Along much of the world’s coastlines, the intersection of vulnerability (e.g., high populations, economic activities supported by the coastal zone) and coastal hazards (e.g., coastal erosion, coastal flooding) produces a risk that is unacceptable to the particular community and society, requiring the use of coastal protection measures to reduce the risk. The following sections detail some of the typical “hard” and “soft” coastal protection measures and how they impact risk and the environment in the coastal zone.
Coastal structures are frequently constructed to prevent erosion of coastal landscapes and infrastructure and mitigate the risks to the populations and economic activities dependent on the coastal zone. Coastal structures, sometimes referred to as “hard” structures, are usually built using materials (at least for certain coasts and beaches) that do not form naturally, such as of concrete, large armor stone, steel, or timber, are relatively permanent (typical 50-yr design life), and are spatially-fixed within an otherwise dynamic coastal zone. The most important hard structure types are dikes (levees), seawalls, breakwaters, groins, and jetties. The following descriptions are taken directly from the USACE Coastal Engineering Manual, the primary reference for coastal structure design in the U.S. Next, read about the hard structures and what each one does to protect the coast.
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).
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 placed 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).
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.
Read over Marine Biodiversity Wiki: Detached Breakwaters [1] to learn about these features.
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.
*(for a definition of Diffraction visit the Coastal Wiki website [3]).
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.
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.
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:
Look at the figure below and then answer the questions below.
After examining the Coastline figure, consider how you would answer the questions below.
It is the stabilization of the shoreline using environmentally friendly techniques used to protect property and uses from shoreline erosion. The main objective of soft shoreline stabilization is to achieve a balance between the need for protection against erosion while maintaining and enhancing shoreline functions.
Contrary to shorelines that are completely hardened with structures (described earlier), soft stabilization methods seek to incorporate key features into the design that either maintain or enhance functions of the shoreline, or those that allow natural processes to continue. But, natural processes, such as the movement of sand along a beach or barrier island or sediment moving along cliff coasts, headlands, etc., can vary widely between sites, making soft stabilization methods quite variant as well. Soft stabilization methods are highly dependent on local environments, and processes governing sediment pathways in each system. As such, additional planning for these methods may be required because of differences in coastal geomorphology, physical processes governing sediment transport, and because local ordinances vary across state boundaries.
As our understanding of the effects of hard stabilization methods increases, many traditional coastal engineering practices are slowly being phased out, especially where soft stabilization methods can replace or restore the ecological function, establish energy continuity, and offer sufficient protection. But, soft shoreline stabilization is a complex topic. Many federal and state agencies, including the National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center, have been working for many years to implement programs to facilitate such protection practices with a fair amount of success. However, we still have a lot to learn before we can completely abandon hard structures. This is especially true if relocating communities that are at immediate risk is not possible (recall concepts in coastal vulnerability, exposure, and rising seas, from previous modules).
Useful information on some approaches and guidelines:
Required:
Recommended:
Protection or mitigation of shorelines using soft approaches has some simple objectives and three basic principles.
The first principle is, try to imitate nature. Within each geomorphic environment, sediment characteristics, shoreline slope, and terrestrial and submerged habitat will be specific, hence using native plants and sediments that have already been exposed and shaped by forces within the specific coastal zone are critical to the success of soft mitigation methods. Plants help retain the soil matrix with their roots, and often offer good protection to erosion. On the other hand, if an area is subjected to higher energy conditions where vegetation is not naturally found, such as a beach, trying to steady the shoreline using vegetation along the high energy environment of the beach might not be a good idea. The fast-moving water and energy resulting from tidal currents and breaking waves will uproot the plants and quickly render the plants ineffective.
The second principle is, maintain gentle slopes. Unless we are in rocky coasts or regions with bedrock exposure, natural slopes where sediment is stable under gravity (less than the angle of repose) are relatively gentle. Maintaining a gentle slope allows for gradual dissipation of wave energy across a longer distance, hence the energy acting on each unit area is much lower compared to a vertical wall.
