Background Information on Adelaide Coast Protection
Why the Adelaide Coast Needs to be Managed
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North Haven,
1949
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North Haven,
1999
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Coastal development
Large quantities of sand have either been 'locked up' or removed from the beach system as a result of coastal development. Many of Adelaide's coastal suburbs were built on an extensive system of coastal dunes or, alternatively, dunes were used to infill coastal backswamps to provide land for housing and Adelaide's airport. Land was also reclaimed along the Lefevre Peninsula to establish the suburb of North Haven.
Had development been restricted to areas further inland, the natural recession of the coast would have been able to continue for many years from Brighton through to Semaphore, with the coastline alignment progressively adjusting inland at southern beaches. The natural littoral drift of sand to the north would have been maintained, but the rate of drift would have gradually reduced as the coast became parallel to the prevailing wave crests.
Stormwater and wastewater discharges
With coastal urbanisation comes the necessity for stormwater infrastructure and sewerage. Much of Adelaide's stormwater is discharged to the coast through the Patawalonga, Torrens and Port River systems, as well as via over 85 smaller outfalls that discharge stormwater within the dunes or directly onto the beach.
Four wastewater treatment plants located in and around Adelaide – Bolivar, Glenelg, Christies Beach and Port Adelaide – have discharged effluent into Gulf St Vincent. Effluent from the Port Adelaide plant is now diverted to the Bolivar plant for treatment rather than being discharged into the Port River. Up until 1993, sludge from the sewage treatment process was also discharged offshore from the Glenelg and Port Adelaide plants. These discharges increase nutrient and sediment levels in nearshore waters.
For more information on stormwater management, see the 'Stormwater' section of the Local Government Association of South Australia website: http://www.lga.sa.gov.au/site/page.cfm?u=575.
For more information on wastewater management, see the 'Environment' section of the SA Water website: http://www.sawater.com.au/SAWater/Environment/.
For more information on Adelaide's coastal water quality, see the 'Water Quality' section of the EPA website: http://www.epa.sa.gov.au/water.html.
For information on the Adelaide Coastal Waters Study, see the EPA
website: http://www.epa.sa.gov.au/acws.html.
Alternatively, see the CSIRO website: http://www.clw.csiro.au/acws/.
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Aerial
photograph, 1949
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Aerial
photograph, 2002
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Seagrass loss and seabed instability
A third of seagrass meadows along the Adelaide metropolitan coast have died since 1950. The aerial photographs to the right show the seagrass loss offshore from Somerton Park and Glenelg between 1949 and 2002 (the seagrass meadows are the dark patches in the water).
Poor water quality resulting from stormwater run-off and effluent disposal has most likely been the initial cause of seagrass loss. Once there are gaps in the seagrass meadows, the sand below the meadow edge can be eroded by waves. This is thought to have increased the rate of seagrass loss and made it difficult for plants to recolonise the seafloor, even though water quality has been improved.
Finer-grained sand that was once trapped by seagrass meadows has been released and washed ashore. Because the sand is fine, it accumulates in the sandbars and washes north to Largs Bay. Although in the short term this sand has added to protection of the coast, it is unsuitable for replenishing Adelaide's beaches in the longer term. This is because it tends to remain in the underwater part of the beach and is moved too quickly by waves.
As a result of the loss of sand from the seabed, the level of the seabed has steadily become up to one metre deeper and the wave energy reaching our beaches has increased. This causes a larger quantity of sand to drift north along the coast.
A laboratory and field study on seagrass rehabilitation techniques undertaken in conjunction with the South Australian Research and Development Institute (SARDI) is in its second year and is producing encouraging results. However, even if the study is successful, rehabilitation of seagrass meadows will not be able to replace the lost sand nor restore the seabed to its former level.
