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

Noosa Shire Council Coastal Hazard Mapping                                                                28
Erosion Prone Area Assessment




3.7      Factor of Safety (F)
         A 40% factor of safety has been applied to the erosion hazard area calculations for this study, as a
         conservative provision to acknowledge the uncertainties and limitations of the adopted methods
         and assumptions. In addition, there are many other conservative assumptions that underpin
         erosion prone area width assessment, including:

         • The design storm erosion event being characterised by coincident 100 year ARI storm tide and
             100 year ARI wave conditions. It is assumed that the probability of this event occurring in any
             given year is less than one percent;

         • The assumption that the coastal barrier only contains erodible sands;

         • No recognition of existing (or possible future) shoreline erosion management activities such as
           dune revegetation, beach nourishment, revetment seawalls and/or other man-made structures
             designed to limit shoreline recession; and

         • Accurate cross-shore profiles, measured from the 2016 topography LiDAR survey and the 2012
           bathymetric LiDAR survey.

3.8      Dune Slumping (D)
         Immediately following storm erosion events on sandy beaches, a near vertical erosion scarp of
         substantial height can be left in the dune or beach ridge. An area of reduced bearing capacity can
         exist on the landward side of sand escarpments. This can impact on structures founded on sand
         within this zone and the sand escarpments pose a hazard associated with sudden collapse.

         Over time the near vertical erosion scarp will slump to the natural angle of repose of the sand.
         Nielsen et al. (1992) outlined the zones within and behind the erosion escarpment on a dune face
         that is expected to slump or become unstable following a storm erosion event (see Figure 3-5),
         namely:

         • Zone of Slope Adjustment (ZSA): the area landward of the vertical erosion escarpment crest
             that may be expected to collapse after the storm event; and

         • Zone of Reduced Foundation Capacity (ZRFC): the area landward of the zone of slope
           adjustment that is unstable being in proximity to the storm erosion and dune slumping.

         Amongst other factors, the width of the dune slumping and reduced bearing capacity behind the top
         of an erosion escarpment is dependent upon the angle of repose of the dune sand and the height
         of the dune above mean sea level.




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