A NEW 1km-long seawall, from the end of the existing rock wall through to Flat Rocks and a 145-metre breakwater, located 150m to 200m off the main surf beach at Inverloch...
These are among the mitigation measures proposed in the final (Stage 1) report from the Cape-to-Cape Resilience Project in response to existing coastal erosion and future projections of the impact of sea-level rise, tides, and storm surges, through to 2040.
Desperate times call for desperate measures, it seems.
Where the proposed breakwater is concerned, the report authors acknowledge that it may not be popular with beachgoers because: “Surfing and beach use would be impacted by reduced wave energy.”
What's a surf beach without waves, some have been prompted to ask.
But, overall, the report largely gives Inverloch a fairly clean bill of health, while identifying some problem areas:
“The town of Inverloch is mostly above the coastal inundation extent with the exception of areas around Wreck Creek and the Surf Beach residential area, around the boat ramp and jetty, and around the Screw Creek and Broadbeach Estate. The area around Wreck Creek is considered to be particularly vulnerable due to the recent erosion of the barrier dune. This dune currently limits direct coastal inundation of Wreck Creek and the residential area between Bunurong Road and the Surf Life Saving Club.”
But work is needed to protect degraded dune systems, roads and residential areas, impacted now and likely to be impacted in the future.
The report also acknowledges massive sand movement:
"Significant change has occurred on the sea floor of the entrance to Anderson Inlet between data captured in 2008/09 and present day. A massive amount of sand has been lost resulting in a net change of -1.8 million m3 in the overlapping surveys. Change is not even across the entrance. Some areas have significant volume loss (eg the Surf Beach) and others have volume gain (Ayr Creek lagoon). Entrance channels have in general, become deeper and wider. However the main channel is wider and flatter across the outer bar. Significant loss of sand is noted upstream. It is unknown if this reduction continues within the Inlet or is limited to the entrance area."
A range of engineering solutions have been proposed, although sadly, not specifically recommended or costed, all with positives and negatives, and there are likely to be legislation and planning controls to follow to limit exposure to present-day and future risks.
The lengthy and technically complex report was released on Wednesday, October 12, 2022, more than two months after the reports were promised, leaving little time to lobby political parties for funding commitments ahead of the November 26 state election.
The measures proposed in the report will need annual allocations of significant maintenance funding as well as major capital works funding if rock walls, breakwaters and groynes are to be part of the solution.
South Gippsland Conservation Society representative on the Cape-to-Cape stakeholders reference group, Phillip Heath said his group welcomed the report but was disappointed it didn't provide its recommendations on preferred actions.
"The fact that the study group hasn't provided recommendations indicates it will have to go through another process. None of the proposals have been costed either which makes it tough to get commitments from the political parties and candidates.
"We're encouraged that a number of the proposals include dune nourishment works but we would like to see some of that work going ahead sooner rather than later, while we wait for a plan to be formulated.
"The work that Parks Victoria has carried out on either side of the rock wall has been excellent and we'd like to see more of that.
"A lot of the erosion and damage to the beach and dunes has been caused by the combination of south-westerly storms and high tides and while that has slowed a bit in recent times, after more easterlies in the past year, we're concerned this has only delayed what is an ongoing sitiation.
"We're still losing mature coastal banksias at Flat Rocks. We need to be taking action to at least save those trees as the only thing protecting Cape Paterson-Inverloch Road."
Mr Heath said he personally saw merit in the proposal to construct a long groyne, up to 1/2km out into the sea from Norman Point, but the project team has listed a large number of "negatives" including complexity of construction and the need for an on-going investment in dune nourishment.
He said the South Gippsland Conservation Society would develop a comprehensive response to the report, with its own priority suggestions.
While the long-version of the reports are yet to be published, several summaries about hazard assessment and possible adaption measures have been provided.
The Adaptation Actions Technical Assessment Report Summary (October 2022) notes that erosion poses a high-level risk to the dunes, impacted roads and parts of the existing residential area:
“Coastal hazards along the Inverloch coastline pose a high level of risk to Bunurong Road and the Surf Beach dunes between Flat Rocks and Point Norman. Natural erosion and variation of the dunes along this coastline is driven by eastward sediment transport and storm activities.
“The coastal environment between Flat Rocks and Point Norman is close to 100% natural with only the (relatively) short seawalls at Bunurong Road and at the Inverloch Surf Life Saving Club interfering with the existing coastal processes. Any engineered adaptation actions implemented along this coastline would be designed to alter these natural processes to protect values, assets and infrastructure located in the hazard zone.
“Adaptation actions to address this coastal erosion for the current planning horizon and sea level (present day to 2040) have been assessed to allow for future coastal adaptation.”
