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

 Noosa Spit SEMP Final Report                                                                               75
 Shoreline Erosion Mitigation Options




4.3.1    Option 1a: sand plug at either end of Dog Beach
         Figure 4-2 illustrates the model representation of the sand plug at either end of Dog Beach. The sand
         plugs are located where scour and dominant flow paths have developed adjacent to the geofabric
         sand container structures. 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 plugs are represented by adjusting the model bathymetry in the areas shown;

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

         • The estimated minimum volume of sand needed to construct the plugs is (additional volume would
           be required to nourish the shoreline):

             ○ Northern plug: 14,000 m3

             ○ Southern plug: 17,000 m3

         The impacts to peak tidal current figures show:

         • A minor reduction (~0.1 to 0.2 m/s) in peak flood current magnitude within the Dog Beach channel.
           This is balanced by increases to current magnitude at the edges and over the crest of each sand
           plug. This suggests that over the time the plugs will be gradually modified by the flood tide currents
           with sediment eroded from the crest and likely deposited in the adjacent channel area. This could
           be described as a gradual ‘smoothing’ of the constructed sand plugs.

         • During peak ebb conditions the impact to currents is widespread with a significant reduction in
           magnitude at the Dog Beach channel and shoreline, and an increase in magnitude through the
           central channel. This suggests that the option is likely to relieve erosion pressure at the Dog
           Beach shoreline and promote the development of a central channel.

         • It is noted that the patterns described above are expected to diminish over time as the sand plugs
           are gradually eroded and smoothed. It is assumed that the sand plugs would need to be
           maintained for the option to continue performing as intended.

         The whole of life cost estimate for this option is $3.4M with annualised costs around $170,000/year.
         This estimate assumes three campaigns between 2020 and 2040. Each campaign would seek to
         relocate up to 80,000 m3 of sand.

         The potential for impact to environmental values is primarily related to the extractive dredging
         activities and disturbance to the estuarine habitats. Dredging campaigns would need to be designed
         to avoid the declared Fish Habitat Area and migratory bird season. Opportunities to enhance
         terrestrial environmental values exist through revegetation and dune management programs to help
         stabilise the nourishment material.

         The social benefits are related to the restoration and maintenance of a recreational beach and the
         redirection of boating traffic away from the Dog Beach channel.

         The risks to Council are considered low to medium manageable. Full details of the risk assessment
         results are provided in Appendix D.




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