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

 Noosa Spit SEMP Final Report                                                                               61
 Numerical Modelling of Entrance Configurations




3.6.3    Assessment Summary
         The river entrance is an important controlling factor on the tidal regime in the estuary with the shoals
         and sand bars causing attenuation of the ocean tidal amplitude. Stable tidal channels, unless
         otherwise geologically restricted or subject to strong external influences such as input of sediments
         by other processes or a fluvial flood, have been shown to exhibit a well-defined relationship between
         the volume of tidal flow (tidal prism) and the cross-sectional area of the flow; a consequence of
         needing to adjust their geometry until a certain equilibrium is achieved. The assessment of the 1987,
         2010 and existing entrance channel and shoal configurations has shown that tidal water levels within
         the estuary are relatively insensitive to the entrance condition when the ‘dynamic equilibrium’
         condition exists.

         The equilibrium tidal regime is disrupted after a significant flood and/or Dog Beach breakthrough
         event. These scenarios cause an increase to the cross-sectional area of the flow, tidal prism and
         peak high tide levels in the lower river. It is noted that the prevailing sediment transport processes
         would be expected to gradually infill the scour associated with post-flood condition and return the
         entrance to the dynamic equilibrium condition. The morphological response following a breakthrough
         event at Dog Beach is less predictable; however, the impact to the estuary tidal regime is likely to be
         permanent and potentially worsen if there was further of loss of land that attenuates the ocean tidal
         amplitude before it reaches existing development at Noosa Sound and at the back of Hastings Street.

         The current patterns vary significantly with entrance condition. The current magnitude and shoreline
         erosion potential along the middle section of the Dog Beach channel is greater in 2019 compared to
         the other historical conditions assessed. In terms of shoreline stabilisation, the works completed in
         2013 have only benefited the western end of Dog Beach. The lowest current magnitude and shoreline
         erosion potential at Dog Beach is when the entrance channel is aligned east-west (i.e. the 1987
         condition) and peak tidal flows are directed away from The Spit.

         The distribution of currents following a breakthrough event suggests the potential for ongoing
         morphological change at The Spit and further loss of coastal land and open space that shelters
         existing development.




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