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

Noosa Shire Council Coastal Hazard Mapping                                                                  15
Erosion Prone Area Assessment




         Beach Protection Authority 1998). Despite this annual variation, periods of erosion are generally
         followed by accretion and therefore the Noosa region beaches would appear to be ‘dynamically
         stable’ in the short to medium term. Persistent erosion at Noosa Main Beach has been an issue
         since the 1970s and has been somewhat mitigated through a combination of coastal protection
         structures (groynes and seawalls) and a permanent sand replenishment program. While this beach
         remains vulnerable to the impacts of short term storm erosion, shoreline recession due to long term
         trends is currently managed.

         There is no strong indication of shoreline recession along the Noosa Eastern Beaches based on an
         analysis of historical aerial imagery (e.g. BMT WBM 2013). However, it should be noted that only a
         relatively short period of historical records exists and it begins at a time when the beaches were
         known to be recovering from severe cyclonic erosion in the early 1970’s. Furthermore, periodic
         exposure of coffee rock at many locations throughout the wider Sunshine Coast region may be
         anecdotal evidence of a receding shoreline on a geological timescale (e.g. Lord and Burgess
         1987).

         Due to the difficulties in determining long term trends from intermittent historical datasets, BMT
         WBM (2013) estimated the rate of long term shoreline trends using wave and longshore sediment
         transport modelling techniques. A minor rate in long term recession was identified by dividing the
         annual sediment loss per metre of shoreline (averaged over the Sunshine Coast study area and
         taken to be 0.55 m3/m/year) by an estimate of the active profile height at each location. The active
         profile is defined as the vertical distance from the dune crest to the depth of the closure. The dune
         crest was obtained from the measured beach profile data and the depth of closure was estimated
         from the local wave conditions. The depth of closure is the theoretical depth limit for sediment
         transport and is discussed further in Section 3.6.2.

         Adopting the average annual sediment deficit per metre of shoreline and the approach described
         by BMT WBM (2013), the annual rate of long-term erosion (R) for the Noosa CHAP planning
         horizons has been estimated and are presented in Table 3-1. These results, together with
         consideration of the available historical data, suggest that the rate of long term shoreline recession
         is very low.

         For the Noosa CHAP, an allowance of 10 m to account for long-term erosion (N x R) has been
         adopted for all planning horizons, since the annual rates of long-term erosion presented in Table
         3-1 (obtained using modelling techniques described in BMT WBM 2013) and are less than the 10
         m minimum allowance required by DEHP.




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