6. Noosa River DMP Ver0 (Combined)

5.2 Marine Plants
The extraction area does not presently support seagrass meadows. However, mangroves and
Casuarina glauca adjacent to the both the replenishment and extraction areas have the potential
to be impacted.

Where trees are to be retained, it is necessary to limit the impacts within the tree protection zone
(TPZ) to ensure their long-term survival. Damage to the roots or degradation of the substrate
within the TPZ is likely to cause serious damage to the tree. It is important to limit the placement
of any sediment that may adversely affect soil permeability, especially around mature trees,
which are more prone to suffer than young trees. Ideally, all areas close to the tree trunk should
be kept at original ground level (Barrell Tree Consultancy, 2010).

The main mass of mangrove roots is generally within the top 2 m. Mangroves do not grow deep
tap roots, probably because of the poor oxygen supply below the surface. The breathing roots
of Avicennia marina grow as cluster of pencil roots (pneumatophores). Under normal conditions
sediments build up around the pneumatophores at the rate of 1.5–2 cm a year and may become
covered as sediments accumulate. However, Avicennia species growing in poorly aerated soil
are known to die when the pneumatophores are covered (Department of Environment and
Science, 2013).

Two mangroves (Avicennia marina) are within the replenishment area along Gympie Terrace.
One mangrove (tree number 3 on the tree survey plan in Appendix A) located within the intertidal
zone along the Gympie Terrace foreshore has numerous pneumatophores (Figure 5.1). This
specimen is likely to be impacted by the replenishment operations as it is growing where
evidence of bank erosion is significant. However, survivability of this specimen is uncertain if
the pneumatophores are completely covered. To minimise impacts to this specimen, two
options are suggested:

     1. Replenishment material be place at a depth so that the pneumatophores are not
        completely covered (i.e. leave at least 2 cm of the pneumatophores exposed); or
     2. Place replenishment material on either side of the tree protection zone of this specimen
        and allow natural processes to move the sediment. It is recommended that the area to
        be avoided is pegged prior to the commencement of replenishment activities.

The other mangrove within the impact area (tree number 26 on the tree survey plan in Appendix
A) had no visible pneumatophores at the time of the site inspection and was located within an
area subject to current erosion control measures (i.e. coir logs) (Figure 5.2).

The tree protection zones of all other marine plants within the replenishment area along Gympie
Terrace can either be avoided or will benefit from sand replenishment around the root zone.
Many of the Casuarina glauca along Gympie Terrace have had the roots undermined by erosion
(Figure 5.3). Without replenishment of the sand along this reach of the river, there is a risk these
trees will fall.

A group of mangroves, comprising seedling Rhizophora spp. and a mature Avicennia marina
were recorded at the interface of the existing and proposed extraction area at Hilton Esplanade.
The existing approved extraction area avoids this group of plants and it is recommended that
the proposed extraction area do the same. The location of these specimens is shown on the
tree survey plan in Appendix A.



Noosa River                                                                                 Page 10
Marine Plant Impact Assessment
Project Number: BC-20067