07/0198 - Op Works - Prescribed Tidal Work - Retaining Wall [As Published] · 142007.198

At Noosa, it is anticipated that groynes would not need to extend far into the channel. However it
is important that they are founded sufficiently deeply to prevent undermining through channel
movement and localised scour which can be expected to occur at the heads of the groynes. The
existing channel depth would provide a guide to the required founding depth. Clearly, recession
will occur between the groynes as pocket beaches gradually form. The extent of recession will
depend on the length and spacing of the groynes. The allowable recession therefore would be a
governing factor in the design layout of the groyne field. It may also be necessary in future to
extend the groynes inland if the recession threatens the landward ends of the groynes. Any groyne
field, revetment wall or combination of both should be designed where practical to encourage the
‘channel to adopt a fairly smooth meander.

With regard to the submitted proposal comprising the rock wall extension and submerged
geotextile groyne, concern is raised regarding the founding depth of the wall and the
effectiveness of the geotextile groyne.

The rock wall appears to be founded on the intertidal shelf characteristic of the bank profile in
this reach of the river. The founding depth is significantly above the bed level of the channel.
There appears to be no reason why the river channel would not continue to move towards the
bank removing the sand from the toe of the wall, resulting in the inevitable failure of the wall. In
order to prevent this occurrence, it would be necessary to found the wall at a depth commensurate
with that of the existing channel. .

Given the relatively small dimensions of the geotextile groyne, the significance of its likely
impact on the behaviour of the river is questionable. Given that the area is popular for swimming,
public safety concerns, similar to those which potentially existed for the proposed artificial reef at
Main Beach, could be expected.

In summary, while the Agency acknowledges the need for some form of river bank hardening,

Council is advised to further investigate the concept of river groynes, more substantial seawalls
or a combination of both. Should you wish, Agency officers are available for on site discussions.

Yours sincerely

Tara Collins
Operations Manager Coastal

Environmental Protection Agency ~ Reference: BNE24889 Page 2 of 2