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

          4.1.6. Flooding

Numerical modelling indicates that should be no significant change to upstream velocities or levels during a normal tidal
cycle as a result of the proposed works. Flood modelling has not been undertaken, however previous floods have resulted in
the development of a dominant middle channel and the proposed works would not affect this development during future flood
events. As the works are essentially within the footprint of the 2002 shoreline, any potential impact of the works is
considered to be within the natural variability of the estuary and should not result in increased tidal levels or flood risk to
areas upstream.

          4.1.7. Flushing

While areas within the lagoon are typically calm, previous sand bunds experienced adequate flushing and it is expected that
this would also be the case with the proposed closure. Velocities within the lagoon are likely to change in response to
changes in river configuration. If necessary, consideration could be given to changing the crest level as part of the
monitoring works.



          4.1.8. Swimmer safety near submerged closure

Velocities over the closure are higher than within the lagoon, but are typically limited to the comparatively narrow width of the
structure itself. This will not compromise the structure itself, but may be of concern with regards to swimmers using this area,
particularly if velocities could result in users being swept into the edge of a dominant navigation channel. It is noted that
there were no apparent safety concerns as a result of the sand bunds and velocities would be expected to be similar. Safety
considerations should continually be reassessed with consideration given to actual velocities, location of the dominant
navigation channel and usage of the lagoon. If swimming within the lagoon or near the structure is not recommended,
suitable precautions may need to be taken. This may include signage and/or barriers at the shoreward ends of the
structures to reduce or control usage and may also include placement of buoys at some distance from the structures to limit
the proximity of vessels to the structure.

          4.1.9. Navigation

As the works involve closure of a channel, there is the potential for vessels to impact with the structure. As such, the
structure will need to be suitably marked in accordance with MSQ requirements / recommendations. It is anticipated that this
could include provision of a beacon at the end of the training wall and buoys at points along the length of the submerged
closure. A Notice to Mariners would also need to be issued.

          4.1.10.Vandalism /Vessel impact with the structure

While the containers are to be fabricated using a robust 1209RP geotextile (or equivalent), there is the potential for
vandalism, large debris or vessel impact with the structure to result in damage to the containers. Emergent sections of the
structures will be particularly vulnerable. Tears or cuts in the geotextile will allow migration of material out of the container
and gradual deflation of the container over time and eventual removal of the container from the structure. If damage is
identified, maintenance [patching] should be undertaken as soon as practicable. If a partially deflated container cannot be
patched, it should be replaced. Both patching and removal of containers can be undertaken with relative ease and minimal
expense by Council staff. In the event that maintenance becomes a burden, consideration may be given to armouring the
structure with rock [alternative materials may also be considered] once the structure is accepted.

          4.1.11. Structural aspects

Risks associated with the individual structures [e.g. undermining and overtopping] have been addressed as part of the
preliminary design review.