Nature based solutions for coastal protection

Nature based solutions generally refer to the sustainable management and use of nature for tackling societal challenges such as climate change, water security, food security, human health, and disaster risk management. Natural infrastructures and nature-based measures are increasingly used to reduce impacts of coastal storms and sea level rise to coastal communities. Therefore, these approaches should be routinely considered as viable options by decision-makers. To perform engineering design for these solutions is not easy, given the complexity and randomness of the related processes and the challenges related to predicting the performances of interventions whose behaviours are not easy to understand and model. When designing these approaches it may be useful to adopt a multi step approach, where sequences of interventions are planned along with monitoring of their performances.

Implementation of nature based solutions can be facilitated by developing engineering guidelines that provide functional and structural design guidance as well as report from previous experiences. The value of natural infrastructure and nature-based methods does not rest solely in risk mitigation and protection, as these solutions offer other valuable ecosystem services, which should be incorporated into cost-benefit and environmental impact analyses.

A classification of nature based solutions has been proposed by Eggermont et al. (2015) and is presented in Figure 1, including a wide variety of contexts and goals. We describe here below the nature based solutions that are most frequently applied in coastal protection.

Figure 1. Schematic classification of the Nature Based Solution (NBS), according to Eggermont et al. (2015). By Hilde Eggermont - received from the original article author, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=54260680

Berms can be effectively protected by displacing cobbles above them, to reduce erosion during extreme events. This strategy is also named as "dynamic revetments". The approach is developed by displacing a gravel or cobble beach at the shoreline where protection against erosion is sought. A cobble berm can be seen as an intermediate strategy between a conventional riprap revetment of large stones and a beach nourishment project, which in fact is frequently realized with sediment of larger size with respect to the natural material. The name "dynamic revetment" reflects that the material is expected to move to some extent, therefore resembling the natural behaviours of beaches. This solution is therefore contrasting with the static behaviours of ripraps and coastal infrastructures. An advantage of a dynamic structure is that it does not fail when movement occurs.

These interventions should be designed to look as close as possible to the natural environment and should favour the development of spontaneous ecosystems. A possible obstacle to overcome is that there is a general lack of consensus on the effectiveness of such measures. Hard infrastructures are massive and generally more visible and therefore induce the belief that they are more effective. Cobble berms have been put in place already in the second half of the XXth century in several locations, initially in the form of beach nourishment with sediment of increased size, therefore obtaining a gravel beach. This basic strategy has then evolved into the displacement of a cobble beach for the purpose of erosion mitigation. The construction of cobble beaches turned out to be simple and not particularly expensive and turned out to be particularly appropriate in environments where the natural configuration of the beach is dominated by the presence of boulders and gravel.

The cobble berm may either front directly into the water or be displaced into the back of a sandy beach that is providing inadequate protection from the forces of extreme waves and currents. Such morphologies are common on coasts, so the placement of a cobble berm constitutes a more natural and aesthetic solution than a conventional revetment or seawall. A cobble berm does not protect the shoreline as a conventional structure. Cobbles may be moved by waves, and may be transported alongshore or offshore by extreme waves. Monitoring and maintenance is therefore required. The cobble berm itself may become a hazard if the cobbles are thrown landward during a storm.

The design of cobble berms is mainly based on empirical relationships that were derived through laboratory experiments and the study of natural beaches. The basis for the design is the stability analysis for rock and concrete armour units, which is discussed here. Shields criteria or other empirical formulas can be applied to check in what conditions armour units may be moved by waves. The results of the experimental and field studies have shown that cobble beaches are indeed a highly dynamic environment, especially when a sandy beach is present seaward. In these conditions there is a continuous interaction between sand and cobbles, with the sand portion typically accreting during summer therefore partially submerging the cobbles. During the winter season, the stronger action of waves induces erosion of sand and therefore cobbles emerge to light and the slope of the beach increases. Cobble beaches may be coupled with artificial dunes.

Strengths of cobble beaches are (Dare, 2003):

• Placement and construction are simple and less expensive than traditional infrastructures;
• They appear as natural environments;
• They are Generally less expensive;
• They are flexible under the attack of waves, and therefore do not fail as static structures do;
• Small gravel or cobbles are less of an obstacle to beach access than large armor stone;
• They may become buried by sand during the summer.

Dare (2003) also mentions the following potential weaknesses for cobble beaches:

• They provides less protection than a revetment or seawall;
• They require frequent maintenance as material can be expected to move;
• Cobbles may be thrown landward during a storm, therefore originating a potential risk;
• Cobbles and gravel do not provide the same recreational opportunities as a sand beach.

Artificial dunes may be created by using sand-filled geotextile containers that are displaced in the back of the berm, to mimic the natural protection of land against sea storms (Dare, 2003). This type of solution is interesting because it looks natural and is relatively inexpensive with respect to infrastructures.

