Project Supporting Document

Transcription

The tidal characteristics and shallow-marine
seagrass sedimentology of Robbins Passage
and Boullanger Bay, far northwest Tasmania
A technical report to Cradle Coast Natural Resource Management
September 2012
Paul Donaldson, Chris Sharples & Robert J Anders
Blue Wren Group
School of Geography and Environmental Studies
University of Tasmania
Page 1 of 88
Blue Wren Group, University of Tasmania
The tidal characteristics and shallow-marine seagrass sedimentology of
Robbins Passage and Boullanger Bay, far northwest Tasmania
Acknowledgements
This project was supported by Cradle Coast NRM through funding from the Australian
Government’s Caring for our Country.
The authors of this report would like to especially thank:
Chris Watson, UTAS, for assistance with tide data processing and analysis; Dave Shaw,
for designing, constructing and operating the sediment corer; Matthew and Niko
Campbell-Ellis, for helping to deploy, download and retrieve the remote water level
loggers, as well as providing general field support and warm hospitality; and Richard
Mount, BOM, for initiating the research project, providing advice on seagrass and blue
carbon and reviewing the draft report.
Thanks also go to:
John Gibson and Jeff Ross, IMAS, for practical advice on the operation of the water level
loggers; Mike Johnson, for assistance with the engineering design for the sediment
sampling platform; Mark Underwood and David Hughes, CSIRO, for advice regarding
stilling well design requirements; Brett Greene and Shane Underwood, Black Reef
Fisheries, for boating assistance during our sediment sampling program; Cradle Coast
NRM staff, including James Shaddock for his enthusiasm towards this project and early
assistance in with the contract management and Sue Botting for her critical comments
on the draft report; and ESRI Australia, for assistance with ArcGIS.
Photo credits
Cover photos: Western Robbins Passage tidal channel and Boullanger Bay (top, Vishnu
Prahalad), Posidonia australis meadows, Boullanger Bay (lower-middle, Richard Mount),
fibrous rich seagrass sediment core from Boullanger Bay (bottom, Paul Donaldson).
All other photos by Paul Donaldson and Chris Sharples, with the exception of Figure 8c
(p 24) by Matthew Campbell-Ellis and Figure 20 (p 48) by David Shaw.
Cover design
Paul Donaldson
Citation
DONALDON, P., SHARPLES, C., ANDERS, R.J., 2012: The tidal characteristics and
shallow-marine seagrass sedimentology of Robbins Passage and Boullanger Bay, far
northwest Tasmania. A technical report to Cradle Coast Natural Resource Management.
Blue Wren Group, School of Geography and Environmental Studies, University of
Tasmania, Hobart.
Page 2 of 88
The tidal characteristics and shallow-marine
seagrass sedimentology of Robbins Passage
and Boullanger Bay, far northwest Tasmania
A technical report to Cradle Coast Natural Resource Management
Paul Donaldson, Chris Sharples & Robert J Anders
September 2012
Blue Wren Group
School of Geography and Environmental Studies
University of Tasmania
Page 3 of 88
Tide and seagrass sedimentology report, Robbins Passage–Boullanger Bay, Tasmania
SUMMARY
This natural resource management research project was initiated by the Cradle Coast
NRM, in response to the knowledge gaps identified by the Blue Wren Group in
understanding elements of Robbins Passage-Boullanger Bay (RP-BB) coastal processes.
The purpose of this study was to:


Improve the understanding of RP-BB tides, based on observational data, and
Investigate the carbon sequestration potential and palaeo-environmental evolution of
RP-BB shallow seagrass beds, based on a set of shallow marine sediment cores.
The key findings of this report are:






RP-BB receives strongly semi-diurnal meso-tides which vary in their range and time
of arrival
Predicted mean spring tide ranges and total tide ranges were found to be 2.80 m and
3.15 m at Howie Island, 2.20 m and 2.63 m at Kangaroo Island, and 2.01 m and
2.42 at Welcome Inlet
The National Tide Centre’s modelled tide range was found to underestimate the tide
range for eastern Boullanger Bay by approximately 30%
Three unique sedimentary deposits (i.e. facies) were identified in the sediment cores,
interpreted as a Late Pleistocene alluvial/lacustrine deposit (SF1), Mid-Holocene
intertidal or shallow subtidal sand flats (SF2), and Mid-Late Holocene seagrass
associated deposits (SF3)
Large carbon rich sediment deposits exist beneath the subtidal seagrass meadows at
RP-BB
RP-BB Posidonia australis dominated subtidal seagrass meadows are highly effective
at sequestering carbon.
ROBBINS PASSAGE-BOULLANGER BAY TIDES
The tides in far northwest Tasmania are highly variable and poorly documented. This
study recorded about 5 months of sea level observation from three locations across RPBB, and analysed this data with T-TIDE in MATLAB, to better define the range and timing
and astronomical components of the regions tides.
A comprehensive literature review was initially conducted to provide background
information on the science of tides, the available methods for collecting and analysing
tide data, and the current understanding of the tidal characteristics of the far northwest
Tasmania.
Tide data collection
Sea level observations from three locations were recorded using pressure gauges
deployed in ‘remote stilling wells’ purposefully constructed for this project. These were
designed for deployment in meso-tidal conditions and constructed from relatively cheap
and readily available materials. The stilling wells functioned to protect the loggers and
improve the data accuracy by mechanically filtering out the effect of waves on the
observed water level data.
Page 4 of 88
Summary
Data was collected at 5 minute intervals with the HOBO U20 water level loggers from
November/December 2010 to May 2012. The loggers were surveyed to the Australian
Height Datum (AHD) with differential GPS.
Tide data processing and analysis
The total pressure data collected from the three sea level observation sites were
converted to water level using the Barometric compensation assistant in HOBOware Pro
and the regional air pressure observations collected in this study. These water levels
were subsequently corrected to AHD. A tidal analysis was conducted on the corrected
data using T-TIDE in MATLAB to define the astronomical constituents for the three sites.
This data was then used to predict the astronomical tides (i.e. those driven by the
gravitational interactions between sun, moon and earth) and define the environmental
effects on the observed tides (such as weather and shallow water).
Various tidal planes were defined for each observation site, using standard observationand harmonic- based definitions. Prediction-based definitions (which apply observationbased definitions to the predicted data) were also applied, to avoid the issues associated
with applying observation- and harmonic - based definitions on our dataset.
Tide characteristics and behaviour
RP-BB receives strongly semi-diurnal meso-tides which vary geographically in its height
and time of arrival.
Tide range
RP-BB receives strongly semi-diurnal (F ≤ 0.2) meso-tides which vary geographically in
their height. Tide range increases from west to east – a trend which is consistent with
the regional gradient experienced across the greater northwest coast.
Table S1: Key tide ranges for Robbins Passage – Boullanger Bay, derived from harmonic-based2
and prediction-based3 tide planes.
Tidal Plane
Total tide range3
Indian Spring Water tide range
Mean Spring tide range
Mean Neap tide range
3
3
2
Howie Island
(m)
Kangaroo Island
(m)
Welcome Inlet
(m)
3.151
2.626
2.422
2.866
2.410
2.122
2.796
2.200
2.094
1.839
1.312
1.311
The tide planes defined for Kangaroo Island and Welcome Inlet show that teh tide range
in eastern Boullanger Bay is greater than previously realised (e.g. NTC standard tide
range model).
Tide timing
Howie Island is the first observational location to receive high tide by 20 minutes,
occurring approximately 55 minutes after Stanley. Welcome Island is the last site to
experience ebb tide, some 40 minutes after Howie Island and Kangaroo Island. All three
measured sites experience asymmetrical tides with lagged ebb times due to shallow
water conditions.
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Table S2: Relative timing of the predicted tidal wave at Robbins Passage-Boullanger Bay.
Kangaroo Island
Arrival time after
Howie Island
Welcome Inlet
High Tide
Low Tide
High Tide
Low Tide
Maximum (minutes)
30
30
35
60
Minimum (minutes)
10
-20
05
10
Average (minutes ± 1sd)
20 (±05)
00 (±10)
20 (±05)
40 (±10)
Astronomical tides
Tidal harmonic analysis solved between 21- 23 harmonic constituents for all
observational sites, with the Principal Lunar semidiurnal constituent accounting for ~50
% of the astronomical tidal variation. Higher order - shallow water harmonics (such as
M4) were also found to be significant for all three sites. Predicted astronomical tides
based on the summation of the solved tidal harmonics account for ~97% of the
observed data.
Tide behaviour
The tides timing indicate that two separate tide waves enter Robbins Passage, from east
and west. The decrease in tide range experienced from west to east suggest a net
hydrodynamic flow occurs in the same direction. This observation supports Mount et al.’s
(2010a) interpretation of the long term sediment transport dynamics through Robbins
Passage.
The timing of tides from the two eastern Boullanger Bay sites suggests that northwest
propagating flood tides are experienced here. This may indicate that two separate tide
waves enter the bay from the north and west, but more tide observations are needed.
Management applications
The improved tide range data defined in this study shows that the National Tide Centre’s
(NTC) tide range model underestimates eastern Boullanger Bay tides by approximately
30%. Additionally, the second stage of the Coastal Inundation Mapping for Tasmania
(Lacey et al., 2012) which maps the onshore sea level rise hazard based in part on a
derivative of the NTC’s tide range data has subsequently underestimated the flood
hazard for this region. Both the NTC’s tide range model and future inundation mapping
should incorporate our improved tide range data for this region.
Table S3: Indian Spring High Water level comparisons between various sources of tide data.
Tide level source
Welcome Inlet
(B. Bay – centre)
Kangaroo Island
(B. Bay – east /
R. Passage – west)
Howie Island
(R Passage – east)
This study
1.06
1.19
1.44
0.76
0.76
1.48
0.74
0.81
1.30
1
NTC
Lacey et al. (2012)
2
1
The closest National Tide Centre data points to the observational sites in this study are ~7km NE
of Welcome Inlet, ~4.5km NNW of Kangaroo Island and ~9km E of Howie Island.
2
Tide data used for the second stage of the Coastal Inundation Mapping for Tasmanian (Lacey et
al., 2012).
Page 6 of 88
Summary
RP-BB SEAGRASS SEDIMENTOLOGY
Six sediment cores were extracted from RP-BB shallow marine environment to
investigate the carbon sequestration potential of the regions extensive seagrass
meadows, based on recent literature which highlights the significant role which
seagrasses play within the global carbon cycle.
Sediment coring
Continuous shallow marine sediments cores penetrating depths of up to 3 m were
sampled from the RP-BB seagrass beds using a purpose built double tube percussion
corer. The coring program focussed on sampling sediments from the extensive subtidal
Posidonia australis dominated meadows, which were hypothesised to form valuable
carbon sinks. Four cores (BB1- BB4) were extracted from Boullanger Bay and two (RP1RP2) from eastern Robbins Passage. The stratigraphy and relative carbon content of the
six cores were visually ‘logged’ (i.e. examined and described) and interpreted.
Core stratigraphy and interpretation
Stratigraphic analysis identified three unique sedimentary deposits (i.e. sedimentary
facies) within the sampled sedimentary record, including a:



