Shoreline Structures

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Contents

Why We Should Care

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Seawalls, groins, jetties and other shoreline stabilization structures have had tremendous impacts on our nation's beaches. Shoreline structures are built to alter the effects of ocean waves, currents and sand movement. They are usually built to "protect" buildings that were built on a beach that is losing sand. Sometimes they are built to redirect rivers and streams. Other times they are constructed to shelter boats in calm water. In many cases, seawalls, jetties, breakwaters and groins have caused down-coast erosion problems with associated costs that have greatly exceeded the construction cost of the structure.

Every surfrider knows that there are groins and jetties that have incidentally improved wave riding. However, in many other areas shoreline construction has ruined wildlife habitat, destroyed surfing waves and caused beaches to erode. As beach lovers and environmentalists, we need to understand the consequences of shoreline structures so that we may be able to effectively influence decisions on the impacts, placement or necessity of these structures. As an environmental group committed to maintaining the natural shoreline and beach equilibrium, we are usually opposed to construction that will disrupt the balance of forces that shape our coastline.

The Basics

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Erosion: Where Has All The Sand Gone?

Every winter, the newspapers show pictures of oceanfront buildings falling into giant surf. Beaches are not static piles of sand. Ocean currents cause beaches to move constantly. Beach sand is primarily a product of the weathering of the land. Sand can also come from ocean organisms such as coral. Sand can come from the erosion of coastal bluffs. However, most of the sand along the world's beaches comes from rivers and streams. When natural processes are interfered with, the natural supply of sand is interrupted and the beach changes shape or can disappear completely. Sand production stops when coral reefs die from pollution, when coastal bluffs are "armored" by sea walls and when rivers are dammed upstream for flood control and reservoir construction. The sand that collects behind upstream dams and reservoirs is often "mined" and sold for concrete production. It then never makes it to the beach. A public resource is sold for private profit.

In the face of eroding beaches, owners of beachfront property will often try to use their political influence to demand that "something be done." The intelligent action would be to move the building away from the ocean. Unfortunately, what has often been done in the past has been to armor the coastline with rocks, concrete and steel. This does not protect or maintain the beach - it only protects the buildings.

Millions of taxpayer dollars have been wasted subsidizing beachfront building. Federal flood insurance and expensive Army Corps of Engineer projects have done very little to make oceanfront buildings safe and have hastened beach erosion. In many cases, it would be more cost-effective for taxpayers to have the government buy the coastal property, condemn the buildings and allow the area to act as a buffer between the ocean and the remaining buildings. In urbanized areas with expensive real estate, a more cost effective and environmentally sound alternative to shoreline structures may be to periodically "nourish" the beach with sand.

The Littoral Cell

On the West Coast of the U.S., beach sand moves from river mouths to the beach. It then moves along the coast in the direction of prevailing currents and eventually it moves offshore. This sand transport system is called a littoral cell.

When waves break at an angle to the shoreline, part of the wave's energy is directed along the shore. These "longshore currents" flow parallel to the shore. Surfers call this the "drift". This current will move sand along the shore and a beach will be formed. The same current that transports a surfer down the beach from the point of entry will also move beach sand down the shoreline. When this longshore current turns seaward, it is called a rip current.

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Some areas have underwater canyons near the beach. These submarine canyons were prehistoric river mouths. Sometimes the longshore current will be interrupted by one of these canyons. In this case, the sand is lost from the beach in water too deep to be returned to shore. The littoral cell system, from the river mouth to the underwater canyon, will always lose beach sand. If the sand supply from the river is cut off, the beach will lose sand causing the beach to become narrower.


