Frequently Asked Questions About Adelaide Coast Protection

1. Isn't beach replenishment interfering with nature?
2. Doesn't beach replenishment simply benefit one beach at the expense of another?
3. Doesn't the sand used to replenish the beaches simply get washed away during the next storm?
4. Is there any evidence that beach replenishment actually works?
5. Shouldn't we stop taking sand from beaches if it results in damage to sand dunes and dune vegetation?
6. Why don't we focus more on dune rehabilitation and revegetation?
7. Why don't we dredge sand from offshore rather than taking it from the beach?
8. Why don't we use a field of groynes to slow the drift of sand along the coast?
9. Why don't we focus on preventing stormwater and effluent entering the ocean?
10. What is the Government doing about restoring seagrass meadows?

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Henley Beach after a storm in 1953

1. Isn't beach replenishment interfering with nature?

Yes, beach replenishment limits the effects of the natural processes taking place on the Adelaide coastline. However, if beach replenishment weren't carried out, the natural processes would result in southern beaches quickly losing their sand and the shoreline eroding inland. Storms would progressively erode dunes on the southern beaches, resulting in extensive damage to reserves, roads and buildings built on the foreshore.

2. Doesn't beach replenishment simply benefit one beach at the expense of another?

Under natural conditions, some Adelaide beaches are constantly building up, some beaches are 'stable' and other beaches are eroding. Beach replenishment takes sand from beaches that are building up and places it at beaches that would otherwise erode away. There is only a finite amount of sand on Adelaide's beaches, so it must be spread in an equitable manner along the coast.

3. Doesn't the sand used to replenish the beaches simply get washed away during the next storm?

The sand put on a beach during replenishment is 'sacrificial'. It is intended that some of it be washed away during the next storm. If this replenishment sand weren't available to be eroded away, then beaches, dunes and foreshore reserves would be eroded instead. Much of the sand that is washed offshore during storms is washed back onshore during calmer periods, so it is not 'wasted'.

4. Is there any evidence that beach replenishment actually works?

Henley Beach in 2002

There are numerous examples worldwide of successful beach replenishment programs. The success of Adelaide's beach replenishment program is demonstrated by the build-up of dunes and increase in beach width in the Brighton and Henley Beach areas, where previously there were only seawalls and no beach at high tide.

Since 1975, a network of beach profiles along the Adelaide coast has been regularly surveyed and evaluated for long-term changes in sand levels. In addition, brass rods installed in the seabed have been used to measure depth changes due to sand movements. A surface modelling technique is used to map and measure large sand movements due to replenishment projects or erosion.

Data from the monitoring program shows that the width of the beach at Brighton has built up by approximately 50 metres since 1975, despite an estimated 70,000 cubic metres of sand moving out of the area each year. Similarly, the width of the beach at Henley has built up by about 20 metres since 1975, despite an estimated 50,000 cubic metres of sand moving out of the area each year.

Figure – Variation in beach width along the Adelaide metropolitan coast 1975–2003 (800Kb PDF)

5. Shouldn't we stop taking sand from beaches if it results in damage to sand dunes and dune vegetation?

Semaphore in 1923

Erosion of the dunes near the Semaphore jetty after removal of sand from the beach, June 2004

It is true that when sand is removed from beaches for recycling purposes, dunes may be eroded and dune vegetation disrupted. However, it is important to realise that most of the pre-European dunes along the Adelaide coast have been covered with development, and that the only large areas of remnant vegetation are the Tennyson and Minda dunes. These are the only dune areas in the metropolitan region that can be managed for their landscape, cultural and conservation values, because their significant landward extent provides for such longevity.

In recently formed dune areas (e.g. at the Torrens Outlet and Semaphore), the majority of the vegetation is non-indigenous, of low ecological significance, and often weed-infested. The primary role of these dunes is to provide reserves for sand recycling and to buffer coastal infrastructure from the sea. The dunes need to be vegetated to prevent wind-blown sand drift and this provides a secondary benefit as a habitat for dune flora and fauna. Even so, it must be recognised that the seaward portions of these dunes could be eroded away at any time in a storm event.

Some dunes (e.g. at the Torrens Outlet and Semaphore) are much wider than would be affected by even very extreme storms, while other parts of the coast have inadequate dunes to provide a buffer to storms. Artificially moving sand from one beach to another attempts to improve the overall extent and health of the beach system for the entire Adelaide coast. Where possible, the landward portions of dunes that form part of the buffer to storm damage will be rehabilitated or preserved in line with coastal vegetation management plans developed by the Urban Forest Million Trees Program in conjunction with local councils.

