Impact of coastal erosion in Australia
As a follow up to the Impending Coastal Crisis feature we posted earlier this week, Senior Coastal Scientist at Coastalwatch Professor Andrew Short has compiled a comprehensive piece focusing on coastal erosion in Australia.
For the 50% of the Australian coast that is composed of sand and in some places mud, the shoreline is prone to change, building seaward and in some places eroding landward. In most locations this is a natural process with usually no impact on human settlement. Coastal protection of the shoreline is rarely required in Australia, however in a few locations the dynamic shoreline has become a problem, in some cases a major and expensive problem, and in almost all of these cases the problem is related to human interference or encroachment on the shoreline. Coastal protection works, such as breakwaters, groynes, or seawalls, are usually built to guard against erosion. In doing so they harden the coast and reduce its ability to adjust naturally. As a consequence, these defences can exacerbate further erosional problems, with seawalls reflecting and concentrating wave energy and erosion, and groynes starving downdrift the coast of sediment thereby leading to further erosion. There are areas where human have encroached into the dynamic beach environment only to suffer the consequences, and others where they have interfered with coastal processes leading to accelerated coastal erosion.
Much of the shoreline of the rapidly expanding tropical city of Cairns appears especially vulnerable. The popular marina area and the hub of Cairns has encroached on Trinity Inlet and formerly mangrove-covered mudflats along the seafront are now bare and lined with seawalls, backed by development close to sea level. Much of Cairns is also built on a coastal plain, part of the Barron River delta, with the mangrove-lined tidal creeks that meandered through these plains all that remain. These are inundated at highest tides and demonstrate that critical infrastructure, such as the airport, lie close to present high tide level (as is often also the case with other airports, such as those at Brisbane and Sydney). At this level much of the coastal plain and city would be inundated by a 2.5 m storm surge calculated to accompany a 1 in 100 year storm, whereas dating sequences of coral rubble ridges deposited by large storms on nearby islands indicate that considerably larger events than those known from the past 200 years have recurred frequently over past millennia. The north Cairns coast along Yorkys Knob-Machans-Holloway beaches is lined with seawalls because much of the shoreline has been retreating, on a coast that in the long-term is building seaward as the Barron River delta progrades. The problem here is both the dynamic shoreline of the delta, which switches channels, has major floods and accompanying pulses of sediment, on a coast exposed to periodic tropical cyclones, all of which produces a highly dynamic shoreline. Roads and houses have been built on parts of the coast which will periodically be reclaimed by the sea, as well as periodically protected by wide beaches. Finally, when climate change is considered, the already perilously low-lying developments in Cairns are at considerable risk to inundation, as discussed in the next section.
Australia’s best known strip of defended coastline, and a major holiday destination for local, national and international tourists, is the 35 km between the New South Wales-Queensland border at Point Danger and the Nerang Inlet, better known as the Gold Coast.
Here is a system that is part of a northerly conveyor belt of sand moving north from New South Wales across the border at a rate estimated at 500 000 m3/yr. In 1962-64 the Tweed River training walls were extended for navigation, 400 m out to sea. The southern wall blocked the movement of sand, which built out the adjacent Letitia Spit by 250 m, in the process trapping millions of cubic metres of sand and preventing it from moving across the border and along the Gold Coast beaches.
As the sand supply was depleted, combined with a series of severe cyclones (Dinah, Barbara, Dulcie, Elaine and Glenda) in 1967, 8 million cubic metres of sand was eroded from the beaches and threatened the backing Gold Coast roads, houses and hotels. The solution has been threefold. First a continuous terminal seawall was built the length of the coast and covered with sand and dunes. Second between 1995 and 2000 3.5 million cubic meters of sand was dredged from the Tweed Bar and placed offshore of the southern Gold Coast beaches. Third, a permanent pumping system was built just south of the training wall, which since 2000 has pumped more than 500 000 m3 of sand each year from New South Wales across the border onto the Gold Coast beaches. In 2007 these beaches were as wide was they have ever been. However it has all come at a cost in the tens of millions of dollars.
A similar training wall built between 1960-66 at the mouth of the Brunswick River, 50 km to the south, to service the small fishing fleet. Studies have shown that the walls impacted the beach to the north and south. The beach built it out for 8 km updrift, while erosion was observed to extend up to 17 km downdrift, with the shorelines not stabilising until 1987. The largest impacts were close to the wall with the small beachfront community of Sheltering Palms, located 2 km north of the wall, experiencing up to 90 m of shoreline erosion which resulted in some houses, roads and telegraph poles ending up in the surf zone and finally abandonment of the entire village by the mid 1970’s. Unlike the Gold Coast, this area did not warrant the massive expenditure on protection, so it was sacrificed.
Coastal erosion, particularly associated with a cluster of east coast cyclones, has occurred at several points along the New South Wales coast. Dressing sheds at Manly were broken up by waves in 1913 and the North Steyne SLSC in Manly was severely damaged in 1950. The jetty at Byron Bay was removed in 1972, and ad hoc coastal protection was attempted at Belongil Spit with car bodies dumped to try to halt the erosion of sand. The 1974 storms, estimated as a 1:200 year event, destroyed the harbour side Manly pier and resulted in loss and damage of property and roads being cut at several points along the Sydney and south coast; elsewhere they were the first stage of erosion, with subsequent storms actually undermining property, as with the three houses that were destroyed during a storm in 1978 at Wamberal.
