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

 Noosa Spit SEMP Final Report                                                                             87
 Shoreline Erosion Mitigation Options




4.3.4    Option 3: extension of submerged breakwater and two new groynes
         Figure 4-11 illustrates the model representation of the extension to the submerged breakwater and
         addition of two new groynes. This option represents the full implementation of the strategy proposed
         by Sunshine Coast Council. Key features of the option include:

         • The submerged breakwater crest elevation at 0.6 m below AHD

         • The groyne crest elevation at 0 mAHD (approximately mean sea level)

         The impacts to peak tidal current figures show:

         • Minimum change to peak flood and ebb tide currents along the shoreline bounded by the
           submerged breakwater.

         • An increase to current magnitude on the seaward side of the breakwater, expected to promote
           scour and channel development (as observed against the existing submerged structure).

         • A reduction in current magnitude between the groynes suggesting small sections of beach could
           be maintained in these locations.

         • An increase in current magnitude at the head of the small groynes suggesting scour local potential
           that the design would need to accommodate.

         The whole of life cost estimate for this option is $2.0M with annualised costs around $100,000/year.
         This estimate is based on the cost for similar works in 2012 (i.e. the existing submerged structure)
         and costs to construct small groynes at Munna Point, with adjustments for inflation. This includes
         beach nourishment as part of the capital costs.

         The potential for impact to environmental values is primarily related to the construction phase. There
         would also be an impact to visual amenity associated with the two groynes.

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

         There are some high and extreme risks associated with this option. Like Option 2, there are
         pedestrian slip and submerged hazards which could result in injury to the general public. In addition,
         there are also political risks associated with continuing to support an expensive and unsuccessful
         ‘legacy’ design. Full details of the risk assessment results are provided in Appendix D.




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