PA3660-Coastal Hazard Assessment - 4 arakoon

                                                  Project related




a=     rise in mean sea level (m) 0.8
D=     vertical height of the active beach profile (m)
The Bruun Rule assumes that:

     •     the beach and dune consist of a uniform and loose sand material. It cannot be applied to rocky
           headlands or hard coasts;
     •     the beach responds to an elevated sea level such that an equilibrium profile is always attained;
     •     the beach is stable in the long term (i.e. there is no net accretion or recession). Gradients in
           longshore sediment transport are ignored; and,
     •     all the eroded sediment is redistributed along the profile in the cross-shore direction.

The geotechnical assessment, which is supported by aerial photography from 1974, has shown the
underlying soil profile includes indurated sand layers (i.e., coffee rock), which may slow recession rates.
Further, as discussed in Section 5.3, net northerly sediment transport is observed along this section of the
coastline and Noosa Head is a control feature that blocks the northern passage of sand.

The assessment's determination of dune height relied on an iterative approach, which is discussed in
more detail in Section 6.2.3.3. The resultant dune height determined through this iterative process was
12m. The profile width from the dune height to depth of closure (-12m) is 550m. Therefore, a profile width
of 550m, a depth of closure of -12m together, and a dune height of 12m AHD provide a recession due to a
sea level rise of 18m (Table 6-4).


Table 6-4: Summary of Shoreline Recession due to Sea Level Rise.
                    Profile Width     Dune Height     Depth of Closure   2100 Shoreline Recession
 Beach
                         (m)            (m AHD)            (m AHD)                 (m)

 Sunshine Beach          550               12                -12                   18

6.2.3.2 Dune scarp component method
Immediately following storm erosion events on sandy beaches, a near vertical erosion scarp of substantial
height can be left in the dune or beach edge. Over time the erosion scarp will slump to the natural angle of
repose of the sand. Since the short-term erosion only accounts for shoreline recession due to wave runup
for cases where the foreshore is not overtopped, the Queensland Hazard Technical Guide (DEHP, 2013)
recommends adding this dune scarp component when calculating erosion width.

RHDHV determined the dune slumping component by retracing the long-term erosion plus the short-term
erosion plus the recession due to SLR (47m) and applying the angle of repose for dune slumping (35
degrees) based on the dune toe level. Using this method, the dune scarp component of 14m was adopted
for our site (refer Figure 6-7 below).

6.2.3.3 Dune height
In this particular coastal stretch, the dunes are classified as 'high dunes'. Determining the recession
caused by sea level rise (S) necessitated identifying the 'active' erosion profile to ascertain the dune
height at this specific location. The assessment employed an iterative methodology to determine the dune
height, involving an initial calculation of S using a test dune height. Subsequently, D was calculated based
on S, C, and NxR to assess if the active profile (horizontal distance from NxR+C+S) intersected the profile
at the same dune height utilised in the S calculation. If there was a disparity, S was recalculated with
different dune heights until it aligned with the dune height of the active profile. For this evaluation, a dune
height of 12m was established (Figure 6-7).



11 April 2024        4-6 ARAKOON CRESCENT, SUNSHINE BEACH                  PA3660-RHD-XX-ZZ-RP-Z-0001   31