Seawalls: Allowing humans to build closer to water, but altering processes along shorelines.

By Abby Tinari, SRC intern

Participating in the Shark Research and Conservation Program’s Urban Shark Project, I have spent a decent amount of time on the water throughout downtown Miami. In this time, I have noticed the concrete shoreline that shapes Miami’s shores. Of course, there are sandy beaches, but much of the barrier between land and water is a hardened seawall. It made sense, considering 20 feet from the seawall is a high-rise building, that needs the support and the seawall provides. This realization, at how much humans have changed and destroyed the coastlines, intrigued me. I then started to wonder how has shoreline hardening and urbanization effected the marine environment?
Coastal cities around the world are hardening their coasts. As the desire to be along the water increases there is a higher pressure to create structures that support buildings. These hard structures replace the natural shoreline, creating a barrier between land and water. Depending on the area and the infrastructural need, a variety of structures can be used. An alternative to natural shorelines, including seawalls, are used to provide stability for sediment and infrastructure built upon the sediment through erosion prevention (Gittman, Scyphers, Smith, Neylan, & Grabowski, 2016). Seawalls are also used as protection from wave energy and storm surges. Seawalls allow humans to complete their desire of living along the coast when they otherwise would have to move inland when the land erodes away. If designed properly, they can be used to prevent sea level rise in a low lying area. The vertical nature of seawalls occupy considerably less space than other shoreline armoring structures, making it an ideal structure in cities and areas where space is limited. Seawalls can last over 30 years in saline environments if designed and maintained properly.

Example of engineered-shore structures: seawall (Gittman et al., 2016)

Although seawalls are important coastal structures they produce adverse effects on the coastal environment. Removing the natural shoreline disturbs and changes longshore currents, wreaking havoc on properties and the shoreline farther along the current. Seawalls remove areas where erosion and deposition would otherwise take place. The sediment in water is deposited somewhere else, altering sediment transport and causing sediment starvation. Sediment starvation usually occurs at the end of a seawall where water velocity speeds up, eroding sediment. Because there is a lack of sediment transferred along the wall the sediment eroded at the end does not have anything to replace it, creating a “starved” area. Erosion still occurs around seawalls, just not along the shoreline. It is prevalent at the base in front of the wall and can weaken the structure due to the change in forces acting on the wall (Figure 2). Concrete does not absorb and reflect energy as a beach or rocky shoreline would. Much of the wave energy is reflected, creating potential boating hazards. These are just the physical process seawalls change.

Erosion occurring at the base of the seawall. Scour erodes sediment that leads to an unstable structure and possible collapse. (ctmirror.org)

How are seawalls affecting organisms that were once present in urban environments? Are they changing biodiversity? One obvious effect seawalls have is they remove natural habitat, reducing the amount of shoreline conducive for organisms. Plants and animals are unable to travel between terrestrial and marine environments reducing the connectivity between the two communities. In the past, as sea level rises, ecosystems have been able to adjust, slowly moving inland. With a concrete barrier, inland migration is extremely difficult, if not impossible for plants and animals. This could lead to a loss of nursery and foraging grounds for birds and fish (Bulleri & Chapman, 2010). These vertical structures reduce the size of the intertidal zone, crowding intertidal species into smaller areas. This zone is the bridge for energy exchange between marine and terrestrial environments (Sobocinski, Cordell, & Simenstad, 2010). Abundances and assemblages of organisms are changing due to the difference in substrate. The lack of crevices and protection from predators and wave energy reduce the likelihood of larval survival (Bulleri & Chapman, 2010). Many of the species sea walls directly affect are sessile, meaning they are immobile. If larvae are not able to survive on the seawall then adults will not be able to either. The lack of native organisms has the potential for exotic species to flourish. In certain situations seawalls do benefit the marine community. Seawalls in Sydney Harbor, Australia provide shading, which has led to an increase in species diversity (Bulleri & Chapman, 2010). Bulleri and Gittman through literature searches found that seawalls do significantly decrease the biodiversity of abundance of species when compared to natural shorelines.
Seawalls are essential to coastal urban infrastructure, but how can we reduce the impact they have on the ecosystem while still having the same structural benefit? In areas where space is available living seawalls can be an excellent alternative. Living seawalls are typically layered or terraced. Each layer provides habitat for different organisms. These work bridge the connectivity gap between terrestrial and marine environments. Fake mangrove panels are a new alternative for South Florida’s sea walls. The panels are concrete but mimic red mangroves, an important habitat for many of South Florida’s marine organisms. University of Kansas assistant professor, Keith Van de Reit, designed these panels which, so far, are proving to be successful in creating habitat for organisms. Riprap, while not providing the same strength as a sea wall can increase the biological activity of the structure. If the stability cannot be compromised, walls such as one in Sydney Harbor (Figure 3), can be useful to mimic a rocky tidal pool environment. The openings in the walls are placed so they are flooded at high tides but retain water at low tides. This provides an environment for rocky tidal species, a marine environment common in Sydney.

