By Christian Flinn
The concept of natural barriers and nature-based defense is highly appealing to coastal resilience researchers. Natural resilience implies a degree of sustainability that may not be achievable with man-made protection alone. This resonates well with communities concerned with preserving their natural beauty while not compromising their safety. What, then, are the advantages of nature-based defenses against sea-level rise? What costs do they have? How well can they actually protect our communities? Answering these questions requires analyzing the different options available to various topographies and climate zones and drawing conclusions from current test sites.
One study published in the scientific journal, PLOS ONE, allows us to do just that. The study measures the effectiveness and viability of nature-based coastal protection by analyzing two main functions: (1) the protection’s comparative effect on wave-height and strength and (2) its cost vis-à-vis more traditional – or artificial – methods. Before delving further into this discussion, however, it is key to understand how nature-based coastal protection is defined. While taking many forms, some of the most popular iterations of natural coastal protection are coastal habitats, often including, but not limited to mangroves, coral reefs, oyster reefs, wetlands, salt marshes, and seagrass or kelp beds. In recent years, a greater understanding of the dangers of sea-level rise has prompted the crucial need to comprehend their value to and role in coastal defense.
By geographic incidence, coastal habitats are at the frontline of any hurricane, monsoon, tropical storm, or tidal wave. Similar to beaches, these habitats often have a mitigating effect on the strength of waves and tides and can help greatly diminish damages to life and property on the shore. The study details formulas and observational data that corroborate the effectiveness of each different type of habitat in accomplishing this. In order to illustrate the value of conservation and restoration efforts, the following paragraphs will discuss two of the habitats mentioned above: mangroves and coral reefs.
According to the data collected in the study, mangroves are one of the cheapest, if less effective with regards to wave-height reduction, mitigating habitats. Reefs and salt-marshes are the most effective depending on certain environmental factors. The low-cost of preservation and restoration efforts associated with mangroves, however, merits further scrutiny. The average potency of wave height reduction among different habitats lies somewhere between 30 and 70 percent. As mentioned above, coral reefs are the most effective (70%) followed by salt marshes (72% on average), mangroves (31%), and seagrass or kelp beds (35% on average). These measurements vary by climate zone. A particular habitat’s position in this hierarchy is indicative of the lower or higher levels of variability in their effectiveness at mitigating against wave height and other storm surge related factors. Mangroves – aquatic trees that grow in areas with low-oxygen soil – grow only in tropical climates and have roots that grow above the level of the water, allowing the trees to absorb excess water and build up sediment to prevent erosion. Generally, they are more effective at flood mitigation than wave height reduction, but their contribution to coastal defense is nevertheless significant.
The reasons for this again depend on the topography of the area in question. Mangroves usually grow in more sheltered coastlines – allowing them to expand rather extensively. Their contribution to securing the coastline is thus constantly increasing and, due to their natural development, can readily adapt to changes in sea-level. This reality, coupled with the fact that mangrove restoration efforts can cost three to five times less than a traditional breakwater (manmade aquatic barrier), create a strong case for their inclusion and expansion along more sheltered coastlines.
With regards to another natural defense, coral reefs, analyzing the benefits of restoration programs is important because reefs come with their own set of advantages and complications. While reefs are the most consistently effective natural defense in terms of wave height reduction, they are also the costliest to restore and maintain. Their effectiveness as nature-based defenses is also subject to a number of factors, such as: reef width, reef depth relative to wave height, and reef width relative to average wavelength. Other factors may relate to the type of reef in question: fringing (grows directly seaward from the shore), atoll (circular in shape, usually limited to submerged volcanic areas), and barrier (bordering a shoreline but separated from land by an expanse of water). In general, “the most effective reefs are at least twice as wide as the wave-length, and located at depths that are at most, half the incoming wave height.” Most efforts involving reefs are restoration-oriented rather than coastal defense based but it could be prudent to begin combining those two efforts in order to promote a healthier and more sustainable coastline.
The inference above, while supported by observational data that describes coral reefs as functioning similar to breakwaters in terms of wave reduction and tidal protection, is not conclusively backed by cost estimates. Essentially, it is unclear how cost-competitive reef restoration efforts are with artificial protection projects. Concluding that natural reefs and breakwaters are thus equal in terms of value, however, obviates several key factors about the importance of reefs to their surrounding ecosystems as well as their value to tourism and the health of the ocean in a given area. Such factors call for a more comprehensive analysis of cost that takes into account positive and negative externalities associated with nature-based defenses and artificial ones.
Ultimately, the coast is a complex and diverse living ecosystem that varies greatly from climate zone to climate zone. What is effective at defending one area may not be as effective elsewhere. While such considerations are important, they should not be employed to rule out nature-based defenses as ineffective in any given geographic area. Instead, they should be indicative of the need for greater research into that coastal region’s needs and capabilities and what mix of artificial and natural defenses will prove most effective. Restoring the environment will almost invariably have some level of positive outcome for the health of the coast and if we as a species can benefit as well that is all the more reason to continue looking into the advantages of nature-based defenses against sea-level rise.
For more information, please contact Christian Flinn at Christian.Flinn@warwickconsultants.net.