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

Noosa Shire Council Coastal Hazard Mapping                                                                 31
Erosion Prone Area Assessment




3.9      Assessment Results
         Open coast erosion hazard distances have been calculated following the methodology and formula
         described in Section 3.2.1 and 3.2.1.1 for locations throughout the study area where sufficient data
         is available. The results (beach compartment averages) presented in Table 3-6 include the 40%
         factor of safety that is assumed to account for the uncertainties associated with the assessments
         (see Section 3.7).

         As previously discussed, the calculated erosion distances do not account for local erosion controls,
         such as the engineered coastal protection structures or erosion resistant bedrock slopes. Properly
         designed and maintained seawalls are expected to limit the landward extent of the erosion. The
         design standard of the Noosa Main Beach seawall, concealed within the sand dune, remains
         uncertain. It has been assumed the existing structure may not withstand a design storm event but
         would be sufficient to limit long term recession and the shoreline response to sea level rise. This
         assumption has been incorporated to the coastal hazard mapping produced for Noosa Main Beach.

         As noted in Section 3.4, it has been difficult establishing a clear long-term trend in shoreline
         recession from the available historical datasets but it is assumed to be very low and/or mitigated
         through ongoing management activities. The adopted value of R = 10 m for all planning horizons is
         based on the minimum allowance required by DEHP.

         The short-term erosion component (see Section 3.5) varies between 7 m and 63 m (n = 25, mean =
         25 m, SD = 16 m) relative to the seaward toe of the frontal dune. The alongshore variation is
         primarily due to differences in the height and volume of the coastal barrier and the nearshore slope.
         The short term erosion component accounts for approximately 30% of the coastal erosion hazard
         area under the 2100 timeframe.

         Considering the SLR projection of 0.8 m by the year 2100 adopted by the Queensland Government
         for planning purposes (DEHP 2015), the shoreline response to sea level rise (S, see Section 3.6)
         component accounts for approximately 35% of the coastal erosion hazard area by 2100.

         The coastal erosion hazard area also considers slumping of the dune scarp (D, see Section 3.8)
         and this component accounts for approximately 15% of the total erosion prone area under the 2100
         timeframe. Dune instability was estimated using the zone of reduced foundation capacity zone
         schema by Nielsen et al (1992).




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