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

     3.7.   Option D3 - Training Wall with Submerged Upstream & Downstream Closure

In Option D3 [Figure 32], a section of the upstream closure was raised to provide a fully emergent training wall and the
downstream closure was relocated to the end of the existing rock wall to reduce the potential impact on the estuary.
Dredging was also included to source material for construction and for nourishment of the beach.




Figure 32: Sketch of works forming Option D3

Numerical modeling [Figure 33 and Figure 34] showed a reduction in ebb tidal velocities in the southern channel similar to
Option D2 [see section 0], both between the closures and further downstream along the existing rock wall. On the flood tide,
the velocities were typically similar near the existing rock wall, but further upstream the partial training wall significantly
reduced velocities adjacent to the shore, although the modeling still showed some locations further from the groyne which
were subject to velocities reaching 0.8m/s.

In the main channel, velocities were generally the same on the flood tide, although the areas of high velocity adjacent to the
northern shoals were increased in size and the velocities adjacent to the upstream closure were slightly increased. On the
ebb tide, velocities were very similar for the bulk of the channel, but were slightly increased (by 0.1 - 0.3m/s in places) where
the channel is narrowest between the shoals. Areas of high velocity adjacent to the southern and northern shoals were
increased in size. Velocities were increased slightly [by 0.1 - 0.3m/s] in the secondary channel between the shoals adjacent
to the downstream closure. Velocities adjacent to the shore at the end of the Noosa River Spit were also increased
[although this location would be protected by rock wall as part of this option].

Velocities over the upstream closure peaked at >lm/s on the flood tide at the end of the training wall, but are close to
existing velocities (-0.5m/s) on the ebb tide. The downstream closure experienced velocities >lmis on the ebb tide and
-0.5m/s on the flood tide. These areas of increased velocity were typically limited to the footprint of the structure itself.

In the medium term, while it is possible the southern closure may help to slightly reinforce a dominant middle channel, it is
likely that the channel will migrate to the south as it has in the past. This could result in the channel breaching the sandbar
that presently exists between the two closures, resulting in the submerged closures becoming submerged groynes. This
could potentially result in velocities that would reintroduce an erosion threat along the shoreline of the Noosa River Spit. This
risk could be addressed by linking the two closures, either during initial construction or in the event that erosion once more
became a threat. This configuration may have some impact on the river processes, but it will be substantially less than the
D2 option and it is unlikely that it will substantially affect the tendency of the river to meander to the north.

The potential impacts of Option D3 include:
                •   Reduced erosion adjacent to the existing rock wall
               •    Generally reduced erosion along upstream sections of the Noosa River Spit [although some erosion may
                    persist in some locations]