Sand dunes protect the land from storm surges and waves. They may also supply sand to the beach. Although sand dunes are naturally created by wind, waves and tides play an important role in shaping the dunes. Like most coastal features, sand dunes are naturally dynamic and the extent of dune development depends on the fronting beach profile and sediment type. Dunes tend to migrate landward in response to sea level rise, but are unable to do so when backed by developments and infrastructures. Accordingly, the extent of dune development largely depends upon human impact. In some locations dunes may have been removed for development, therefore disabling the buffering role they play. In addition, construction and human foot traffic result in loss of vegetation, reducing a dune's ability to trap wind-blown sand that would naturally replenish the sand that is eroded by wave activity. Construction of artificial dunes requires a lot of sand that may be difficult to collect.

When designing artificial dunes an important design variable is the height of the dune crest. A wave overtopping the dune is a reason of concern as it may originate erosion and flooding of the land behind the dune with impact on properties and vegetation. Therefore extreme wave analysis is the basis for the design of an artificial dune. The return period of the design wave is selected basing on the value of the land and the properties.

Dare (2003) lists the following weaknesses for artificial dunes:

• Geocontainers resistance to punctures and abrasion is low;
• Geocontainers may be vandalized;
• Geocontainers can be weakened by ultraviolet radiation from sunlight;
• If fill material consolidates over time, the height of the geocontainers will decrease;
• A geocontainer may twist or roll to one side while filling, leaving the filling port on the side;
• A geocontainer cannot be expected to reach more than 5 feet in height.

An application of cobble berm and artificial dune is presented by Komar and Allan (2010), which is available here.

Beach nourishment, also called beach replenishment, consists in the artificial replacement of sediment (usually sand) that was eroded. Beach nourishment is in most cases integrated into a coastal defense scheme, where it is typically coupled with hard infrastructures. In fact, nourishment alone does not remove the physical forces that cause erosion. Therefore, to avoid repetition of the intervention on a regular basis the causes for erosion may be mitigated by using other type of interventions.

The most relevant issues for beach nourishment are related to the selection of the sediment size and the original location of the material. While in the past there was a preference for material of increased size with respect to the natural configuration of the beach, recent findings and experiences show that the material should closely match the pristine conditions. Excess silt and clay fraction (mud) versus the natural turbidity in the nourishment area disqualifies some materials. Nourishment sand that is only slightly smaller than native sand can result in significantly narrower equilibrated dry beach widths compared to sand the same size as (or larger than) native sand. Furthermore, an unmatched grain size may imply a reduced touristic appeal.

With regard to the location where the material is taken, possible alternatives are offshore areas, locations close to inlets, accretionary beaches, riverine areas, lagoons, or uplands, which usually is the easiest location to obtain permits.

Cunniff and Schwartz (2015) discuss the following strengths for beach nourishment:

• Reduces erosion, flooding, and wave attack;
• May favour the development of an evolution of beaches that is close to natural, especially if the sand fill areas are located updrift with respect to the beaches of interest. In fact, beachfill might protect not only the beach where it is placed, but also downdrift stretches by providing an updrift point source of sand;
• Coastal risk reduction projects can be designed to provide increased ecological value.

Cunniff and Schwartz (2015) also discuss the following weaknesses for beach nourishment:

• It may requires periodic to continual sand resources for renourishment, especially if it is not coupled with hard infrastructures;
• It can be eroded by extreme events surge and waves; no high water protection;
• It has possible impacts to regional sediment transport;
• It can lead to removal of large volumes of offshore sand;
• Even though beach nourishment is generally considered as an environment-friendly option for coastal protection and beach restoration, sizeable impacts on several beach ecosystem components may occur;
• There might be environmental impacts where the sand is taken from;
• It can lead to steeper beach profiles, which can increase wave energy on the beach, therefore increasing beachside erosion.

As we already know, beaches are a dynamic environment whose evolution includes a seasonal component. In particular, sand beaches may be subjected to erosion during winter, as a consequence of stronger wave action, and accretion during summer when mild conditions prevail. Several beaches, including those along the Adriatic coast in Italy, may be eroded during winter while during summer mild conditions prevail therefore favoring accretion and the formation of the berm.

Beach scraping manipulates the beach profile by removing accreted material from the lower part of the beach and transferring it to the upper part of the beach or to the dune toe, where it may better serve coastal protection during strong wave action (Dare, 2003). The material may be in surplus during the summer season and may therefore be transported landwards at the closure of the touristic season, to save sand from erosion and to protect the dune. The technique is also thought to encourage additional sand to accrete on the lower beach, which can then be scraped in the future, leading to a net gain of sediment on the beach. Beach scraping is easy to apply and is relatively inexpensive.

Research appears to indicate that the techniques used to scrape the beach may ultimately determine its degree of success. One major criticism is that beach scraping adversely steepens the beach profile, making subsequent erosion more likely and severe especially if a very extreme event occurs. A further concern is that beach scraping will have an adverse effect on the natural nourishment of downdrift beaches (Dare, 2003). Another issue is the ecological impact of this disturbance to the beach. The sand beach ecosystem is characterized by dense populations of burrowing macro-invertebrates that serve as the prey base for shorebirds and commercially important surf fishes (Dare, 2003).

