2019-12-16 GC Agenda - Item 5 - Attachment 1 - Noosa Spit Shoreline Erosion Management Plan-Part 1

 Noosa Spit SEMP Final Report                                                                                83
 Shoreline Erosion Mitigation Options




4.3.3    Option 2: extension of the existing groyne at Dog Beach
         Figure 4-8 illustrates the model representation of the extension to the existing groyne. Key features
         of the option include:

         • The crest elevation of the groyne is 1 m above AHD (not overtopped under normal tidal conditions)

         • The extension is similar in length to the existing geofabric sand container groyne (effectively
           doubling the length of the existing structure).

         The impacts to peak tidal current figures show:

         • A moderate reduction (~0.2 to 0.4 m/s) in peak flood current magnitude at the Dog Beach channel
           and shoreline.

         • Increases to flood tide at the head of the proposed extension, effectively transferring the scour
           currently observed at the existing structure. There is also a minor increase in peak flood current
           magnitude through the central channel adjacent to the groyne structure.

         • The peak ebb tide impacts are like Option 1a and 1b, with a significant reduction in magnitude at
           the Dog Beach channel and shoreline. An increase in ebb tide current magnitude through the
           central channel is also observed.

         • The groyne extension option is considered ‘permanent’ and would be developed with a goal to
           minimise ongoing maintenance requirements. The design would need to account for local scour
           potential at the structure.

         The whole of life cost estimate for this option is $1.8M with annualised costs around $90,000/year.
         This estimate is based on the cost for similar works in 2012 (i.e. the existing training wall) with
         adjustments for inflation. This includes beach nourishment as part of the capital costs. It is noted that
         the extended structure is not expected to stabilise the entire length of shoreline and therefore
         additional management actions and costs may be required toward the eastern extent of the Dog
         Beach shoreline.

         The potential for impact to environmental values is primarily related to the construction phase. There
         would also be a notable impact to visual amenity.

         The social benefits are primarily related to stabilising a longer section of shoreline and providing a
         width of beach for recreational opportunities.

         The risks to Council are generally low to medium and manageable. The exception being pedestrian
         slip hazard at the crest of the structure and the submerged hazard which could result in injury when
         swimming (diving) or boating near the structure. Similar risks are associated with the existing
         structure, so the training wall extension does not represent a significant change in risk profile. Full
         details of the risk assessment results are provided in Appendix D.




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