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

     6.3.   Revetment Wall

Given the difficulty of placing smaller containers at an adequate depth, only rock and mega-containers [Figure 45] are
considered to be suitable. It was originally preferred to construct the wall using rock given that the structure was emergent
and therefore potentially vulnerable to vandalism.




Figure 45: Wall constructed using rock or sand-filled geotextile containers

Preliminary design has been prepared [Figure 46].



                                            HAT

                                            EM
                               1 V:1.5H
                                  RL -3,5m AHD




                                                                  HAT
   Natural surface levelLAT
     [varies]
                                                       Dredge
                                                             1 V:2H
                                      15m                 RL-5 6m AHD



Figure 46: Preliminary design of revetment wall

The top of the existing bank at the end of the Noosa River Spit ranges from RL 0.3m to 1.Om AHD. As such, much of the
end of the Noosa River Spit is inundated during king tides and flood events. As construction of a full-height wall would be
-1.5m above natural surface levels, it has been decided to construct a wall that was more in keeping with natural levels and
can accommodate some inundation during king tides and flood events. As such, there is the potential for some erosion
behind the wall during flood events. While the structure can accommodate some scour at the crest, excessive scour may
result in slumping of the crest. It is assumed that required maintenance [including backfill] will be handled through post-flood
maintenance works.

The channel is very close to the bank along the southern end of the Noosa River Spit and ongoing erosion has resulted in
the majority of the foreshore being relatively steep (approx. 4H:1V) to a channel depth of approx. -3.5m AHD. Despite this,
there is the potential that the channel will migrate northward and become entrenched adjacent to the structure. This
potentially could result in deepening of the seabed adjacent to the toe of the rock wall in excess of current maximum channel
depths in this location. A rock design is flexible and "top-up" is simple and effective, so the preliminary rock design has been
minimized with a toe at -3.5m AHD [-1m below existing levels]. As "top-up" of mega-container structures is not as simple
and slumped structures are not as visually appealing, it was necessary to take the base of this option deeper [to -5.6m AHD]
to allow for additional scour.