Submarine Canyon in Santa Monica Bay and San Pedro (image from Dartnell, and Gardner, 1999, U.S. Geological Survey Digital Data Series DDS-55 (CD-ROM)


On the East Coast of the U.S., the shore formed differently. Sand comes from the erosion of headlands, bluffs and cliffs. The underwater coast (continental shelf) of the east is broad and flat. East Coast beaches are generally wider. Barrier islands run along the coast. In contrast to the West Coast, submarine canyons are rarely near the beach and seldom act as conduits for sand loss. A notable exception is the Hudson Canyon at the southwest end of Long Island, New York. Sand that moves south here is lost down the canyon. On the East Coast, sand "loss" is primarily from the movement of barrier islands. Barrier islands naturally migrate landward due to sea level rise, but this migration is accelerated during storm events. Powerful hurricanes deposit sand inland by washing it over the dunes. Sometimes these storms will create strong currents that take sand too far offshore for it to return to the beach. The depth where sand is moved so far offshore that it cannot return is known as the "closure depth". The precise depth is under scientific debate and varies with time, wave and weather conditions. When humans try to interfere with the natural migration of barrier islands, it is usually at their long-term peril.

Erosion is a process, not a problem. Beaches are dynamic and natural. Buildings, bridges and roads are static. The problem occurs when there is a static structure built on a dynamic, moving beach. If buildings and roads were not built close to the shore, we would not have to worry about shoreline structures or sand erosion.

Responses to Erosion

Seawalls

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See the full article: Seawalls

When coastal buildings or roads are threatened, usually the first suggestion is to "harden" the coast with a seawall. Seawalls are structures built of concrete, wood, steel or boulders that run parallel to the beach at the land/water interface. They may also be called bulkheads or revetments. They are designed to protect structures by stopping the natural movement of sand by the waves. If the walls are maintained they may hold back the ocean temporarily. The construction of a seawall usually displaces the open beach that it is built upon. They also prevent the natural landward migration of an eroding beach.

See this gallery of photos of seawalls, revetments and other attempts at shoreline armoring from around the world.

When waves hit a smooth, solid seawall, the wave is reflected back towards the ocean. This can make matters worse. The reflected wave (the backwash) takes beach sand with it. Both the beach and the surf may disappear.

Seawalls can cause increased erosion in adjacent areas of the beach that do not have seawalls. This so-called "flanking erosion" takes place at the ends of seawalls. Wave energy can be reflected from a seawall sideways along the shore, causing coastal bluffs without protection to erode faster. When it is necessary to build a seawall, it should have a sloped (not vertical) face. Seawalls should also have pockets and grooves in them that will use up the energy of the waves instead of reflecting it.

Usually the most cost-effective, environmental solution is to move the building away from danger. Building seawalls will buy time against natural processes, but it will not "solve the problem" of erosion by waves.

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Groins

Groins are another example of a hard shoreline structure designed as so-called "permanent solution" to beach erosion. A groin is a shoreline structure that is perpendicular to the beach. It is usually made of large boulders, but it can be made of concrete, steel or wood. It is designed to interrupt and trap the longshore flow of sand. Sand builds up on one side of the groin (updrift accretion) at the expense of the other side (downdrift erosion). If the current direction is constant all year long, a groin "steals" sand that would normally be deposited on the downdrift end of the beach. The amount of sand on the beach stays the same. A groin merely transfers erosion from one place to another further down the beach.

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Groins occasionally improve the shape of surfing waves by creating a rip current next to the rocks. The rip can be a hazard to swimmers. The rip can also divert beach sand onto offshore sand bars, thereby accelerating erosion. Groins can also ruin the surf. If the waves are reflected off the rocks, the waves may lose their shape and "close-out."

As soon as one groin is built, property owners downdrift of it may start clamoring for the government to build groins to save "their" beach. Eventually, the beach may become lined with groins. Since no new sand is added to the system, groins simply "steal" sand from one part of the beach so that it will build up on another part. There will always be beach erosion downdrift of the last groin.

Breakwaters

A breakwater is a large pile of rocks built parallel to the shore. It is designed to block the waves and the surf. Some breakwaters are below the water's surface (a submerged breakwater). Breakwaters are usually built to provide calm waters for harbors and artificial marinas. Submerged breakwaters are built to reduce beach erosion. These may also be referred to as artificial "reefs."