Clay rich terrestrial facies (SF1: Late Pleistocene alluvial/lacustrine deposit),
Well sorted sandy marine facies (SF2: Mid-Holocene intertidal or shallow subtidal
sand flats), and
Silty sand to sandy marine facies variably rich in organic fibres (SF3: Mid-Late
Holocene seagrass associated deposits).
The seagrass associated facies (SF3) was further divided into four sub-facies comprising:




Organic silty sands rich in cellulose fibres (SF3a: subtidal P. australis seagrass
platform),
Organic silty sands with silt rich laminae (SF3b: palaeo-tidal channel infill deposit),
Organic sands rich cellulose fibres (SF3c: subtidal P. seagrass flats), and
Organic sands with variable organic fibres (SF3d: intertidal mixed seagrass flats).
Table 1: Sedimentary facies summary description.
Facies &
subfacies
Sedimentary facies &
sub-facies summary
description
Present
in cores:
Overlies
facies:
Underlies
facies:
Relative
‘blue
carbon’
abundance:
SF1
Mottled silty clayey sand to
clay, cohesive, massive, no
organic fibres of shell material
present.
RB1, RB2
?
SF3c
None
SF2
Well sorted grey quartzcarbonate sands. Sands fine to
medium.
BB1, BB2,
BB3
?
SF3a
Low
SF3a
Olive grey organic silty quartzcarbonate sand, variable rich in
cellulose fibres. Moderately
sorted. Sands fine. Some
broken and whole shells.
BB1, BB2,
BB3, BB4
SF2,
SF3b
-
High
Page 7 of 88
SF3b
Dark grey organic rich silty
sand with regular silt rich
laminae, minor cellulose fibres.
Moderately sorted. Sands fine.
Broken and whole shells
present.
BB4
?
SF3a
Moderate
(to high?)
SF3c
Olive grey organic quartzcarbonate sand, variable rich in
cellulose fibres. Moderately
sorted. Sands fine. Some
broken and whole shells.
RP1, RP2
SF1
SF3d
High
SF3d
Olive grey quartz-carbonate
sand, with dark organic fibres.
Moderate-well sorted. Sands
fine. Some broken and whole
shells.
RP2
SF3c
Moderate
Carbon sequestration potential of RP-BB seagrasses
All seagrass associated sedimentary deposits were found to have significant carbon
content in their sediments, with the subtidal P. australis seagrass platform and the
subtidal P. seagrass flats notably rich in organic carbon. Thus the two key finding were:


RP-BB Posidonia australis dominated subtidal seagrass meadows are highly
efficient at sequestering carbon, and
Large carbon stocks exist beneath the subtidal seagrass meadows at RP-BB.
Coastal habitat mapping indicates that RP-BB subtidal seagrass cover an area of 61km2,
and the cores found the subtidal P. australis seagrasses to form carbon rich deposits
approximately 1.25 m thick, thus the total volume of carbon rich sediments is likely at
least 76.25 km3. However the cores did not sample towards the outer edge of the
seagrass meadows where deeper carbon rich profiles have likely formed. Further
analysis is needed to estimate the total amount of carbon storage in these sediments,
and the rate at which it is sequestered.
Management implications
RP-BB seagrasses have the capacity to continue sequestering significant amounts of
carbon with rising sea levels if properly managed, but like all global seagrasses they are
highly susceptible to degradation from human disturbances. Management effort should
focus on reducing nutrient loads in the coastal waters, and preserving water clarity
through conserving, managing and improving the regions coastal and riparian
vegetation, and minimising physical disturbance of coastal sediments. Mismanagement
of the regions wetlands could result in seagrass health degradation and erosion of
carbon rich sediments which would have long term global implications for atmospheric
carbon concentrations.
RECOMMENDATIONS
Recommendation are made for both studies, which advise to provide the datasets
created in this project to appropriate authorities, and detail the need for, and steps
required to, undertake follow up detailed investigations for these two preliminary
studies.
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Summary
RP-BB tides

Tide data should be provided to the National Tide Centre for archiving

This report should be provided to the National Tide Centre to highlight the errors
associated with their tide range model for eastern Boullanger Bay

Additional sea level observations should be made throughout the greater (northern
and western) Boullanger Bay to better define the tidal characteristics of this poorly
understood and documented region

Redeploy a tide logger at one existing RP-BB survey location to increase the
observational period, as this will allow longer period tidal constituents to be resolved
RP-BB seagrasses

Manage seagrass health, to ensure carbon sequestration continues by:
o
limiting human practices which increase nutrient and sediment input into the
passage/bay, including those which physically disturb the nutrient rich RP-BB
coastal sediments
o
conserving and improving RP-BB coastal and riparian vegetation to improve
coastal waters nutrient levels and clarity

Disseminate our ‘blue carbon’ findings to the greater scientific community by
providing this report to appropriate research bodies (e.g. UNESCO Blue Carbon
International Scientific Working Group)

Undertake additional research to better define RP-BB seagrass carbon stocks by:

o
Measuring carbon content of the existing core samples
o
Undertake additional coring programs and/or seismic survey to better resolve
the 3D geometry of the blue carbon stocks
Radiocarbon date the existing seagrass sediments to define the long term rate of
carbon sequestration
Page 9 of 88
TABLE of CONTENTS
SUMMARY ........................................................................................................... 4
1
2
INTRODUCTION .......................................................................................... 12
1.1
Project aims ........................................................................................... 12
1.2
Study area ............................................................................................. 13
1.3
Work undertaken and report structure ....................................................... 13
1.4
Scope .................................................................................................... 13
TIDAL OBSERVATIONS, ANALYSIS AND CHARACTERISTICS ....................... 14
2.1
Introduction ........................................................................................... 14
2.2
Background ............................................................................................ 14
2.2.1
Causes of sea level variation .............................................................. 14
2.2.2
Ocean tides ...................................................................................... 15
2.2.3
Tidal analysis and prediction............................................................... 16
2.2.4
Tidal variability in far northern Tasmania ............................................. 17
2.2.5
Tidal measurements .......................................................................... 18
2.3
Methods ................................................................................................. 20
2.3.1
Data collection.................................................................................. 20
2.3.2
Data analysis ................................................................................... 28
2.4
Results .................................................................................................. 32
2.4.1
Observed tides ................................................................................. 32
2.4.2
Tide analysis and prediction ............................................................... 32
2.4.3
Tide characteristics ........................................................................... 36
2.5
Discussion .............................................................................................. 38
2.6
Management applications ......................................................................... 39
2.6.1
Improved tide range ......................................................................... 39
2.6.2
Management recommendations .......................................................... 41
2.7
Data outputs .......................................................................................... 41
2.8
Future work ........................................................................................... 41
3 STRATIGRAPHIC AND BLUE CARBON INVESTIGATION OF THE SEAGRASS
BEDS ................................................................................................................ 42
3.1
Introduction ........................................................................................... 42
3.2
Background ............................................................................................ 42
3.2.1
Blue Carbon: natural coastal carbon sinks ............................................ 43
3.2.2
Blue carbon in seagrasses meadows .................................................... 44
3.2.3
Palaeo-environmental investigations using soft sediment cores ............... 45
Page 10 of 88
Table of contents
3.3
Methods ................................................................................................. 46
3.3.1
Site selection.................................................................................... 46
3.3.2
Soft sediment coring ......................................................................... 47
3.3.3
Visual logging of sediment core .......................................................... 50
3.4
Results .................................................................................................. 51
3.4.1
Site location and description............................................................... 51
3.4.2
Core stratigraphy .............................................................................. 53
3.4.3
Sedimentary facies ........................................................................... 66
3.4.4
Sedimentary facies interpretation ....................................................... 68
3.5
Carbon sequestration potential of Robbins Passage – Boullanger Bay seagrass
meadows ......................................................................................................... 70
4
3.6
Management implications ......................................................................... 71
3.7
Management recommendations ................................................................ 71
3.8
Future work ........................................................................................... 72
References .................................................................................................. 73
Appendix .......................................................................................................... 76
Page 11 of 88
1 INTRODUCTION
This technical research project investigates aspects of Robbins Passage – Boullanger Bay
wetlands environmental functioning. This study was initiated by the Cradle Coast NRM in
response to the lack of research into, and understanding of, two specific coastal
processes in the region. These knowledge gaps were identified by the Blue Wren Group,
University of Tasmania, following our earlier study into Circular Head’s coastal habitat
(Mount et al., 2010a).
1.1 Project aims
The aim of this study was to investigate two elements of the Robbins Passage Boullanger Bay (RP-BB) environment and produce new datasets that both strengthen the
current understanding of the regions coastal processes and contribute to ongoing
environmental monitoring and management of the region.
The specific objectives were:

To collect 5 months of sea level observational data from RP-BB (Figure 1) for
the purpose of investigating the tide range and characteristics of the region,
and;

To obtain a set of shallow marine sediment cores (Figure 1) for the purpose
of:
o Investigating the carbon sequestration potential of the RP-BB seagrass
meadows, and
o Improving the understanding of the modern sedimentary processes
and palaeo-environmental evolution of the RP-BB region.
Figure 1: The Robbins Passage – Boullanger Bay study areas, far northwest Tasmania.
Page 12 of 88
Introduction
1.2 Study area
The Robbins Passage and Boullanger Bay area is located off the far western end of
Tasmania’s north coast (Figure 1). This region’s coastline is complex, comprising many
inlets and offshore islands which are inter-dispersed with vast sandy and dense seagrass
inhabited intertidal and shallow subtidal flats. These areas are dissected by an extensive
network of tidal channels. The region is hydrodynamically complex, experiencing mesotides which vary geographically in their range and time of arrival. The RP-BB coastal
setting is sheltered from ocean swell, but experiences strong tidal currents and regular
wind derived waves.
1.3 Work undertaken and report structure
This present study was undertaken by the Blue Wren Group, School of Geography and
Environmental Studies, University of Tasmania, in response to the knowledge gaps we
indentified in the understanding of the Circular Heads coastal processes during an earlier
study of the coastal region (see Mount et al., 2010a).
Within this study we completed two independent technical research projects, each
addressing specific research aims. These two projects were:

Improving the tide range observations for RP-BB (see Section ‎2). This was
achieved by measuring the regions tides and analysing the observational data.