Collaroy Beach on Sydney’s northern beaches is a classic example of inappropriate planning and shoreline subdivision that took place more than 100 years ago. The original property boundaries extend, and still do, down across the dune onto the beach, with most of the houses and now some high rises built on the beach-dune area. The consequences were entirely predictable, every time the beach retreated during high seas, the then beach shacks were undermined. Major erosion occurred in 1920, seven shacks fell into the sea in 1944-5, and one was washed out to sea in 1967. Following the 1945 storms the council voted to resume the remaining houses. Instead, within ten years the first block of flats was built and soon after the first high-rise, which in turn was undermined by the 1967 storms. More high rises followed, the next built just in time to be undermined by the 1974 storms. Here the council has allowed initial development in a hazard zone, and later massive over-development even after houses had been washed away. Collaroy remains a problem area with most of the affected properties fronted by makeshift seawalls. The council has started slowly buying back some properties and hopes the state government will allow massive beach nourishment at some time in the future. In the meantime every big sea removes the narrow beach and exposes the unsightly and hazardous seawalls on one of Sydney’s premier beaches.
Warilla Beach just south of Port Kembla was sacrificed to protect a row of low cost houses. Like Collaroy the houses had been built too close to the shoreline on the formerly active foredune. The volume of sand on the beach, already depleted through mining for export in the 1960s, was further reduced because longshore processes carried a proportion into the entrance to Lake Illawarra when the entrance lay to the south of a tombolo connected to an offshore island. Any sand returned by tidal processes leaked onto Perkins beach to the north when the tombolo closed off Warilla Beach, acting as a trapdoor. Finally when the 1974 storms threatened the houses, the council responded by building a rock seawall along the southern half of the beach, replacing the public beach with rock, even the surf club had to be moved. This example of poor planning has only been remediated recently when further works at the lake’s entrance, in an effort to keep it permanently open, led to significant nourishment of the beach. A series of storms generated by east coast lows in June and July 2007 however severely truncated the beach as the sand was being pumped south, providing an ominous warning of the likely long-term fate of the rebuilt beach.
The rocky shores of New South Wales appear much more resistant against erosion. Locally however there are concerns about rock falls and public safety, as at Bilgola, Newport and Narrabeen headlands in Sydney. An exception is the steep cliffs at Coalcliff, first named by Flinders and Bass, where landslides are associated with the claystones and shales that are interbedded with the sandstones. The continual damage to Lawrence Hargrave Drive and the threat of falling rocks, led to the construction of the spectacular Seacliff bridge, a 665 m stretch of road that is built out from the cliff face over extensive rock platforms.
Most of the Victorian coast is protected by a foreshore reserve, however at Portland the reserve was not wide enough to protect a stretch of beach known as the Dutton Way. Here problems started when a breakwater was completed in 1960 to expand the port of Portland, thereby interrupting the easterly movement of sand to the downdrift Dutton Way beach. As the beach began retreating threatening a road a seawall was commenced in 1970. Since then the wall has continue to follow the erosion to the east and now winds it way along the shore for 4.5 km. Low-lying areas around the Gippsland lakes are subject to flooding, particularly after the artificial opening of the Gippsland Lakes in 1889. Flooding in 2007 after intense rainfall over the catchments that drain into the lake was exacerbated at high tide and inundated much of Lakes Entrance.
The Adelaide metropolitan beaches have been experiencing erosion for decades, as a result of the natural 40,000-50,000 m3/yr northerly sand transport, exacerbated by dieback of nearshore seagrass meadows as a consequence of sewage pollution; and further aggravated by some roads and structured located too close to the shore. The erosion has been managed both by the construction of 14 kilometres of seawalls and the trucking of sand from the northern end of the system back to the south, and more recently pumping sand onshore from nearshore sand deposits. Maintenance of these metropolitan beaches continues at significant cost; but it has been possible to sustain the natural values of the coast, even re-establishing dunes in front of the esplanade at Brighton.
In a political decision that will ensure South Australia has generations of coastal problems the South Australian government in 1990 voted to freehold hundreds of beach shacks, many built close to or on the beaches and in low lying erosion and flood-prone areas. Now the shacks are freeholded it will be up to the government and taxpayers to maintain these unsightly ribbon developments and to try and protect these properties as they become increasingly exposed to shoreline erosion and sea level rise.
In contrast the Western Australian government has been successively removing the many hundreds of beach and fishing shacks that dotted the coast north of Perth as far as Geraldton. These have been removed along with the associated myriad of 4WD tracks and replaced by coastal reserves with well planned and designed access points along the coast, while neighbouring coastal towns are being developed as nodes for the increasing coastal population. Western Australia’s Geographe Bay is a relatively sheltered sandy embayment with a northerly drift of sand from Busselton north to Mandurah.
In the early 1990s this northerly drift was interrupted by the construction of a series of training walls and groynes associated with a canal estate called Port Geographe. Not only sand but also piles of rotting seagrass built out 100 m against the updrift wall, while the beach on the northern side eroded back 250 m, threatening a road then houses. A combination of makeshift seawall and sand bypassing has been used with limited success; meanwhile the rotting seagrass continues to pile up and waft across the development. Just 120 km further north the Port Bouvard development built a similar training wall, but also added a permanent sand bypassing system, the result, no uplift build up and no down drift erosion.
These few examples serve to illustrate that the coast is a dynamic and at times very inhospitable environment. When we develop the coast it is essential that we first understand the nature processes and hazards, including longterm rates of shoreline movement and change, and extent of inland erosion and inundation. We must then plan with this information in mind, so that no inappropriate structures or development are placed in this hazards zone, and if they have to be such as ports and airports, then they are properly defended. Likewise if we interrupt the long-shore movement of sand we need to make contingencies such as sand bypassing, otherwise nature will realign the shores and place any down-drift development at risk.
- Professor Andrew Short, Senior Coastal Scientist, Coastalwatch
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