A breakwater can be offshore, underwater or connected to the land. As with groins and jetties, when the longshore current is interrupted, a breakwater will dramatically change the profile of the beach. Over time, sand will accumulate towards a breakwater. Downdrift sand will erode. A breakwater can cause millions of dollars in beach erosion in the decades after it is built.

Beach Nourishment

In recent years, the hard structures described above have fallen somewhat out of favor by communities due to the negative impacts we have discussed. Beach nourishment (or beach fill) is becoming the favored "soft" alternative. Beach nourishment is simply depositing sand on the beach in order to widen it. Although paid for by all taxpayers, it is frequently undertaken to protect private oceanfront buildings. Occasionally the taxpaying public is refused access to beaches that they have paid to protect. Sand nourishment is a costly, temporary solution. The projects are not intended to have a long life span and must be renourished on a regular basis, creating a cycle that will go on until the money runs out or shorefront buildings are relocated.

There are many considerations that must addressed when designing a nourishment project. If the grains of sand are not exactly the same size as that of the natural beach, the newly nourished beach may erode faster than the natural beach was eroding. Beach nourishment can cause bottom organisms and habitats to be smothered by "turbid" water that has sand and mud suspended in it. The shoreline is moved seaward into deeper water, causing the beach to drop off quickly, posing a hazard to swimmers. This may also impact the surf for a period of time, causing the waves to break as shore break, until the beach and sandbars can reestablish a level of equilibrium.

Navigation Structures

Harbors, Natural and Artificial

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On the West Coast of the U.S., artificial harbors have been constructed by building a series of breakwaters and jetties. When an artificial harbor is built in an area that is subject to high-energy wave action, it will invariably interrupt the longshore flow of sand. This will cause serious downdrift erosion. Some harbor designs force the longshore current to make a 90-degree turn towards the ocean. This causes a large rip current that may carry sand offshore that might otherwise remain in the surf zone. This will have the effect of completely changing the shape of the ocean bottom. An artificial harbor mouth can act as a trap for the longshore sand transport causing it to clog up with sand, which makes costly periodic dredging projects necessary.

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Natural harbors, like San Francisco Bay, are protected from the ocean's fury but are still subject to tidal and wave energy. This causes water mixing and circulation. Stagnant artificial harbors are easily polluted by boating activities: paint, oil, grease, garbage and illegally dumped sewage. These wastes can poison the living creatures that swim in these waters. When the harbor is dredged, the sand and contaminated sediments cannot be returned to the beaches and must be disposed of in a safe place. Often, the sediments are dumped in deeper waters, poisoning the marine life food web.

Some harbors have been built by dredging wetland areas. Wetlands are habitat for birds and marine life. They can also provide water storage capacity to prevent coastal flooding during rains. Wetlands are natural water filters that purify land run-off before it enters the ocean. Dredging a wetland to build a boat harbor should never be done. We have lost over half the wetlands in the U.S. to human development. In California, we have lost over 94% of our wetlands.

Jetties

Jetties are large, man-made piles of boulders or concrete that are built on either side of a coastal inlet. Whereas groins are built to change the effects of beach erosion, jetties are built so that a channel to the ocean will stay open for navigation purposes. They are also built to prevent rivermouths and streams from meandering naturally.

Jetties completely interrupt or redirect the longshore current. Just as a groin accumulates sand on the updrift side, so do jetties. The major difference is that jetties are usually longer than groins and therefore create larger updrift beaches at the expense of the smaller downdrift beaches.

On East Coast barrier islands, ocean tidal inlets migrate naturally with the longshore current. A jetty system will permanently disrupt the equilibrium of the beach. This may seriously affect the tidal circulation and the health of the wetlands between the barrier islands and the mainland.

Inlets with short jetties that don't quite reach the surf will clog up with sand. The sand must be dredged on a regular basis. A "sand by-passing" system may be built to pump sand around the jetties. The sand pumping may come from within the inlet or from the updrift beach. These methods are expensive and must be maintained indefinitely.