The third principle is, employ combined or mixed material approaches. Along many shorelines, we see a variety of terrestrial plants, various sediment sizes ranging from mud to sand or gravel, and shorelines are often lined with trees and other plants, and slopes can vary widely. Therefore, using a combination of approaches that imitates nearby natural shorelines is the best recipe for the successful implementation of soft approaches.
Methods of protection also often involve integrated approaches that include a combination of soft and non-traditional hard structure approaches.
There are many methods of soft shoreline stabilization with complex construction methodologies and materials. Here we will list a few examples for illustrative purposes in order to introduce the concept, and examine case studies that demonstrate how these methods work.
Stream and riverbank protection efforts in populated areas are expected to address issues such as habitat, aesthetics, and water quality, as much as they address needs such as flood control and erosion protection. Therefore, integrated streambank protection designs that include vegetation are likely to satisfy these multiple objectives. Soil bioengineering is a method routinely used to address erosion and can be achieved in many ways across different systems. These systems utilize hybrid approaches that use geotextile fabrics and/or vegetation and can provide sound streambank protection while maximizing ecological and water quality benefits. These methods are used in place of riprap, concrete, or other inert structures alone. The Jacques-Cartier Park case study describes soil bioengineering systems that have been used to meet specific aquatic and riparian habitat objectives, and the procedures developed for this project are considered or have been used elsewhere where environmental concerns are placed high on the priority list, such as Alaska and the Ottawa River in Canada, which divides the Provinces of Ontario and Quebec.
Soil bioengineering methods have a common geotechnical benefit of providing root reinforcement in the soil and can help modify drainage patterns of the soil, help stabilize soils at steeper angles if desired, help keep grasses, and bushy vegetation in place resisting erosion, and support woody debris or other types of vegetation. The species of woody vegetation selected for inclusion in soil bioengineering systems can have a significant effect on the habitat benefits. While various species of willow are the most common woody plants used in soil bioengineering because of their excellent rooting ability, good overhanging cover and shade for streams, good nesting habitat for some species of birds, and some cover for mammals, it is not noted as an excellent food source for land animals, nor it is suitable for saline systems and may have limited applications in coastal settings.
As part of the remediation of the Jacques-Cartier Park site, contaminated soils and materials were excavated and replaced with a sand/clay subsoil mix (A) and the resulting embankment was then topped off with a new topsoil blend. The steepness of the constructed slope and the presence of the river below required using live fascines (bundles of brush) on the contour with erosion control fabric made of coir (fiber from coconut husk) to provide surface stability (B). Other project objectives for this case study included preparing a foundation where, over time, a natural community of indigenous plant materials for upland and riverine habitat would evolve, thereby improving aesthetics and establishing a long-term, maintenance-free natural slope along the Ottawa River within its highly urbanized context. The success of this project to meet the desired goals enabled Public Works to designate the area as an extension of Jacques Cartier Park (C).
In this more coastal example, the Massachusetts Office of Coastal Zone Management Stormsmart Coasts Program provides guidance to property owners wishing to use bioengineering techniques to stabilize shorelines on their property. The use of a matting material made of coconut fiber called coir logs is used in combination with planting native vegetation. This is described in StormSmart Properties Fact Sheet 4: Bioengineering - Coir Rolls on Coastal Bank [9], a public information brochure provided to the public on the Mass.gov website. In another example, Wilkinson Engineering was employed to stabilize a shoreline on Waquoit Bay in Massachusetts. They used a variety of methods, including changing the angle of slope, installing "coir" matting to stabilize the bank, and establishing fringing marsh at the base of the slope.
Bioengineering Strategies: Reducing Coastal Erosion and Coastal Storm Damage While Minimizing Impacts [10] illustrates the series of steps employed in bioengineering to stabilize a coastal shoreline on Waquoit Bay. Take a look at this richly illustrated document explaining the process.