Brochure – Seagrasses of South Australia (400Kb PDF)
SARDI website: http://www.sardi.sa.gov.au/
Sea level rise
One of the consequences of climate change and rising global temperatures is that mean sea levels around the world are also rising. Parts of the Adelaide coast are also subsiding. A relative sea level rise of one to two millimetres per year has been recorded for Adelaide. The effects of this sea level rise along the coast are gradual though highly significant over the long term. Increases in sea level will cause greater erosion of the dunes and loss of beach width. Sea level rise will also be likely to alter the angle at which waves strike the shore, leading to changes in rates of littoral drift and, consequently, changes in the locations at which beaches build up or erode. Extra sand needs to be added to the beaches to counter the effective loss of sand as a result of sea level rise.
Coast Protection Methods Used
to Date
Beach replenishment
The Coast Protection Board first implemented beach replenishment in 1973 as its main strategy for protecting Adelaide's beaches. Over the last 30 years, sand has been needed to replenish the beaches at Brighton and Glenelg North in particular, and this has been taken mostly from Glenelg, the Torrens Outlet, Grange and Semaphore. Sand has also been dredged offshore from North Haven. In all, the average quantity of sand moved along the Adelaide coast has been over 100,000 cubic metres each year.
While recycling has been an effective method in maintaining sandy beaches, it does have its pitfalls. Highest among them is the necessity for trucks and other earthmoving equipment to remove, cart, dump and level sand on the beaches when replenishment programs are in progress. Trucks also increase greenhouse gas emissions and traffic congestion in affected coastal areas. The community has rightly voiced concerns about safety issues, noise pollution and beach-use interference.
Sand has also been imported from sources outside the metropolitan beach system to counter sand lost offshore, impounded onshore (locked up by development, seawalls and sand dunes), or simply blown inland by the wind. Ideally, sand from elsewhere should be similar to that originally deposited on the beach. The sand considered most suitable for beach replenishment should be of a similar grain size or coarser than sand from Brighton beach. The sand should consist of silica, and at least 50% of it should be coarser than 0.2 millimetres, with less than 5% silt and clay content. Preferably, sand grains should also be rounded and off-white or pale in colour.
Investigations undertaken by the Coast Protection Board have located
several sand sources of these or similar specifications in previous
years. In 1989, nearly 190,000 cubic metres of sand was removed
from the Torrens Island dunes and taken to Glenelg North. A particularly
large deposit was also located offshore from the Port Stanvac oil
refinery. This has so far been the major external replenishment
source for Adelaide's beaches. Over one million cubic metres of
sand was dredged and pumped onshore at Brighton during the 1990s.
Much of this, over 600,000 cubic metres, was dredged to Brighton
between October 1997 and February 1998.
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The trial
breakwater at Semaphore South
during construction, 2004 |
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One of
the groynes constructed
at Somerton Park in 2005 |
Sand trapping
Sand trapping can occur either directly due to purpose-built breakwaters and groynes, which slow sand movement along particular areas along the coast, or indirectly as a result of coastal developments such as marinas or stormwater outlets obstructing sand movement.
For many years, the Semaphore Park and Tennyson foreshores have undergone considerable erosion associated with seagrass loss. A trial breakwater was constructed in 2003–04 at Semaphore South as Stage 1 of the three-stage Semaphore Park Foreshore Protection Strategy. The breakwater is designed to trap a proportion of the sand drifting north along the coast. Modelling has predicted that up to 40,000 cubic metres of trapped sand can be removed from the area near the breakwater each year and taken south to replenish the foreshore at Semaphore Park. This will minimise the need for sand to be carted from Semaphore beach to Semaphore Park. The results from monitoring during the four- to five-year trial period will inform the decision on proceeding to Stage 2 of the Semaphore Park Foreshore Protection Strategy. For more information see Semaphore Park Foreshore Protection Strategy.