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What You Can Do

Environmental Impacts

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Before a shoreline structure is built, the local community must be informed of its environmental impacts. The National Environmental Protection Act (NEPA) mandates that an Environmental Impact Statement (EIS) must be prepared to identify environmental impacts of the project. This document must spell out all effects that a new structure will have on the surrounding area. It is during the scoping of and subsequent public comment period of preparing an EIS that Surfrider Foundation activists can have the greatest impact on the proposed project.

The EIS process allows activists to educate the public about the project's impacts on the environment. Written comments on the draft EIS are crucial for legal purposes. Oral comments at hearings are even more important because they are picked up in the media, which allows more of the public to become informed.

Our goal is to make sure that the long-term effects and the true costs of the project are carefully spelled out for both the public and the decision-makers. If there are environmental impacts, the developer must provide ways to "mitigate" the damage. For instance, if the project will cause downcoast erosion, the developer may be required to install and maintain a sand replenishment system or promise to post a bond that will pay for periodic sand replenishment as long as the structure exists. This may be impractical. If there is wildlife habitat destroyed, the developer may be required to restore habitat on site if feasible.

The Only Permanent Solution: Retreat from the Beach!

"Hard" shoreline structures have severe environmental impacts on the longshore current and the natural processes of beach sand distribution. "Soft" solutions like sand nourishment are expensive and temporary. Marinas should be built in natural harbors away from the energy of the waves. Building on our ocean's shore is not a good idea. NATURE WILL ALWAYS PREVAIL.

Shoreline construction means that taxpayers pay the bills when the ocean behaves as expected. Whether it is fire department rescues, the Public Works Department placing sand bags, the police guarding vacant buildings from looters or the Army Corps of Engineers spending millions to "correct the problem," taxpayers are the ones who pay. Shoreline protection is, often, "welfare for the rich."

Shoreline property owners frequently limit the public's access to the beach by refusing to let the public cross their property to get to the beach.

Shoreline building also means habitat destruction. Birds, plants and animals that call coastal dunes and beaches their homes are slowly becoming extinct.

As humans continue to overpopulate our coastal areas (and the planet) we will have to be more thoughtful about our relationship with the ocean. Surfrider Foundation activists will continue to educate the public about the natural processes that create and maintain our shoreline. Sometimes shoreline structures must be built, but the public must know the impacts. Society will have to continually pay to maintain the structures and correct the environmental damage caused by them. The best solution is to retreat from the beach and allow nature to replenish, maintain and change the beach as she sees fit.

Surfrider Foundation Beach Preservation Policy

Surfrider's official policy regarding beach preservation and shoreline structures.

Restore the Shore Video

Video produced by the San Diego Chapter of Surfrider Foundation discussing beach erosion, shoreline structures and ways to respond to the changing coast.



North Carolina's Summary of the Effects of Shoreline Structures

Since 1985, North Carolina prohibited shoreline armoring. The following text, from the state's 2010 Habitat Protection Plan does a good job explaining the physical and ecological effects of shoreline armoring:

"Shoreline hardening, or hard stabilization, involves construction of hard immovable engineered structures, such as seawalls, rock revetments, jetties, and groins. Seawalls and rock revetments run parallel to the beach. Seawalls are vertical structures, constructed parallel to the ocean shoreline, and are primarily designed to prevent erosion and other damage due to wave action. Revetments are shoreline structures constructed parallel to the shoreline and generally sloped in such a way as to mimic the natural slope of the shoreline profile and dissipate wave energy as the wave is directed up the slope. Breakwaters are structures constructed waterward of, and usually parallel to, the shoreline. They attempt to break incoming waves before they reach the shoreline, or a facility (e.g., marina) being protected. Jetties and groins are manmade structures constructed perpendicular to the beach, with jetties usually being much longer, and are located adjacent to inlets with the purpose of maintaining navigation in the inlet by preventing sand from entering it. In contrast, terminal groins are structures built at the end of a littoral cell to trap and conserve sand along the end of the barrier island, stabilize inlet migration, and widen a portion of the updrift beach. Terminal groins are designed so that when the area behind the groin fills in with sand, additional sand will go around the structure and enter the inlet system.

