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

 Noosa Spit SEMP Final Report                                                                              79
 Shoreline Erosion Mitigation Options




4.3.2    Option 1b: sand plug at southern end of Dog Beach
         Figure 4-5 illustrates the model representation of the sand plug as the southern end of Dog Beach.
         Like Option 1a, it’s assumed a small cutter suction dredge and slurry pipeline would be used to
         relocate the sand from a nearby borrow area within the Noosa River estuary. Key features of the
         option include:

         • The sand plug is represented by adjusting the model bathymetry in the areas shown

         • The crest elevation of the sand plug is 0 mAHD (approximately mean sea level)

         • The estimated volume of sand needed to construct the plug is 17,000 m3 (additional volume would
           be required to nourish the shoreline).

         The impacts to peak tidal current figures show:

         • Very little change to peak flood current magnitude within the Dog Beach channel. Increases to
           current magnitude are predicted at the edges and over the crest of the sand plug. As described
           above for Option 1a, this suggests that the flood tide currents will gradually erode and smooth the
           constructed sand plug.

         • The pattern of impact to ebb tide currents is like Option 1a, with a significant reduction in
           magnitude at the Dog Beach channel and shoreline. Option 1a and 1b are similar in terms of the
           immediate benefit to Dog Beach erosion associated with ebb tide currents, but with less sand
           required to implement Option 1b.

         • An increase in ebb tide current magnitude through the central channel is observed, assumed to
           promote the development of a dominant ebb tide flow path through this area.

         • It is assumed that the sand plug would need to be maintained for the option to continue performing
           as intended. In comparison to Option 1a, the single sand plug at the southern end of Dog Beach
           may need to be reinstated at more regular intervals, potentially offsetting the benefit of the
           reduced sand volume needed for construction.

         The whole of life cost estimate for this option is $4.1M with annualised costs around $200,000/year.
         This estimate assumes five campaigns between 2020 and 2040. Each campaign would seek to
         relocate up to 50,000 m3 of sand.

         Like Option 1a, the potential for impact to environmental values is primarily related to the extractive
         dredging activities and disturbance to the estuarine habitats with opportunities to enhance terrestrial
         environmental values through revegetation and dune management programs to help stabilise the
         nourishment material. The increased frequency of dredging activities (i.e. more regular disturbance)
         is the reason Option 1b receives a lower score in comparison to Option 1a. The increased frequency
         of disturbance is also the reason for the lower score with respect to social benefits and risks to
         Council. Full details of the risk assessment results are provided in Appendix D.




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