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Surfside Sinking

The issue of the sinking Surfside subdivision is a complex one but which has at its roots the interference to the Batemans Bay sediment distribution system through construction works including the bridge, the south bank river training wall, the north bank sea wall defending the caravan park, and the dredging of the channel(s).

It is obvious that what has happened has been a slow process over 100 plus years which has been exacerbated by shot term storm events made worse by natural defences being weakened over those many years. You cannot remove some 1.3 million cubic metres of sand over the past 120 years without it having consequences elsewhere such as collapsing sand banks. This is still happening with the Council and the NSW being oblivious to the effects of bad decisions such as the dredging of the bar over the past few years. This is supported by Government initiated studies and consultancies - in particular the 1996 Batemans Bay Vulnerability Study. The Batemans Bay Vulnerability Study reached the following conclusions: Coastal processes within the bay are controlled or influenced by the following factors: • the Clyde River and its fluvial discharge, in particular periodic flood discharge and accompanying sedimentation; • twice-daily tidal flows into the Clyde and within the bay; • ocean wave and local wind wave attenuation across the entrance and into the bay including periodic high wave events; • existing river mouth and tidal shoals within the bay entrance; • the funnel-shape nature and general morphology of the bay; • the construction of the southern training wall; and • offshore islands and reefs. The nature and processes within the bay are described by PWD (1989) and Short (1995). A sandy beach can experience large changes in its morphology in response to the forces of wind, waves, and currents. These currents can be short-term fluctuations or long term trends. Short-term erosion is associated with coastal storms, floods, or both. This erosion is part of the natural response of a beach to changing wave and water level conditions. The sand is moved offshore and alongshore on some beaches and deposited in a bar or shoal which gives the beach some protection. The sand is generally transported back onshore during calmer swell conditions. If a proportion of the sediment eroded from the beach during a storm is not returned, or if there is some mechanism which causes a nett loss of sediment from a beach, then that beach will experience long-term recession. Offshore losses are a likely contributor to shoreline recession at Long and Maloneys Beaches, while at Corrigans, Surfside and Cullendulla the erosion escarpment and shoreline recession is due to a combination of longshore and offshore sediment transport. In addition to these natural mechanisms, Batemans Bay beaches have been subjected to human interference in the form of: • construction of the rock training wall; • dune reconstruction at the northern end of Corrigans Beach; • the dredging of channels and shoals; and • terminal revetments at various locations. In order to estimate future behaviour it is necessary to quantify the past behaviour of the subaerial beach both in terms of long-term trends and short-term fluctuations. As historical ground surveys are very seldom available, the most reliable information is in the form vertical aerial photography. The photographs are analysed photogrammetrically to yield “survey” data of the sub-aerial beach and back beach area. In addition to the historical photogrammetric information, land surveys carried out in 1989, 1990, and 1995 provide recent information on beach volumes and shoreline movement. Hydrographic surveys of Batemans Bay are available 1864 to 1995 and these have been used to describe the movement of shoals and channels within the bay. There is no evidence of storm demand in the areas of Batemans Bay that are not exposed to east or south-east storms. Beaches that are protected by headlands or nearshore reef systems have not experienced significant storm cut. Exposed beaches show long-term recession as a series of episodic storm cuts, eg the western end of Long Beach. From 1990 to 1995 the inner bay accreted by approx 0.4 million m3. The extension of the training wall resulted in increased scouring along the northern side of the wall. Also resulting from the extension of the sea wall is seaward movement of the river mouth bar. Further accretion of northern Corrigans Beach occurred, except for the area adjacent to the south side of the training wall where significant erosion occurred. It should be noted that the morphology of the bay is highly dynamic and is capable of pronounced changes over short periods of time. Hence long term trends are masked by short term fluctuations. Hydrosurveys may not indicate the changes that have occurred between survey dates. In order to more fully understand the processes operating in the inner bay, accurate information is required on the following processes described by Short, (1995) inter alia: • transport of marine sands from the outer bay; • the impact of major storm wave events on sediment transport within the inner bay and the impact of the waves and associated set-up and run-up on the bay shores; • the sequential modification of the depth and morphology of the bay associated with such events; • the impact of this modification of adjacent coastal processes and sedimentation; • the impact of the southern training wall on processes and sedimentation within the inner bay; • the interaction of all these processes over years and over decades. There have been a number of attempts to model the likely sediment paths around the inner bay. The first attempt was the Sinclair, Knight and Partners (SKP) conceptual model of sediment movement during flooding and in the absence of flooding. A second attempt by Patterson, Britton and partners (PBP) (1992) included the SKP model, with the addition of waves, tidal effects, etc. The conceptual model shows that inner bay to outer bay movement of beach-size sediment is not significant. However, isopac analysis between 1922 and 1978 shows some sediment build up may have occurred in the outer bay. 580,000 m3 removed from the inner bay between 1901 and 1964, the accretion behind Corrigans Beach since the construction of the training wall and losses to the outer bay. Initial estimates of sediment accretion behind Corrigans Beach using the 1989 and 1995 shoreline give an approximate volume of 800,000 m3. This is about 8,000 m3 yearly accretion to Corrigans Beach. Although sediment movement analysis within the control area shows little change in volume in the inner bay from 1899 to 1995, the shoreline of Corrigans Beach continued to prograde. This progradation is limited by the capacity of the training wall to prevent longshore sediment transport. When this capacity is exhausted, sediments will bypass the wall and the inner bay will start to infill. This is what is causing the bar to shallow.

Images found and shared by Harry Watson Smith of the Batemans Bay Boaters Association Inc

Readers may be interested in the following aerial picture of the Corrigans Beach and Hanging Rock precincts that shows the massive accretion to those locations from sand dredged from the river or re-directed by the training wall and its extension some 30 years ago.

According to the Batemans Bay Vulnerability Study 1996, the sand would have come from the Inner Bay - mainly North Batemans Bay, Surfside and Cullendulla

The red-line shoreline depiction circa. 1890 is from Professor Andrew Short and based upon NSW Government historical records.

It was with some amusement that I read some years ago the “Corrigans Beach Reserve Plan of Management 2002” prepared by Council Planning and Environment experts for the NSW Government where it said in Section 2.2: “The site (Corrigans Beach Reserve) is basically flat land which is extensively cleared of natural vegetation”. What natural vegetation?

If Council and the NSW Government can get this so wrong then how could they be trusted to provide expert advice on the sinking of the Surfside sub-division?

#Opinion #BatemansBay

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