Amended Noosa Project Restoration Plan V12_5

8.4 Erosion control

TNC’s consultant engineers have confirmed:

The purpose of these works is not to provide erosion protection but to restore rock oyster ecosystems
throughout the Noosa River.

Natural erosion can be the result of tidal or flood flows, but in many locations within the Noosa River
erosion is strongly influenced by wind waves and vessel wake. Hard structures have the potential to
result in reduced (or increased) erosion because of changes to river flows or wave action, changes to
sediment transport patterns as well as end effects. Hard structures can also result in shoreline
protection that reduces erosion.

Even though the works are not intended to provide erosion protection, the nature of the works are
generally expected to result in reduced severity of coastal erosion adjacent to the structures. The oyster
reef patches in this project are engineered to reduce any potential for these structures to result in
increased erosion of adjacent shorelines, including shorelines upstream and downstream of the works.

Features of the oyster reef patches (the works) that consider coastal processes include:

Concept module layout - Concept Layouts 2 & 3 (segmented longshore modules) show how oyster reef
patches are configured to minimise impacts to littoral drift and coastal processes by being aligned with
river flows and providing gaps between oyster reef patches. Despite this, the oyster reef patches are
anticipated to act as a segmented reef and provide some sheltering to the shoreline from wave action
and vessel wake and result in reduced erosion. Reef has been segmented to limit this impact and allow
penetration of waves and flows without resulting in pockets of increased erosion.

Concept layout 1 provides habitat that extends out perpendicular from the shore similar to observed
development of oyster reef habitats. This layout is only suitable in locations with low sediment
transport and not within Restoration Zone 1 Main Channel. In these locations there is limited risk of
the works acting as a submerged groyne, which would have the potential to impact on littoral drift,
erosion, and accretion patterns (as well as result in undesirable burial of sections of the structure).

Shaped oyster reef patches – Oyster reef patches may be curved or shaped to reflect the local
bathymetry and site-specific conditions. Alignment with the shoreline or directing flows away from
the shoreline reduces the potential for focussing of tidal or flood flows. Tapering of the reef ends
reduces the potential for flow disturbances (such as eddies) to develop.

Crest level – Crest levels are dictated by desirable water depths for the target species and do not extend
above MHWS. As with many natural oyster reefs, they are exposed during lower tides and inundated
during higher tides (or flood events). When water levels are close to or over the crest of the reef, wave
action and vessel wake would be expected to overtop the structure. As such, wave energy reaching the
shoreline (especially for Layout 2 & 3) will be reduced but not eliminated. While natural erosion may
continue to occur, it is expected to be reduced.

Access space – The provision of an access space facilitates monitoring of the reef habitat, but also
provides space for natural flows to occur between oyster reef patches.

Porous rock substrate – waves impacting with the substrate are dissipated into the voids in the
substrate, with reduced wave energy impacting on the adjacent shoreline and reduced wave reflection
compared with solid structures.
                                                                                                       71