Development Application - Part 2 of 2 - Report & Plans · OPW11/0116

3.   OPTION ASSESSMENT

The objective of the present study was to assess a range of options to allow for the design, approval and construction of
structures to alleviate the erosion of Noosa River Spit and re-establish a beach. While recreational aspects of this area are
key for the community, there were also concerns raised about continued erosion and the potential for breaching of the Noosa
River Spit into Noosa Sound and the increase in wave propagation into the Noosa River estuary [particularly at Munna Point].

The following options were considered:

     A.     Do Nothing.
     B.     Establish erosion limit control lines [present strategy].
     C.     Stabilise present channel location and beach:
                 C.1 Wall [with embayed beach].
                C.2 Groyne field [with embayed beach].
     D.     Relocate main channel away from the beach:
                D.1 Submerged upstream closure.
                D.2 Submerged upstream & downstream closure.
               D.3 Training Wall & submerged upstream & downstream closure.
     E.     Combination.

Further consideration was not given to ongoing dredging due to the undesirability of the works in such close proximity to the
Noosa River Fish Habitat Area nor the use of a fluidizer since the technology is not well proven and the Noosa River estuary
is so highly dynamic, although further consideration could be given to this option in future if necessary.

Options were assessed in two ways - hydrodynamic modelling for short-term assessment and conceptual analysis for
longer-term assessment.

Hydrodynamic modeling was undertaken using a partially calibrated MIKE21HD model developed for this project by Griffith
Centre for Coastal Management & DHI Australia. Full details of modeling and data collection are presented in Appendix B.
For the purposes of this report, results relating to maximum spring tidal velocities associated with ebb and flood tides are
presented as well as differential plots [showing the change in velocity between the existing situation and the option being
considered]. This modeling provides an assessment of the immediate impacts of the proposed works - a "snapshot" of the
river response.

Sediment transport modeling was considered, however this type of modelling would be expensive [of the order of $200,000]
and time-consuming [>12 months]. Given the variability of the river, it is likely that by the time modelling was undertaken [let
alone the works constructed], the river configuration could have changed and the evolution could be very different. Given
that the model would be essentially uncalibrated, the results would be scenario-based and the Noosa River estuary is
naturally variable, it is highly unlikely that this type of modeling would provide an accurate representation of the actual
evolution of the river over time. It would also be of limited benefit in providing a realistic worst-case scenario for impact
assessment or design purposes. As such, sediment transport modelling has not been undertaken.

Long-term evolution of the river in response to the proposed works has been undertaken using conceptual analysis. This is
based on the aerial photo analysis undertaken by Chamberlain & Tomlinson that was extended by Griffith Centre for Coastal
Management as part of these works [Appendix A].

While the approach taken is best practice, it is noted that the coastal environment is inherently complex and variable
[increasingly so with climate change] and there are limitations and uncertainties inherent in these techniques. As a result, it
is recommended that the proposed works be staged where possible and the design be robust and flexible. A comprehensive
monitoring strategy [Appendix C] should also be implemented to allow for ongoing assessment of the performance and
impacts works. It is also preferable that the works be able to be modified or removed if monitoring shows that this is
necessary.



     3.1.   Option A - Do Nothing

Meandering of riverbanks and associated erosion and accretion is a natural event [despite the fact that the Noosa River Spit
was artificially reclaimed and is not a natural feature in the river]. The BPA report noted that Noosa River Spit would be
"subject to erosion and accretion as part of the normal beach system", although they also noted that it would "require
maintenance ... [although] the timing and extent of maintenance cannot be estimated ... because of the unpredictable nature
of the sed'.