It is well accepted that hard stabilization techniques along high energy ocean shorelines will accelerate erosion in some location along the shore as a result of the longshore sediment transport being altered (Defeo et al. 2009). The hydromodifications resulting from coastal armoring modifies sediment grain size, increases turbidity in the surf zone, narrows and steepens beaches, and results in reduced intertidal habitat and diversity and abundance of macroinvertebrates (Walton and Sensabaugh 1979; NRC 1995; Dolan et al. 2004: 2006; Pilkey et al. 1998; Peterson et al. 2000a; Miles et al. 2001; Dugan et al. 2008; Walker et al. 2008; Riggs and Ames 2009). A study looking at the effect of a short groin (95m) on the benthic community found that the groin created a depositional condition on one side of the structure and erosion on the other, and macroinvertebrate diversity and abundance was significantly reduced within 30m of the structure, as sand particle size and steepness increased (Walker et al. 2008). The change in benthic community was attributed to the change in geomorphology of the beach. Hard structures along a sandy beach can also result in establishment of invasive epibenthic organisms (Chapman and Bulleri 2003). A secondary impact of hardened structures is that the areal loss of beach resulting from hardening of shorelines is often managed by implementing nourishment projects, possibly having additional damage to subtidal bottom (Riggs et al. 2009). Anchoring inlets also prevents shoal formation and diminishes ebb tidal deltas, which are important foraging grounds for many fish species. Recognizing that hardened structures are damaging to recreational beaches and the intertidal zone, four states have prohibited shoreline armoring: Maine, Rhode Island, South Carolina, and North Carolina (effective in North Carolina since 1985).

Perhaps the greatest impact of terminal groins and jetties results in the long-term effect on barrier islands and the effect that will have on marine and estuarine ecosystems. By stabilizing the inlet, inlet migration and overwash processes are interrupted, causing a cascade of other effects (Riggs and Ames 2009). In the case of Oregon Inlet, the terminal groin anchored the bridge to Pea Island and stopped the migration of the inlet on the south side. But the continuing migration of the north end of Bodie Island led to an increased need for inlet dredging. The combination of reduced longshore transport of sediment due to the groin and the post-storm dune construction to open and protect the highway prevented overwash processes that allow Pea Island to maintain its elevation over time. With overwash processes disrupted, the beach profile has steepened, and the island has flattened and narrowed, increasing vulnerability to storm damage (Dolan et al. 2006; Riggs and Ames 2009; Riggs et al. 2009). At Oregon Inlet and Pea Island, the accelerated need for beach replenishment is further aggravated by the need to maintain Hwy 12 on the narrowing beach. From 1983 to 2009 approximately 12.7 million cubic yards of sand have been added to the shoreline within three miles of the terminal groin (Riggs and Ames 2009). Dolan (2006) documented that the large volumes of sand replenishment in this area, required to maintain the channel, protect the road, and maintain a beach have resulted in a significant reduction in grain size and reduction in mole crab abundance. Mole crabs are considered an important indicator of beach conditions due to their importance in the food web as prey for shorebirds and surf fish. In addition to causing erosion on downdrift beaches, altering barrier island migration processes, and accelerating the need for beach nourishment projects, jetties obstruct larval fish passage through adjacent inlets (Blanton et al. 1999)."



This article is part of a series on Shoreline Structures looking at types of structures commonly built along shorelines, and the policies, laws, and regulations which can affect where and under what conditions they are built.

For information about laws, policies and conditions impacting shoreline structures in a specific state, please visit Surfrider's State of the Beach report to find the State Report for that state, and click on the "Shoreline Structures" indicator link.