A sediment coring investigation into the shallow marine coastal stratigraphy and
carbon sequestration potential of RP-BB (see Section 3
‎ ). This was achieved by
collect a set of shallow marine sedimentary cores form the extensive seagrass
beds.
This report is split into two stand alone sections, each detailing these two independent
studies.
1.4 Scope
We should highlight that both bodies of research undertaken on the RP-BB
environmental functioning are preliminary in nature: the tides have been investigated
from a relatively short period of sea level observations from three locations; and the
seagrass bed sedimentology was analysed for the region from six sediment cores by
simply visually analysing and documenting their sedimentary record.
Despite the preliminary nature of this project, our study has produced new insight into
the environmental processes at RP-BB, which have important implications for the natural
resource management of this region. Additionally, the datasets compiled from our
research can be further studied or built on, and we outline the required steps to do so.
Page 13 of 88
2 TIDAL OBSERVATIONS, ANALYSIS AND CHARACTERISTICS
2.1 Introduction
Far northwest Tasmania experiences diurnal tides which vary substantially in their range
and time of the arrival. Measured observational tide data from this region is sparse and
details of the sharp gradient and behaviour of the tides experienced between Tasmania’s
northern western tip at Cape Woolnorth and Stanley have not been previously measured
to our knowledge. The Robbins Passage – Boullanger Bay (RP-BB) region falls within this
poorly documented region in an area where the tide range gradient is likely the greatest.
This knowledge gap was highlighted by the Blue Wren Group following on from our
detailed study into the vulnerability of the Circular Heads coastal foreshore habitats to
sea level rise (Mount et al., 2010a). We also identified that future coastal management
for the far northwest would benefit from improving the understanding of the regions tidal
characteristics by:

Allowing more accurate projections of sea level rise to be made and mapped onto
the regions shores (i.e. more precisely defining future sea level rise coastal
footprints);

Having the tide data available for future process studies (such as hydrodynamic
modelling which would provide valuable information about ecological,
physiochemical and geomorphological processes in the area).
Our study described here measured the local tides at 3 locations in RP-BB over a 5
month period. The datasets created from these sea level observations has improved the
understanding of the areas coastal processes and will benefit the natural resource
management of the region.
2.2 Background
The process of measuring tides is complex, requiring not only technical equipment and
knowledge of the methodology required to collect accurate sea level data, but also a
sound knowledge of the physical sources which contribute to sea level variation.
Knowledge of these sources is necessary to enable meaningful tidal data to be extracted
from the sea level observations. The following sub-sections briefly review some
important background information which needs to be considered when conducting a tidal
survey. Further information on this topic can be gained from technical manuals such as
Forrester (1983) and IOC (2006).
2.2.1
Causes of sea level variation
Sea level variation occurs due to the combination of a number of different physical
sources, ranging from short term (1 - 20 second) variations in ocean water levels caused
by wind generated swell and waves, to long term (10’s of thousands of years) variation
in sea level primarily driven by changes in global temperatures. However, the most
predominant and regular variations observed are caused by (1/2 – 1 day) periodic tidal
waves which are driven by gravitational interactions between the earth, sun and moon
(IOC, 2006). A summary of the various components that contribute to the sea level
variation, which include both tidal and non tidal sources, are provided in Table 2. To
Page 14 of 88
Tide observations and analysis
understand the true contribution (and characteristics) of the astronomical tides to a sea
level observational dataset, other contributing sources of change (listed below) must be
accounted for by undertaking tidal analysis on the data.
Table 2: Causes of changes in sea levels, modified from IOC (2006).
Causes of sea
level variation
Period
Description
Surface waves
(wind or swell
waves)
1-20 seconds.
Surface waves created from winds, including wind
waves and swell which can vary in amplitude from
centimetres to metres.
Seiches
Minutes to hours.
Local periodic variation in sea level due to causes
such as strong wind or current, or a sudden change in
atmospheric pressure.
Tsunami
Minutes to hours.
A series of long wavelength waves created from the
vertical displacement of the water column, nominally
due to tectonic faulting of the ocean floor, but also
from other disturbances such as landslides and
meteorite impacts.
Astronomical
tides
½ - 1 day.
Predominant changes in sea level due to oceans
response to gravitational attraction of predominantly
the moon and sun, and also solar radiation.
Meteorological
effects
(e.g. storm surges)
Several days to
inter-annual and
decadal.
Changes in sea level due to transfer of energy
between ocean and atmosphere, due to the changes
in air pressure (where a drop in barometric pressure
slightly increases sea levels, and vice versa) and the
stress of wind on the sea surface (where onshore
wind drag sets up sea level, and vice versa).
Long term trends
in sea level
(including eustatic
and isostatic
changes in SL)
Long term
changes observed
over years to 10’s
of thousands of
years.
Long term changes in mean sea level in relation to a
fixed point, or datum, due to changes in the ocean
water volume (eustatic changes), and/or changes in
the shape of the earth surface (isostatic changes).
2.2.2
Ocean tides
Ocean tides are the periodic rise and fall in sea levels, produced by the gravitational
attraction of the sun and moon. Tide levels are also influenced by non astronomical
forces collectively known as ‘environmental effects’, which include the influence of
meteorological (i.e. weather) conditions on sea levels, as well as factors such as seabed
bathymetry, water depth and coastal topography (UCAR, 2006; ICSM, 2011). The
combination of astronomical forced tides and environmental effects on sea level produce
tides that vary significantly in behaviour (i.e. range, timing and periodicity) throughout
the world’s oceans and estuaries.
Astronomical tides
The primary daily variations in ocean water levels are caused by the gravitational
interactions between the sun, earth and moon, which create very long wave length
waves that traverse the global oceans. These tidal waves have wave lengths of 100s to
1000s km’s in the open ocean, where the wave crest forms the high tide and the trough
form the low tide. Tides periodicity can vary between diurnal tides: one high and one low
tide per day; to semi diurnal tides: two high and two low tides per day.
Astronomical tides are driven by the combined result of a number of physical
astronomical forcing mechanisms, also known as tidal constituents. Each tidal
Page 15 of 88
constituent has an individual periodicity that is constant across the globe, but varies in
strength and timing depending on the geographic location. For example, the “principal
lunar semi-diurnal” gravitational force (known as M2 tidal constituent) forms the most
important astronomical force on the ocean tides which is commonly expressed as two
high and two low tides each day. However differences in the timing and amplitude of the
high and low tides produced by the principal lunar semi-diurnal constituent varies across
the globe. Furthermore, differences experienced at any one location between
consecutive high and low tides water levels occur due to the varying influences of the
additional astronomically forced mechanisms. For example the regular larger tidal ranges
(spring tides) occur once every two weeks on the full or new moon, when then Earth,
Moon and Sun become aligned. Conversely, the regular smaller tide range of the same
period (neap tides) occurs when the Earth, Moon and Sun are at positioned at 90
degrees (i.e. at half moon). A complete cycle of all the tide-producing forces, known as a
tidal epoch, has a ~19 year return period (NOAA, 2012).
Tide characteristics also vary with time and space due to a number of non astronomical
effects, such as meteorological effects and sea floor bathymetry.
Meteorological (weather) tides
Sea level elevations vary due to the meteorological effects of barometric pressure and
wind, which can superimpose their effect on astronomical tide to increase or diminish the
water levels (NOAA, 2012). Air pressure variations produce what is known as the reverse
barometric effect, where increases in air pressure depress the ocean surface thus
lowering water levels, and vice versa. Additionally, the frictional force of winds can
increase water levels by literally pilling up the ocean in the down wind direction, most
notably occurring when onshore winds push water into enclosed embayments. The most
dramatic variations in meteorological tides occur during storm surges, where deep low
pressure systems and prolonged onshore winds combine to significantly increase water
level heights (Pugh, 1987).
Bathymetry and shallow water effects
Tide behaviour is complex and affected by geographic factors such as seabed
bathymetry, water depth and coastal topography. Tide form varies when traversing from
the deep to shallow water; tide wave propagation slows, wave lengths shorten and
amplitude increases (UCAR 2006). Conversely, tide waves become distorted when
moving into shallower waters through restricted embayment openings with ranges
diminishing and waters levels rising more rapidly rate than they fall (ICSM, 2011).
Additionally, shallow water conditions also affecT-TIDE propagation as it becomes lagged
due to friction with the seafloor (ICSM, 2011).
2.2.3
Tidal analysis and prediction
Sea level observations can be analysed in a way which enables future astronomical tides
to be predicted. The most common method of tide analysis is called the harmonic
method. This method analyses the tidal constituents that comprise the total astronomical
tide for a particular location, based on simultaneous linear regression which solves the
amplitudes and phases of the astronomical constituents for their set of known harmonic
frequencies. This harmonic data (i.e. the frequencies, phases and amplitudes of the tidal
constituents) can then be combined to simulate the astronomical tide (ICSM, 2011;
Table 3, Figure 2). Longer observed tidal records allow higher numbers of tidal
constituents to be solved, and more accurate predictions to be made. All tidal
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constituents can only be solved when analysing observation records equal to, or greater
than a full tidal epoch of approximately 19 years length. (NOAA, 2012). However,
appropriate tidal analysis and predictions can still be made from analysing shorter
observational datasets (C. Watson pers. comm., 2012), with a minimum of one month’s
data required to permit proper tidal analysis (Forrester, 1983).
Table 3: Major tidal constituents (modified from ICSM, 2011 and NOAA, 2000).
Harmonic
constituent
Definition
Major semi-diurnal constituents
M2
Principal lunar semidiurnal
constituent
Principal solar semidiurnal
constituent
Larger lunar elliptic semi
diurnal constituent
S2
N2
Major diurnal constituents
K1
O1
P1
Principle Lunisolar diurnal
constituent
Principle Lunar diurnal
constituent
Solar diurnal constituent
Figure 2: Schematic example of how a number of defined harmonic tidal constituents can be
summed to produce a predicted astronomical tidal curve (modified from DONPS, 2012).
The harmonic analysis method also defines the contribution to the total tide record from
environmental (i.e. non-astronomical) sources which equals the non linear component of
the observed data. These sources mostly comprise weather effects (i.e. meteorological
tides), but can also include other site specific environmental influences such as the time
lag caused by friction due to shallow water conditions.
2.2.4
Tidal variability in far northern Tasmania
Far northwest Tasmania experiences diurnal tides which varies significantly in their range
and time of arrival (Short, 2006; Figure 3). The northern-west coast receives microtides, with Granville Harbour experiencing a mean spring tide range of 0.9 m. This is in
stark contrast to the western-north coast which receives meso-tides, with Stanley
experiencing a mean spring tide range of 2.6 m from tides that arrives ~1 ¼ hrs after
Granville Harbour (Short, 2006; tide timing obtained from the Bureau of Meteorology,
Figure 3).
The strong variation in Tasmania’s far northwesT-TIDEs results from the interactions
between the offshore approaching tidal wave and the regional complex bathymetry.
Tasmania’s tides initially arrive from the east and meet towards the western end of the
north coast some 5 hours later. The tides first enter Bass Strait from the east, and
subsequently refract around Tasmania’s south and west coast to finally enter the strait
from the west (Short, 2006; Figure 3). The funnel like nature of Bass Strait’s shallow
seafloor and its surrounding coastline geography leads to a significant increase in the
tidal range toward the middle of the north coast. The strongest gradient in tides is
experienced along the Tasmania’s far northwest shores where the tidal wave becomes
substantially amplified over relatively short distances (Figure 3).
Page 17 of 88
Figure 3: The behaviour of Tasmania’s tides, showing co-range (left) and co-tidal (right) lines,
where: co-range lines indicates the variation in spring tide range in metres; and co-tidal lines
indicate the relative time of arrival. Note how: the tidal range is significantly amplified in Bass
Strait with the gradient greatest in the far northwest; and the tide moves clockwise around
Tasmania to enter Bass Straight in the west 5 hours later than in the east.
Sea level observation data defining the far northwest coasts tides is limited.
Observations are routinely measured at Burnie and Devonport only and historically from
Stanley. Prior to our research the actual range and timing of tides west of Stanley
(where the gradient is likely greatest) was not studied.
2.2.5
Tidal measurements
Measuring sea level variation is a technical process, requiring specialist instrumentation
and equipment. Of the four main types of technical equipment used for measuring sea
levels (Table 4), pressure sensing systems are arguably the most affordable and
versatile method for recording suitably accurate observational data. Pressure sensors
can be used for sea level studies in a variety of ways. One such method includes
deploying an absolute pressure sensor offshore in a stilling well. The sensor records
absolute pressure variation through time, which is then converted to sea level based on
the knowledge of the barometric pressure and seawater density. The stilling well
provides the important function of increasing the accuracy of sea level observations by
mechanically dampening out high frequency water level variations due to waves. We
employed this method to monitor the tides at RP-BB, and it is reviewed in detail below.
Table 4: The four fundamental types of sea level measuring technology, as described in IOC
(2006).
Methods for directly measuring sea level variation:


