Ask Dr Dazza | Bribie breakthrough

OUR coastlines are always changing naturally, particularly near surf bars. It is part of what makes our coastline unique. There are plenty of places that I grew up fishing on Moreton and North Stradbroke Islands that simply no longer exist.

Coastlines can change rapidly in response to heavy weather. Last year, Bribie Island broke through opposite Golden Beach, but the opening proved to be short-lived.

Spring tides in combination with swells from ex-tropical cyclone Seth have resulted in a breakthrough in the same area and a permanent North and South Bribie Island is now a distinct reality. It was always a question of not if – but when. Previous research by QUT identified that during the past 60 years, the eastern shoreline of the northern part of Bribie Island has retreated landward by approximately 75m. There are also several other potential locations on (south) Bribie Island where further breakthroughs may occur.

Pumicestone Passage itself is relatively young in geological terms and was formed when a river valley that was fed by Elimbah, Bullock and Ningi Creeks was flooded by the rising sea. Since its formation, Pumicestone Passage has been becoming generally shallower as sediment comes in from the various creeks as well as well as from Moreton Bay and the ocean.

The new breakthrough if it remains open is likely to have several impacts on Pumicestone Passage including to the mainland shoreline directly adjacent to it. The potential impacts are highly unlikely to be catastrophic and impacts to property and infrastructure should be able to be managed. Pumicestone Passage will still exist, but parts of it will just be a little different.

An earlier modelling report by the Pumicestone Passage Advisory Taskforce identified the types of impacts from the type of breakthrough that has occurred. These include enhanced sanding up of the existing Caloundra bar and the area directly around it, increased tidal range between Golden Beach and Halls Creek, and increased wave activity in the passage directly adjacent to the new entrance.

In terms of fish and fish habitats, there are likely to be changes in the distribution of seagrass in Pumicestone Passage and this may impact the distribution of juvenile fish. Shoreline erosion may result in some areas causing more trees to fall into the water which may provide some additional fish habitat. Some coffee rock may be exposed which is great habitat for many species. More sandy shoals are likely to form in and around the new entrance.

Time will tell whether the new breakthrough will remain open, but long-term modelling has always predicted ongoing erosion that will lead to a new entrance forming.