Pumicestone Passage is a double ended Barrier Estuary between Bribie Island in the East and the Mainland. The narrow neck of Bribie has always been subject to potential tidal breach and in geological terms the island is ephemeral, or short lived so it is expected that the spit will vanish at some time.
Pumicestone Passage was created about 6-7000 years ago as sea level rose from the last Ice Age. The Island was formed during the transfer of sand from south to north with the changed current direction. During this formation it is possible that an opening had existed in a number of locations along the island. Some of these positions are probably indicated by the location of islands in the Passage which may have generated at the outlet to the ocean or simply at the head of a creek.
The Caloundra Bar was discovered by John Bingle in 1822 and proved the Passage was open to the ocean and was indeed a Passage, not a river as named by Matthew Flinders. In recent history during European occupation and local knowledge, the opening, or Bar from the Passage, has been situated at the north end with a wider opening from Deception Bay in the south creating dynamics that result in a slow northerly movement of water from south to north. The Bar position has been known to move south from its’ current position and then relocate back to the north. This being a natural process has created no problems with the dynamics of the Passage. Aerial photos would indicate that the last time the bar was considerably south of its’ current position was in the 1960’s.
Does the current potential breakthrough of the Island therefore pose a risk? There are a number of potential outcomes, some of limited concern, however, overtime a trend of increased and rapid erosion could develop resulting in the loss of the Island Spit. If no-one lived here and no manmade changes had been introduced to the Passage, infrastructure did not exist and we had not become dependent on the character of the area, or the tourist asset it has become, the loss of the island would not matter. Now, however, all of the above are factors that need consideration when assessing the ultimate consequences of a breach that may rapidly lead to a wide opening, even the possibility of change to the flow dynamics of the Passage. A wide Bar could neutralise the northerly flow, silt the central area and ultimately create two separate estuaries. It could simply create higher tidal levels for the Golden Beach and Diamond Head regions or rougher water where we currently enjoy smooth water recreation. It is a barrier against rough seas and storm surges and adds to the visual character of the Passage defining the top end as a barrier estuary. It will impact estuarine ecosystems that have developed behind the top end of Bribie Island with changes to marine and terrestrial vegetation that has developed over recent years.
There are some basic things that could be tried to prevent ongoing loss of the spit. These should be considered under the policy of “do no harm” so that if failure occurs, no serious changes to the natural process would occur. The Island is currently subject to “wash-over” in a number of places but the major location is likely to become a “breakthrough” if current high tides persist or if a following storm season creates excessive tides, winds and storm surge.
Due to all the man-made structures in and along the mainland shoreline, the character of the Passage can no longer be considered natural in the purest terms. As such, a little human assistance to nature on the island may help preserve the dynamics and the ambience for a longer period. It is now or never.
Written by Ken Mewburn, Take Action for Pumicestone Passage
Bar position 1967
Tide running over from the beach, exiting into the Passage and the second channel running to the Passage, December 2020