Geotextiles or geosynthetics have become very popular methods for several streambank stabilization projects, dune stabilization, and generally when earthen stability is required. A geotextile material that can be chosen varies in thickness and porosity and will depend largely on soil properties or whether it is necessary to improve a soil property – for instance, to increase surface soil strength, increase erosion resistance, or stabilize weak soils on steep slopes. For the case study in Grand Isle, LA, the objective was to protect the island from storm waves and storm surge, a function that is typical of a dune system. However, the island is highly exposed to the Gulf of Mexico storm waves that frequently overwash and erode the dune system, with increasing dune rebuilding costs after each storm. To protect against this erosion, coastal engineers employed geotubes, which were filled with native material excavated from the existing storm-damaged dune system. Once put in place, the tubes were covered with a top layer of sand and were vegetated for added soil stability. Over time, wind-blown sand from the beach system accumulates at the seaward side of the dune system and organizes into smaller dunes, where additional vegetation growth takes place and provides additional protection from waves and storm surges approaching the island.
Sand dunes are common features in coastal zones and desert environments. Along the coast, dunes can protect beaches from erosion during storms and supply sand to a beach that is eroding. Dunes also provide habitat for highly specialized plants and animals, including rare and endangered species. Because of threats by both intentional and unintentional human activity and because of the benefits they offer, such as storm protection and sediment cycling between dune and beach environment, many countries such as the Netherlands, United Kingdom, and the United States employ dune protection programs. These include stabilization programs and restoration efforts centered on building or re-building dunes. Protection, stabilization, and restoration methods utilize measures to reduce the transport of sand by wind and water, such as planting vegetation, constructing sand fences, and selecting access areas that avoid damage to dunes and dune vegetation from foot traffic.
It is important to consider dune structure when planting dune vegetation. Dunes are composed of the foredune (the part that faces the ocean), the sand plain (the dune crest or top), and the backdune (the side facing away from the ocean). The micro-environments of these dune components limit the types of plants that can thrive on them. For instance, foredune plants need to be tolerant to salt spray, strong winds, and some burial by wind-blown sand from the berm and beach environment, while sand plain and backdune plants can be less tolerant of these stresses because they are typically protected from salt spray and sand burial.
Managing coastal dunes for use as part of a flood protection and mitigation strategy involves an integrated management approach or plan, which follows closely with some of the principles we introduced for soft shoreline engineering. These are to:
In the Cape Cod area of Massachusetts, erosion caused by winter storms, in particular, results in loss of beach area. And one solution put into practice to address this issue is dune restoration using sand fencing to trap sand and build new dunes. This technique, in concert with planting suitable beach grass species to hold the sand in place, can be a very effective method.
The document "Coastal Dune Protection and Restoration [12]" from Woods Hole Sea Grant and Cape Cod Cooperative Extension provides excellent details and illustrations of the process.
Alternative soft stabilization approaches can offer many benefits over typical hard stabilization structures. Often these approaches are referred to as living shorelines because they offer added ecological benefits. Some of the benefits of soft stabilization approaches include:
While there are many benefits associated with living shorelines, they are not appropriate for all geomorphic environments. Drawbacks for living shorelines include:
Links
[1] http://www.marbef.org/wiki/Detached_breakwaters
[2] https://earthexplorer.usgs.gov/
[3] http://www.coastalwiki.org/wiki/Diffraction
[4] http://earthexplorer.usgs.gov
[5] http://cirp.usace.army.mil/
[6] http://explorebeaches.msi.ucsb.edu/beach-health/coastal-armoring
[7] http://www.dec.ny.gov/docs/permits_ej_operations_pdf/shorestabil.pdf
[8] http://www.dec.ny.gov/docs/permits_ej_operations_pdf/shorelinestable.pdf
[9] https://www.mass.gov/files/documents/2018/05/29/ssp-factsheet-4-coir-rolls-new.pdf
[10] http://www.waquoitbayreserve.org/wp-content/uploads/H_LocalCommunitySite_Shoreline_SWilkinson_3-28-17.pdf
[11] http://www.mvn.usace.army.mil/About/Projects/GrandIsle.aspx
[12] https://www.whoi.edu/fileserver.do?id=87224&pt=2&p=88900
[13] https://commons.wikimedia.org/wiki/File:Spencer_Park.JPG
[14] http://creativecommons.org/licenses/by-sa/3.0/
[15] https://commons.wikimedia.org/wiki/File:Spencer_Park_2.JPG