Among other things, the Inundation Report Summary (October 2022) says this about the risk posed to the Inverloch township:
Inverloch township
“Flooding within Inverloch from direct coastal inundation is limited to the low-lying areas (at or near sea level) around Screw Creek and Wreck Creek where there is also limited stormwater drainage.
“The Ayr Creek catchment is steeper with limited low-lying areas, which means that there are fewer areas exposed to coastal flooding.
“Localised flooding can occur within the town where the elevated coastal water levels prevent the free drainage of the stormwater system. This is particularly evident around Wreck Creek where the land is flatter and stormwater drainage is limited.
“Flooding as a result of elevated coastal water levels is also observed around the Inverloch Jetty and Boat ramp and Screw Creek/Broadbeach estate. Prediction of future flooding associated with Wreck Creek is complex. Currently, the dunes protect the backshore areas from direct coastal inundation but ongoing existing erosion and predicted future erosion will likely reduce or completely remove these dunes allowing direct inundation to occur.”
Summary of Findings: Inundation Hazard
This investigation has provided us with a better understanding of the factors that influence coastal inundation and the potential extent of flooding in the Cape-to-Cape area. Coastal inundation is driven by elevated tidal levels from the ocean, combined in some locations with runoff from the catchment.
The catchment to Anderson Inlet is extensive and inflows from the Tarwin River can cause widespread flooding without the addition of elevated coastal waters. Tidal levees within Anderson Inlet are unlikely to be enough to prevent flooding during combined future coastal and catchment extreme flood events.
Additional inundation over or around the levees is expected as sea levels rise across the coming century. However, the major residential areas within the inlet at Tarwin Lower and Venus Bay are almost completely above the predicted coastal inundation extents for all scenarios assessed.
Inundation is limited to agricultural land and some isolated properties, however the main roads connecting Tarwin Lower and Venus Bay to major services are likely to be flooded during inundation events.
The town of Inverloch is mostly above the coastal inundation extent with the exception of areas around Wreck Creek and the Surf Beach residential area, around the boat ramp and jetty, and around the Screw Creek and Broadbeach Estate.
The area around Wreck Creek is considered to be particularly vulnerable due to the recent erosion of the barrier dune. This dune currently limits direct coastal inundation of Wreck Creek and the residential area between Bunurong Road and the Surf Life Saving Club. Inundation along the open coast is limited due to the height of the Bunurong Road and sand dunes from Point Smythe to Cape Liptrap
Measures proposed
Bunurong Road Beach Renourishment:
Between 75,000 to 100,000m3 would be required for a “sacrificial” beach to prevent erosion in 2% AEP storm. Ongoing re-nourishment of up to this volume may be needed annually.
* Positives: Natural alongshore transport of sand would benefit Surf Beach. Can be short term whilst adaptation planning is progressed. Less chance of long-term impacts (reversible). Allows natural processes to take place
* Negatives: Risk of impact on Anderson Inlet entrance dynamics with the continued large volume of imported sand for nourishment. High risk of impacts on intertidal habitat at Flat Rocks. Potential to increase inundation risk landward of Bunurong Rd through blocking of existing drainage paths. Change in beach amenity from wide flat beach to high dune. Requires investment in regular maintenance nourishment, possibly 100% of the original volume/year. Community could view the need to regularly renourish as a failure of the action
Seawall
Bunurong Rd Seawall 1,020m of seawall would be required to protect the entire length of the beach/road interface. Design would need to address drainage.
* Positives: Would prevent structural failure of road and protect utilities in the road reserve Can be short term whilst adaptation planning is progressed.
* Negatives: Would need to be up to 1km long; difficult to remove in future. Over wash in a storm could still occur, damaging the top of the wall making the road unsafe. Potential to increase inundation risk landward of Bunurong Rd if drainage not maintained/improved. The beach/seabed in front would be lowered (toe scour); beach/dune at the ends would also scour. Significant impacts on beach views, and use/access, particularly at high tide.
Surf Beach Groynes and Nourishment
Three groynes of 180 – 210m along Surf Beach, with additional groynes likely into Anderson Inlet over time. Groynes would need to be around 4m high and 14m wide at seaward end. Initial beach nourishment of 100,000 – 200,000m3 could be required to allow for realignment to provide an ongoing sacrificial beach.
* Positives: Would prevent erosion of existing beach and dune, maintain vegetation buffer Can be constructed using geotextile containers for reversibility & reduced visual impact.
