Semaphore South dune restoration

A severe storm in 2016 impacted the area, resulting in the loss of the dune and its vegetation. Works to restore eroded dunes at Semaphore South were completed in 2020. Replenishment of this area is required periodically to keep the dunes topped up. This was last done in March/April 2021.

The dunes will be topped up again in June 2021. More than 4000 additional plants will also be planted in the area later in 2021.

Frequently asked questions

Why is the area to the north of the Semaphore South breakwater an erosion hot spot?

The area immediately north of the Semaphore South breakwater (from approximately Noonies Cafe to Hart Street) is prone to erosion. 

A severe storm in May 2016 caused widespread coastal erosion throughout South Australia. At Semaphore South, much of the dune and its vegetation was lost.

Semaphore South storm damage May 2016

Major replenishment works to restore the dunes and protect the area from further damage by storms was undertaken in 2020. The project aimed to rebuild the eroded dunes and stabilise them with vegetation and drift net fencing.

The project was led by the Department for Environment and Water in partnership with the City of Port Adelaide Enfield. As part of the restoration works the City of Port Adelaide Enfield installed a new fence and reinstated the turf and irrigation to its former state. 
Semaphore South dune erosion
A section of eroded beach at Semaphore South prior to the dune restoration works commenced. Photo taken in April 2020

What has been done to restore the dunes?

Sand was moved from north of the Largs Bay jetty, where there is a large accumulation to rebuild the eroded dunes at Semaphore South between Arthur Street and Hart Street. More than 8000 plants were planted and drift net fencing was installed to stabilise the dunes. Sprinkler irrigation was installed to help the plants establish and grow and also help to reduce wind-blown sand.

Semaphore South dunes after restoration

When was the work done?

Dune restoration works commenced in late April 2020 and was completed in August 2020.

  • Sand was moved from the beach between the Largs Bay jetty and Strathfield Tce, Largs North to rebuild the sand dunes at Semaphore South. (Stage 1 – completed end May 2020)
  • Drift net fencing, revegetation and sprinkler irrigation was installed to help stabilise the dunes. (Stage 2 – fencing and irrigation completed July 2020 / revegetation (planting) completed August 2020)
  • The City of Port Adelaide Enfield replaced the boundary fence and upgraded the turf and irrigation in the area.  

This timing ensured that a buffer was in place to protect the area for winter when storms causing further erosion are more likely. 

semaphore-south-dune-restoration-graphic.jpg

How is the sand being stabilised?

The sand was shaped to match the normal dune profile along Adelaide’s metropolitan coast (i.e. a raised fore dune area and lower rear dune / swale behind).

Stabilisation has followed well established methods using drift net fencing and revegetation (refer for example to the Tasmanian Coastal Works Manual for further information).

  • Standard drift net fences, as used extensively in other council areas, has been erected along the dune, parallel to the beach. The drift fences help to trap windblown sand coming from the beach and dune face and provide a more stable environment for establishment of vegetation. 
  • During Stage 1 of the project (moving sand to rebuild the dune) beach-cast seagrass wrack, left on the beach by tides and waves in the vicinity of the works, was incorporated into the replenished dune at Semaphore South. The seagrass wrack provides some organic nutrients, assisting plant growth and it also helps to bind the sand.
  • Extensive revegetation followed, in accord with a species list and planting plan provided by independent ecologists who completed a comprehensive vegetation survey of the Largs Bay coast.
  • Ongoing weed control and further planting will occur in subsequent years to increase the biodiversity and resilience of this area. 

These techniques have been used with great success to re-establish dunes on Adelaide’s southern beaches, such as at Seacliff and South Brighton (example pictured below).

Shorehame Road - dune restoration comparison
Dune restoration examples at Shoreham Road, South Brighton taken in 1985 and 2018. Photos courtesy City of Holdfast Bay.

Is this a one-off project or will it be a regular occurrence?

The project to re-establish the Semaphore South dunes using sand from north of the Largs Bay jetty is a one-off trial project. However, as has occurred in the past, the seaward part of the Semaphore South dunes will erode from time to time and will require periodic replenishment with smaller amounts of sand to maintain the foredune area that is directly exposed to coastal processes. The sand used to replenish Semaphore South in the past has been moved back from the area around Semaphore jetty.

Testing during 2019 showed that sand to the north of Largs Bay jetty is suitable for replenishing the Semaphore South area. This is another possible location where sand can periodically be moved back to replenish and maintain the Semaphore South dunes.

Sand from the Semaphore South breakwater was used to top up the dunes in March/April 2021.

sem-south-april-2021

Semaphore South reserve area looking south towards Noonies Cafe, April 2021

What do I need to do when I visit this area?

The community and beach users are asked to:

  • Take extra care on the beaches and adhere to safety signs and directions. 
  • Stay off the dunes. This is important so that the sand remains stable and there is no damage to plants. 

See: Why have some of the beach access tracks been raised in height?

restored dunes at Semaphore South

Is there a long-term solution planned for the Semaphore South erosion problem?

The project to rebuild and stabilise the Semaphore South dunes using sand from north of the Largs Bay jetty will not negate the need for ongoing periodic replenishment of the Semaphore South area.

The dune will still be subject to coastal processes, including erosion during storm events, and it is expected that replenishment (on a smaller scale) will continue to be required on a periodic basis to maintain the foredune.

