Erosion study in 1977 described sand dynamics

Letters to the editor

In the article “Channel needs science, not amateur ideas, says CEN” (Peninsula News edition 441), the chairman of the Community Environment Network, Mr John Asquith, stated: “CEN could find no credible studies to support a strategy of pumping sand out of the clogged channel into Brisbane Water at Half Tide Rocks and onto Ocean Beach.”

In the same article he claimed dredging needs science engineering and common sense and not ill-considered amateur ideas, opinions and dogma. The problems of erosion at Ettalong and Ocean Beach go back well before the construction of the Diggers Building in 2004. In fact, the problems are well documented back to 1948. Dredging and renourishment of the beach is part of the highly credible 1977 Ettalong Beach Erosion Study and Management Program conducted by the Department of Public Works, Coastal Engineering Department. Summary inter alia: The object of this study was to gain an understanding of the processes governing erosion of Ettalong Beach and to formulate a remedial Beach Management Programme for the area based on this understanding. An appreciation of sand movements in the region and the controlling physical processes was developed progressively. A geological survey was undertaken to define the boundaries of sediment movement within the Ettalong estuary.

A conceptual model was developed to describe the sediment dynamics within these boundaries prior to human intervention. The model was verified by interpretation of historical data, aerial photographs and examination of the impact of human intervention. Data on currents, tides, waves, shoal mobility and so on, was collected in the field and used to evaluate the time scales of erosion processes and rates of sand movements. From this investigation, a number of significant conclusions were reached. Erosion events in the Ettalong area were inter-related and therefore not soluble in the long term by means of isolated remedial works. Erosion is not primarily caused by the natural loss of sand from the estuary as the quantity of sand contained between the Rip Bridge and the outer edge of the Ettalong Point shoal in Broken Bay remains essentially constant. Sand contained within this estuary system is quite mobile, moving from the outer shoal to the beach and thence back to the outer shoal via the tidal channels.

Erosion is attributable to the fact that this circulatory pattern is variable in time, rate and location, resulting in beach fluctuations. These fluctuations can be increased by variation in channel alignment within the estuary. Past works have interfered with this natural sand movement and/or removed sand from the system. Hence Ettalong Beach is more sensitive to erosion cycles which are also more severe in degree. In addition, these works have encouraged the development of a deep channel close to Ettalong Beach which further accelerates the removal of sand from the beach. To retain an estuary beach amenity at Ettalong, a more natural pattern of sand circulation should be restored and the nearshore channel development against the Ettalong shoreline should be curtailed. “Soft” management techniques such as dredging, beach nourishment and dune construction are more suited to achieving this objective than “hard” structures such as rock groynes. In view of these conclusions, a beach management program has been formulated, based on the zoning of the beach according to its sensitivity, to the various identified modes of erosion and the implementation of “soft” management techniques. The programme provided a flexible framework upon which to base future works, maintenance and long-term planning for Ettalong Beach.

It is stressed that there should be continuing management of this fragile beach in the future. The beach management program recommended the interrelated activities of continued monitoring of the system and three stages of works. The recommendations are detailed in Section 8 and summarised below. 1. Institute an ongoing monitoring program. 2. Establish a more natural sand circulation system by the nourishment of the beach and nearshore channel, dredging, partial removal of Ettalong Point groyne, modification of Ettalong Beach groynes etc. 3. Consolidate the preceding works by construction and stabilisation of dunes in light of an assessment of the establishment phase. 4.

Maintain the sand system with possible periodic nourishment as indicated by the monitoring program. As part of the 1977 Department of Public Works survey, sampling was carried out over the entire area of Broken Bay. The Hawkesbury River as far as Patonga Beach and into Brisbane Waters as far as Pelican Island. To examine the dynamics of sand movements on the shoal at the entrance to Brisbane Waters, samples were collected at specified locations in Broken Bay and on the shoal at monthly intervals. Beach erosion has been the subject of many studies since the 1977 report. Beach erosion is not unique to Ettalong. It is a worldwide problem. If a study came up with an answer to natures’ fury which causes the erosion, it would create world headlines. In the meantime, dredging of the Ettalong channel is necessary in keeping with the recommendations of various studies and to help save lives as the dangerous channel at little Box Head has claimed many lives over the years.

Email, 27 Mar 2018 Kevin Woods, Umina Beach

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