14169-001-Rev0 17 Webb Rd -Revised Draft · 132002.220527.1

                               17 WEBB ROAD, SUNSHINE BEACH
                               PROPOSED RESIDENCE

6.2        Stability Assessment
6.2.1       Qualitative Assessment
The risk assessment for this project has been carried out following the AGS 2007 Practice Note Guidelines
for Landslide Risk Management. Relative levels of risk and their implications are given in Table 1 below
and the Qualitative Terminology for Use in Assessing Risk to Property is attached in Appendix C.
Table 1: Stability Risk Levels
 Risk Level            Example Implications(1)
                       Unacceptable without treatment. Extensive detailed investigation and research, planning
         Very High
 VH                    and implementation of treatment options essential to reduce risk to Low; may be too
         Risk
                       expensive and not practical. Work likely to cost more than value of property.
                       Unacceptable without treatment. Detailed investigation, planning and implementation of
 H       High Risk     treatment options required to reduce risk to Low. Work would cost a substantial sum in
                       relation to the value of the property.
                       May be tolerated in certain circumstances (subject to regulators’ approval) but requires
         Moderate
 M                     investigation, planning and implementation of treatment options to reduce risk to Low.
         Risk
                       Treatment options to reduce to Low risk should be implemented as soon as practicable.
                       Usually acceptable to regulators. Where treatment has been required to reduce the risk
 L       Low Risk
                       to this level, ongoing maintenance required.
         Very Low
 VL
         Risk                                 T
                       Acceptable. Manage by normal slope maintenance procedures.

 Note: (1) The implications for a particular situation are to be determined by all parties to the risk assessment and may
                                           AF
 depend on the nature of the property at risk; these are only given as a general guide.



Given the geology and topography of this site, particularly the relative position of the residence (to be
renovated) to the foredune slope (20 m to 30 m west of the slope), it is unlikely that landslide could affect the
                                  R

proposed development. Sand dune slopes are generally close to the ‘limit of equilibrium’ where the surface
gradient is about 30°. Our measurements indicate that the dune slope is typically 30°. The limit of equilibrium
is the ‘tipping point’ to instability, where the factor of safety (FOS) of the slope is approximately 1.0. The FOS
                        D


of a slope is the balance (ratio) between the forces driving instability (e.g. soil weight, groundwater pressure)
and the resisting forces (e.g. soil friction and cohesion).

Steep sand dune slopes generally remain marginally stable (i.e. FOS just greater than 1.0) when they are:

        well vegetated (mix of ground cover, shrubs and small trees);
        not subjected to concentrated stormwater flows;
        not surcharged with additional loads (e.g. fill); and
        when the ‘toe’ of the dune is not disturbed.

Where dunes are well vegetated, the plant roots can provide a surface reinforcing layer which may enable
dunes to stand up relatively steeply. Direct discharge of stormwater onto dune slopes may cause channel
erosion, undermining and disturbance of vegetation, and development of generally localized instability. It is
critical that stormwater runoff be collected (or re-directed) and disposed of off-site prior to reaching the dune
slopes.

The primary trigger for large-scale instability of sand dunes is when the toe of the dune is disturbed. This may
be caused by excavations or natural erosion. At this site, erosion of the dune toe and undercutting of the
slope could occur during high seas when waves surge over the low-height barrier dune. The annual likelihood
of such events may vary, and possibly increase, subject to factors including sea level rise.

On similar sites in the Noosa region, sand dune instability due to undercutting of the toe generally occurs as a
translational-type slide where a ‘slab’ of sand on the dune face slides down slope. The depth of such sliding
may range up to a few metres during one event, but on-going erosion can cause retrogressive failure of dune
slopes leading to multiple sliding events, if left unchecked.


26 February 2015
Report No. 14169-001-Rev0                                   8