2019-12-16 GC Agenda - Item 5 - Attachment 1 - Noosa Spit Shoreline Erosion Management Plan-Part 1

Noosa Spit SEMP Final Report                                                                            21
Noosa Spit Shoreline Erosion




        morphology. Since at least late 2016 (and during other earlier periods), the Dog Beach channel has
        provided a commonly used route for safe navigation.

        Vessels displace water and as they move generate a wake (wash) wave train. In rivers these small
        waves propagate and will often reach a shoreline where the bulk of the remaining energy is
        dissipated. The characteristics of the wake waves depended on several factors relating to vessel
        operation and hull design, displacement, trim, loading, speed, method of propulsion, course and rate
        of change in course (e.g. WRL 2014). The impact of wake waves that reach the shoreline is also
        dependent on the shoreline type and its susceptibility to erode.

        The influence of vessel activity on shoreline recession at Dog Beach has not been quantified as part
        of the SEMP and is assumed to be a secondary influence in comparison to the broader estuarine
        processes. However, it is likely that boat wake waves will exacerbate the erosion, particularly on
        sections of shoreline that are relatively steep and already receding due to channel migration. The
        boat wake waves will have less of an impact on the wider sandy sections of shoreline where the
        energy is dissipated over a larger area.

        Although a secondary influence and difficult to quantify, it’s reasonable to assume that boat wake
        contributes to erosion at Dog Beach when the adjacent channel is being used by vessels. This
        contributor to shoreline erosion is essentially removed when the navigable channels are in other
        locations. This also provides the added benefit of increasing the distance between powerboat activity
        and other recreational uses of Dog Beach such as swimming, canoeing/kayaking, paddle boarding
        and fishing.

2.5     Potential Entrance Configurations for Further Assessment
        To explore how different entrance conditions influence the rate of shoreline recession at Dog Beach
        and lower river hydrodynamics, the following scenarios are suggested for further assessment with
        the numerical modelling tools developed for the SEMP:

        • An east-west aligned channel in the centre of the embayment, such as that observed in 1986/87
          (Figure 2-7, left panel)

        • A northeast-southwest aligned channel in the centre of the embayment, such as that observed
          (Figure 2-7, right panel).

        Testing the other historical scenarios and comparing them to the present condition provides a basis
        for understanding the potential ‘benefit’ of alternative channel and/or entrance configurations with
        respect to reducing the erosion pressure at Dog Beach. Re-establishing or ‘encouraging’ a channel
        to remain in a particular location or alignment via carefully planned and designed works (such as
        dredging, beach nourishment and/or structures) is likely to form part of the strategy for mitigating
        erosion at Dog Beach. The initial assessment of these and other lower river conditions is described
        in Chapter 3 and strategies to relive the erosion pressure at Dog Beach are presented and assessed
        in Chapter 4.




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