(a) Intertidal ‘rock-pools’ constructed in the vertical dace of a seawall in Sydney Harbor (Australia). These features of habitat were introduced to seawalls to mitigate efforts of loss or degradation of rocky platforms on intertidal biodiversity. (b) Details of a rock-pool retaining water during low tide. (Bulleri & Chapman, 2010)

Although there are some studies on the biological impact of seawalls, more research needs to be done, especially with the rising coastal population. Other alternatives, like the mangrove panels, should be researched. A healthier and more ecologically connected water way helps human in numerous ways. It would increase water quality and create habitat. It would also provide for more tourism and an economic benefit for cities.

Works Cited
Bulleri, F., & Chapman, M. G. (2010). The introduction of coastal infrastructure as a driver of change in marine environments. Journal of Applied Ecology, 47(1), 26-35. doi:10.1111/j.1365-2664.2009.01751.x
Gittman, R. K., Scyphers, S. B., Smith, C. S., Neylan, I. P., & Grabowski, J. H. (2016). Ecological Consequences of Shoreline Hardening: A Meta-Analysis. Bioscience, 66(9), 763-773. doi:10.1093/biosci/biw091
Sobocinski, K. L., Cordell, J. R., & Simenstad, C. A. (2010). Effects of Shoreline Modifications on Supratidal Macroinvertebrate Fauna on Puget Sound, Washington Beaches. Estuaries and Coasts, 33(3), 699-711. doi:10.1007/s12237-009-9262-9
Spiegel, Jan Ellen. “Connecticut’s Trouble with Seawalls.” The CT Mirror. N.p., 17 Feb. 2014. Web. 24 Mar. 2017

By Kevin Reagan, SRC Intern

As the ocean becomes more of a “hot topic” in the media and public forums, planners and designers of marine reserves must increasingly factor in socio-economic factors alongside biological principles when discussing policy. The human aspect that is usually responsible for the initial need for conservation and things like no-take marine reserves is undeniable, and as populations continue to increase, the negative impact many human activities have on marine environments does as well. These effects are particularly strong in coastal cities or estuarine flows, where a river flows into the ocean. However, this may not necessarily mean that marine reserves would not be beneficial in these areas though. In his 2015 study, Chantal M. Huijbers and colleagues analyzed whether being located close to major influences like cities on the coast affected the efficacy of marine reserves in enhancing the number of organisms that were present, relative to other areas outside of the reserve.

Diagram representing the 3 major categories of determinants expected to influence the performance of marine reserves.

Marine reserves are usually placed in deeper, offshore locations that have little to no commercial value. These areas are generally remote, and partly chosen because they offer the least chance of the reserve interfering with human activities such as fishing. Though this leaves areas near coastal cities poorly protected, these decisions are made because of impacts like habitat degradation and fishery overexploitation that occur in coastal areas that increase pressure on marine ecosystems. Native marine diversity has been shown to decrease with increasing human population density, and coastal areas generally have greater levels of biodiversity than offshore areas; marine reserves can play a crucial role in protecting this diversity, but both positive and negative impacts have been seen. There is, though, a chance that the extent of positive effects from things like fishing restraints is overshadowed by negative effects from sources outside of the boundary of the reserve.

Edgar et al. demonstrated that for a marine reserve to be effective, it must be four out of these five things:

No-take zone
Large
Have high levels of compliance with regulations,
Have been protected for a long period of time
Separated from fished areas by geographic boundaries or channels

These factors demonstrate the importance of human-related factors in planning future marine reserves, and intuitively it would seem that the best framework for planning marine reserves is the current one that delegates them to remote offshore locations. But, after compiling a database of peer-reviewed studies that reported the biological effects of marine reserves and examining 150 articles published between 1977-2012 and included 113 different reserves, Huijbers found that there was a greater abundance of organisms in marine reserves compared to control areas regardless of the reserves proximity to coastal influence. Essentially, the reserves close to urban centers were indistinguishable from those that were more remote.

Example of what a healthy, protected reef can look like. Flynn’s Reef, Phillips Island, Australia, part of the Great Barrier Reef Marine Park.

Huijber et al. only included studies that measure fish, invertebrate, and algal abundance in formally designated reserves that were also fully protect no-take areas, and each study included one or multiple sites that were both in and outside the reserve, or before/after the reserve was established. After using statistical analysis to examine the relationship between eleven predictor variables that measure the effect of coastal influence and marine reserves, they showed that coastal marine reserves were equally as effective as less-developed offshore locations. Though variations in the methodologies of the studies included in the analysis may affect the strength of the findings, the data examined and presented is compelling and at the very least warrants further research on the subject.

For any of these policies to work, efficient enforcement of rules and regulations is paramount. Planners cannot assume that people will comply with regulations out of the goodness of their hearts. Enforcement is key and studies have shown that there is a greater effect of reserves on ecosystem health and biomass where there is less encroachment. That being said, placing marine reserves near coastal areas will also raise awareness of biodiversity and can potentially increase the success of marine reserves. Coastal marine reserves can provide the crucial link between the social and ecological environments in these areas and end up fostering a sense of admiration and appreciation for what the ocean has to offer for generations to come.

Huijbers, Chantal M., et al. “Conservation benefits of marine reserves are undiminished near coastal rivers and cities.” Conservation Letters (2014).

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Seawalls: Allowing humans to build closer to water, but altering processes along shorelines.

Marine Reserves Still Beneficial for Conservation Near Coastal Rivers and Cities