In its most recent report, the United Nations Intergovernmental Panel on Climate Change (IPCC) continued to emphasize the speed with which global sea levels are rising, contributing to coastal flooding and erosion. High sea levels that historically have occurred once-a-century may become once-a-year by 2100. The US is no exception to this trend. The map below, a screenshot from NOAA’s interactive Sea Level Trend viewer, reveals quickly to the eye that sea levels are rising, at varying speeds, on all of our coasts.

As fellow SMEA student Leah Huff and I discussed in our summer article, coastal zones face rising sea levels combined with stronger storms. These hazards are exacerbated by climate change and the longstanding removal of coastal wetland buffers against inland weather impacts. The result: increased flooding in major US coastal cities, from San Francisco to New York.

An old technique (and I’m talking 7,000 years old) to protect coastal areas from flooding and erosion has been the seawall. Seawalls are walls built along the shore to hold back water. Their construction varies, but they are now often large, heavy structures made of concrete, which creates its own set of issues. For example, while these structures protect the land and settlements directly behind them, any shoreline between them and the sea, such as a beach, continues to erode. Water reflects off of the wall and scours, or digs into, the seafloor before it. Seawalls can also increase flooding in other areas not protected by the wall, as the re-routed water flows along the coast. Check out some great graphics of this process here from the Michigan Natural Shoreline Project.

From an ecological perspective, beach erosion associated with seawalls removes coastal sandy bottom habitat. Seawalls often cannot become habitat themselves, as most marine life cannot attach itself to the smooth concrete surface. This shortcoming has drawn the interest of researchers worldwide. Scientists from Singapore to Washington are asking: how can we make these concrete seawalls work as habitat, creating an artificial rocky reef?

A walk along the Ballard Locks gives a hint as to how this is being accomplished. Look down into the Locks as they drain, and you can see algae growing in the rectangular depressions in the walls. On the seaward side of the smaller set of locks, at the moment you can also spot (what I believe are) barnacles growing on the gridding. Even where they are meant to be smooth, the walls of the Locks have also developed enough texture over the last century that barnacles need to be cleaned off the walls yearly; this prevents them from scraping the skin of migrating salmon. In this case, the growth of barnacles is not an intentional addition to the Ballard Locks, but a nuisance. When marine life grows on artificial surfaces, such as boats, buoys, and locks, and impedes their function, this is called biofouling. Such biofouling does not usually represent a diverse ecosystem, but rather a few hardy adaptor species.

This is where intentionality comes into play: how can complexity and depth of surface, which allow barnacles to colonize the walls of the Locks, be used to nurture healthy habitats? Intentionality in building artificial habitat is not a new concept. One term to describe this intentionality is ecological engineering, when structures are designed to provide function both for human and non-human life. In fact, the Locks themselves provide an example of a different kind of ecological engineering: the fish ladder. Over a hundred years ago, engineers modified the structure of the Locks to create better habitat for migrating salmon, allowing them to pass through the Locks on their way upstream to spawn.

By adding habitat to seawalls, researchers like Jason Toft of the University of Washington Wetland Ecosystem Team are not just adding a key habitat feature for a single species, but rather building an ecosystem for a diverse set of organisms. As seen in the photo below, Wetland Ecosystem Team’s seawall in downtown Seattle, completed in 2015, has a complex surface. In line with the Ballard fish ladder, this feat of ecological engineering also provides habitat for migrating salmon, but this time in the other direction. Juvenile salmon heading for the ocean can use the seawall for both protection and feeding, as the structure creates habitat not only for the salmon, but also for other species. The seawall provides function both for the habitat and for the infrastructure, protecting the popular downtown waterfront from erosion while creating rocky marine habitat in the heavily urbanized area. Check it out for yourself – a good viewing point at low tide is Pier 62.

Beyond the US, researchers in the UK, Singapore, and Australia are further experimenting with different shapes and surfaces to add to seawalls. These range from flowerpots to honeycomb-like medallions. Almost all have been observed to increase the species richness of marine life that can survive on a seawall. These strategies are able to borrow from adjacent fields in marine conservation, such as oyster restoration. Restoration scientists and nonprofits already use oyster reef balls, for example, hollow concrete domes with holes that increase the surface area for settlement of oyster spat. Techniques to boost oyster restoration are themselves part of the larger restoration field of artificial reefs. Some fun further examples in this field include reefs made of retired subway cars, or even accidental reef ecosystems that form around offshore oil rigs.

With such a range of successful strategies, increasing the habitat of a seawall need not be prohibitively expensive, and can be tailored to different locations. The Seattle seawall habitat modifications only required a 2% increase in the overall cost of the seawall, which was already in need of replacement. This low cost is critical because sea level rise and strengthening storms due to climate change will only exacerbate coastal flooding and erosion, as the most recent IPCC report warned. Choosing to hold off the floods, be it through parks, seawalls, or other design strategies, rather than moving our settlements back from the water, is going to be expensive. Yet by adding these relatively low-cost modifications to seawalls, shoreline armoring has the potential to integrate better into the local ecosystem. Some species may even increase the longevity of the infrastructure itself; for example, barnacles seem to reduce rates of deterioration in concrete they colonize.

A notable caveat is that even with ecological engineering modifications, protective artificial structures such as seawalls will not return the habitat to its former state. Particularly when it is replacing a sandy bottom habitat, an ecologically engineered seawall will mimic a different habitat, a rocky reef, and will not perfectly mimic it at that. Nevertheless, an ecologically engineered seawall has the potential to alleviate certain issues that can occur with traditional seawalls. For example, already, without ecological engineering, New York’s Department of Environmental Conservation recommends that a seawall slope down to meet the natural contours of the seafloor, rather than abruptly stopping the waterflow and exacerbating the scouring and flooding problems mentioned earlier. This gradual extension can help marine life as well by increasing shallow water habitat. If you looked underwater, you could see an elevated seafloor implemented at the Seattle seawall.

The settler colonial mindset that we can be separate from natural forces has resulted in climate change and widespread habitat degradation. Integrated re-thinking of urban waterfronts as valuable habitat is an important perspective and strategy to bring to the fore. And a final thought: public engagement is critical to this work. Moving “towards an urban marine ecology” should not be restricted to the realm of scientists, but rather must be done with the participation of the human inhabitants of that urban ecosystem, be that through educational outreach, or even seawall design collaboration.

For access to journal articles cited in this piece that are not publicly available, contact Currents’ Editor-in-Chief:

Coombes, M. A., Viles, H. A., Naylor, L. A., & La Marca, E. C. (2017). Cool barnacles: Do common biogenic structures enhance or retard rates of deterioration of intertidal rocks and concrete? Science of The Total Environment, 580, 1034–1045. https://doi.org/10.1016/j.scitotenv.2016.12.058

Sawyer, A. C., Toft, J. D., & Cordell, J. R. (2020). Seawall as salmon habitat: Eco-engineering improves the distribution and foraging of juvenile Pacific salmon. Ecological Engineering, 151, 105856. https://doi.org/10.1016/j.ecoleng.2020.105856