusing tidal currents. However, their presence also establishes a major hazard for vessels. Hence, a navigation structure must be designed with regard to functional concerns.

Depending on the physical characteristics and uses of each particular harbor and port, different types and configurations of structures may be implemented.

Breakwaters are used to protect a harbor, anchorage, basin, or area of shoreline from waves. Breakwaters reflect or dissipate wave energy and thus prevent or reduce wave action in the protected area. These structures must be designed to effectively serve competing requirements for wave blockage and safe vessel passage from fully exposed waters through a constricted entrance into tranquil harbor waters.

Breakwaters are often constructed as shore-connected structures, thereby allowing access from land for construction, operation, and maintenance. But this design may have an adverse impact on water quality or sediment transport, so detached offshore breakwaters are used in certain situations. Many systems utilize a combination of shore-connected and offshore breakwaters to protect anchorage or mooring areas.

It is preferable for breakwaters to prevent wave energy from entering a harbor rather than try to dissipate excessive wave energy inside the harbor. It is also important that breakwaters be constructed to limit wave reflection, which can cause hazardous navigation conditions.

Most breakwaters built on the open coasts of the United States consist of rubble-mound construction. Other structural types include concrete caisson, timber crib, sheet pile, composite, and floating. All breakwaters must be high enough to prevent excessive wave overtopping and sufficiently impermeable to deter wave transmission through the structure.

A jetty is a shore-connected structure, generally built perpendicular to the shore, extending into a body of water to direct and confine a stream or tidal flow to a selected channel and to prevent or reduce shoaling of that channel. Jetties at the entrance of a bay or river also serve to protect the entrance channel from storm waves and crosscurrents. When located at inlets through barrier beaches, jetties help to stabilize the inlet.

Jetties are usually built in pairs, with one on either side of a channel entrance. Sometimes jetties are used in combination with a breakwater. A single jetty may also be used, located on the updrift side of the entrance. This single-jetty configuration, however, can allow a channel to migrate and is therefore not preferable in some situations.

Jetties should be long enough to prevent littoral transport around the jetty ends and into the navigation channel. Jetties built parallel to each other will also confine flood and ebb flow, thereby raising flow velocities and providing adequate sediment flushing into the flood and ebb deltas.

While jetties provide the important function of reducing sedimentation in navigation channels, they also can contribute to erosion downdrift of stabilized inlets. By blocking longshore sediment transport, jetties can impound sediment into updrift fillets and starve downdrift beaches of sand, which leads to erosion. Thus, sediment transport must be addressed in jetty construction, and mitigation measure may be implemented.

Structures besides breakwaters and jetties can help reflect and dissipate wave energy at harbors and ports. These structures are used in conjunction with breakwaters and/or jetties, depending on the conditions at and configuration of the harbor.

Training dikes serve to direct current flow in a desired path. A common application is the use of training dikes interior to a jettied inlet to confine currents to the navigation channel and help prevent channel shoaling and erosion of nearby banks and shores. Dikes are usually constructed of stone, timber pile clusters, or piling with stone fill.

Wave absorbers are sometimes constructed inside harbors to dissipate short-period wave energy. Rubble slopes are stone wave absorbers that perform the same energy dissipation function as beaches, but they do not take up as much area as beaches.

Armoring structures are included in, or adjacent to, most harbors. Vertical structures, such as seawalls and bulkheads, are useful as quay walls or docking/mooring areas. Wave reflection from vertical structures, however, can create hazardous conditions for small craft, erosion of adjacent shorelines, and beach profile changes. Therefore, bulkheads to provide vessels with direct access to shore should only be placed in areas with little exposure to wave energy. Concave-curved or rubble-slope structures can also be practical design alternatives. Revetments are typically less reflective than vertical structures.

Breakwater

Breakwater
Source: National Oceanic & Atmospheric Adminstration (NOAA)

Structures

Breakwaters and jetties are the most commonly used structures for harbor protection.

Breakwaters

Physical Models: Because they can accurately reproduce many of the complex, interacting hydrodynamic effects on a harbor, physical models provide the most reliable method for optimizing breakwater layout.

Jetties

Most jetties are rubble-mound structures. Materials used for jetty construction include stone, concrete, steel, and timber. Unlike breakwaters, jetties are usually designed to allow some wave overtopping. Also, jetty cores may be lower and more permeable than breakwater cores, provided the jetty sufficiently blocks passage of sediment into the navigation channel.

Other Structures

Rubble revetments may be effective wave absorbers but can hinder access to the beach. Smooth revetments built with concrete blocks generally present little difficulty to pedestrians, but they are more reflective than rubble revetments. Thus, the use of these armoring structures must be dictated by harbor characteristics and usage.