Published on 07 November 2021

Responding to rising seas in Onkaparinga

The City of Onkaparinga’s Coastal Adaptation Study has revealed how Onkaparinga’s 31-kilometre coastline might be affected by sea level rise over the next 80 years.

The study shows how people, the natural environment and built assets might be impacted by rising sea levels so that council and other stakeholders, such as state and federal government and private landowners, can plan now for any future changes.

City of Onkaparinga Mayor Erin Thompson says the study puts council in a strong position to respond to future climate risks.

“We’ve been taking action on climate adaptation for decades, but this study will ensure we’re on the front foot in understanding sea-level rise impacts across our coastline,” she says.

“Our coastline is generally elevated and therefore most areas won’t be vulnerable to sea-water flooding, but the study does reveal our main challenge will be responding to erosion issues, and limited areas may experience some inundation.

“Getting ahead of the game by taking a planned response to the study will allow us to keep our communities and environment safe and save us and our ratepayers money in the longer term.”

“Partnering with both the state and federal governments will also be important in addressing the long-term issues of coastal impacts.”

Coastal adaptation involves adjusting council’s practices in response to the impacts of the current and expected climate.

This could include ensuring assets or services aren’t in areas that could be impacted, progressively moving assets or services away from these areas, modifying construction of assets or how services are delivered, or installing protection or defence infrastructure to reduce the impact of coastal hazards.

The study splits Onkaparinga’s coastline into 12 distinct sections or “cells” in recognition of the fact that our coastline varies from place to place, to make it easy for residents to learn more about the section of coastline near them, and to help council focus on the different adaptations that might be required for each section.

“Releasing this study is the first phase in a longer-term program of community engagement, and our next step is to work with coastal experts to identify adaptation options before we come back to the community to get their feedback,” Mayor Thompson says.

“This information will feed into a Coastal Adaptation Plan, which the community will also have an opportunity to comment on.”

You can read the study and a summary of its findings at City of Onkaparinga’s Your Say page, where you can also find a fact sheet for each of the 12 coastal cells along Onkaparinga’s coast.

Key findings

• Sea levels have been rising at 4-5mm in Onkaparinga since the 1990s, which is faster than the previous decades (1950s to 1980s), but the rate of sea level rise isn’t projected to significantly accelerate until after 2050.
• While most of Onkaparinga’s coastline won’t be vulnerable to sea-water flooding (inundation), exceptions are Onkaparinga River, Pedler Creek and the Aldinga Washpool, if seas rise as projected after 2050.
• In most places, public roads are positioned between the coast and private assets. This means most risks will initially impact council-owned assets, but private infrastructure, public safety, and the health of Onkaparinga’s ecosystems could also be impacted in the longer term.
• If seas rise as projected, then beaches and soft sediment cliffs will increasingly undergo recession and eventually place infrastructure at risk.
• Onkaparinga’s coastline is characterised as moderate to high erodibility, depending on the location. Exceptions are the Lonsdale region, which is characterised as low-erodibility, and the Seaford Cliffs, which are characterised as very high erodibility.
• The coastline has been relatively stable over the past 70 years, but there are some pockets of erosion, for example Snapper Point near Aldinga Reef.
• The study has created a baseline understanding as to how the coast has operated over time. Ongoing monitoring of the coast will provide the basis for making timely, cost-effective adaptations to the coast where feasible.