Are seawalls the best answer to rising sea levels – or is retreat a better option?
While barriers are the most widely used method of protecting coastal homes and infrastructure in Australia, landscape architects must consider other options
| June’s devastating storms damaged beachfront homes on Sydney’s northern beaches. |
Some are calling for a seawall to be put in place. Photograph: Dean Lewins/AAP
The extraordinary pictures of subsumed gardens and a swimming pool wrenched from the ground by the giant waves that battered Sydney’s northern beaches last month have revived debate about seawalls and the impact of human attempts to keep the rising ocean from our doors.
Given their spectacular locations, the homes in the frontline of raging waves are usually valuable property. Attempts to fortify them are met with resistance from ecologists and other beach users, who say the houses should not have been built there in the first place.
They object to seawalls because they stop the beach from being a dynamic system, in which wind and waves continually reshape the shore. Natural processes will usually redeposit much of the lost sand back on to beaches in the weeks after a storm. But where there is a seawall, heightened waves run up the shore and slam against it. The beach can’t move backwards, so the sand disappears.
But seawalls, like dykes, levees and berms, have been used across the world for centuries to protect homes and other assets. From Sydney to São Paulo to New York, a sea-level rise of even a few centimetres can threaten to swallow homes and highways, inundate sewage treatment plants, and contaminate water supplies. Whether faced with king tides or swelling rivers and lakes, planners in coastal and estuarine urban settings must find a way to brace against the impacts of climate change.
Some schemes address the wider landscape. New York’s Big U, an ambitious berm project, will effectively raise the bank of the East river almost three metres – well above Hurricane Sandy’s high-water mark. Others approaches consider making individual structures more adaptive, such as buildings on stilts, or houses that float – either permanently or when the land is inundated.
In Australia, seawalls are the most widely used method of protecting waterfront properties and infrastructure. While many object to their use, moves to minimise their visual and ecological impact are continually being refined.
David Rissik, deputy director of the National Climate Change Adaptation Research Facility which supports decision makers in managing sea-level rises, says while Australians have become used to being able to hold back the ocean, the location, structure and materials used for seawalls are under much greater scrutiny than in the past.
“Where people have taken over places where there is natural migration of riverbanks or open coast, seawalls have allowed them to live,” he says.
Rissik is sanguine about the concentration of infrastructure around Australia’s coast. “From a pathway from the beach, a local economy grows and, once that is established, it anchors people to that area and change is very difficult.”
The row about whether a seawall should be built on a stretch of coast in Collaroy has raged since the shoreline of the northern Sydney suburb was subdivided more than 100 years ago. When the beach retreated during high seas, the then beach shacks were undermined. Records show major erosion occurred in 1920, seven shacks fell into the sea in 1945 and one was washed away in 1967. But, as Rissik says: “People forget and they build there again.”
While Collaroy is among few beaches worldwide where researchers have a 40-year unbroken record of the changes to the coast through survey techniques, Rissik says coastal experts don’t have a great knowledge of where sediment supplies and erosion are in many other parts of the country.
Queensland’s Gold Coast is an exception. Maintaining the beach’s amenity is so vital for tourism that planners have established a comprehensive strategy for coastal management. Under its A-line scheme, the city council allows seawalls under strict regulation. When developers want to build one it must be effectively behind the dunes, acting as a last line of defence, but buried by sand and hidden from view. The costs of dredging and distribution of sediment are enormous but compliance keeps protests to a minimum.
David Hatherly, a landscape architect whose company Vee Design has won acclaim for its redesign of the waterfront at Yeppoon on Queensland’s central coast, says the prospect of rising sea levels was completely ignored in his brief from the council that commissioned him. “I don’t think we in Australia have done a very good job of understanding our coastal environment,” he says.
Cyclone Sid in 1998 caused the worst floods in Townsville’s history, destroying much of the infrastructure on the Strand, the city’s foreshore focal point. As part of the restructure, artificial headlands were built to hold the sand and prevent longshore drift. It’s a typical response, Hatherly says. “We’re coming up with these hard engineering solutions to fight nature. Artificial headlands, seawalls … they are Band-Aid solutions. The most cautious thing to do would be to maintain proper buffer zones.”
Responses to sea-level rises fall under the three broad categories, Rissik says: defence, accommodate and retreat. Seawalls and levees are the most obvious defences, along with “soft options” such as mangroves and vegetation. Human-made dunes can be used to lessen wave energy.
Accommodation approaches include the development of areas in cities that allow us to continue to live, work and play where we do now, while rising seas and occasional floods take place around us. Futuristic versions of this could see communities built on piled foundations, where structures are screwed into the ground, surrounded by wetlands and beaches.
Rissik describes the retreat option as the point when we recognise we cannot cope with the impacts of a rising ocean and move to areas better suited to habitation. Hatherly points to the 2011 Queensland floods as an example of an extreme weather event which he expected would make a strong case for the retreat option.
“But people are already flocking back to the river’s edge with limited concern for the possibility of flood,” he says. “They won’t relinquish those locations as long as governing authorities allow rebuilding.”