24-01-2020 - Appeal No. 245 of 2019 - Final Order of Judge Rackemann · 51987.2951.01

Working Notes:
M Design and Fourteen Enterprises Pty Ltd as trustee v Noosa Shire Council -
42 Seaview Terrace, Sunshine Beach

        “Although the abovementioned probability of occurrence may appear to be fairly
        high, it is considered that this choice is reasonable when considered in conjunction
        with the method used to determine erosion prone area widths. This method implies
        that the erosion prone area has sufficient width to accommodate the design storm
        erosion in 100 years time, when all of the estimated long-term erosion has occurred.
        Therefore, it also follows that for much of the 100-year planning period, the erosion
        prone area is sufficient to accommodate a larger storm than the one selected for
        design purposes. In fact, in the most critical last 10 years of such a planning period,
        there is only a 10% probability of occurrence of the design storm. Thus the risk of a
        storm breaching the entire erosion prone area at some time during the 100-year
        period will be significantly less than 63%.”

Sunshine Beach is a wave-dominated coastline and based on the methodology for calculating short-
term erosion the combination of storm tide and wave conditions must be based on local conditions.

It is noted that BMT WBM (2018) adopted a combination of a 100 year ARI Storm Tide level and 100
year ARI significant wave height as the 100 year ARI storm event for the entire Noosa Shire Council
area. The combination adopted by BMT WBM (2018) is more extreme than the combination
recommended by the Department of Environment and Heritage Protection (2015) as discussed
above. BMT WBM (2018) assessment is therefore more conservative than the requirements
specified by the Department of Environment and Heritage Protection (2015).

For this assessment a combination of a 100 year ARI storm tide and 20 year ARI significant wave
height have been adopted as the parameters for a 100 year ARI Storm Event for the following
reasons:

         The tide range at Sunshine Beach is relatively low (i.e. 1.4 metres at Spring Tides) and as
          such the effect of water level on beach erosion is likely to be more significant. The Beach
          Protection Authority (1984) found that water level rather than waves has a greater influence
          on beach erosion during storm events.
         Sunshine Beach is located immediately south of Noosa Headland which will somewhat
          protect the site from larger waves from the north. Previous work by Cardno (2009) had
          shown that from Moreton Bay north, 100 year ARI storm events are dominated by tropical
          cyclones with the more frequent events (i.e. 20 or 50 year ARI) being associated with east
          coast lows. Tropical cyclones will form in the warm waters off the north Queensland coast
          and as they travel south generate large waves. Noosa Headland will provide a level of
          protection for Sunshine Beach from these northerly waves.

For these reasons the parameters chosen for the design storm event are a 100 year ARI storm tide
level of 1.74 metres AHD, consistent with BMT WBM (2018) and a 20 year ARI significant wave
height of 5.5 metres. The probability of occurrence of this event during the 30 year lifespan of the
swimming pool is estimated to be 25% based on Patterson (1986) and this is therefore a
conservative assessment.

Figure 5-2 and Table 5-2 present the results of the modelled storm erosion associated with the
above design event. This shows a storm erosion cut of 41 metres is predicted to occur. This
represents a conservative estimate of storm erosion for the life of the swimming pool.




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