Shore-parallel structures (offshore breakwaters) are generally
preferred over perpendicular structures (groynes) because the latter
would interrupt the mostly continuous beaches we are so fortunate
to have today. Nevertheless, small groynes can be useful for raising
beach levels on a minor scale. An example of this is the small geotextile
groyne constructed at Somerton Park in 2001, which is approximately
1.5 metres high and 25 metres long. This groyne has collected enough
sand to raise the beach level to the south of it above the normal
high-tide level on a beach that was previously submerged at high
tide. There is no observable adverse effect on the beach to the
north of the groyne. Similar groynes have been installed at Somerton
Park in 2005 to raise upper beach levels and provide beach access
over longer periods of the day.
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Sand bypassing
at the Torrens Outlet
(Photo: City of Charles Sturt) |
Sand bypassing
Sand bypassing is the manual movement of sand to overcome an obstacle that has caused it to become trapped, with the intent that sand continues to move as if unhindered by the obstacle. Along the Adelaide coast, sand bypassing presently occurs from south to north at the Holdfast Shores marina at Glenelg, the Adelaide Shores boat haven at West Beach, and the Torrens Outlet.
One of the most marked changes to the Adelaide coast in recent years has been the construction of the Holdfast Shores and Adelaide Shores harbours. Dredging and sand bypassing are undertaken at each facility to maintain channel depths and enable sand to continue to drift northward. Sand from the channel areas is dredged and pumped offshore, while sand from the salients (the build-up of sand between the harbour breakwaters and the beach) is carted by truck and dumped either immediately north or, in the case of Holdfast Shores, periodically to Brighton and Seacliff. At the Adelaide Shores boat haven, seagrass wrack is also carted by truck and used as a buffer to reinforce the base of the West Beach dunes.
Much of the sand that accumulates at the Torrens Outlet is bypassed
by the City of Charles Sturt to the north at Henley Beach South,
although several large amounts have been used on an 'as needs' basis
to replenish Glenelg North, Somerton Park and Seacliff. In 2004,
sand was taken to protect the dunes at West Beach, and in 2005,
sand was used to replenish the West Beach dunes, Glenelg North,
Brighton and Seacliff.
Seawall protection
Seawalls currently protect a length of 14 kilometres – about half of the metropolitan coastline. Most of the seawalls are found south of Grange, in particular at Henley Beach, West Beach and from Glenelg North to Seacliff (apart from the Minda and West Beach dunes areas). Seawalls act as the last line of defence in protecting coastal infrastructure and property during storm events.
While early seawall designs were often solid concrete structures,
most seawalls since the 1960s have been constructed and repaired
using large boulders and rocks. This has enabled seawalls to better
absorb wave energy. Nevertheless, many of the seawalls are inadequate
and the forces of the sea are such that, in the future, most seawalls
will need repairs. Had beach replenishment not been implemented
in the 1970s, thereby creating dune buffers, many of Adelaide's
seawalls would by now have been undermined at their base. Continued
beach replenishment has therefore reduced the risk of seawall damage
and reconstruction costs.
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Drift fencing
at Grange
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Dune management
The Coast Protection Board undertook widespread dune stabilisation work in the 1970s when large areas of dune were affected by wind-blown sand drift. More recently, dune management has been implemented by coastal councils. This has included drift fencing, dune revegetation and access controls, together with the installation of educational signs and viewing areas at some locations. Drift fencing and dune revegetation are primarily carried out to prevent the loss of wind-blown sand inland and to maintain dune stability. The dunes are further maintained by ensuring the public uses formal pathways to access the beach and providing suitably located car parking and public amenities. A number of volunteer dune care groups as well as contractors undertake revegetation and dune weeding programs. Since 2003, the Urban Forest Million Trees Program (http://www.urbanforest.on.net/omtmain.htm) has been working with coastal councils on a series of vegetation management plans to guide councils and volunteer groups in the implementation of revegetation works in coastal reserves.
