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

     3.6.   Option D2 - Submerged Upstream & Downstream Closure

Option D2 includes the submerged closure of the southern channel at both the upstream and downstream ends (Figure 29],
similar to the previous closures achieved using sand bunds. The same crest height as for Option D1 was used [RL -0.6m
AHD or approximately low tide].




Figure 29: Sketch of works forming Option D2

Numerical modeling [Figure 30 and Figure 31] showed a reduction in ebb tidal velocities for the full length and width of the
southern channel. Adjacent to the shore, velocities were typically <0.25m/s, although they were as high as 0.4m/s in places.
On the flood tide, the structures impacted very little on velocities in the southern channel, although areas of high velocities
adjacent to shore at the upstream sections of the Noosa River Spit were generally reduced [although some areas of higher
velocity remained].

In the main channel, velocities were hardly affected on the flood tide. On the ebb tide, velocities were very similar for the
bulk of the channel, but were slightly increased (by 0.1 - 0.3m/s) where the channel is narrowest between the shoals. Areas
of high velocity adjacent to the southern shoals were substantially increased in places. Similarly, there were increased
velocities adjacent to the shore at the end of the Noosa River Spit [although this location would be protected by rock wall as
part of this option].

Velocities over the upstream closure peaked at >1 m/s on the flood tide and 0.5m/s on the ebb tide. The downstream closure
experienced velocities >1 m/son the ebb tide and slightly lower (0.8 -1 m/s) velocities on the flood tide. These areas of
increased velocity were limited to the footprint of the structure itself.

In the medium term, while it is possible that a submerged closure of the existing southern channel could reinforce the middle
channel such that it remained the dominant channel, the response of the system is difficult to predict and it is possible [if not
likely] that at some point in the future the channel will attempt to migrate to the south as it has in the past. This would result
in the channel breaching the sandbar that presently exists between the two closures. It is difficult to assess whether this
would 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. With migration of the channel to the south effectively prevented, it is possible that there will be an increased
likelihood of the northern channel becoming dominant [as it has done on occasion in the past] in the event that the middle
channel does not remain dominant.

The potential impacts of Option D2 include:
               •    Reduced erosion adjacent to the existing rock wall
               •    Potentially continued erosion along upstream sections of the Noosa River Spit
               •    Larger impact on river processes
               •    Increased potential for meandering to the north [potential impact on Fish Habitat Area]