Stilling well and float

Acoustic systems
Pressure systems

Radar Systems
Stilling wells
Stilling wells are a technical requirement for a number of the sea level monitoring
methods. Their function is ‘to still’ (i.e. mechanically filter out) the high frequency
fluctuations in sea level (e.g. waves) to improve the accuracy of measuring the low
frequency tidal signal (i.e. astronomical plus weather tides; IOC, 2006). Their use is
important when measuring tides in open, to semi open waters which are commonly
exposed to waves. Figure 4 below shows the damping effect of stilling wells on sea level
processes of varying frequencies (i.e. astronomical tides, harbour seiches, and wave
swell; modified from Forrester, 1983).
Page 18 of 88
Figure 4: Damping effect of stilling well for sea level oscillations with a period of 12 hours (top), 6
minutes (middle) and 6 seconds (bottom), which are representative of varying sources of sea level
variation including (diurnal) astronomical tides (top), harbour seiches (middle), and wave swell
(bottom). Modified from Forrester (1983).
The stilling wells reduce wave activity by housing the water level logger in a vertical
enclosure (i.e. the well), which is long enough to adequately cover the tidal range of the
site, and has only limited access to the outside sea at its base (Forrester, 1983; IOC,
1985; IOC, 2006). This configuration acts as a ‘low pass’ filter by forcing the sea to
slowly infiltrate into, and out of, the well, mechanically damping out the effect of high
frequency processes such as waves. Detailed design requirements of stilling well are
described in various technical manuals (e.g. Forrester, 1983) and are summarised in
Table 5 and Figure 5.
Pressure sensor water level loggers
Pressure gauges are commonly used for measuring sea level variation. This is achieved
by deploying the pressure sensor in the sea, measuring the variation in total subsurface
pressure (due to the combination of sea level and atmosphere) through time, and
subsequently converted to the total pressure time series dataset to sea level. Converting
the absolute pressure to sea level requires knowledge of barometric pressure, seawater
density and gravitational acceleration, according to the law:
h=(p-pa)/(ρg)
where
h = height of sea level above the pressure sensor
p = measured pressure
pa = atmospheric (barometric) pressure
ρ = seawater density
g = gravitational acceleration (IOC, 2006)
Page 19 of 88
Table 5: Generic design requirements for pressure gauge stilling wells to mechanically dampen
out wave action (modified from Forrester, 1983; IOC, 2006).
Stilling well requirements:

The well should stand vertical and
extend below the lowest water level to
above the highest water level (including
wave action outside of well).

The well should be water tight over the
portion that is submerged, except for
the intake hole(s) at, or near the base
of the well.

The intake holes should not be too close
to the bottom that they may become
blocked by sediments.

The well should be sturdily constructed
and secured to minimise motion of the
logger from wave action.

The top of the well should be vented, to
equalise pressure differences between
the inside and outside of the well.

The pressure transducer sensor must be
secured in position, relative to a
predefined stilling well datum over
duration of survey.
Figure 5: Schematic description of a stilling well design for the measurement of sea levels with
pressure transducer data logger, such as that required for the HOBO U20 water level loggers used
in this study.
Atmospheric pressure and seawater density must also be measured and so an identical
pressure sensor must be deployed in a position exposed to the open atmosphere and
measure barometric pressure (IOC, 2006). Additionally, the elevation of the pressure
sensor deployed in the sea must also be surveyed, to relate the measured water level
data (which is relative to the pressure sensor) to a specific benchmark, or datum (e.g.
the Australian Height Datum, or AHD).
2.3 Methods
2.3.1
Data collection
A number of stages were involved in collecting tide data from RP-BB. These included:
selecting 3 key (and practical) survey locations; designing and constructing appropriate
stilling wells to house the water level loggers; programming the data loggers; deploying
the barometric and water level loggers (and their stilling wells); surveying the elevation
of the water level logger pressure sensors; and periodically downloading the data
loggers. These stages are detailed below.
Site selection
We selected three key tidal survey sites and one barometric survey site from our target
RP-BB region, based on their geographic location, accessibility and local infrastructure
availability for stilling well attachment. Figure 6 shows the location of the data logger
sites, including the three tide survey sites at ‘Howie Island’, ‘Kangaroo Island’ and
‘Welcome Inlet’, and the barometric survey site at ‘Stony Point’ (see Table 6).
Page 20 of 88
Howie Island and Kangaroo Island water level sites for Robbins Passage were determined
by the availability of navigation posts within the passage which were used to secure both
the loggers and stilling wells to. These sites were accessed by boat. The Welcome Inlet
site was located at the head of Boullanger Bay and was accessed on foot. This site had
no available infrastructure for stilling well attachment. The onshore barometric site was
placed at ‘Stony Point’ (Montagu) for accessibility reasons.
Figure 6: Logger location map, Robbins Passage-Boullanger Bay, Tasmania (image modified from
Google Earth, accessed May 2012).
Table 6: Tide survey site location details.
Site name
Coordinates
(GDA94)
Description
Howie Island
(Robbins Passageeast)
328, 821 mE
5, 488, 136 mN
Attached to channel marker, immediately adjacent (north)
Robbins Passage main easterly channel on shallow
subtidal sandy seagrass flats. Approximately 700 m
southeast of Howie Island.
Kangaroo Island
(Robbins Passagewest)
318, 580 mE
5, 492, 736 mN
Attached to channel marker, immediately adjacent (east)
Robbins Passage main north-westerly channel on shallow
subtidal sand flats. Approximately 1 km east of Kangaroo
Island.
Welcome Inlet
(Boullanger Bay)
321, 250 mE
5, 490, 914 mN
Secured by a concrete anchor adjacent the Welcome River
channel, on the eastern shallow sandy-muddy sub tidal
seagrass flats. Located towards the mouth of the inlet,
bordering Boullanger Bay.
Stony Point
(Montagu)
328, 696 mE
5, 487, 140 mN
Housed in dry observation well secured below the ground
surface, near sea level (<10 m), on the foreshore slopes
between Stony Point and eastern Montagu Beach.
Page 21 of 88
Water level loggers
We measured tidal water levels with HOBO U20 Water Level Loggers (i.e. pressure
gauges). Three tidal loggers were deployed offshore to measure total subsurface
pressure and one barometric logger was positioned onshore to measure air pressure at
~sea level. All loggers were programmed to record pressure and temperature at 5
minute intervals, beginning at 09:00:00 Eastern Standard Time (EST) on 27/11/2011
(i.e. 23:00:00 Coordinated Universal Time, or UTC, on 26/11/2011), however the
deployment of the Welcome Inlet logger failed, and was relaunched at 15:25:00 EST on
23/12/2011 (i.e. 05:25:00 UTC on the same date).
Data was periodically retrieved onsite using a HOBO U-DTW-1 Waterproof Shuttle and 2B coupler. On downloading the data the logger’s memories were automatically cleared
and relaunched using the existing survey parameters. On each data download the
logger’s internal clocks were reset to match the shuttles clock, which was in turn
synchronised with the host computer. Preceding the final logger launch, the shuttles
clock was mistakenly set to local daylight saving time (i.e. EST + 1 hr), which resulted in
the last data files for all the loggers being offset from their preceding files by + 1 hour.
This error was corrected in the data processing stages; however the original HOBO data
files have not, and cannot be corrected.
Stilling wells
The three water level loggers deployed offshore were individually housed in stilling wells
purposefully built for the local conditions. Each stilling well was designed and constructed
to emulate the ‘high-tech’ stilling wells commonly used for long term monitoring of sea
level (e.g. GLOSS network), whilst complying with the criteria in Table 7.
Table 7: Robbins Passage-Boullanger Bay remote stilling well design criteria.
Robbins Passage-Boullanger Bay remote stilling well design criteria:

Effectively filters high frequency variations in sea level (i.e. wave action), by including the
stilling well requirements outlined in Figure 5 where practically possible.

Can be constructed from relatively cheap, and readily available materials

Is mobile (i.e. can be easily transported to remote location by small boat1 or foot2 and
removed at completion of the survey)

Can be secured to navigation aid1 or sandy subtidal seafloor2

Is able to withstand local environmental conditions (i.e. semi-open coastal environment,
strong tidal currents, meso-tidal sea level fluctuations)

Provides easy access to data logger for periodic retrieval, data downloads and redeployment

