OLD BAR BEACH - STABILISATION INVESTIGATION: FEASIBILITY STUDY


FEASIBILITY STUDY EXECUTIVE SUMMARY The objective of this investigation has been to determine the feasibility and efficacy of using an MPR(s) for beach protection/stabilization by development of a locally wider beach at Old Bar. This has been achieved through the combination of data collection, data compilation, data analysis, wave transformation of the offshore wave-climate to the inshore site, and the application of empirical and numerical modelling tools.

The ongoing erosion at Old Bar is believed to be the effect of cross-shore sediment transport during storm activity moving sand offshore and out of the beach system faster than it can be naturally replenished (Wright and Short, 1983; Goodwin, 2005; Worley Parson, 2010). Detached breakwaters, emergent breakwaters and submerged breakwaters (reefs) are used typically along coastlines with small tidal fluctuations to control the cross-shore sand transport processes (USACE, 1995; Nielsen, 2001), such as the situation at Old Bar Beach, which has a maximum tidal range of only ~1.6 m.

The ongoing beach erosion at Old Bar is thought to be due to the erosion of the "old bar" itself; that is the submerged river delta offshore. Offshore of Old Bar patch reefs comprised of river stones cemented together with mud stone are elevated above the sandy seabed, while occasional pieces of this "old bar" have been reported to wash up on the beach and break apart since the first settlement of the area in the 1940's. Loss of elevation of the offshore "old bar" would lead to shoreline retreat. Thus, reinstating some of the offshore old bar with an offshore submerged reef would likely have a positive effect on beach stability. Through the process of data collection and compilation to quantify and understanding the physical processes at Old Bar, potential multi-purpose reef (MPR) positions and crest heights were considered at the site using empirical relationships and calibrated numerical modelling.

Considering the results of the various investigations along with the experience of previous designed and measured results for MPRs, it has been shown that an MPR placed 250 m offshore with a crest height of -0.5 m (below MSL), an alongshore length of ~110 m and a volume of ~18,000 m3, will lead to the development of a salient with a volume of 20-25,000 m3. Since the salient asymptotes to zero width at each end along its ~800 m length, it is considered that some 500-600 m of beach length will be widened by between 10 m (each end of the 500-600 m length) and >40 m (in the lee of the reef).

With the application of two MPR's, in front of Lewis Street/Badger's Beach and in front of the school, an approximately 1,200 m length of beach could be widened in the area of most aggressive erosion. To reduce the total project cost, the southern reef would be built with reduced amenity incorporated into its design. A rough order cost estimate of construction of an MPR using sand-filled geotextile containers (as was done in 1999 for the Gold Coast MPR) is presented, and the next step forwards for this option of coastal protection at Old Bar are also provided. Since the focus of this study is coastal protection/beach widening, the other aspects of MPR's have not been considered in detail. However, a range of information and a recent review on the 7 MPR's built in the past 12 years are included in the Appendices of this report.

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FUNDING FOR STUDIES

These studies are financed with funds raised over 5 years - over $47,000.
• $25,400 on "Old Bar Beach Stabilisation Investigation: Feasibility Study" (together with Federal Grant of $30,000 - study completed)
• $10,000 on "The Economic Benefits and Costs of Beach Protection" (still to be completed)
• $12,000 still in kitty.


FEASIBILITY STUDY PROPOSAL

Thumbnail example (This is the Introduction to the Feasibility Study Proposal for Old Bar Beach)
This proposal describes the steps needed to undertake a feasibility assessment and produce a preliminary design for the construction of a Mutli-Purpose Reef, at Old Bar Beach, New South Wales, Australia. The ongoing erosion at Old Bar is believed to be the effect of cross-shore sediment transport during storm activity, which moves sand offshore and out of the beach system faster than it can be naturally replenished. Detached breakwaters, emergent breakwaters, and submerged breakwaters (reefs) are typically used along coastlines with small tidal fluctuations to control the cross-shore sand transport processes. As the maximum tidal range at Old Bar Beach is relatively small, it is expected that an offshore detached structure will be effective in providing a new beach control point and widening Old Bar Beach.
In the present case there are several studies that indicate submerged structures will be effective for salient developement of the presently eroded beach. The larger scale "example" of Urana Bombora, which results in the large salient that is Old Bar, provides additional evidence to support this approach. Thus, the focus of ASR's study will be to investigate Multi- Purpose Reefs as a feasible option for beach protection/replenishment by the development of a locally wider beach at Old Bar.
This proposal outlines the benefits of Mutli-Purpose Reef technology and how this technology can be applied to Old Bar Beach. This is followed by a description of the specific steps required to perform a feasibility study and preliminary design for the site.
To read the full Propsal click here to download the Pdf file.
Dr. Shaw Mead
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