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

          4.1.2. Variability in shoreline response

Given the variability of the river, it is anticipated that the performance of the structures will also vary to a certain extent.
Initially, the structures will behave as two river closures, but after the river evolves and the dominant channel moves to the
south, velocities will increase and the downstream 'closure' will become a training wall and the upstream 'closure" will
become a channel guide separating the lagoon, as intended. The location and dominance of the adjacent channel will
impact on the final stable beach alignment and the length of foreshore protected by the groyne. The beaches will continue to
be subject to some level of erosion and scarps may still form along the foreshore, although the incidence and severity should
be significantly reduced and the threat of the Spit breaching should be effectively eliminated. Evaluating the performance of
the structures will form an essential component of the monitoring program and help determine if construction of the shorter
groynes or modification of any of the structures is necessary.

          4.1.3. Channel Evolution 1 River Processes

Numerical modelling of short-term impacts indicates that there is no significant pressure for the river to meander to the north
(any more so than it has in the past) in the short term. In the long term, the fact that the footprint is essentially within the
2002 shoreline [Figure 38] indicates that the structures should influence the river configuration in a similar way to the natural
shoreline in 2002. It should be no more likely to create a dominant northern channel than it has in the past. It is noted,
however, that the river has meandered to the north in the past and it is entirely possible (if not likely) that it will do so at some
point in the future. A northerly dominant channel should not be interpreted as an "impact" of the works without significant
investigation.

          4.1.4. Impact on Shoals

The works are anticipated to result in erosion of sand banks in some locations [middle channel and secondary southern
channel] in the short term. This increased erosion pressure will reduce as the channel changes in response to the works.
The works will also have some impact on the evolution of these shoals as the river develops. These shoals are already
highly mobile, however, and their evolution is unpredictable in any case. Given that the works are essentially within the 2002
shoreline footprint, long-term influence on evolution should be similar to that experienced in 2002.

          4.1.5. Erosion of Mangroves

The proposed revetment wall at the end of the Noosa River Spit will prevent significant further erosion along its extent. At this
stage, it is not proposed to extend the revetment wall such that it would provide protection to the adjacent mangrove stands
[although this could be considered if necessary] and these will remain vulnerable to natural erosion processes. While
erosion rates will vary in response to natural conditions, the high level of erosion experienced in recent years (averaging
15mtyr) suggests that erosion at the end of the Spit may continue as part of natural processes [Figure 40]. A high level of
erosion of the mangrove populations has been a natural feature in this location and loss of mangroves should not
automatically be considered to be a result of the construction works without further investigation. Numerical modelling
indicates that the works will not exacerbate the velocities in this area during the more critical ebb tide, so there should be
minimal impact as an immediate consequence of the works. As erosion occurs, a step in the shoreline will develop [Figure
40] and while the dominant transport direction is such that any groyne effect should be beneficial, end effects and eddies
associated with the wall return may result in locally increased erosion at the end of the revetment wall. The impact of the
works on the adjacent mangrove area will be a focus for monitoring activities. Works may need to be extended or modified if
erosion is significantly exacerbated.




Figure 40: Natural Mangrove Erosion [assuming ongoing erosion at recent average rates]