Has a small to no environmental footprint
1
Refers to Howie Island and Kangaroo Island locations;
Table 6 for location details)
2
refers to Welcome Inlet locations (see Figure 6 and
Page 22 of 88
Figure 7: Remote navigation post stilling well at Howie Island and Kangaroo Island; (a) schematic
design of a stilling well that extends above the highest water level and beneath the mean low
water level; (b) a saddle clamp securing the well to the navigation post; (c) details of the low tide
logger access with a ‘Y connector’ which allows the logger to be retrieved from part way up the 4.5
m stilling well; (d) the well’s base, with 5 inlet holes and two protruding threaded rods which form
a stable platform for the logger (and logger housing) to sit on when deployed. These rods also
form the external datum for surveying the logger in with the DGPS.
Page 23 of 88
Figure 8: Remote anchored stilling well at Welcome Inlet; (a) schematic design of a stilling well
with self supporting concrete anchor secured to the seafloor with star pickets; (b) details of well
cap, wire line and logger housing; (c) details of concrete anchor; (d) the Welcome Inlet survey site
(looking north) located on the outer margins of the intertidal seagrass flats.
Page 24 of 88
Two different well designs were required, due to the variable local conditions, and
infrastructure availability (or their lack of), at each survey site. The Howie Island and
Kangaroo Island survey sites are relatively remote, located ~1 km and ~16 km by boat
from Stony Point located on marginal inter-subtidal sand flats, adjacent the eastern and
western Robbins Passage channel, respectively. The stilling wells for both these sites
were designed identically, to be affixed to their available navigation posts (Figure 7). The
remote Welcome Inlet survey site is located 100 m from the adjacent shoreline, on
marginal inter-subtidal sandy seagrass flats. No infrastructure was available in this
region to secure the stilling well to, thus we designed the well to be self supporting
(Figure 8). Building a self supporting stilling well which extended the full range of the
tide would have required a significant investment of time, money and infrastructure that
was outside the scope of the project. As such, a relatively cheap, self supporting, low
environmental impact well was alternately designed that extended to mid tide height
only. The two stilling well designs are described below.
Remote navigation post-stilling well design: Howie and Kangaroo Island
The Howie Island and Kangaroo Island stilling wells (Figure 7) are constructed from 2
inch diameter ABS pipe and ABS pipe fittings and joined with ABS cement. They consist
of an outer, vertical 4.5 m stilling tube with an adjoining pipe and screw cap located part
way up the well for low tide access of the logger. The wells are mostly water/air tight,
with the exception of five 6 mm Ø basal inlet holes, and an air pressure vent on the
access cap and at top of the well.
The data logger is housed in an inner 178 mm section of 1 inch Ø ABS pipe which is in
turn deployed within the stilling well. The logger sits in the housing on a bottom screw
and is secured by a zip tie. The housing is attached to the outer wells low tide access cap
via a wire line, connected to an upper screw in the housing and an eye bolt on the
access cap, allowing for easy periodic retrieval, download and redeployment of the
logger. The housing sits near the bottom of the well on two 200 mm lengths of 6 mm Ø
threaded rods which protrude perpendicularly from the well to one side, serving as the
stilling wells fixed datum, which can be easily surveyed at low tide.
All the hardware components of the stilling well and logger housing are marine grade
stainless steel (SS-316). The well is attached to the navigation pole with a series of
galvanised steel saddle clips for 2 inch Ø pipe.
Remote anchored-stilling well design: Welcome Inlet
The Welcome Inlet stilling well (Figure 8) is identical to that described above, with the
exception of the vertical outer well being 1.6 m in height only due to practical limitation
of the site; the logger and housing are accessed by the vented screw cap at the top of
the well and the well is secured to a ~25 kg concrete anchor at its base. The anchor is a
rectangular prism, with two bent protruding reinforced steel bars handles and the wells
basal cap concreted into place. Note that this well does not extend above the highest
water levels, and thus its ability to dampen out the effect of wave action is reduced.
Page 25 of 88
Logger elevation survey
GPS equipment and software
The water level logger’s elevation was surveyed using differential geodetic grade Leica
Viva GPS receivers and AS10 antennas (with quoted accuracy of 10 mm + 2 ppm).
Measured GPS data were processed with Leica GeoOffice version 7 software (LGO),
using AUSGeoid09 to calculate Australian Height Datum (AHD) heights from the GPS
derived ellipsoidal heights (http://www.ga.gov.au/ausgeoid/nvalcomp.jsp). This data
was additionally processed by AUSPOS (http://www.ga.gov.au/earthmonitoring/geodesy/auspos-online-gps-processing-service.html) for a less accurate but
independent verification of the LGO processing.
Differential GPS survey
Standard differential GPS (DGPS) methods were applied to measure the tide loggers
elevation, with a reference base station GPS setup measuring an onshore benchmark
(Control Point 730/70 at Stony Point Caravan Park, Montagu), whilst a rover GPS was
simultaneously measuring the offshore tide loggers. GPS data was recorded at 2-second
interval in standard RINEX format. The rover GPS antenna was mounted on a pole and
secured to the stilling well datum using a purpose built mounting bracket. This pole was
rigidly fixed to the navigation post to eliminate movement of the GPS antenna (Figure
9).
Figure 9: Differential GPS (DGPS) elevation survey of water level loggers at (a) Kangaroo Island
and (b) Welcome Inlet. The DGPS pole mounting bracket (c) secured the roving GPS antenna to
the stilling wells (and data loggers) external survey datum.
Tide loggers at Kangaroo Island and Welcome Inlet were surveyed for 1 hour over a
baseline of 12 and 17 km respectively, and the Howie Island tide logger was measured
over a 1 km base line for 16 minutes1 only due to unavoidable time and weather
limitations experienced. All ambiguities in processing the GPS data in LGO were resolved
for each survey.
Position of the onshore reference benchmark 730/70 was subsequently determined by
differential connection to a GPS Primary Control Point at the Smithton Airport (ST1087).
Over a 13 km baseline, this connection was measured for 4.5-hours and processed in
LGO using AUSGeoid09 for a AHD height determination.
1
The 16 minute measuring period for Howie Island didn’t allow for a comparison to an AUSPOS
solution.
Page 26 of 88
Potential sources of error
Excluding gross errors, potential sources of error in surveying of the logger heights are
attributed to systematic and random errors associated with height datum’s and the GPS
equipment. We estimate an error margin of ±0.138 m for the surveyed water level
loggers AHD heights. These errors are discussed below.
All heights were surveyed in the Australian Height Datum (AHD). The AHD is Australia’s
official national height datum and is classed as a 3rd Order Network only, as it has
inherent known systematic errors (Featherstone et. al., 2010; Featherstone and Kuhn,
2006). The AHD was defined in 1971 by the simultaneous adjustment of 97,230km of
two-way levelling and setting mean sea-level height of 30 tide gauges Australia wide to a
height of 0.000m (ICSM, 2009). Known errors in the AHD include gross errors with the
two-way levelling, using limited tidal data (2-3 years), not accounting for ocean
temperature differences between northern and southern Australia (introduces a ~1 m tilt
in the north-south direction), uncertainties of measuring gravity, and associated issues
with developing a reliable geoid model (Featherstone et. al., 2010; Featherstone and
Kuhn, 2006).
Brown (2010) estimates an accuracy of ±0.050m across most of Australian using the
AUSGeoid09 model to convert ellipsoidal heights derived from GPS measurements to
AHD heights. This newly available geoid model (released in April 2011) provides greater
accuracy than preceding models for converting GPS measurements to AHD and combines
the Earth Geopotential Model 2008 (EGM2008; Pavlis, 2012) with Australian land gravity
data and a ‘correction grid’ between AHD and the gravimetric geoid (Figure 10).
Figure 10: The various height datums used to compute AUSGeoid09, using the following
equation: O = h – Nag – Hahd, where O = the difference between the AHD and gravimetric geoid; h
= ellipsoidal height from GPS observations; Nag = gravimetric geoid-ellipsoid separation; Hahd =
AHD height (modified from Brown, 2010).
Instrumental errors were minimised by using high accuracy equipment (10 mm + 2
ppm). Issues associated with excessive baseline distances were minimised by using the
730/70 as a reference GPS base station, approximately mid-way between the primary
GPS control point at Smithton Airport and the far field sites at Welcome Inlet and
Kangaroo Island. Additionally, GPS data was measured at a high frequency and
sufficiently long periods to ensure satisfactory processing of the data in LGO. Over the
maximum baseline length of 17 km for Welcome Inlet, a horizontal and vertical accuracy
of the order of ±0.044 m and ±0.088 m is expected. Combined with a ±0.050m error
associated in converting ellipsoidal heights derived from GPS to AHD heights using
AUSGeoid09, the accuracy of AHD heights of the water level loggers is ±0.138 m.
Page 27 of 88
2.3.2
Data analysis
Data processing
All individual data logger time series datasets were converted to Coordinated Universal
Time (UTC) to correct for the time offset error (UTC+10 to UTC+11) which occurred
between the second and third download. The Howie Island, Kangaroo Island and
Welcome Inlet total pressure data was converted to water levels relative to their
pressure sensor using HOBOware Pro Barometric Compensation Assistant and the
Montagu barometer dataset. The default salt water fluid density input of 1025 kg/m3 was
used for this conversation. In reviewing the literature for available data on the locally
measured salinity levels we found that this default fluid density was sufficient2, with
minor variations between the local salinity levels and the default salinity density found to
have an insignificant influence on the calculated water levels (i.e. < 0.1 cm). Corrected
water level data files were merged and exported for subsequent tidal harmonic analysis.
Tidal analysis – Harmonic constituents and tidal predictions
Dr Chris Watson (UTAS) conducted a tidal analysis on the corrected observational water
level data, using T-TIDE, a MATLAB based harmonic analysis and prediction program
which uses a least squared approach to solve the amplitude and phase (and compute
error estimates) for the known tidal constituent frequencies that could be determined for
each data set . Signal to noise ratios (e.g. amplitude/amplitude error) were determined
for each solved tidal constituent, and those with values less than 1 were discarded.
The astronomical tide was subsequently modelled for the duration of the observed data
in T-TIDE by summing the solved tidal constituents (see Figure 2 for schematic example
of this process). The predicted astronomical tide was then subtracted from the observed
total tide to compute the non-tidal residual (i.e. the non-linear environmental effects on
the observed total tide, including meteorological tides and local physical effects such as
