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

Noosa Shire Council Coastal Hazard Mapping                                                                25
Erosion Prone Area Assessment




         • The shoal zone, which “extends from the seaward edge of the littoral zone to a water depth
           where expected surface waves are likely to cause little sand transport” and “waves have neither
             strong nor negligible effects on the sand bed” ; and

         • The offshore zone, which is seaward of the shoal zone and water depths are relatively deep
             with respect to surface wave effects on the sea bed.

         Hallermeier (1981) stresses that sediment motion can and does occur seaward of the shoal zone,
         however the seaward boundary (di) defined by Hallermeier (1981) aims to provide “a physically
         meaningful seaward limit to the usual wave-constructed shoreface”.

         Hallermeier (1981) then identifies two depths that define the landward and seaward boundaries of
         the shoal zone:

         • Depth dl which is the “maximum water depth for sand erosion and seaward transport by an
           extreme yearly wave condition”; and seaward of this; and

         • Depth di which is the “maximum water depth for sand motion by the median wave condition”,
           corresponding to the seaward limit of the usual wave-constructed profile.

         Patterson (2012; 2013) identified that the time-scale of profile response, the time required for the
         profile to achieve equilibrium, increases with depth and needs to be considered in determining
         closure depth. Nicholls et al. (1996, 1998) and Cowell et al. (2001) both refer to the closure depth
         in terms of the time scale considered. That is, they note that profile “closure” occurs at greater
         depth as the time scale increases. Nicholls et al. (1998) adopt a version of the Hallermeier (1977;
         1981) relationship for depth of closure of the form:

         𝑑𝑙,𝑡 = 2.28 𝐻𝑒,𝑡 − 68.5 (𝐻𝑒,𝑡2 / 𝑔𝑇𝑒,𝑡 2 )
         Equation 3-7

         Where dl,t = the predicted depth of closure over t years, referenced to Mean Low Water; He,t = non-
         breaking significant wave height exceeded 12 hours per t years; and Te,t = associated wave period.

         Following Equation 3-7, the depth of closure to cater for SLR over a planning period of 100 years
         will be greater than that adopted for shorter durations. Adopting a representative regional 1% AEP
         design wave height of 6.2 m (see Table 3-2) and an associated wave period of 12 seconds in
         Equation 3-7 suggests a 100 year planning period depth of closure around 11-12 m for Noosa
         beaches. However, it should be noted that this does not provide for the concept of accumulation at
         the lower part of the equilibrium profile translation to balance upper profile erosion on which the
         Bruun Rule is based.

         Considering the above, the horizontal distance offshore to the depth of closure varies between
         approximately 525 m and 950 m throughout the study area. Considering the variation in dune
         heights and offshore bathymetry, the measured Bruun Rule slope factor ranges from 1:32 to 1:56.

3.6.3    Shoreline Response to Sea Level Rise Assessment Results
         The shoreline response to SLR assessment results are presented in Table 3-4. It must be
         recognised that the Bruun Rule does not account for the influences of longshore sand transport
         processes on the profile response to sea level, nor does it consider the potential changes to


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