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

     3.5.   Option D1 - Submerged Upstream Closure

Option D1 includes a submerged closure of the southern channel at the upstream end only [Figure 26]. While previous
campaigns have focused on the formation of these closures using loose sand, the rapid erosion and the environmental
impacts of ongoing dredging make this undesirable and a structural closure is being considered in this case, which also
allows an equivalent height to be achieved with a substantially smaller footprint. The submerged closure is set at a height of
-0.6m AHD [approximately low tide] such that the structure will be fully submerged the majority of the time and partially
emergent only on the lowest of tides.




Figure 26: Sketch of works forming Option D1

Numerical modeling [Figure 27 and Figure 28] showed that on the ebb tide, the upstream closure resulted in little change to
velocities in general. Although some areas (including some high 'spots' of velocity) were significantly reduced, these were
generally not in the 'problem' areas. The velocities on the flood tide were very similar to the existing situation with high
velocities adjacent to the shore persisting [if not increased] in upstream areas. It may be that the potential impact of the
upstream submerged closure is being masked by the fact that the southern channel is currently partially closed in this
location, such that the change to the seabed in this location is not as pronounced as it may have been in the past.

In the main channel, velocities were generally the same on the flood and ebb tides, although the areas of high velocity
adjacent to the shoals were increased in size on both the ebb and flood tides. Velocities adjacent to the shore at the end of
the Noosa River Spit were increased on the vulnerable ebb tide.

Velocities over the closure peaked at >lm/s on the flood tide and 0.75m/s on the ebb tide. The area of increased velocity is
limited to the footprint of the structure itself.

In the medium term, while it is possible that the closure will reinforce the middle channel, it is likely that the channel will
attempt to migrate to the south as it has in the past. This would result in the channel closure becoming a submerged groyne
type feature. Since velocities have not been substantially reduced by the structure as a closure, it is unlikely that it would
perform substantially better as a submerged groyne and it is likely that erosion issues will not be fully resolved, although they
may be mitigated if the structure performs better under increased flows associated with a dominant southern channel. The
structure could be extended [either initially or as required after breaching] to reinstate the closure.

It appears that provision of a single submerged upstream closure would be of limited benefit in the short term and unknown
benefit in the medium term. It seems likely that either a secondary closure would need to be introduced downstream [as
done previously using artificial sand bunds, see Option D2] or the height of the structure will need to be raised in order to
have a greater impact on velocities and erosion.