Sea dikes are onshore structures with the principal function of protecting low-lying areas against flooding. Sea dikes are usually built as a mound of fine materials like sand and clay with a gentle seaward slope in order to reduce the wave runup and the erodible effect of the waves. The surface of the dike is armored with grass, asphalt, stones, or concrete slabs (USACE, 2005).

Armored sea dike in the Netherlands at low tide.

Credit: Photo courtesy of K. Hanegan, 2010.

Seawalls are onshore structures with the principal function of preventing or alleviating overtopping and flooding of the land and the structures behind due to storm surges and waves. Seawalls are built parallel to the shoreline as a reinforcement of a part of the coastal profile. Quite often, seawalls are used to protect promenades, roads, and houses seaward of the crest edge of the natural beach profile. In these cases, a seawall structure protruding vertically or close to vertically from the beach profile must be built. Seawalls range from vertical face structures such as massive gravity concrete walls, tied walls using steel or concrete piling, and stone-filled cribwork to sloping structures with typical surfaces being reinforced concrete slabs, concrete armor units, or stone rubble (bulkheads, revetments, and Rip Rap are different types of seawalls).

Waves breaking on the Galveston, Texas, seawall.

Credit: By Trube, Galveston, Texas (Photo postcard by Trube, Galveston, Texas Via [1]) [Public domain], via Wikimedia Commons.

Waves breaking on the Galveston, Texas seawall – note the added toe protection using large rip-rap stones to reduce waves run-up and undercutting of the structure.

Credit: Ioannis Georgiou © Penn State is licensed under CC BY-NC-SA 4.0

Detached breakwaters are small, relatively short, non-shore-connected nearshore breakwaters with the principal function of reducing beach erosion. They are built parallel to the shore just seaward of the shoreline in shallow water depths, using solid concrete structures, piles of stone/concrete blocks, or rubble mound. Multiple detached breakwaters spaced along the shoreline can provide protection to substantial shoreline frontages. The gaps between the breakwaters are in most cases on the same order of magnitude as the length of one individual structure. Each breakwater reflects and dissipates some of the incoming wave energy, thus reducing wave heights in the lee of the structure, interrupting transport along the shore, and reducing shore erosion. Beach material transported along the beach moves into the sheltered area behind the breakwater, where it is deposited in the lower wave energy region. The nearshore wave pattern, which is strongly influenced by diffraction* at the heads of the structures, will cause salients and sometimes tombolos to be formed, thus producing a shoreline similar to a series of pocket beaches. Once formed, there is positive feedback: the pockets will cause wave refraction, which helps to stabilize the pocket-shaped coastline. Breakwaters can also be constructed with one end linked to the shore, in which case they are usually classified as sea walls.

The figures below illustrate the formation of salients (cusps of sediment protruding from the shore) and tombolos (larger cusps that attach to the breakwater), to form the pockets described above.

High-resolution orthoimagery of the offshore breakwaters and tombolos along the East Ocean View, Norfolk, VA shoreline, taken in 2009.

Credit: USGS: EarthExplorer (Public Domain)

Image showing a tombolo in Grande Isle Louisiana, at the terminus of the barrier island. This tombolo has grown over time (compared to the insert in the top right) and has now completely disrupted longshore sediment transport at this location.

Credit: Mark Kulp © Penn State is licensed under CC BY-NC-SA 4.0

*(for a definition of Diffraction visit the Coastal Wiki website).

Groins are built to stabilize a stretch of natural or artificially nourished beach against erosion that is due primarily to a net longshore loss of beach material. Groins function only when longshore transport occurs. Groins are narrow structures, usually straight and perpendicular to the pre-project shoreline. The effect of a single groin is the accretion of beach material on the updrift side and erosion on the downdrift side; both effects extend some distance from the structure. Consequently, a groin system (series of groins) results in a saw-tooth-shaped shoreline within the groin field and a differential in beach level on either side of the groins.

Figure 8.9: High-resolution orthoimagery of the Groins along the Ocean City, NJ shoreline, taken 15-March-12.

Credit: USGS: EarthExplorer (Public Domain)

Jetties are used for the stabilization of navigation channels at river mouths and tidal inlets. Jetties are shore-connected structures generally built on either one or both sides of the navigation channel, perpendicular to the shore and extending into the ocean. By confining the stream or tidal flow, it is possible to reduce channel shoaling and decrease dredging requirements. Moreover, on coastlines with strong longshore currents and longshore sediment transport, jetties also function to arrest the crosscurrent and direct it across the entrance in deeper water where it represents less hazard to navigation. When extended offshore of the breaker zone, jetties improve the maneuvering of ships by providing shelter against storm waves. Jetties are constructed using methods and materials similar to breakwaters.

Figure 8.11: Jetties at Indian River Inlet, DE with updrift accretion and downdrift erosion.

Credit: US Army Corp of Engineers CIRP - Coastal Inlets Research Program (Public Domain)

While hard coastal structures can be the most effective option for flood protection and/or mitigation, or for stabilizing a shoreline at a fixed position, there is a price to pay. Hard structures partially hinder the recreational use of the coastal zone and can cause adverse ecological effects within the coastal zone. For example, when seawalls are constructed on eroding beaches, the erosion continues so that the beach in front of the seawall can become very narrow or disappear completely. And while groins and jetties trap sediment on the updrift side, resulting in shoreline accretion, there is corresponding shoreline erosion on the downdrift side due to the interruption in longshore transport. Some of the disadvantages of hard structures include:

• Visual impacts
• Horizontal and vertical access restrictions
• Loss of sand supply to beach from armoring backshore
• Placement losses with construction of revetment or seawall
• Passive erosion
• Active erosion