Coastal Hazard Assessment · MCU25/0087

                                                    17 The Esplanade, Sunshine Beach                                    Report


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;
        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 investigation, supported by aerial photography from 1972, has shown the underlying
soil profile within the dune system may include indurated sand layers (i.e., coffee rock), which may
slow recession rates. Further, as discussed in Section 4.3, net northerly sediment transport is
observed along this section of the coastline and Noosa Head is a control feature that restricts the
northern passage of sand.
Recent research undertaken by Geoscience Australia (2021) indicates that 76% of the north-eastern
coast of Australia has been stable from 1988 to present with 11.3% retreating by more than 0.3m/year
and 12.8% accreting by more 0.3m/year. Similar trends are observed at all locations around Australia,
except for the western coast and north-western coast. During this period, sea level rise is reported to
be approximately 2-3mm/yr (5-10cm from 1988 to present). The observations from Geoscience
Australia does not match to hypothesis of long-term recession due to sea level rise predicted by the
Brunn Rule. Therefore, there is uncertainty and potentially conservatism in this approach. However, it
is consistent with the methodology outlined in the ‘Coastal Hazards Technical Guide’ (Department of
Environment and Heritage Protection, 2013).
The site is located on a high dune and an iterative approach to determining the dune height for the
assessment has been adopted. The recession due to sea level rise was calculated based on an initial
dune height and combined with the long-term and short-term erosion components to provide an
overall erosion width. The dune scarp component was then calculated and the intersection with the
dune profile was assessed. The dune height was adjusted until the overall erosion width coincided
with the dune height adopted.
The resultant dune height determined through this iterative process was 15m AHD. The variables
adopted for the Bruun Rule calculation are provided in Table 5-4.
Table 5-4 Summary of Shoreline Recession due to Sea Level Rise

 Beach                              Profile Width      Dune Height       Depth of Closure    2100 Shoreline Recession
                                        (m)              (m AHD)             (m AHD)                   (m)

 17 The Esplanade, Sunshine Beach       542                 15                 -12                     15


5.2.4    Dune Slumping (D)
Dune Scarp Component
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 near vertical 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 Coastal
Hazard Technical Guide (DEHP, 2013) recommends adding this dune scarp component when
calculating erosion width.
M&N determined the dune slumping component by retracing the long-term erosion plus the short-term
erosion plus the recession due to SLR (41m) 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 25m was
adopted for our site (refer Figure 5-8, below).
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



                                                                        252662-MN-RP-CO-0001B | 10 November 2025 | Page 28