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

6.   PRELIMINARY DESIGN & COSTING

     6.1.   Submerged "Closure"

While placement of bulk sand to form a natural bar has successfully been undertaken in the past, rapid erosion of the
material and turbidity associated with ongoing dredging makes this undesirable. Sand-filled geotextile mega-containers are
preferred because rock is less user-friendly and smaller units may not be stable. As the structure will be predominantly
submerged, there will be no visual impact the majority of the time and during lower tides the structure may be partially
emergent [Figure 41].




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Figure 41: Low-crested breakwater constructed using sand-filled geotextile mega-containers

Preliminary design has been prepared [Figure 421.


                                     Crest                                         Lagoon
 HAT                             RL-0,6m AHD ,4                     Mid-tide -wi     vanes   -
                                                                                                                             5m
                                                                                                            - 1 V:3H
 LATE                                                                                                       Nourish
                                                       Dredge
             1V:2H
                                                       RL-5 . 8m AHD
                                        12m
Figure 42: Preliminary design of submerged "closure"

The crest level of the "closure" is RL-0.6m AHD [as per the modelling]. This means that the structure will be predominantly
submerged and only partially emergent on lower tides. The temporary sand bunds constructed previously were at a similar
or higher level and did not experience any water quality issues associated with poor flushing. As such, flushing is expected
to be adequate for the proposed structure. Crest height can be raised in future, if monitoring indicates that this is desirable.

Preliminary design has allowed to take the closure to a depth of approx. RL-6m AHD, over 1m lower than surveyed
maximum channel depths within the Noosa entrance (and 2 - 6m below current depths). The only location which is in
excess of this depth is adjacent to the water inlet for the sand recycling and this is not in close proximity to the structure.
Still, given the variability of the river and the proposed staging of the works, it is possible that at some point during the life of
the structure the "closure" could be subject to undermining due to channel deepening or local scour. In the event that
monitoring identifies that levels are approaching the foundation depth directly adjacent to the structure, placement of an
additional container on the river side may be undertaken at this point. Sand-filled geotextile containers are flexible and can
accommodate some settlement, although this would result in an equivalent lowering of the crest. Significant undermining
should be avoided as this could lower the stability of the crest container.

The alignment of the closure should follow that allowed for in the numerical modelling. For ease of construction, curved
closure may incorporate a short return direct into shore at the upstream end to minimize required excavation while
accommodating a reasonable level of erosion without breaching landward of the structure. The works may be terminated at
the existing low tide mark and allow for flows into the lagoon landward of the structure provided monitoring is in place and the
closure is extended if required. There is also the potential to raise the level of the return to create a third groyne if monitoring
indicates that this would be desirable.

Preliminary estimate of the associated costs is as per Table 2.