bathymetry). Additional 10 year tide predictions (until June 1, 2022) for each site were
computed using their determined tidal constituents.
Tidal classification – harmonic based classification
The common harmonic based method for classifying tides, known as the ‘form factor’,
was used to characterise the tides for each site (Table 8).
Table 8: Harmonic based Form factor scheme for used to classify Robbins Passage – Boullanger
Bay tides (as outlined in the Australian Tide Manual; ICSM, 2011).
Form factor (“F”) – harmonic tide classification scheme
F = (K1 + O1) / (M2 + S2)
where:
F = form factor, and
K1, O1, M2 and S2 = amplitudes of the 4 main tidal constituents, as
derived from the tidal analysis.
when:
F < 0.5, the tides are considered semi-diurnal, and
F > 0.5 the tides are considered diurnal
note:
Semi-diurnal tides = 2 high and low tides/day, and
Diurnal tides =1 high and low tide/day.
2
Edgar et al (1999) measured a single surface salinity at Welcome Inlet in February, 1997 at
34.8‰, equalling a fluid density between ~1027 to ~1025 kg/m3 at 10 to 20 °C, and sixteen
salinity samples from 4 sites in Robbins Passage at Montagu measured by DHHS (2011) through
2010 averaged 33.6‰, equalling a fluid density of ~1026 and ~1024 kg/m3 at 10 to 20 °C.
Page 28 of 88
Tidal planes
The various tide levels which can be derived from sea level datasets (e.g. high water,
mean sea level) are collectively known as tidal planes. Tidal planes were defined for each
site, based on the observation-based, harmonic-based and prediction-based definitions
as outlined in, or modified from, the Australian Tide Manual (ICSM, 2011).
Observation-based definitions are the only universally accepted method of defining tidal
planes, however they require “sufficiently long” periods (preferably 19 years) of sea level
observations to adequately define each level (ICSM, 2011). The use of short records
(such as our RP-BB dataset) may result in an over or underestimation of the tidal planes
due to the variable influence of seasonal, annual or inter-annual environmental effects
on short term sea level records.
Harmonic-based tide plane definitions are considered “convenient simplifications” of
these levels and are based on tidal harmonics (such as those we derived in our RP-BB
tidal analysis). These definitions are commonly used in Australia, however they assume
that M2, S2, O1 and K1 are the dominant four astronomical components of the local tide,
which is not necessarily true for all locations (ICSM, 2011), and was also not found to be
the case at RP-BB.
Due to issues associated with applying the above methods to our RP-BB tide data, we
developed alternate prediction-based tidal plane definitions, which include applying the
observation-based definitions to sufficiently long periods of predicted sea levels (≥5
years).
Details of the observation-based, harmonic-based and prediction-based definitions we
applied to the RP-BB data are outlined in Table 9.
Table 9: Tidal planes.
Tidal Plane
Definition
1
observation-based
2
harmonic-based
3
prediction-based
Method applied in this
study
Notes
HAT3
Highest Astronomical
Tide3:
The highest level of
water which can be
predicted to occur under
any combination of
astronomical conditions.
Highest astronomical
water level predicted
from 20 year period of
predicted astronomical
tides modelled from the
harmonic constituents
defined in this study
(only).
Our results are indicative
only and underestimate
the true HAT levels, as
not all astronomical
constituents were solved
in our tidal analysis,
hence our tide
predictions do not model
‘any (possible)
combination of
astronomical conditions’.
ISHW2
Indian Spring High
Water2:
ISHW = Z0 + (M2 + S2 +
K1 + 01);
where Z0 = mean sea
level and M2, S2, K1, 01 =
their usual harmonic
constituents.
Same as the definition
This is a tidal plane
designed for regions with
mixed tides; ISPW * 2 is
the standard NTC
Australian tide range
model, and is used to
map SL inundation
footprints in Tasmania
(e.g. Mount et al.,
2010b, Lacey et. al.
2012).
Page 29 of 88
MHWS1
Mean High Water
Springs1:
The average of all high
water observations at the
time of spring tide over a
sufficiently long period of
time.
The average of all spring
high water observations
over our ~5 month
period of data collection.
only, due to the relatively
short time period of
observations used in this
analysis.
MHWS2
Mean High Water
Springs2:
MHWS = Z0 + (M2 + S2);
where Z0 = mean sea
level and M2 & S2 = their
usual harmonic
constituents.
Same as the definition.
only, as the harmonic
based definitions assume
that M2, S2, O1, and K1
are the dominant four
components, which is not
the case at RP-BB.
MHWS3
Mean High Water
Springs3:
The average of all
predicted high waters at
the time of spring tide
over a sufficiently long
period of time.
The average of all the
high waters predicted at
over a 5 year period.
only, as not all
astronomical constituents
were solved in our tidal
analysis, hence our tide
predictions do not model
every combination of
MHW1
Mean High Water1:
waters observed over a
sufficiently long period.
water observations over
our ~5 month period of
data collection.
analysis.
MHW3
Mean High Water3:
waters predicted over a
predicted high water over
a 5 year period.
only, due to issues
previously noted in
MHWS3 regarding tide
predictions derived from
limited harmonics.
MHWN2
Mean High Water
Neap2:
MHWS = Z0 + (M2 - S2);
where Z0 = mean sea
usual harmonic
constituents.
only, due to the issues
regarding the harmonic
definitions previously
noted under MHWS2.
MHWN3
Mean High Water
Neap3:
The average of all
the time of neap tide
period.
only, due to issues
previously noted in
limited harmonics.
MSL1
Mean Sea Level1:
The arithmetic mean of
hourly heights of the sea
at the tidal station
observed over a period of
time (preferably 19
years).
The average of all sea
level observations over
our ~5 month period of
data collection.
analysis.
MLWN2
Mean Low Water
Neap2:
MLWN = Z0 - (M2 - S2);
where Z0 = mean sea
usual harmonic
constituents.
noted under MHWS2.
Page 30 of 88
MLWN3
Mean Low Water
Neap3: The average of
all predicted low waters
at the time of neap tide
period.
predicted low waters at
only, due to issues
previously noted in
limited harmonics.
MLW1
Mean Low Water1:
The average of all low
waters observed over a
NA
This level cannot be
extracted from our data,
as our tide loggers were
deployed in shallow
conditions above the
lowest water levels and
thus these levels were
not observed.
MLW3
Mean Low Water3:
waters predicted over a
The average of all
predicted low water
observations over a 5
year period.
only, due to issues
previously noted in
limited harmonics.
MLWS1
Mean Low Water
Springs1:
water observations at the
time of spring tide over a
NA
This level cannot be
extracted from our data,
as our tide loggers were
deployed in shallow
conditions above the
lowest water levels and
thus these levels were
not observed.
MLWS2
Mean Low Water
Springs2:
MLWS = Z0 - (M2 + S2);
where Z0 = mean sea
usual harmonic
constituents.
noted under MHWS2.
MLWS3
Mean Low Water
Spring3:
The average of all
predicted low waters at
over a period of time
(preferably 19 years).
low waters predicted at
only, due to issues
previously noted in
limited harmonics.
ISLW2
Indian Spring Low
Water2:
ISPW = Z0 – (M2 + S2 +
K1 + 01);
where Z0 = mean sea
level and M2, S2, K1, 01 =
their usual harmonic
constituents.
Same as the definition
See ISHW notes.
LAT3
Lowest Astronomical
Tide3:
The lowest level of water
which can be predicted to
occur under any
combination of
Lowest astronomical
water level predicted
from 20 year period of
predicted astronomical
tides modelled from the
harmonic constituents
defined in this study
(only).
Page 31 of 88
only and overestimate
the true LAT level, (thus
underestimating the true
tide range) due to the
tide prediction issues
notes in HAT3.
2.4 Results
2.4.1
Observed tides
Selected site location and tide characteristic data for Howie Island, Kangaroo Island and
Welcome Inlet are given in Table 10, and the observed total tides for these sites are
presented in Figure 11 – 13.
Table 10: Selected data associated with water level observations.
Howie Island
Kangaroo Island
Welcome Inlet
Easting (MGA)
328,821 mE
318,580 mE
312,250 mE
Northing (MGA)
5,488,136 mN
5,492,736 mN
5,490,914 mN
Start date + time
(UTC + 0hrs)
26/11/2011
23:00:00
26/11/2011
23:00:00
23/12/2011
05:30:00
No. obs. (5 mins)
47016
46755
39282
Period obs. (days)
163.25
162.34
136.4
Max obs. height
(m > AHD)
1.792
1.451
1.318
Mean obs. height (m
> AHD)
0.009
0.018
0.001
Min obs. Height1
(m > AHD)
-1.409
-1.261
-0.947
Pressure sensor
height1 (m > AHD)
-1.390
-1.226
-0.920
1
Note the minimum observed water levels pressure sensor height, as these loggers were not
deployed below the lowest water levels. Minor discrepancies (up to ~3.5 cm) are likely due to the
artefacts associated with using regional barometric pressure to correct local total pressure to water
levels.
2.4.2
Tide analysis and prediction
Harmonic analysis conducted on the observed sea level datasets solved 21 tidal
constituents for both Howie and Kangaroo Island (Table 11 and Table 12), and 23 tidal
constituents for Welcome Inlet (Table 13). The Principal Lunar semidiurnal constituent,
M2, was found to have the greatest amplitude for each site, accounting for ~50 % of the
astronomical tidal variation. Higher order-shallow water harmonics (such as M4) were
also found to be significant for all three sites and accounted for at least 14 % of the
astronomical tides at Welcome Inlet.
The predicted tides, formed by combining the solved tidal constituents, accounted for
~97 % of the total observed tidal variability at RP-BB (Figure 11, Figure 12 and Figure
13). The remaining 3 % of observed total tide variability not accounted for is largely due
to the environmental effects on sea levels and is defined by the non-tidal residuals
computed for each site (see below). The standard deviation of the non-tidal residual is
~12 cm for all three sites and the maximum variation in observed sea levels from the
predicted tide was -67.7 cm at Welcome Inlet.
Page 32 of 88
Figure 11: Observed sea levels and predicted astronomical tides (top), and environmental effects
(i.e. non tidal residual; bottom) for Howie Island, Robbins Passage-east.
(i.e. non tidal residual; bottom) for Kangaroo Island, Robbins Passage-west.
Page 33 of 88
(i.e. non tidal residual; bottom) for Welcome Inlet, Boullanger Bay.
Table 11: Physical attributes for the 21 tidal constituents solved for Howie Island, derived from
the harmonic analysis of 163 continuous days of observed tide data.
Frequency
Amplitude
Amplitude
error
M2
0.0805114
1.0871
0.012
73.2
0.6
8.00E+03
N2
0.0789992
0.2115
0.013
48.57
3.67
2.80E+02
S2
0.0833333
0.1399
0.012
218.26
5.4
1.30E+02
Tidal
constituent
Phase
Phase
error
Signal to
noise ratio
K1
0.0417807
0.1316
0.013
314.8
6.54
1.00E+02
O1
0.0387307
0.0836
0.014
299.67
10.18
34
M4
0.1610228
0.083
0.005
81.72
3.79
3.00E+02
L2
0.0820236
0.0718
0.012
82.97
8.33
38
MU2
0.0776895
0.0596
0.011
44.49
10.11
31
MN4
0.1595106
0.0365
0.005
59.86
7.82
52
MK3
0.1222921
0.0229
0.004
336.89
11.49
28
M6
0.2415342
0.0219
0.002
268.38
6.33
1.00E+02
MS4
0.1638447
0.02
0.005
223.03
15.34
18
EPS2
0.0761773
0.0184
0.013
20.04
32.97
2.1
Q1
0.0372185
0.0167
0.013
293.36
46.6
1.6
MO3
0.1192421
0.0149
0.004
338.47
16.33
12
2MN6
0.2400221
0.0135
0.002
227.13
10.89
31
2MS6
0.2443561
0.0097
0.003
54.33
12.62
14
M3
0.1207671
0.0081
0.005
209.55
30.91
2.9
2MK5
0.2028035
0.0073
0.003
37.17
20.66
7.2
M8
0.3220456
0.0048
0.001
269.21
13.01
24
3MK7
0.2833149
0.0021
0.001
121.32
30.81
4.5
Page 34 of 88
Table 12: Physical attributes for the 21 tidal constituents solved for Kangaroo Island. derived from
