The reason I chose Australia to study seawalls was because of the studies the Centre for Research on Ecological Impacts of Coastal Cities has done, specifically in Sydney. I met with Professor Ross Coleman who gave me some great info on their studies:
The Centre for Research on Ecological Impacts of Coastal Cities is doing the necessary research to improve our understanding of the ecological processes affected by coastal changes so that we can make better progress in our protection and use of coastal habitats. Their team includes some of the world’s leading researchers in various related fields. The Centre is able to do the research to integrate ideas and models across habitats, to understand the processes of re-colonization after disturbances, to interpret the way populations persist when they are disturbed and to predict the patterns of occupation of new habitats when they are built.
In the New South Wales system, the natural urban coast consists of rocky ledges backed by vertical slopes. The study done by the University under the leadership of Prof Gee Chapman wanted to focus on what would happen with sea level rise when the habitat would have a sudden switch from horizontal to a vertical habitat (e.g. reduction in area etc). What they noticed was that what went on on rocky shores was radically different from what went on on vertical concrete seawalls, which led to engineering for biodiversity. The scientific process makes certain predictions: if I change the habitat (or structural complexity) I predict that the numbers of these organisms will increase compared to places that don’t have the habitat changed.
McMahon’s Point Seawall
An extensive seawall at McMahon’s Point was in disrepair and needing to be rebuilt as part of North Sydney Council’s program of repairing and redesigning many of their seawalls. The original seawall was made of sandstone blocks, but, in order to provide adequate support and resistance to wave-action in the long term, a precast concrete seawall was constructed. It was then faced with large sandstone blocks to resemble the original wall. Blocks at different tidal elevation were removed to provide tide pools to increase habitat.
At McMahons Point they built an old seawall (vertical) and a new seawall (vertical with tide pools) side by side so you could look at the relative abundance of organisms. The Sydney tidal floor range is under two meters, making all the seawall habitats be shallow. This points out a large difference between Sydney and Seattle, as the sea floor along the seawall in Seattle is as deep as 30 feet in some places. I asked Prof Coleman if the depth of the sea floor would affect the outcome. Prof Coleman replied that anything that increases habitat will increase biodiversity. What they don’t know is how the relative scale at which you manipulate the habitat will affect the scale at which the system responds. For example, is it good to build a bunch of big holes, or do you need lots of little ones? That sort of ratio is largely dictated by the system that you are in. All the variables for the specific habitat will come into play and it is hard to predict the actual scale of habitat change without doing tests in the actual system itself. The things that control abundance are usually very contingent on where you are. He concluded that you can’t predict the exact percentage of biodiversity increase but if you do change the seawall to have nooks and crannies, you will see some increase in habitat. It largely depends on what the management goals are.
At the McMahons Point seawall the biodiversity part is not structural. All the structural work is done by the reinforced vertical concrete wall behind the sandstone seawall panels along the front, which is similar to the studies being done by the UW on the Alaskan Way seawall. One thing is for sure: If you increase structural complexity you increase biodiversity. But they don’t know by how much and by what. The actual raw data of the % of habitat increase at McMahons will not be released until the publication of Gee Chapman’s seawall study. Until the scientific publication goes through, the quantitative data will not be released.
The tide pools are more like caves rather than rock pools so you get some animals that you would find in rock/tide pools, but since they don’t get full sunlight, you get other animals as well because it tends to be more sponge dominated. The alternative is to have things sticking out instead of receding, and what they are studying at the moment is bolting flower pots on the side of a vertical seawall. Various councils have the obligation under federal and state laws that they have to increase the biodiversity of their area, so if you don’t want to knock a seawall down but you want to increase the biodiversity, you can attach these flower pots to create tide pools.
A couple of facts about the Sydney Harbour:
-The tidal floor range is under two meters, making all the seawall habitats be shallow.
-There is little or no commercial fishing in the harbour because of contaminated surroundings.
-There is no migratory salmon along McMahons seawall, or Sydney Harbour
-There is very low tidal action, just boat wash.
-Councils (or cities) have state regulations that require to increase biodiversity if rebuilding a seawall.
Here is Ross Coleman speaking about seawalls:
They’ve also studied how a fish is attracted by light and not by eyesight. In Seattle, this means that when the migratory salmon swims north out of the Duwamish River, it will swim around the piers to be near light. This exposes them to predators. Maybe there can be something incorporated at the joint between the seawall and the pier. Perhaps a metal grid so light can travel downwards or perhaps artificial light to attract the fish as they travel. This could also provide a visual connection for the human to the water.
As people we tend to homogenize environments and make them cleaner and sleeker, when in fact biodiversity is accomplished by complicated structures (nooks and crannies). A system where the estuary would have normally had rocky shores, sandy beaches, muddy coves, tree material… that all accounts as structure and accounts as habitat for lots of organisms. So imagine if you replace all of that with a vertical concrete wall – you destroy all the complicated structure that creates life.
The Uni did research on artificial reefs and found there is no detectable difference between an artificial versus a natural habitat. You technically could get cleaned up rubbish and it would produce the same habitat as using clean stone and expensive materials. Creating topographical diversity, from large to small holes, even receding the grout between the sandstone blocks about 3 mm, can create different surfaces for habitat. There is benefits in creating both caves and stick outs. But, invasive species occur more on artificial habitat. So you may increase biodiversity but some of those species will be non natives. You have a higher probability of non natives than native species. There are issues on what you term biodiversity and it might be a double edged sword.
One great study they have recently embarked upon in a project names Iconic Landscapes. The University was finding that the general public and some legislations were not responding to the terms biodiversity or environmentally-friendly seawalls. The general public sees an experiment as something that will not happen, something they have to pay for but will not yield something for sure. And ecologist, however, knows it might not happen but sees the process as beneficial. How each type of person views ecological language largely affects how much they will be used. Iconic Landscapes is a group of social scientists that are surveying and interviewing different teams of people to see their reactions to different environments and their terms. One of these environments is seawalls. They are assembled three groups of people: scientists, legislators, and Joe Public. Do they feel the same way about the environment as the scientists do? Do they care about improving biodiversity or just catching more fish? They have a website and blog that I will link to shortly. This study is increasing awareness about biodiversity. Click here for link to website.
Associate Professor Ross Coleman is the Director for the Centre for Research on Ecological Impacts of Coastal Cities for the School of Biological Sciences for the University of Sydney.