Bribie Island Breakthrough

Georgia Keeshan

The Bribie Island National Park is a sand barrier island separating the Pumicestone Passage from the Coral Sea. Other similar islands off south-east Queensland are Moreton Island, the Stradbroke Islands and Fraser Island.

The northern end of the island spit is narrow and historically there was a significant risk of overtopping and breakthrough of the spit which is categorised by a narrowing of the land over recent times. The northern entrance of Bribie Island has changed location over the years, ranging from extreme south in 1962 to its current extreme northern position that was in place during the recent breakthrough event.

Erosive pressures on the sand island spit are at their highest during summer months when moist tropical air flows generate storm events such as ’East Coast Lows‘ and occasional cyclones which typically increase the loss of sand on the eastern side of the island. During accretion season, typically the winter months, calmer conditions will increase volumes of sand on the eastern beaches. However, the rates or erosion have typically been higher here than the rates of accretion, leading to a narrowing of the spit and increasing the risk of the breakthrough scenario.

Another factor impacting the northern spit of Bribie Island is the prevailing sediment transport directions at this location. Sediment investigations in the northern area of Bribie Island, and around the Kings Beach headland, show that the typical net northerly transport of sediment which dominates the east coast of Australia is not the directional transport flow here. This net southerly transport can be seen by the increasing width of Bribie Island in the south, and the groyne at Kings Beach which is on the southern side of the beach and has accreting sand on its northern side.

The northern spit is managed by different government and public stakeholders and these stakeholders play an important role in how this area is to be managed in the future. The Queensland Government is responsible for the management and monitoring of Bribie Island as a National Park, with Council responsible for the management of the Golden Beach foreshore which is a designated Coastal Management District. Golden Beach is situated south of Caloundra and features a significant population of residents on the foreshore and in the canal and lake estate of Pelican Waters. Golden Beach and the Caloundra headland have also been a major tourism destination for the Sunshine Coast since the early 1900s and the associated businesses are major stakeholders in this region.

In the 2020/2021 storm season a wash over occurred on the sand spit during a significant swell event. The wash over consisted of ocean swells breaching the sandy dunal system at the northern spit and these ocean generated waves entered the passage. The vegetation at the wash over location was lost and the sand levels reduced to just above the highest astronomical tide (HAT), however a full breach did not occur during this time.

In January 2022 a large swell event affected south-east Queensland and the ocean swells broke through the wash over location on the spit and breached the Pumicestone Passage. This event created the current southern breakthrough location that is visible on site today. The breakthrough began as a 50m wide channel that entered the passage however with the persistent La Nina weather pattern during this storm season the breakthrough has now increased to almost a 500m wide channel.

The new breakthrough at this location is now becoming the dominant entrance to the passage and many changes are beginning to be observed along the foreshore of Golden Beach. Some changes that have been observed since the breakthrough are: the predicted tidal heights and timeframes, the formation of a sandy delta on the western side of the new breakthrough site, channel locations in the passage, narrowing of the northern entrance channel, and swell propagation to areas of the foreshore that were typically only affected by wind-generated waves from the passage. As the breakthrough locations stabilise these new impacts will change over time, and it is possible that the conditions we see today may not persist into the future.

Bribie Island National Park is managed by the Queensland State Government’s Department of Environment and Science, therefore Council’s planning focus is on the foreshore along Golden Beach. In 2009 Council commissioned a risk assessment which identified various breakthrough scenarios and the potential risk that these posed to the foreshore along Golden Beach. The report also detailed the State Government’s position at that time to let natural processes run their course and for Council to manage the impacts to the foreshore.

This report was followed in 2014 by the Shoreline Erosion Management Plan (SEMP) which was intended as a document to provide detail around Council’s preferred management options for addressing the current and potential coastal erosion issues and the management responses for these sites. The SEMP focused on a 10-year time period and considered current erosion issues associated with a defined 2% AEP storm event (significant wave height). The SEMP identified that the beach units along Golden Beach were all susceptible to changes and erosive pressure in the event of a breakthrough scenario occurring.

An action of the SEMP was to conduct regular monitoring of the Golden Beach foreshore which was funded and conducted since 2015. Prebreakthrough the sand beaches along this foreshore were typically impacted by channel migration and wave generated waves. An additional action was to conduct more investigations regarding the potential Bribie Island Breakthrough and provide a detailed response plan should a breakthrough occur.

The Golden Beach and Bribie Island Breakthrough – Options, Design, Approvals and Investment Plan was completed in January 2015 as a recommendation of the SEMP. The plan was intended to provide: identification of viable options for addressing the potential impacts from a permanent breakthrough, a clear preferred approach by council to shoreline management implementation, a document for public consultation, information for statutory bodies to evaluate proposed actions, and supporting information for any future development approval applications. The key threat identified in this report was the change to tidal regime within the passage which would lead to increased risk of tidal inundation associated with storm tide events, with greater emphasis on lowlying land and assets in this location.

Implementation of the Plan began after the report was endorsed by council, and the main driver of implementation was the inclusion of the Plan’s recommendations in the 10year capital program. Although a breakthrough had not been realised at the time of the report, the threat posed by a potential breakthrough was significant enough to warrant inclusion in the capital program to ensure upgrades of ‘at risk’ assets were provided in a timely fashion, and for reactionary works to be limited if the breakthrough scenario was realised.

The process implemented by council along the Golden Beach foreshore has shown the importance of planning for risks associated with naturally occurring events with unpredictable timeframes. Early identification of the hazard was an important first step in the development of this issue as without identification of a hazard there could be no risk assessment produced. The risk assessment was then used to further identify and scope the potential hazard and to provide council with a document detailing the four most likely breakthrough sites for monitoring. From this position council was able to model which sites provided the greatest risk, which breakthrough site was most likely, what the short- and long-term threats to the foreshore were, and then recommend the production of an erosion management plan. Once the risk assessment had been completed council was able to produce the detailed document (the Plan) that was utilised to implement funding and works. These steps proved vital for council to provide a proactive response to the breakthrough.

The immediate future of the Pumicestone Passage, Bribie Island and the Golden Beach foreshore is still uncertain. It is worth noting that the reports listed in this article do not discount the possibility of further breakthroughs occurring at another of the four sites identified in the 2009 report (Site 1 is the current breach). The tidal regime is unlikely to stabilise in the short term and channels in the passage are likely to modify over the medium term. It is still uncertain how much of the sandy delta will protect the foreshore from ocean swell propagating into the passage and the final form of the northern entrance is changing every day. Council faces significant challenges in managing this dynamic situation but the progression of the planning documentation over the last 15 years has facilitated a scientific and proactive approach for council to utilise. Further engagement of both state government and affected residents and businesses will prove vital in future management decisions along this dynamic coastline and estuary environment.