Tidal
constituent
Frequency
Amplitude
Amplitude
error
Phase
Phase
error
Signal to
noise
ratio
M2
0.0805114
0.8753
0.01
75.24
0.71
7.40E+03
N2
0.0789992
0.1693
0.01
49.7
4.05
2.80E+02
K1
0.0417807
0.1154
0.013
295.91
6.52
84
S2
0.0833333
0.1139
0.012
195.52
5.92
93
O1
0.0387307
0.0824
0.013
284.48
8.78
39
L2
0.0820236
0.0486
0.011
89.4
14.95
19
M4
0.1610228
0.039
0.004
107.92
5.71
1.00E+02
MU2
0.0776895
0.0385
0.01
40.99
16.42
15
M6
0.2415342
0.0348
0.003
294.44
5.1
1.30E+02
Q1
0.0372185
0.0204
0.013
265.1
32.3
2.5
2MN6
0.2400221
0.0199
0.003
252.1
9.56
41
MN4
0.1595106
0.0178
0.004
87.17
13.38
16
2MS6
0.2443561
0.0139
0.003
88.6
12.87
21
EPS2
0.0761773
0.013
0.012
10.47
49.95
1.2
MK3
0.1222921
0.0111
0.002
31.71
11.34
25
MO3
0.1192421
0.0065
0.002
50.45
20.1
7.4
MS4
0.1638447
0.0061
0.004
276.96
34.88
2.1
2MK5
0.2028035
0.0046
0.002
19.58
31.93
3.5
M3
0.1207671
0.004
0.002
233.79
31.75
2.5
SK3
0.1251141
0.0025
0.002
228.1
60.6
1.2
3MK7
0.2833149
0.0024
0.001
178.36
39.45
2.7
Table 13: Physical attributes for the 23 tidal constituents solved for Welcome Inlet. derived from
Tidal
constituent
Frequency
Amplitude
Amplitude
error
Phase
Phase
error
Signal to
noise ratio
M2
0.0805114
0.797
0.01
81.26
0.77
6.00E+03
N2
0.0789992
0.1517
0.01
59.26
3.58
2.40E+02
S2
0.0833333
0.1041
0.011
202.9
5.19
93
K1
0.0417807
0.0946
0.012
299.85
7.51
62
M4
0.1610228
0.0945
0.006
103.05
3.66
2.30E+02
O1
0.0387307
0.0664
0.012
286.05
10.3
29
L2
0.0820236
0.0527
0.009
84.03
12.24
37
MN4
0.1595106
0.0423
0.007
82.11
8.84
42
M6
0.2415342
0.0356
0.004
345.21
6.9
76
MU2
0.0776895
0.0344
0.009
46.85
18.74
13
MK3
0.1222921
0.0216
0.004
357.29
13.42
26
MS4
0.1638447
0.0191
0.007
230.54
19.68
7
2MS6
0.2443561
0.0189
0.005
125.54
13.17
16
2MN6
0.2400221
0.0188
0.004
313.31
13.09
20
Q1
0.0372185
0.0171
0.013
268.2
45.41
1.8
MO3
0.1192421
0.0158
0.005
352.08
15.5
11
M8
0.3220456
0.0153
0.002
341.84
9.76
46
EPS2
0.0761773
0.0116
0.009
27.17
50
1.6
M3
0.1207671
0.0065
0.004
199.88
40.8
2.5
2MK5
0.2028035
0.0058
0.002
301.97
23.95
5.5
3MK7
0.2833149
0.0057
0.002
211.05
21
6.3
2SM6
0.2471781
0.0042
0.004
267.56
57.21
1.1
2SK5
0.2084474
0.0021
0.002
118.33
62.68
1
Page 35 of 88
2.4.3
Tide characteristics
Tidal Planes
The RP-BB region receives strongly semi-diurnal (F ≤ 0.2) meso-tides, with Howie Island
(Robbins Passage-east) receiving the greatest predicted maximum tide range of 3.15 m,
compared with 2.63 m at Kangaroo Island (Robbins Passage-west) and 2.42 m at
Welcome Inlet (bay head of Boullanger Bay). Observed3 and predicted mean spring tide
ranges were found to be 2.97 and 2.80 m at Howie Island, 2.37 and 2.20 m at Kangaroo
Island and 2.22 and 2.09 m at Welcome Inlet. A greater number of tidal planes derived
for these three sites by observation-based, harmonic-based and prediction-based
definitions are detailed in Table 14, with key tide ranges summarised in Table 15.
Table 14: Tidal planes of Robbins Passage – Boullanger Bay, derived from observation-based1,
harmonic-based2 and prediction-based3 definitions.
Tidal Plane
HAT3
Howie Island
Kangaroo Island
Welcome Inlet
(m > MSL)
(m > AHD)
(m > MSL)
(m > AHD)
(m > MSL)
(m > AHD)
1.512
1.521
1.296
1.278
1.186
1.187
2
1.442
1.451
1.187
1.169
1.062
1.063
1
1.484
1.493
1.187
1.169
1.110
1.111
2
1.227
1.236
0.989
0.971
0.901
0.902
3
MHWS
1.375
1.384
1.169
1.151
1.047
1.048
1
1.200
1.209
0.958
0.940
0.910
0.911
MHW3
1.180
1.189
0.944
0.926
0.873
0.874
MHWN
2
0.947
0.956
0.761
0.743
0.693
0.694
MHWN
3
1.006
1.015
0.734
0.716
0.714
0.715
ISHW
MHWS
MHWS
MHW
MSL
1
0.000
0.009
0.000
-0.018
0.000
0.001
MLWN
2
-0.947
-0.938
-0.761
-0.779
-0.693
-0.692
MLWN
3
-0.833
-0.824
-0.578
-0.596
-0.597
-0.596
MLW
1
NA
NA
NA
NA
NA
NA
MLW
3
-1.110
-1.101
-0.891
-0.909
-0.808
-0.807
1
NA
NA
NA
NA
NA
NA
2
-1.227
-1.218
-0.989
-1.007
-0.901
-0.900
3
-1.421
-1.412
-1.031
-1.049
-1.047
-1.046
2
- 1.442
-1.433
-1.187
-1.205
-1.062
-1.061
-1.639
-1.630
-1.330
-1.348
-1.236
-1.235
MLWS
MLWS
MLWS
ISLW
3
LAT
Table 15: Key tide ranges for Robbins Passage – Boullanger Bay, derived from harmonic-based2
and prediction-based3 tide planes.
Tidal Plane
Total tide range3
Indian Spring Water tide range
Mean Spring tide range
Mean Neap tide range
3
3
2
Howie Island
(m)
Kangaroo Island
(m)
Welcome Inlet
(m)
3.151
2.626
2.422
2.866
2.410
2.122
2.796
2.200
2.094
1.839
1.312
1.311
3
The observed mean spring range equals double the observed MSHW, as the observed MSLW
could not be defined from the observed data.
Page 36 of 88
Timing of the tides
Analysis of the predicted high and low tides show the timing of the RP-BB tides to be
variable (Table 16). Howie Island is first to receive high tide, followed by Kangaroo
Island and Welcome Inlet some 20 minutes later. Low tide is generally experienced
concurrently at Howie Island and Kangaroo Island, and is followed 40 minutes later at
Welcome Inlet. Howie tides occur approximately 55 (±5) minutes after Stanley.
Table 16: Relative timing of the tidal wave at Robbins Passage-Boullanger Bay.
Arrival time after
Howie Island
Kangaroo Island
Welcome Inlet
High Tide
Low Tide
High Tide
Low Tide
Maximum (minutes)
30
30
35
60
Minimum (minutes)
10
-20
05
10
Average (minutes ± 1sd)
20 (±05)
00 (±10)
20 (±05)
40 (±10)
All three measured locations experience shorter flood times than ebb times (Figure 14)
due to their shallow water conditions slowing the speed of the receding tidal wave
trough. This behaviour is most pronounced at Welcome River where the rate of water
level change during the ebbing tide abruptly slows at -0.5 m AHD (in water depths of
around 40 cm).
Figure 14: One and a half days of selected observational sea level data characteristic of average
tidal conditions (top), detailing the variation in timing, range and influence of shallow water (and
other environmental effects) on tides at Howie Island (red), Kangaroo Island (blue) and Welcome
Inlet (green). Time lines of the semi-diurnal flood-ebb tidal period (~12.4 hours) spanning 28-29
march, 2012 are shown in the time line (bottom). Note the asymmetrical nature of all three tidal
curves, notably Welcome Inlets, where the frictional force of shallow water conditions on the
receding tide wave lags the timing of ebb tide (see arrow).
Page 37 of 88
2.5 Discussion
The ~5 months of sea level observations collected from Robbins Passage-Boullanger Bay
in this study provides the first detailed measurement of the regions tidal characteristics
and behaviour. These tide observations show that the tide range, timing and tidal
constituents at RP-BB are geographically variable.
In defining the tidal planes for RP-BB, we applied a variety of methods as neither the
commonly used observation-based nor harmonic-based definitions were absolutely
suited to our dataset. The universally excepted observation-based method requires long
periods of data, and thus our short period of observations may have under- or overestimated the true tidal plane heights due to short term environmental effects skewing
the results. Also, the harmonic-based definition assumes that the tidal constituents K1,
O1, M2 and S2 are the four primary tidal constituents, which we found to not be the case
for RP-BB. Due to these potential issues we also applied prediction-based methods to
define the RP-BB tide plane heights. Furthermore, we suggest that these heights are
likely the best approximation of the true tidal planes, as they are not affected by the
issues associated with the observation- and harmonic-based definitions. Additionally, we
found the prediction-based levels to form the median values for those tidal planes where
all three definitions were applied4. The average difference between coincidenT-TIDE
planes heights produced by varying definitions is 10 cm, ranging from a minimum
difference of 1 cm to a maximum of difference 26 cm 5.
The tide planes we produced for RP-BB show the regions tide range to increase from
west to east, with a predicted mean spring tide range of 2.09 at the head of Boullanger
Bay (Welcome Inlet) increasing to 2.80 m in the eastern channel of Robbins Passage
(Howie Island). This increase from west to east is consistent with the regional gradient
experienced across the greater northwest coast which sees the Indian Spring Water tide
range becoming amplified from micro-tides on the west coast (‘standard’ NTC tide range
< 1.5 m) to meso-tides across the mid north coast (‘standard’ NTC tide range > 3.0). In
comparing our RP-BB tide ranges with the NTC tide range models we found their
modelled data for the eastern region of Boullanger Bay to underestimate the tide ranges
by ~30% when compared to our adjacent observational sites at Welcome Inlet and
Kangaroo Island. Here our ‘standard’ and total tide range is 2.12 and 2.42 m for
Welcome Inlet and 2.41 and 2.62 m for Kangaroo Island, compared with the closest NTC
modelled range of 1.53 and 1.72 m. The smaller tide range for this modelled point of
concern was likely skewed by the historical observational data from the relatively nearby
Stack Island which likely experiences micro-tidal conditions similar to the adjacent west
coast. We however have found eastern Boullanger Bay to experience meso-tides, which
are characteristic of Tasmanian north coast. The management implications arising from
the differences between these two datasets are discussed below (see Section ‎2.6:
Management applications).
Inferences can be made on the hydrodynamics of the RP-BB region based on the timing
of our observed tides. As expected, the comparable timing of high tides in the eastern
and western channels of Robbins Passage (at Howie Island and Kangaroo Island)
4
The range of mean spring water levels for derived Kangaroo Island demonstrate how the
prediction-based tide planes forms the median value, where the observation-, prediction- and
harmonic-based levels are 1.17, 1.15 and 0.97 m, respectively.
5
The minimum difference between coincidenT-TIDE planes heights was between the observationbased and prediction-based MHW level at Kangaroo Island; whereas the maximum difference was
between the observation-based and harmonic-based MHWS level at Howie Island.
Page 38 of 88
indicates that two separate tidal waves enter the passage from opposite ends and meet
in the middle of the passage. The coincident arrival times of the tidal wave at Welcome
inlet at the head of the bay, and Kangaroo Island in the far east of the bay broadly
suggests that Boullanger Bay experiences a broadly SSE propagating tidal wave front.
This may indicate that the tides here are sourced from a combination of a southerly
propagating wave sourced from Bass Strait between Hunter Island and Walker Island
and an easterly propagating wave, sourced from west coast between Woolnorth point
and Hunter Island, however more tide observations are required to constrain this
hypothesis (see Section 2
‎ .8: Future work).
The distorted-asymmetrical RP-BB tide curves shows that the regions tides experience
more rapid rise times than fall times. This is due RP-BB’s coastal setting and bathymetry
which produces stronger flood currents and lagged ebbing tides due to the frictional
forces of the shallow water conditions (Pugh, 2004). The Welcome InleT-TIDEs
experience the most asymmetrical tidal signal of all of our three observational sites, with
the timing of ebb tide lagging 40 minutes behind both Howie Island and Kangaroo
Island. Here the falling tide is notably distorted due to shallow water conditions at -0.4
m AHD where the rate of change in water level abruptly deceases in water depths of
~0.75 m (Figure 14).
2.6 Management applications
The tide data produced from our observations has a number of management
applications. These are summarised below:

