Abstract From Dr Kate Panayotou's Thesis
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The Minnamurra River estuary, located on the
southeastern coast of Australia, provides an opportunity to
examine the sedimentary records of the stages of infill of
a barrier estuary through to maturity. By describing and interpreting
the diverse geomorphological and stratigraphic units of Minnamurra
River estuary and comparing the long-term Holocene morphodynamic
evolution with the short-term engineering time scale changes,
the nature of Holocene estuary evolution in a bedrock confined
valley in a microtidal environment is investigated, providing
an evolutionary context for contemporary estuary changes.
Management concerns for the Minnamurra River estuary are also
investigated, in particular the study includes examination
of estuary sediment geochemistry and patterns of recent sedimentation
and erosion.
The Minnamurra River estuary is characterised
by three main depositional environments: the marine depositional
environment including the marine flood-tide delta and sand
barrier, the estuarine depositional environment incorporating
the central mud basin, and the fluvial-dominated environment
including the fluvial delta, riverine channel and the alluvial
plain. During the final stages of the postglacial marine transgression,
the Minnamurra River estuary became a sediment sink that has
been largely infilled by marine and fluvial sediments. Examination
and quantification of past and present patterns and rates
of sedimentation has been assessed through a detailed stratigraphy
constructed from 42 auger drill holes, supplemented by full
core drill holes, shorter PVC cores from recent depositional
environments and surface sediment sampling. A detailed chronology
of the infilling of the barrier estuary was also established
using radiocarbon dating and 210Pb analysis.
210Pb was used to examine sediment deposition
at four sites in the Minnamurra River estuary. Cores were
collected from two cut-off embayments, the edge of the main
channel and from a tributary off the main channel, Rocklow
Creek. Average sedimentation rates ranged from 0.3 - 0.8 cm/year
and provided 52 - 148 years of sedimentation history. These
results provide a means of comparing infilling rates throughout
the Holocene evolution of a mature barrier estuary. In particular,
they have afforded the opportunity to quantify the amount
of sediment that bypasses the estuary, which is now in a mature
form. The reduced accommodation space of a river-dominated
estuary means that the amount of sediment which used to be
deposited in the extensive central mud basin that acted as
a sediment sink, must now be deposited elsewhere. In the Minnamurra
River estuary a small percentage of the sediment is deposited
in the remaining cut-off embayments or in overbank deposition
during time of high flow. The remaining sediment, calculated
to be approximately 90% of the total sediment supplied to
the Minnamurra River estuary, bypasses the system and is deposited
in the nearshore.
Recent trace metal levels recorded in the surface
sediment samples and short cores show there was little change
in the levels of trace metals measured in the sediment. Most
trace metal results were below the ISQG-Low trigger value,
however there were a few exceptions. Arsenic, copper and nickel
levels of several sediment surface samples were between the
ISQG-Low and ISQG-High ANZECC (2000) sediment quality guideline
values. Only copper levels exceeded the ISQG-High ANZECC sediment
quality guidelines in a few sediment samples.
Although geomorphological evolution of wave-dominated
estuaries is relatively well known, the understanding of wave-dominated
estuaries that have reached a river-dominated stage is less
well known and as a result, has been investigated in this
thesis. This research has described and interpreted the Holocene
transformation of the estuarine ecosystem of the Minnamurra
River estuary from a wave-dominated system 8500 years BP to
the present river-dominated system with its restricted estuarine
habitats and geomorphology. A five-stage evolutionary model
has been devised for the Minnamurra River estuary depicting
the changes of infill that occurred during the Holocene. During
the first stage of Holocene evolution, 8500 - 6500 years BP,
the Minnamurra River valley was inundated by rising sea-level.
Initial marine deposition occurred semi-enclosing the valley,
forming a central mud basin. The rate of infill increased
substantially in stage two (6500 - 4500 years BP) when barrier
progradation further enclosed the central mud basin. During
stage three (4500 - 2500 years BP) extensive infilling led
to continued barrier progradation and further infill of the
central mud basin by fluvial deposits, which also may have
resulted in a number of cut-off embayments forming. In the
fourth stage of estuary infill, the central mud basin was
largely filled and characterised by an extensive swamp. During
this stage the estuary progressed towards becoming river-dominated
with a confined channel, extensive floodplain and an increase
in intertidal environments. The fluvial channel form meandered
through the floodplain and the sand barrier sequence. The
last 100 years encompasses the fifth stage of the evolutionary
model for the Minnamurra River estuary in which the estuary
was impacted by anthropogenic influences. During this stage
the swampy floodplain was drained and the meandering channel
in the fluvial and estuarine depositional environments was
straightened. Accommodation space during the final stage of
the model is significantly reduced with only a few cut-off
embayments remaining. The results provided a chronology of
the deposition of transgressive deposits, barrier progradation,
central mud basin development and subsequent fluvial expansion.
They also provided the opportunity to calculate volume of
Holocene estuary infill. Approximately 98,000,000 m3 of sediment
was deposited during the Holocene.
The framework of geomorphology and management
adopted in this study can be applied to other estuaries. The
future success of estuary management requires a combined effort
to integrate management in a comprehensive sense by linking
geological, biological and chemical aspects with social and
economic factors, framed within the context of the longer-term
estuary evolution and recent estuary changes. The evolution
of the Minnamurra River estuary has undergone five geomorphologically
distinct phases associated with rising and later falling sea-level,
estuary confinement from barrier development, fluvial delta
progradation and anthropogenic influence by draining the floodplain
and channel straightening. Sedimentation rates differed in
these five geomorphologically distinct phases. The model of
estuary evolution proposed in this thesis endeavours to synthesise
divergent hypotheses in the literature with three key features.
These three key features are firstly hydrodynamics that combines
the three processes (waves, tides and river); secondly, geomorphology,
which is shaped by the three processes; and thirdly percentage
infill that is controlled by the balance between accommodation
space and sediment supply.
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