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

     3.4.   Option C2 - Groyne Field and Embayed Beach

Option C2 involves the construction of a series of groynes along Noosa River Spit, one of which acts as a training wall to the
flow of the river. A terminal seawall also forms part of this option, although the intent is for this structure to be buried. See
Figure 23.




Figure 23: Sketch of works forming Option C2

Numerical modeling [Figure 24 and Figure 25] clearly showed that the groyne field would be very effective along the
upstream sections of the Noosa River Spit, lowering velocities to <0.25m/s during both the ebb and flood tides. Despite the
intrusion into the channel, adjacent velocities were only slightly increased, indicating that the dredging was adequate to offset
the potential impact on the channel. As per Option C1, however, the region between the two most downstream groynes
remained subject to increased velocities during ebb tides [see section 3.3].

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 in some locations and decreased in others. On the ebb tide, areas of high velocity
adjacent to the southern and northern shoals were increased in size. Velocities adjacent to the shore at the end of the
Noosa River Spit were lower on both the flood and ebb tides.

In the medium term, it is likely that the southern channel will once more become dominant. This will result in a deepening of
seabed levels adjacent to the ends of the groynes and an increase in velocities in the southern channel. While it would be
expected that the groynes would continue to provide protection, the shoreline alignment will vary in response to these
changes to the river system.

The potential impacts of Option C2 [groyne field & embayed beach] include:
               •   Within embayment
                         •    Increased erosion
                         •    Increased potential for breaching of Noosa River Spit in an already vulnerable location
               •   Within upstream groyne field
                         •    Generally stable foreshore, although erosion may still occur
                         •    Improved recreational amenity

Results suggest that a series of groynes could effectively stabilize the foreshore along the upstream areas of the Noosa
River Spit as well as allowing for the creation of a usable beach. The provision of an embayed beach remains problematic
[see Option Cl].