Alternative Management Strategies
In 2000, the Department for Environment and Heritage, on behalf of the Coast Protection Board, initiated a review of the management of Adelaide's metropolitan beaches. During the review, the Department considered a range of alternative coast protection strategies. Most of these alternatives had been examined in previous studies in 1970, 1984, 1992 and 1997. However, all alternatives were reassessed in light of the recent changes to the coast and the results of updated coastal process modelling data commissioned by the Board during the review.
Based on examination of the benefits and costs of all alternatives, along with the results of a series of modelling and feasibility studies and input from the community, the Department has developed an innovative strategy for managing Adelaide's beaches called Adelaide's Living Beaches: A Strategy for 2005–2025. It is based on alternative 11 described in the table below.
Evaluation of alternative beach management strategies for Adelaide
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Match sand movement |
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1. Maintain current strategy |
Maintain the current sand management activities, i.e. beach replenishment and harbour bypassing (including carting and dredging of sand), to not only match the rate of littoral drift but also slowly build up dune buffers in critical areas. |
This alternative is practical, maintains sand on the beaches, builds up dune buffers and provides additional sand to compensate for sand loss as a result of relative sea level rise. However, the cost of coastal management under the existing strategy is continually increasing. |
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2. Reduced level of beach replenishment |
Maintain sand management activities, but reduce the level of beach replenishment to 'just match' the rate of littoral drift. |
This alternative would not build up dune buffers nor provide additional sand to compensate for sand loss as a result of relative sea level rise. Consequently, there would gradually be less and less sand on beaches affected by erosion. Maintaining sand on the beaches is important to the community for both social and economic reasons, so this alternative is unacceptable. |
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3. Major replenishment |
Undertake a large replenishment program that will make further replenishment unnecessary for 20 years. |
The environmental and social impacts of this alternative are unacceptable. Nearshore seagrass would be buried under replenishment sand, stormwater outfalls would become clogged with sand, and beaches would initially be very wide and subject to high levels of sand drift. Furthermore, the replenishment rates necessary to undertake such a major replenishment could only be achieved by dredge, and no suitable offshore sand sources have been identified that are economically viable at present. |
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4. Recycle sand |
Install pipelines and pumping systems to pump sand that accumulates on northern beaches back to southern beaches. |
This alternative is not feasible on its own, because sand accumulating on the beaches north of Semaphore is mostly fine and calcareous and therefore unsuitable for replenishment of the southern beaches. Nevertheless, the concept of a pipeline to recycle sand is valid and is considered under alternative 11. |
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Retreat or no replenishment |
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5. Retreat |
Relocate, 'buy back' or rezone foreshore development allowing the shoreline to recede as a result of erosion, i.e. no replenishment, no new seawalls, and gradual removal of existing seawalls as they are undermined by erosion. |
This alternative would unlock impounded sand within the dunes and maintain beach amenity. However, the cost for purchasing properties alone would be prohibitive, let alone the cost of replacing and modifying public infrastructure such as roads, water and sewerage systems. Assessment of the cost of properties and equating this to the volume of sand released indicates a cost in the order of $400 per cubic metre, which is more than 10 times the current cost of sourcing all sand from Mount Compass. In some areas, sub-surface clay could be exposed, thus reducing beach amenity. In addition, it would be very difficult for retreat to be achieved in a manner that was fair to coastal residents. This alternative is therefore not feasible. |
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Slow sand movement |
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6. Groynes with replenishment |
Construct a groyne field along the coast to minimise net littoral drift, and replenish the beaches between groynes as required. |
This alternative would require an extra two million cubic metres of sand from external sources than a beach without structures, and is therefore very expensive. Once constructed, a groyne field could not be adjusted to cater for ongoing sea level changes or managed for seasonal variations in wave conditions, other than by adding or removing sand. A groyne field would also interfere with the coastal landscape and limit pedestrian access along sections of the beach. This alternative was costed during the review but has been dismissed because of the social impacts and high ongoing capital and operating costs involved. |