6. Why don't we focus more on dune rehabilitation and revegetation?

Dune weeding contractors at Semaphore

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. A number of volunteer dune care groups as well as contractors undertake revegetation and dune weeding programs.

Drift fencing and dune revegetation are primarily carried out to prevent the loss of wind-blown sand inland and to maintain dune stability. They can do little to prevent sand being eroded by waves and water during storms. Without the beach replenishment program, even well vegetated dunes would erode away on the eroding (southern) parts of the coast. Therefore, the Board's primary focus in managing Adelaide's beaches is on beach replenishment, which not only ensures that there is enough sand along the coast and but also enables dunes to form in the first place.

7. Why don't we dredge sand from offshore rather than taking it from the beach?

Sand dredged offshore from Port Stanvac being pumped ashore at Brighton, 1997

Dredging sand from offshore sources and using it for beach replenishment has two main advantages – it reduces the need to cart sand in trucks, and it increases the total amount of sand within the metropolitan beach system.

Unfortunately, dredging can create large sediment plumes that can adversely affect nearby seagrass and algae habitats. The extent of this effect depends on the tide and wave currents during and immediately after a dredging operation, as well as the overall health and ability of the ecosystem to recover from a potential smothering episode. In addition, the substrate is deepened where dredging occurs, which can result in altered wave conditions. This in turn can affect any mangrove and samphire communities that may be present. It can also affect littoral drift rates and therefore beach and dune widths. It is therefore important to design dredge areas so that their depth, distance offshore and configuration take these potential impacts into account.

Over recent years the Coast Protection Board has carried out a range of offshore sand source investigations. Sand suitable for beach replenishment has been discovered offshore from Port Stanvac and at the Section Bank near Outer Harbor.

In the 1990s, over one million cubic metres of sand was dredged offshore from Port Stanvac and pumped ashore at Brighton. The current good condition of this beach is due to the quantity and quality of the sand it received during this period. By the late 1990s, the Port Stanvac source was considered used up. Additional dredging may be possible following the closure of Port Stanvac in 2004, which has freed up a previously restricted area. Even so, the dredge site remains constrained at its boundaries: important seagrass habitats to the south could be at risk from nearby dredging; the north and west borders have poor-quality sediments and/or shallow rock or clay; and on the eastern inshore border dredging in less than 10 metres of water would affect inshore coastal processes.

Recent investigations regarding the use of the Section Bank as a sand source and the potential impacts of dredging have shown that the region is already highly stressed and that dredging the Section Bank may pose, or at least be seen to pose, significantly high risks to the coastal ecosystem.

Future dredging of offshore sand deposits is not proposed unless studies can demonstrate that any environmental impacts are less than those associated with alternative approaches. Investigations of sources including North Haven, the Section Bank, Port Stanvac and Moana are continuing.

8. Why don't we use a field of groynes to slow the drift of sand along the coast?

Groyne field showing buildup of sand due to alongshore drift, with and without replenishment

Groynes are structures built across the coast usually from dry land out into the water. They act to interrupt the alongshore movement of sand by being a physical barrier across the beach, collecting sand on the updrift side in what is called a fillet. They can only trap sand if there is sufficient alongshore drift to bring sand into the fillet.

To completely stop alongshore transport, a groyne needs to extend out far enough to allow the build-up of sand to reach an equilibrium angle (the angle of the beach, as viewed from above, at which the breaking wave crests are parallel to it) with the waves that drive the sand movement. A groyne also needs to be long enough to extend beyond the seaward limit of the littoral zone to prevent sand washed offshore by storms from then being transported alongshore. For the Adelaide coast, this length would be around 300 metres for groynes spaced about 600 metres apart.

A groyne should be of sufficient height to prevent alongshore sand transport by overtopping. For both now and in the future on the Adelaide coast with its large tidal range, that would mean heights of about 1 metre lower than the height of seawalls backing the beach. Such a large structure would probably result in a lowering and recession of the beach to the north (downdrift) of it. There would be significant difficulty in walking along a beach interrupted by such a large structure, since there would be a drop of several metres between the beach on the updrift and downdrift sides of it. This downdrift erosion can be avoided or minimised by adding sand to the fillets during the construction process. However, the amount of sand required to do this is large, and obtaining sufficient sand would be both costly and potentially environmentally damaging.

In other words, a complete groyne field along Adelaide's beaches would be both publicly unacceptable and very costly. Groyne fields are also difficult to alter in response to changing coastal conditions such as seagrass loss, seabed erosion, sea level rise and land subsidence.

