The Alexandrina 25km coastline has significant cultural, social, environmental, and economic value to our local community, the Ngarrindjeri Nation and visitors to the region. Rising sea levels will exacerbate erosion and seawater inundation in many coastal areas.

Horseshoe Bay in Port Elliot is one beach identified as a priority action area in our 2022 Coastal Adaptation Plan

Recent investigations indicate the shoreline will recede by up to 12m by 2050 and 25m by 2100, placing built infrastructure at risk.

An independent coastal engineer appointed by Council has investigated the projected impacts of coastal hazards such as sea level rise and coastal erosion in the Bay.

• Option 1: Raising the breakwater height and length.
• Option 2: Constructing a submerged breakwater - nature-based or artificial reef - in the near shore,
• Options 3A and 3B: Sand nourishment only (from the eastern side of Horseshoe Bay or from external sources)
• Option 4A and 4B: Replacing and extending the existing seawall (with or without sand nourishment)
• Option 5: Managed relocation of assets.

Cape Advisory has also provided initial estimates of possible capital and operational costs for four of the options.

We are seeking community feedback on these presented options.

Council has not formed a view regarding the options and is consulting with key stakeholders and the community to understand their views.

The area of Horseshoe Bay that houses built infrastructure is Crown Land under Council Care and Control. The State Government's Coast Protection Board has ‘power of direction’ over this area and will have the final say.

Option 1 – Raise existing breakwater height and length

Capital cost: $17.9M
30-year operating cost: $0.6M

• Effective at reducing recession due to storm erosion in short term
• Limits disruption to the beach.

• Does not reduce recession due to sea level rise in the longer term
• Does not improve recession on most eastern extent
• Expensive ($18.5M) and challenging to construct
• Impact to sea grass and heritage breakwater.

*None of the options are presented in order of preference or priority.

Option 2 – Offshore underwater breakwater in the nearshore

Capital cost: $9.6M
30-year operating cost: $0.3M

• Nature-based recreation opportunities
• Limits disruption to the beach.

• Does not reduce recession due to storm surge or sea level rise in the longer term
• Challenging design and construction
• Increase erosion risk east and west of the structure.

*None of the options are presented in order of preference or priority.

Option 3A – Sand nourishment (bay sand)

This option intends to increase the volume of sand in the central part of the Bay by borrowing sand from the eastern part of the Bay (beach, not dunes, in the short term) to increase capacity for erosion during storms. In this case, the beach profile on the eastern side of the bay would retreat by 10m to provide the source of sand.
Consultants advised that this option is effective in the short term and moderately effective in the long term.

Capital cost: $0M
Operating cost: $0.6M ongoing every 1-5 years, depending on seasonal weather impacts.

• Effective in reducing recession in the short term
• Moderate reduction in recession due to sea level rise in the longer term
• Lower comparable longer-term costs
• Flexible (for example, Council can pursue this option now and decide to do something else later)
• Increase beach width in high-use area
• Retains beach amenity
• Promotes the environmental value of the foreshore and dunes.

• Disruption to beach during sand operations
• In the long term the eastern part of the Bay will retreat due to the sourcing sand and eventually impact the dunes
• Council requires State Government authority approval for each campaign (every 1-5years).

*None of the options are presented in order of preference or priority.

Option 3B – Sand nourishment (quarry sand)

Capital cost: $0M
Operating cost: $1.25M-$5.4M (every 1-5 years, depending on seasonal weather impacts)

Pros

• No impact to the eastern section of the bay as it is using external sand.
• Effective at reducing recession in the short term
• Moderate reduction in recession due to sea level rise in the longer term
• Lower comparable longer-term costs
• Flexible (for example, Council can pursue this option now and decide to do something else later)
• Increase beach width in high-use area
• Retains beach amenity
• Promotes environmental value of the foreshore and dunes.

Cons

• Disruption to the beach during sand operations
• Need authority approval for each campaign (every 1-5years).

*None of the options are presented in order of preference or priority.

Option 4a – Buried seawall & existing seawall replacement

This option intends to ‘hold the line’ against beach erosion by extending the vertical seawall. This will remove the risk to built infrastructure. However, the beach in front of the wall will continue to erode/deepen and reduce the amount of usable beach over time. Consultants advise this option would be effective in the short term and in the longer term.

Capital cost: $3.8M
30-year operating cost: $0.62M

• ‘Holds the line’ - limits recession past the wall in the short and longer term
• Straightforward approvals, design and construction.

• Loss of useable beach over time (as sand erodes in front of the wall)
• Impact on beach amenity (subjective)
• Impact on the beach environment
• Significant rebuild costs after 30 years
• Disruption during construction
• Significant loss of useable sand beach (impact tourism and use).

*None of the options are presented in order of preference or priority.

Buried seawall and existing seawall replacement with nourishment

This option intends to ‘hold the line’ against beach erosion by extending the vertical seawall but also bringing in approximately 9000m3 of external sand to better maintain a useable sandy beach.

Consultants advised that this option would be effective in the short term and the longer term.

Capital cost: $3.6M
30-year operating cost: $0.2M

• ‘Holds the line’ - limits recession past the wall in the short and longer term
• Straightforward approvals, design and construction
• Maintains useable sandy beach
• Lower comparable rebuild cost due to maintenance of sand in front of wall.

• Impact of sea wall on beach amenity (subjective)
• Impact on the beach environment
• Disruption during construction and sand campaigns.

*None of the options are presented in order of preference or priority.

Option 5 – Managed relocation of assets

This option would seek to progressively move buildings and assets out of the areas were shoreline recession is anticipated. This would need to be done collaboratively with asset users. Due to complexities of this option, it was not costed or assessed in detail by the consultant.

Consultant advises that this option would be effective in the short term and the longer term.

Due to the complexities of this option, it was not costed or assessed in detail by the consultant.

• Removes assets and infrastructure from the recession areas (mitigates the hazard)
• Allows natural bay processes to occur
• Does not interfere with habitat.

• Community may not wish for assets to be moved
• Challenging to implement
• Sea level rise beyond 2100 may still require protection.

*None of the options are presented in order of preference or priority.