* Negatives: Groynes would have a significant visual impact on the beach, especially at low tide. Beach would be very wide in parts, up to 80m. Inundation risks to land behind Surf Beach due to impacts on drainage of Wreck Creek. Significant impacts on beach views. Change in beach amenity from wide flat beach to steep sloping dune. Groyne closest to Point Norman could cause the main entrance channel to move and deepen Potential erosion of beach east of Point Norman, moving channel closer to Surf Parade and Veronica Street. Impact on Point Norman coastline uncertain Requires investment in ongoing renourishment
Nearshore breakwaters
Conceptual configuration of eight nearshore breakwaters, ranging between 80m and 145m long, 4 to 4.5m high, placed 150m to 200m from the present day shore.
* Positives: Would protect the coastline by reducing wave energy reaching the shore. Reduction in alongshore sediment transport. No structures on the beach.
* Negatives: Significant visual impact on the beach Significant change in beach type along the shore (i.e. impact waves coming ashore). Coast to the east of last breakwater would recede due to lack of incoming sediment from the west. Additional coastal protection or groyne field may be required in future to the east. Complex entrance dynamics create uncertainty of significant impacts on eastern side end of breakwaters. Potential for beach to join a breakwater and block sediment transport eastward Very large rock required may be difficult to source. Sacrificial beach would require monitoring and regular renourishment. Inundation risks to land behind Surf Beach due to impacts on drainage of Wreck Creek. Surfing and beach use would be impacted by reduced wave energy. Beach may widen significantly and may not be liked by beach users. Breakwaters would need to be exposed above low tide level by 2 to 2.5m. A lot of seaweed may get trapped between the breakwaters and the beach. Likely erosion of adjacent beaches
Long Groyne Point Norman
Groyne 300m to 500m is required to anchor enough sediment to realign the beach and protect the coastline at Wreck Creek. Beach nourishment volumes required to achieve this protection are unknown and the initial storm demand of approximately 100,000m3 would be required to be managed or increased as the beach realigned.
* Positives: May reduce the rate of dune recession. May provide a buffer to the existing dune during storm events. Minimise loss of sand towards Anderson Inlet.
* Negatives: Beach width 50 – 80m at initial sacrificial nourishment. Potentially a very wide beach may form along the groyne at Point Norman. Significant impact on beach aesthetics. Inundation risks to land behind Surf Beach due to impacts on drainage of Wreck Creek from sacrificial nourishment. May require regular relocation of sand from near the groyne back towards Bunurong Rd. Complex entrance dynamics = uncertainty of significant impacts on eastern side of groyne. Coast to the west would erode, receding toward Surf Parade. Tidal channel may form along east side of groyne. Entrance channel could move closer and deepen at groyne, construction complexity would be high to minimise risk of structural failure. Additional coastal protection or groyne field may be required in future. Significant impacts on beach views. Unlikely to provide stable beach protection, especially at the western end of Surf Beach. Size of rocks/sandbags required would be difficult to source & construction complicated. Potential beach very wide adjacent to groyne. Requires investment in ongoing renourishment and sand relocation
Engineering adaptation recommendations
All five engineered coastal protection actions have advantages and disadvantages. They all require significant capital works and ongoing maintenance costs.
All come with risks and many of the impacts cannot be predicted or modelled because of the entrance dynamics and variability of the future wave and storm climate. The options recommended below allow for community and stakeholder decision making when developing the future adaptation pathways
Flat Rocks to Bunurong Rd
Pathway 1: Keep Bunurong Road in current position
* Design a seawall suitable for future (2100+) conditions. Include allowance for drainage, including tidal gates. Consider height of the existing road, raise the road above future inundation.
Pathway 2: Relocate Bunurong Road
* Design a seawall suitable for short term protection of the road (and services) for the full length. Identify erosion trigger levels for when construction works should be undertaken - ensuring that construction does not create multiple disconnected seawalls. Assess feasibility of stockpiling rock or geotextile bags to prepare for rapid response to erosion.
Bunurong Road Seawall to Point Norman
* Pathway 1: Maintain existing dune and beach amenity for long term planning horizon (2100+)
Groyne field – constructed to create pocket beaches Initial large scale beach renourishment Regular renourishment sand management Include drainage pathways for Wreck Creek.
* Pathway 2: Plan for retreat
Identify triggers for beach renourishment during retreat planning Assess feasibility of annual renourishment and sand management Establish contracts and permits to enable rapid response when required.
Stage 2 options to follow
The project team is now into Stage 2 of the project, and Alluvium (Alluvium Consulting is a leader in the provision of catchment, river and coastal analysis and advice) is continuing to lead the extensive stakeholder and community engagement planned for this phase.
The aim of Stage 2 is to develop the Cape to Cape Resilience Plan, which will include adaptation pathways for the region (out to 2100+) and implementation planning.
The project group will run community engagement events over the next six months, including popping up around the region over summer, open house/drop in sessions, online engagement and stakeholder and community roundtables. They intend to speak to the community and stakeholders over this period to help inform this critical piece of strategic planning for the Cape to Cape region.
