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

Noosa Shire Council Coastal Hazard Mapping                                                                  13
Erosion Prone Area Assessment




3.2.1.1 Application of the Calculated Erosion Distance Formula
         DEHP (2013) guidelines require that the open coast erosion hazard area distance is measured
         landward from the seaward toe of the frontal dune. This is normally approximated by the seaward
         limit of terrestrial vegetation or, where this cannot be determined, the level of present day HAT. The
         coastal zone is highly dynamic and significant fluctuations can occur in the dune profile and
         correspondingly the location of the seaward toe of the frontal dune. At some locations and times
         there can be a large height (and therefore volume) difference between the higher hind dune areas
         and the low foredunes which can be quite wide.

         The calculation of the short term erosion component (C) is volumetric based and where there is a
         broad low foredune, the calculated distance from the seaward toe of that dune can be large. The
         adopted formula for calculating the overall erosion hazard area width also includes a factor of
         safety that is applied to the short term erosion component (C). In situations with a broad low frontal
         dune, this can lead to an unrealistic overestimation of the width of the short term erosion threat as
         measured from the seaward toe of the frontal dune. Accordingly, for the present study, this short
         term erosion component has been split into two sections (C1 and C2) with the revised calculated
         erosion distance formula as follows:

         𝐸 = [(𝑁 × 𝑅 ) + 𝐶1 + 𝑆] × (1 + 𝐹) + 𝐷 + 𝐶2
         Equation 3-2

         Where     C1 = short term erosion from the design storm event, measured from the location where
                   the design water level intersects the pre-storm beach profile (metres)

                   C2 = distance from the seaward toe of the frontal dune to the location where the design
                   water level intersects the pre-storm initial beach profile (metres)

         The modified calculated erosion distance formula is illustrated conceptually in Figure 3-2 using an
         example beach profile. The C1 term in the current study is the modelled short term erosion setback
         distance based on the volume eroded from the main pre-storm dune above the design water level.
         This is the primary short term erosion component to which the factor of safety is applied. The C2
         term is the distance between the seaward toe of the frontal dune and the location where the design
         water level intersects the pre-storm profile. This covers the short term erosion of the low foredune
         area to which the factor of safety is not applied. As outlined above, this minimises the potential
         overestimation of the total short term erosion component when using the standard formula on
         beaches with a broad low dune terrace. The approach for assessing short term erosion is
         discussed further in Section 3.5.

         For the present study, the Department of Natural Resources and Mines (DNRM) state coastline
         definition has been used to estimate the frontal dune toe position along Noosa open coast beaches
         for erosion hazard area assessment and mapping purposes. The DNRM coastline was digitised
         manually, guided by several references including aerial imagery and HAT contours generated from
         LiDAR survey data. A review of the DNRM coastline definition along Noosa beaches indicates that
         digitised coastline is at an elevation above HAT and close to the seaward edge of dune vegetation.




G:\Admin\B22613.g.mpb.Noosa_CHAS\R.B22613.002.04.NSC_CHAP_Mapping.docx