Replace bags as required. Since 2003 most of the bags have been covered by sand meaning little or no deterioration. This means only the top ones and end bags need replacing. This is by far the cheapest option and maximises the value of the existing asset. The Council's own commissioned study by the expert consultants estimates that only $150,000 to $200,000 is required to restore the groynes and extend their life by 8 to 10 years.
Replace the existing structures with geotextile bags, bag for bag. If properly maintained geotextile groynes can now last up to 25 years instead of the 17 years this comparative study uses. If the 25-year figure is used in the comparison then bags become even more cost effective.
Replace the existing structures with rock. The capital cost of rocks is over twice that of geotextile bags. Worth noting is that there is very little difference between the cost effectiveness of rocks ($19.76 million) to bags ($20.19 million) Table 5.4 Pb. 29 of the report. Rock construction raises additional questions which have not been addressed by this study:
Where will the rock come from and what will the cost be? It is believed the cost estimates for the rock are based on local sources that are no longer suitable and therefore underestimate the true costs.
No account has been made for the disruption to the Cotton Tree Precinct with the transport of rock onto the beach.
What will be done to avoid the reduction on natural amenity and user friendliness that the existing geotextile bags provide?