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

Noosa Shire Council Coastal Hazard Mapping                                                                                            17
Erosion Prone Area Assessment




3.5         Storm Erosion (C)
            Storm erosion occurs when increased wave heights and water levels result in the erosion of sand
            from the upper beach ridge. The eroded sand is taken offshore where it is deposited as a sand bar
            located near the wave break area. After the storm event the sediment is slowly transported
            onshore, often over many months or several years, rebuilding the beach.

            The potential for short term storm erosion due to severe wave and elevated sea water levels (storm
            tide conditions) has been predicted using the simple cross-shore equilibrium profile model of
            Vellinga (1983). This empirical model calculates upper beach and dune erosion volume associated
            with the given storm induced extreme water level and wave conditions. The amount of shoreline
            recession is determined from the input parameters and the initial (pre-storm) beach profile shape.
            The model assumes the volume of material eroded from the upper beach/dune system and
            deposited offshore is balanced by a setback of the shoreline. This assessment approach is
            described further below.

3.5.1       Design Event Erosion Assessment

3.5.1.1 Background Information and Datasets
            As discussed in Chapter 2, Aurecon (2013) recently assessed and updated storm tide levels
            throughout the Noosa region. The present-day 100 year ARI ‘surge plus tide’ levels have been
            adopted for the short term storm erosion assessments.

            BMT WBM (2013) assessed design wave heights for the Sunshine Coast region which included the
            Noosa Eastern Beaches. Inputs to the wave modelling was based on the statistical analysis of the
            Brisbane Waverider buoy data (Allen & Callaghan 2001)2 that considered waves generated by
            cyclonic and non-cyclonic conditions. The 100 year ARI significant wave height has been used as
            input to the erosion hazard assessments. In addition, the wave setup contribution to the sustained
            water level at the shoreline has been estimated following Stockdon et al (2006):

            S shoreline  0.35  f H 0 L0 
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            Equation 3-3
            Where βf is the foreshore slope (approximately 1V:25H throughout the study area), H0 is the deep
            water significant wave height and L0 is the deep-water wave length (L0 = gT/2π where T is the
            wave period = 12s).

            The setup allowance has been added to the ‘surge plus tide’ levels reports by Aurecon (2013) and
            used as input to the Vellinga (1983) storm erosion model. This is considered a conservative
            approach since the erosion model only requires input of the ‘surge plus tide’ level and the
            additional contribution of wave setup to the extreme water level leads to an increase in the
            predicted erosion volume.

            An assumed beach sediment median grain size of 0.22 mm (220 µm) was adopted throughout the
            study area (e.g. Delft Hydraulics Laboratory 1970; Jones 1992). This grain size is typical for south

2
 Shand et al. (2011) also report extreme value analysis of Brisbane waverider buoy recordings using approximately 10 years of
additional data. Their results are consistent with Allen & Callaghan (2001) with the 100 year ARI significant wave height Hs ≈ 8 m.


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