One of the new groynes constructed at Somerton Park in 2005

As an alternative, smaller groynes can be useful for raising beach levels on a very small scale and their height can be adjusted to trap enough sand to aid use of the beach without being obstructive. An example is the small, geotextile groyne at Somerton Park built in 2001. At approximately 1.5 metres high and 25 metres long, the structure has collected enough sand to raise the beach level to the south of it above normal high-tide level on a beach that was previously submerged at high tide. Because of its size, the fillet was filled quickly and sand now bypasses and washes over the groyne. There is no observable adverse effect on the beach downdrift of it.

In mid 2005, three additional small groynes were constructed at Somerton Park, with the aim of raising beach levels along that part of the coast to provide beach access over longer periods of the day.

9. Why don't we focus on preventing stormwater and effluent entering the ocean?

Stormwater outlet at Taperoo

Gross pollutant trap

Bolivar wastewater treatment plant (Photo: United Water)

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.

Stormwater and effluent entering the ocean increase nutrient and sediment levels in nearshore waters, which is thought to have caused the death of seagrass meadows along the Adelaide coast. This has resulted in the level of the seabed becoming up to one metre deeper and a larger quantity of sand drifting north along the coast. The health of mangrove communities, tidal wetlands and subtidal reefs has also declined in some locations as a result of discharges into the marine environment.

A number of State Government departments are taking action to reduce the amount of stormwater and effluent entering the ocean. For example, the EPA has established the Adelaide Coastal Waters Study, which will identify key threats to the coastal environment and develop options to minimise impacts. The EPA is also merging its Stormwater Pollution Prevention Codes of Practice into a consolidated Stormwater Code with associated guidelines for specific industries. SA Water is undertaking Environment Improvement Programs, which involve upgrading wastewater treatment plants to minimise high nutrient sewage discharge into the marine environment, preferably by re-using wastewater for irrigation. Catchment water management boards, regional natural resources management boards and local councils are all actively developing integrated water quality and stormwater management strategies to minimise discharges into the marine environment.

Unfortunately, these programs are large and slow to come into effect, and impacts to date such as the large-scale erosion of the seabed are irreparable. In any case, stormwater and wastewater discharges are not the primary cause of erosion on Adelaide's beaches. Even if the health of nearshore waters is improved dramatically, the ongoing management of sand on our beaches will remain necessary.

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/.

10. What is the Government doing about restoring seagrass meadows?

Collection of mature Amphibolis antarctica from a donor meadow

Two bag types previously trialled: 1. hessian bags covered with coarse hessian layer ('double bags') 2. hessian bags with jute mats attached (Photo: Rachel Wear)

Large number of Amphibolis antarctica recruits on a hessian bag (Photo: Rachel Wear)

The main focus of the State Government's work to date on restoring seagrass meadows has been on improving the quality of nearshore waters through reducing stormwater and effluent discharges. Great improvements have already been made and the rate of seagrass loss along the metropolitan coast has reduced in recent years.

In 2001, SARDI Aquatic Sciences and the Coastal Protection Branch of the Department for Environment and Heritage held Australia's first workshop on seagrass restoration to review the current status and knowledge in Australia and overseas. Following the seagrass restoration workshop a program for seagrass rehabilitation was formulated, with the ultimate objective to develop reliable techniques for seagrass restoration and rehabilitation suitable for application along the Adelaide metropolitan coast.

Three approaches to seagrass restoration have been or are currently being investigated:

1. donor-dependent methods, involving the collection of mature seagrass from a donor meadow for transplanting to areas of seagrass loss
2. donor-independent methods, involving the collection, successful germination, grow-out and planting of seedlings
3. recruitment facilitation methods, aimed at enhancing natural seagrass recruitment.

During 2002 and 2003 work focused on donor-dependent and independent methods, but current research is predominantly directed towards recruitment facilitation. Most successful is the use of hessian material to boost natural recruitment of Amphibolis antarctica juveniles. Trial sites are located at Henley Beach and Tennyson. Seedlings are being monitored for survival and growth. If the trials prove successful, then artificial stabilisation of the seabed to assist seagrass regrowth could be contemplated.

This approach not only avoids damaging existing seagrass meadows but also appears to be less time-consuming than the propagation and planting of seagrass seedlings. Notwithstanding this, research into the use of seedlings in seagrass restoration is ongoing.

New seagrass meadows would provide habitat and protect marine life, which could restore a substantial ecology that has been lost off the Adelaide coast. However, it would be many centuries before seabed levels are restored by this means. Therefore, the exacerbated foreshore erosion due to seagrass loss will only partially be reduced. This reduction would be due solely to the increase in seabed friction on wave energy from the seagrass meadows.

SARDI website: http://www.sardi.sa.gov.au/