2019-07-09 P&E Agenda - Item 10 - Attachment 1 - Coastal Hazards Risk, Vulnerability and Adaptation Assessment

Noosa Shire Council Coastal Hazard Mapping                                                               21
Erosion Prone Area Assessment




3.5.1.3 Wave Runup & Overtopping Potential
         A preliminary assessment of the contribution of wave runup processes to the extreme water level
         has been considered at the open coast locations. Within estuaries the contribution of wave driven
         processes to the extreme water level is assumed to be much smaller.

         Wave runup is the intermittent process of advancement and retreat of the instantaneous shoreline
         position on a timescale that is of the order of the incoming wave period. Wave runup can be a
         significant contributor to the peak water levels and inundation associated with the overtopping of
         coastal barriers. Furthermore, the large quantity of energy contained in individual wave runup can
         pose a serious risk to coastal barriers (natural or man-made) within the wave runup zone.

         The wave setup and runup contribution to shoreline water levels within the coastal zone has been
         estimated using an empirical formulation based on 10 dynamically diverse field experiments
         described in Stockdon et al (2006). The runup height predicted with this formula is the level above
         the offshore mean water level that is exceeded by 2% of runup events (R2). The general
         expression for wave setup and wave runup on beaches provided in Stockdon et al. (2006):

         Wave setup

          S shoreline  0.35  f H 0 L0 
                                         1/ 2



         Equation 3-4


         Wave runup

                  
                                                       
                                 H 0 L0 0.563 f2  0.004 
                                                         1/ 2

          R2  1.1 S shoreline 
                                            2                
                                                             
         Equation 3-5
         Where βf is the foreshore slope, H0 is the deep water significant wave height and L0 is the deep
         water wave length (L0 = gT/2π where T is the wave period).

         Adopting a representative regional 100 year ARI design wave height of 6.2 m (see Table 3-2), an
         associated wave period of 12 seconds and foreshore slope of 1V:25H in Equation 3-5 gives a 2%
         runup height of 1.77 m.

         Table 3-3 provides a summary of the present-day runup elevation (mAHD) associated with the
         adopted design storm definition. Table 3-3 also indicates the primary frontal dune height at each
         assessment location and the potential for overtopping of the dune (or coastal barrier). Overtopping
         is predicted to occur at Noosa Main Beach and Teewah Beach, with the latter considered being of
         little consequence due to the well-established hind dune area.

         The consequence of overtopping would be greater at Noosa Main Beach and may significantly
         influence the coastal hazard at this location. Furthermore, since the landward migration of the
         shoreline in response to SLR is constrained by development at this location, the wave overtopping
         potential will increase with SLR if left unmitigated. Overtopping at Noosa Main Beach may need
         further assessment during the options analysis stage of the Noosa CHAP.


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