I
EXECUTIVE SUMMARY
EXECUTIVE SUMMARY
This Estuary Processes Study of Gunnamatta Bay
provides an essential step in the formation of a
long-term Estuary Management Plan for the bay.
The aims of the study were to investigate and
develop
an
understanding
of
the
estuarine
processes of the bay, and the interactions
between
them.
The
estuarine
processes
investigated include the hydraulic, water quality,
sedimentary and ecology processes.
flushing times have been calculated to be less
than 10 days, while the large resident volume and
relatively small freshwater inflows means that the
bay has a high physical assimilation capacity.
Fluvial Sedimentation
A number of fluvial deltas have formed at the
outlets to natural creeklines and stormwater
drains.
The establishment and growth of these
deltas was accelerated during urbanisation of the
Provided below is a brief summary of these
catchment, however, they have now stabilised and
processes, and their interactions.
are starting to become reworked under the
influence of wind-wave action.
Catchment Processes
Gunnamatta Bay receives rainfall runoff and
pollutant loads from catchment sources via several
natural creeklines, and numerous stormwater
outlets.
Although
pollutant
loads
(mostly
Fine sediment discharged to Gunnamatta Bay is
generally deposited within the deeper sections of
the bay, which has an average infilling rate of
about 1 mm/yr.
sediment and litter) are reduced somewhat by
Sediment samples taken from most areas of
existing stormwater devices and street sweeping
Gunnamatta Bay were generally free of excessive
activities, the two main sources of pollutants are
pollutants.
general catchment runoff and sewer overflows. It
sediments from the vicinity of Cronulla Marina and
is also suspected that exfiltration of raw sewage
the fluvial shoal fronting Tonkin Park indicated
from the sewerage system into the stormwater
that concentrations of Copper, Lead, Zinc, Mercury
drains may contribute significantly to catchment
and PAHs exceeded recommended guideline levels
loads.
and are likely to be a threat to ecosystem health.
Bathymetry
Organic material present within the bed sediments
The bathymetry of Gunnamatta Bay is unusual in
that it has a marine shoal across its mouth, with
deeper sections in the upstream reaches.
The
entrance shoal has a typical depth of about 1
metre (below Mean Sea Level), while the northern
sections of the bay are up to 12 metres deep.
The
total
resident
volume
of
the
bay
However, testing undertaken on
of Gunnamatta Bay could potentially be releasing
nutrients back into the water column, thus
providing an additional source of nutrients for
primary producers (eg algae) within the bay.
Entrance Marine Shoal
Although no external sediment is being supplied to
is
the marine shoal at the entrance to Gunnamatta
approximately 4,500 ML, which is about 4.5 times
Bay, wind wave action is slowly reworking the
the average annual runoff from the contributing
shoal in a net northerly direction.
catchment.
marine sediment along the eastern foreshore of
Hydrodynamics
Although no tidal recorder is located in the bay, it
is expected that the tidal range of Gunnamatta
Bay is much the same as that of the ocean. Tidal
Gunnamatta
Bay
is
presently
Transport of
impacting
on
Gunnamatta Baths.
The navigation channel along the western edge of
the marine shoal was initially dredged around
1915, and has been maintained ever since by
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EXECUTIVE SUMMARY
regular dredging. The navigation channel can infill
The extent of aquatic vegetation (ie seagrass) has
quite rapidly following dredging by reworking of
also reduced significantly over the past 50 years.
material off the entrance shoal, or ingress of
A small remnant area of saltmarsh at the end of
material into the channel from Port Hacking during
Shell Road, which has been invaded by weeds
storms.
should be considered for rehabilitation.
Existing Water Quality
The aquatic environment is currently under threat
While some water quality parameters that have
been measured in the bay indicate acceptable
conditions, there are still some issues requiring
from an invasive seaweed, Caulerpa taxifolia,
which
has
the
potential
to
overrun
the
Gunnamatta Bay aquatic habitats.
Although the impacts have not been quantified,
consideration as discussed below.
Elevated bacteria levels in certain sections of the
Bay, particularly adjacent to stormwater outlets
after rainfall events, represent a significant health
risk in terms of primary contact activities (ie.
swimming).
motor vessels and swing moorings may affect
seagrass beds and may assist in the spreading of
Caulerpa taxifolia.
Processes Interactions
There are strong interactions between many of
Sewer overflows contribute approximately 38% of
the estuarine processes investigated, particularly
the catchment-derived total nitrogen load and
between catchment and water quality processes,
approximately 69% of the catchment-derived total
and
phosphorus load to the bay, while catchment
Ecological processes are affected by most other
runoff accounts for the vast majority of sediment
processes,
input.
sediments.
The concentrations of nutrients in the bay do not
Impact of Human Activities
generally
exceed
recommended
levels
and
catchment sources have minimal impact when
considered in the context of the quick flushing
time and large volume of the Bay.
catchment
and
particularly
sediment
water
processes.
quality
and
Post-European human activities have had a
significant
and
detrimental
estuarine
processes.
impact
Urbanisation
on
of
most
the
catchment has resulted in sediment and pollutant
Effective flushing and assimilation of nutrients
loads to the Bay that are many times higher than
appears to prevent algal (phytoplankton) blooms
during pre-European times, which has impacted
within the Bay.
the water quality, sedimentation and sediment
In the past, excessive concentrations of heavy
quality of the bay.
metals have been present in some sections of the
Usage of the bay itself for boating, fishing, and
Bay (ie. around commercial activities at the head
commercial enterprises has also resulted in
of the Bay). However, further testing would be
additional pollutant loads to the bay, and has
required to confirm whether this is still the case.
displaced ecological habitats, such as mangroves,
saltmarsh
Existing Ecology
Little
natural
Gunnamatta
bushland
Bay
remains
catchment,
within
which
and
seagrass
beds.
the
would
significantly limit the ecological value of the area
for terrestrial species. The cutting or poisoning of
remaining native vegetation to enhance views has
been reported.
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EXECUTIVE SUMMARY
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