PA3660-Coastal Hazard Assessment - 4 arakoon

                                                  Project related




It should be noted that the IPCC AR5 sea level rise projections are relative to 2006 mean sea level as
indicated in Figure 6-16. IPCC (2014) has estimated the mean rate of global averaged sea level rise was
3.2mm annually between 1993 to 2010 which would indicate that the mean sea level has risen by
approximately 54mm between 2006 and 2023. Both the BMT (2018) and RHDHV have adopted a 0.8m
sea level rise relative to current sea level which effectively results in adoption of a higher sea level rise
projection due to the inclusion of sea level rise that has occurred to date.




Figure 6-16 Global mean sea level rise from 2006 to 2100 determined by multi-model simulations (IPCC, 2014)


BMT (2018) adopted a dune height of 4.9m AHD to undertake the assessment of recession due to sea
level rise based on a profile width of 525m (ETA602) and depth of closure of 11.5m. We believe that the
dune height proposed by BMT (2018) underestimates the true dune elevation along this particular stretch
of beach. Through our calculations, we've determined that the dune height in the active beach area stands
at 12 meters AHD (Section 6.3.4).

BMT calculated a recession due to SLR of 26m, whereas RHDHV adopted a recession due to SLR of
18m.

6.3.12 Dune Slumping
BMT (2018) have adopted a conservative approach in their assessment of dune slumping based on:

     •     Calculation of the dune slumping from a scour depth of 1m below the eroded dune toe; and,
     •     Calculation of dune slumping based on the short-term erosion width including the factor of safety,
           effectively applying the factor of safety to the dune slumping component.

A schematic of the BMT (2018) approach is presented in Figure 6-13 above.

The ‘Coastal Hazards Technical Guide’ (Department of Environment and Heritage Protection, 2013)
recommends the assessment of dune slumping from the eroded toe of the dune due to short-term erosion.

RHDHV determined the dune slumping component (D) by integrating the erosion associated with both
long-term (NxR), short-term (C) and recession due to SLR (S) factors, which currently do not account for
dune slumping in their calculations.

Therefore, RHDHV derived the dune slumping factor by adding the long-term (10m), short-term erosion
(19m) and recession due to SLR (18m) potentials from the dune toe and applying a 35-degree angle of
repose for dune slumping, based on the dune toe location (Figure 6-7 above).


11 April 2024        4-6 ARAKOON CRESCENT, SUNSHINE BEACH                       PA3660-RHD-XX-ZZ-RP-Z-0001    40