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

Noosa Shire Council Coastal Hazard Mapping                                                                    24
Erosion Prone Area Assessment




                     Figure 3-4    Bruun (1962) Concept of Recession due to Sea Level Rise


         Application of the Bruun Rule has been highly contested within the coastal science community (e.g.
         Ranasinghe et al., 2007), often relating to the method for estimating the depth of closure. The
         depth of closure is the theoretical depth limit at which there is little or no potential for significant
         cross-shore exchanges of sand. Recession estimates can vary by around 500% depending on the
         method used to calculate the depth of closure (Ranasinghe and Stive, 2009). This compounds the
         already high level of uncertainty associated with the future rate of SLR and highlights the
         appropriateness of a risk-based approach to future climate shoreline recession assessments.

         As noted by Woodroffe et al. (2012), the wide application of the Bruun Rule probably reflects its
         simplicity rather than its proven accuracy and recession rate estimates based on the method
         should be considered as only broadly indicative. More robust numerical methods to assess future
         climate shoreline recession exist; however, such methods require extensive historical datasets to
         underpin the modelling assumptions and, despite significant additional effort, will not always reduce
         the level of uncertainty for decision makers over long planning periods.

         The ‘Standard’ Bruun Rule Approach

         The simplified Bruun Rule as shown in Figure 3-30 for the linear recession distance r (in metres) is:
              𝐵𝑎
         𝑟=
              𝐷
         Equation 3-6

         Where: B = horizontal distance offshore from the top of the dune to the depth of closure (d); a = the
         rise in sea level, and D = the vertical distance (height) from the top of the dune to the depth of
         closure (d).

         Depth of Closure

         Hallermeier (1981) divides the nearshore zone into three zones, namely:

         • The littoral zone, which “extends to the seaward limit of intense bed activity”;


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