Semaphore South has previously been replenished using sand from the Semaphore jetty area. Testing during 2019 showed that sand to the north of Largs Bay jetty is suitable for replenishing the Semaphore South area. This is another possible location where sand can periodically be moved back to replenish and maintain the Semaphore South dunes.

semaphore_south_jan2021

What community engagement has occurred?

The department has been working closely with the City of Port Adelaide Enfield and community representatives on the project. Community input was sought in March 2020 on the proposal, and the majority of feedback received supported the work to restore the dunes at Semaphore South.

The department regularly meets with community representatives and council to help guide the Semaphore sand movement works. 

ADDITIONAL INFORMATION

Why have some of the beach access tracks been raised in height?

Some of the beach access tracks at Semaphore South were raised with sand to match the shape of the newly placed dunes. This has been done to ensure that all of this stretch of foreshore is protected from flooding and erosion. The department and council will monitor the area and assess this trial replenishment over the coming months.

The beach access path nearest to Noonies Café has not been raised in height with imported sand and a blue accessible mat is at this location.  

We apologise for any inconvenience caused.  

sem-south-10feb2021

How do you know that the sand to the north of the Largs Bay jetty is suitable? 

Historically, sand at the northern end of Largs Bay has proven to be unsuitable for replenishing beaches further south on the Adelaide coast as it is finer and would wash away more quickly. The most recent sand sampling analysis (2019) indicated that sand in the area immediately north of Largs Bay jetty has become coarser over time and is now suitable for replenishing the dunes at Semaphore South.

Large volumes of coarse sand from external sources were added to the southern beaches in the 1990s. It is possible that this sand has now moved northwards into this section of the coast.

Largs Bay dunes
Largs Bay beach and dunes, April 2020

How much sand has been moved from Largs Bay to Semaphore South?

An estimated 34,000 cubic metres (m³) of sand has been moved to Semaphore South to rebuild the Semaphore South dunes.  This includes approximately 30,000 m³ from the beach north of the Largs Bay jetty to Strathfield Tce, Largs North and approximately 4,000 m³ moved from around the Largs Bay jetty. The new sand dunes have been built to resemble the wider dunes to the north of Hart Street in size and width.

Could seaweed bales be used to help stabilise the newly placed sand?

Not as part of the trial. Baling seagrass is labour-intensive and beach cast seagrass wrack plays an important ecological function in the coastal environment.

During Stage 1 of the project (moving sand to rebuild the dune) beach-cast seagrass wrack left on the beach by tides and waves in the vicinity of the works was incorporated into the replenished dune at Semaphore South. The seagrass wrack will provide some organic nutrients, assisting plant growth and it will also help to bind the sand.

What impact will this have to the dunes north of the Largs Bay jetty?

The approach for managing Adelaide’s beaches is based on expert advice underpinned by decades of collected data, including current and historical survey information on beach profiles and independent technical reports. A team of environmental specialists monitor and assess Adelaide’s beaches regularly. Annual beach surveys are undertaken along the coast to measure the beach profiles and assess if there is a sufficient dune volume buffer in the area.

  • The government has assessed that any environmental impacts on the coastline north of Largs Bay jetty as a result of the proposed removal of the volume of sand required to rebuild the Semaphore South dunes would be minor.
  • The stabilisation and revegetation of the Semaphore South dunes will create significant environmental benefits associated with improved biodiversity outcomes. 
  • At the location north of the Largs Bay jetty, the government’s beach profile data indicates that there is sufficient dune volume buffer to allow removal of the sand required to rebuild the Semaphore South dunes.
  • Based on previous experience and monitoring of areas where sand has been collected, the impact on the dunes of removing sand from the intertidal zone as part of the trial is expected to be minimal.
  • The area will steadily build up again through natural processes.
  • The department engaged independent ecologists to undertake a vegetation survey in March 2020 to assess and map the flora communities along the 5km length of coast between Semaphore Surf Life Saving Club and Strathfield Terrace. See: ‘Has an assessment of the dune vegetation been done?
  •  This baseline information has informed species selection for the planting of the restored Semaphore South dunes, will enable any impacts of the trial to be monitored, and will inform future works.
  • The field assessment includes gathering information about native species present, weed species present and their cover category, native plant life forms, native/exotic understory biomass, tree health, etc. Incidental fauna observations were also recorded.

An independent review is also being undertaken to assess the impacts of moving sand from the northern beaches.

Largs Bay Jetty looking north
Largs Bay jetty and beach looking north, April 2020

Why was sand removed around the Largs Bay jetty?

In May 2020 the department undertook routine maintenance work to remove sand around the Largs Bay jetty on behalf of the Department of Transport and Infrastructure (DIT). This is necessary maintenance work in order for DIT to assess the condition of the jetty and undertake any repairs that may be needed to keep the jetty in good condition. 

The works were undertaken at this time to achieve cost benefits given the contractors were present on site to undertake the works for Semaphore South.

Why is water pooling around the Largs Bay jetty at times?

The nature of the excavation around the jetty, carried out to protect the jetty, means that water may be present from time to time.  This is largely dictated by the adjacent seawater level, and any recent rain. When the tide is very high, water may pool from either rain, or as it rises up from the underlying sea water table. When the tide lowers water is free to drain away. There is also a possibility that storm surges could flow over the low dune at the seaward end of the jetty and deposit seagrass into the lower area behind. This has happened at Semaphore jetty for similar operations.

The department is monitoring the area regularly to assess if pooling water is an ongoing problem.

More information