Strengths of beach scraping are (Dare, 2003):

• Widens the dry beach for recreational use;
• Increased beach width provides improved temporary coastal protection;
• Reduced aesthetical impact;
• Natural and compatible sediment supply;
• May not inhibit the natural cycles of the coast;
• Increased defense without the expense of importing volumes of sand.

Dare (2003) mention the following potential weaknesses for beach scraping:

• Serves only a temporary solution that may need to be repeated frequently;
• Temporarily interrupts sediment supply which could result in downdrift erosion;
• Least effective in front of sea walls, where protection may be most needed;
• Modification or destruction of habitat;
• Offshore borrow areas may increase erosion rates;
• Size of borrow area may adversely steepen beach profile.

Vegetation recovery is an effective measure for coastal protection due to its beneficial effects on sand stability. Vegetation may also trap sand transported by wind therefore favouring the accretion of dunes.

There are generally three zones of vegetation that form on coastal dunes. Each of these zones is exposed to different levels of soil salinity which determines the types of plant species that grow within each zone. The foredune, the angled side which faces the ocean, is the part that is much exposed to salinity. It may host several grasses and other herbaceous plants that are able to tolerate high exposures to salt spray, strong winds, and burial by blowing/accumulating sand. Typical vegetation includes Ammophila arenaria, Honckenya peploides, Cakile maritima, and Spartina coarctata.

Landward with respect to the sand plain at the top of the dune, which may or may not be present, we find the backdune, the angled side that faces away from the ocean. Plants which thrive on the broad dune plain and backdune grow together into dense patches termed dune mats that hold the dune together. Vegetation typical of the plain and backdune include Hudsonia tomentosa, Spartina patens, Iva imbricata, and Eregeron glaucus. Introduced species can out compete native plants and disrupt animal life, making them formally "invasive species". These species generally have lower salt tolerance. Farthest from the ocean is the maritime forest zone, which supports pines and hardwoods.

Recovery of vegetation may be obtained by planting selected species, with preference for autochthonous ones. Growing vegetation needs to be fertilized and protected by strong weather and disturbances caused by wildlife and other animals (see Figures 1 and 2).

Figure 2. Vegetation recovery at Spencer Park (Michigan). By Emma Kelland - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15058331

Figure 3. Protecting plants with chicken wires at Spencer Park (Michigan). By Emma Kelland - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15058531

In the back dune it is important to recovery vegetation by mimicking the natural evolution. First, herbaceous plants should be installed. After they have rooted and developed fully, a second stage, the "shrub stage", can begin. During this phase, larger plants with deeper root systems can be planted. Examples are Empetrum nigrum, Ilex vomitoria, and Vaccinium ovatum. The shrub stage is usually the final phase in the back dune and may last for short or long periods of time depending on microclimatic conditions such as distance from the shoreline, availability of groundwater, or salt spray effects.

An important role is played by water. In coastal dunes, ocean water may enter soils via salt spray through the surface or by ocean water intrusion into deeper vadose layers (Greaver, 2005). The interaction between freshwater and salt water in coastal dunes is still poorly understood. Fresh water may come into the dune from the land or the top of the dune that collects rainfall. Eco-hydrological models attempt to study the distribution of water in coastal areas. Inland groundwater pumping, climate change and sea level rise may change the form of the habitat in coastal areas.

Finally, vegetation is highly impacted by human activities. Human access and walking over the vegetation may threaten ecosystems that are already very fragile. Interdiction to access may be a necessary measure.

Cunniff, S., & Schwartz, A. (2015). Performance of natural infrastructure and nature-based measures as coastal risk reduction features. Environmental Defense Fund.
Dare, J.L. (2003), Alternative Shore Protection Strategies: Innovative Options and Management Issues, Project Report Submitted To Marine Resource Management Program College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis.
Eggermont, H., E. Balian, J. M.N. Azevedo, V. Beumer, T. Brodin, J. Claudet, B. Fady, M. Grube, H. Keune, P. Lamarque, K. Reuter, M. Smitt, C. Van Ham, W.W. Weisser, X. Le Roux. 2015. Nature-based solutions: New influence for Environmental Management and Research in Europe. GAIA Ecological Perspectives 24/4: 243-248.
Greaver, Tara L., "Eco-hydrology and physiological water relations of vegetation along coastal dune ecotones on subtropical islands" (2005). Dissertations from ProQuest. 2309. http://scholarlyrepository.miami.edu/dissertations/2309
Komar, P. D., & Allan, J. C. (2010). “Design with Nature” Strategies for Shore Protection: The Construction of a Cobble Berm and Artificial Dune in an Oregon State Park. Shipman, H., Dethier, MN, Gelfenbaum, G., Fresh, KL, and Dinicola, RS, eds, 117-126.

Last modified on May 2nd, 2017