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7. Offshore breakwaters with replenishment |
Construct a field of offshore breakwaters along the coast to minimise net littoral drift, and replenish the beaches in the lee of the breakwaters as required. |
This alternative is similar to alternative 6, but with higher construction costs offsetting a relative advantage in terms of continued pedestrian access along the coast. This alternative has been dismissed for similar reasons as alternative 6. |
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8. Hybrid field of structures |
Construct a field of groynes and offshore breakwaters, tailored to local coastal values and uses, and replenish the beaches along the field as required. |
This alternative has been dismissed for similar reasons as alternative 6. |
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9. Use coarser sand |
Replenish the beach with coarser sand, which drifts less under Adelaide's wave conditions. |
This alternative could not by itself prevent erosion of Adelaide's beaches. The large-scale replacement of the vast quantity of sand on the beaches is not feasible, even were a sand source of this size available. Furthermore, littoral drift would continue to occur at substantial levels, requiring ongoing recycling and replenishment. The use of coarse sand is therefore best combined with other methods, as considered under alternative 11. |
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Fusion approaches |
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10. Sand recycling and/or replenishment combined with structures |
A combination of approaches, managing sections of the coast with sand recycling and/or minor replenishment and sections of the coast with structures. |
This alternative is a combination of the best aspects of alternatives 4 and 8. While this alternative is feasible, it is less effective than alternative 11, and of a similar or slightly greater cost. |
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11. Sand recycling combined with structures and replenishment with coarse sand |
A combination of approaches, managing sections of the coast with sand recycling and sections of the coast with structures, but also adding coarse sand from external sources. |
This alternative draws on the best aspects of alternatives 4, 8 and 9. It is more effective than alternative 10 because it incorporates the use of external sand sources to counter the ongoing loss of dune volume and beach width caused by sea level rise and other factors. This alternative forms the basis of Adelaide's Living Beaches: A Strategy for 2005–2025. |
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Other approaches |
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12. Seawalls |
Protect foreshore development from erosion when and as needed by constructing seawalls. |
This alternative would quickly result in the loss of sand from beaches if not combined with beach replenishment. Maintaining sand on Adelaide's beaches is important to the community for both social and economic reasons, so this alternative is unacceptable on its own. However, seawalls are important as the last line of defence against storms, so their continued maintenance has been included in Adelaide's Living Beaches: A Strategy for 2005–2025. |
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13. Do nothing |
No further sand management or coast protection works. Remove seawalls, roads, pipelines, other infrastructure and houses when damaged by erosion. |
This alternative would quickly result in the loss of sand from beaches and progressive damage to foreshore infrastructure and buildings. The cost due to loss of beach value alone would be very high, with further costs incurred for the management of subsequent debris and pollution. This alternative is therefore not feasible. |
Information Resources
Brochure –
Do Adelaide's Beaches Need Help? (150Kb PDF)
Coastline 26 – Coastal Erosion,
Flooding and Sea Level Rise Standards and Protection Policy (700Kb
PDF)
Coastline 27 – The Adelaide
Metropolitan Coastline (250Kb PDF)
Coastline 28 – Maintaining the
Adelaide Coastline (550Kb PDF)
Coastline 29 – The Value of
the Adelaide Beaches (150Kb PDF)
Coastline 31 – Coastal Vulnerability
Assessment (100Kb PDF)
Coastline 32 – Monitoring Sand
Movements along the Adelaide Coastline (350Kb PDF)
Coastline 36 – Recreational
Beach Widths along the Adelaide Coastline (1.1Mb
PDF)
Coast Protection
Board Policy Document 2002 (2.3Mb PDF)
Educational Cartoons
Coast management (800Kb
PDF)
Beach values (900Kb
PDF)
The living coast (850Kb
PDF)
Human impacts (850Kb
PDF)
Sand management (900Kb
PDF)
See Adelaide's Living Beaches: A Strategy for 2005–2025 for downloadable A3 posters on the strategy.
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| This page was last modified 2006-05-15 | ||
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