Tide planes – the improved understanding of the regions tide range will allow
more accurate projections of sea level rise to be made and mapped onto the
adjacent foreshore of the area. This will assist in more precisely defining sea level
rise inundation footprints for the region (see below).

Tide observations and tidal constituents – can be used in future tide studies,
including as input into 2D (and 3D) hydrodynamic modelling. Such modelling
would provide valuable information about a large number of physical and
ecological processes, including:
o Nutrient and toxicant concentrations (dilution rates, flushing time)

o
Water quality modelling (algal bloom modelling)
o
Geomorphological studies (sediment transport e.g. through Robbins
Passage)
o
Habitat responses (seagrass depth ranges, wetting and drying regimes)
Tide predictions – can be used for a multitude of purposes, including for persons
and authorities engaged in recreation, tourism and marine resource related
industries to name a few.
2.6.1
Improved tide range
The tide ranges we defined for RP-BB show that adjustments need to be made to the
NTC modelled tide range grid for Tasmania in eastern Boullanger Bay. The single NTC
data point for this region models the ‘standard’ tide range and total tide range heights
~30% less than our more rigorously defined tide range data at Kangaroo Island and
Welcome Inlet (Figure 15).
Page 39 of 88
Figure 15: Comparisons between our tide range data and existing modelled ranges for the
Robbins Passage-Boullanger Bay region. Note the ‘standard tide range’ shown equals the water
level height variation between Indian Spring Low Water and Indian Spring High Water.
The NTC’s models underestimation of the true tide range for Boullanger Bay has
implications for the natural resource management of adjacent coastal regions. These
underestimated data have been incorporated into the coastal flooding hazard
identification methodology being applied by Tasmanian Department of Premier and
Cabinet (DPAC) to map the future sea level rise inundation footprints onshore for a
variety of different sea level scenarios (Mount et al., 2010b; Lacey et al., 2012). As such
the most recently mapped flood hazard area underestimates the true flood hazard.
Comparisons between NTC modelled tide ranges, the tide ranges used in the latest
Tasmanian inundation mapping and our calculated tide ranges are provided below (Table
17). We recommend that future updates to the Tasmanian inundation mapping should
incorporate our Indian Spring High Water (‘half tide range’) levels for the RP-BB region.
Table 17: Comparison between our harmonically defined Indian Spring High Water level, the
National Tide Centre modelled heights and those being used for the updated Tasmanian DPAC
Coastal Inundation Mapping (Lacey et al., 2012).
Welcome Inlet /
Boullanger Bay
Tide level
source
Kangaroo Island /
Robbins Passage west
Howie Island /
Robbins Passage east
ISHW
(½ tide)
range (m)
HAT (m)
ISHW
(½ tide)
range (m)
HAT (m)
ISHW
(½ tide)
range (m)
HAT (m)
1.06
1.19
1.19
1.28
1.44
1.52
NTC
0.76
0.93
0.76
0.93
1.48
1.75
Tas
Inundation2
0.74
0.91
0.81
NA
1.30
NA
This study
1
1
The closest NTC data point is compared with our observed data, which is ~7km NE of Welcome
Inlet, ~4.5km NNW of Kangaroo Island and ~9km E of Howie Island (Figure 15).
2
Data to be used in updated Tasmanian Inundation Mapping (Mount et al., 2010b; Lacey et al.,
2012).
Page 40 of 88
2.6.2
Management recommendations
We recommend that both the raw sea level observations and predicted astronomical
tides produced for our three RP-BB sites be supplied to, and archived at, the National
Tide Centre.
2.7 Data outputs
Data outputs, included raw sea level observations, tide analysis results and predicted
tides are provided in digital appendices. These include:





Raw HOBO water level logger (total and barometric pressure) observations
Water level logger observations and corrected sea levels
Observed sea levels, predicted astronomical tides and non tidal residuals
Predicted astronomical tide cycle
Predicted astronomical high and low tides
The meta data for these data files are provided in the Appendix: Metadata for sea level
observations and tide predictions data files, Robbins Passage and Boullanger Bay,
Tasmania which follows this report.
2.8 Future work
Given the significant discrepancies we have highlighted between the NTC modelled tide
ranges and our observational based data for eastern Boullanger Bay, we recommend
that additional sea level observations be made for this region. Future observational
points should include towards the western end of the bay (e.g. Murkay Islets), and the
north-eastern end of the bay (e.g. Petrel Islands). This greater spread of data would
allow the spatial variation of Boullanger Bay’s tide range to be more accurately defined.
Also, these additional data points would provide information on how the tide wave
propagates through this tidally complex area.
Additionally, it would be useful to redeploy a water level logger at one of the existing
sites (e.g. Howie Island) for the purpose of extending the period of observation. Such
data would allow an additional tide analysis to define the longer period tidal constituents
which could not be resolved from our ~5 month dataset, and subsequently allow more
precise tide predictions to be made from the existing and future tide data in the region.
We also recommend that future data be collected at 15 minute intervals only (in
comparison with the 5 minute intervals used in this study) to reduce the time and
resources required to periodically download the loggers.
Page 41 of 88
3 STRATIGRAPHIC AND BLUE CARBON INVESTIGATION OF THE
SEAGRASS BEDS
3.1 Introduction
The Robbins Passage – Boullanger Bay (RP-BB) coastal marine environment forms a
unique and complex wetland system, comprising a diversity of habitat types ranging
from beaches and sand flats to tidal channels and seagrass beds (Mount et al., 2010a).
Seagrass meadows dominate the RP-BB intertidal flats and shallow subtidal areas,
covering intertidal and subtidal areas of ~50 km2and ~60 km2, respectively6. These two
habitat types were identified by the Blue Wren Group (Mount et al., 2010) as highly
significant for their carbon sequestration potential - based on the recent global studies
into the capacity for seagrasses to deposit and accrete significant amounts of biogenic
carbon (e.g. Kennedy and Björk, 2009).
This study was undertaken as a follow up to the hypothesis of Mount et al. (2010a) that
the RP-BB seagrass beds house a significant carbon sink. The main aim of this study
therefore was to investigate the sedimentary record of the hypothesised carbon rich
seagrass sediments and collect preliminary data on the nature of these sediments. This
was achieved by extracting a set of six shallow marine cores from the RP-BB seagrass
meadows and visually analysing their sedimentology and relative abundance of carbon.
It is envisaged that this study will have management outcomes for the RP-BB wetlands,
by:

Improving the knowledge of the regions environmental history and informing current
understanding of ongoing, present day geomorphic and ecological processes.

Confirming the carbon sequestration value of these RP-BB seagrasses and providing
a stronger argument for their future protection and management.
The preliminary results obtained from this research will also contribute to the
understanding of the role which cool temperate seagrass meadows play within the global
carbon cycle.
Additional objectives of this study were to develop a relatively cheap and functional
shallow marine coring rig which could be used for similar studies and identify follow-up
investigations needed to further improve the scientific understanding of northwest
Tasmania’s ‘blue carbon’ sink.
3.2 Background
The value of managing and conserving ecosystems with high carbon sequestration
capabilities (in addition to reducing anthropogenic CO2 emission) is becoming globally
recognised as a practical and cost effective strategy for mitigating climate change
(Canadell and Raupach, 2008). Recent research has highlighted the significant role which
coastal ecosystems play in sequestering carbon dioxide, which subsequently prompted
the undertaking of this study. The carbon stored within coastal ecosystems has been
termed blue carbon (Mcleod et. al., 2011).
6
Aerial extent for Robbins Passage – Boullanger Bay region only, based on GIS habitat mapping
by Mount et al., (2010a) of the Circular Head coastal foreshore region.
Page 42 of 88
Seagrass bed sedimentology
The capacity of seagrasses to accumulate and store carbon over the long term is high,
exceeding that of many terrestrial ecosystems (Kennedy and Björk, 2009).
Investigations into the carbon stocks of a seagrass ecosystem require their subsurface
sediments to be extracted with a sediment corer and their carbon content analysed (e.g.
Fourqurean et al., 2011).
In the following section we provide background information for our study into the RP-BB
seagrass sedimentology. We initially overview the importance of natural coastal ‘blue
carbon’ sinks and then focus on the carbon sequestration capabilities of seagrass
meadows. We finish this section with a brief review of principles behind soft sediment
coring as a method for investigating palaeo-environments and processes.
3.2.1
Blue Carbon: natural coastal carbon sinks
Vegetated coastal habitats have relatively recently become recognised as being superior
at fixating carbon in comparison with land based (terrestrial) vegetated ecosystems
(Laffoley and Grimsditch, 2009). Three coastal ecosystems found to be particularly good
as sequestering carbon include mangrove forests, tidal salt marshes and seagrass
meadows. These coastal vegetation communities are known as ‘blue carbon’ ecosystems,
and their role in the global carbon cycle is summarised in Table 18.
Table 18: Overview of the carbon sequestration potential of coastal vegetation communities
(Modified from Laffoley and Grimsditch, 20091 and Mcleod et al., 20112).
Fast facts: Blue carbon

Coastal ecosystems have recently become well recognised as being highly important global
carbon sinks1.

Notable carbon sequestering coastal vegetated communities include mangrove forests,
seagrass meadows and tidal salt marshes, and are termed ‘blue carbon’ ecosystems2.

Hectare for hectare, the long term carbon sequestration contribution of coastal blue carbon
ecosystems is greater than terrestrial vegetated ecosystems, however their global areal
extent is significantly less1, 2.

Blue carbon ecosystems sequester carbon in their:
o
living biomass (leaves, stems and roots),
o
non-living biomass (e.g. litter, dead wood)2, and
o
underlying sediments.

Vegetated coastal ecosystems can sequester carbon from internal (organic) and external
(inorganic, i.e. calcium carbonate sediments) sources1, 2.

The C-rich biomass produced by blue carbon ecosystems is sequestered over the short
term (decennial), whereas the C-rich sediments they fixate are sequestered over the long
term (millennial)2.
The long term rate and size of blue carbon sinks continue to increase over time, because
unlike terrestrial vegetation their soils/sediments have the capacity to accrete vertically in
response to sea level rise (see Figure 16 below for comparison of coastal and terrestrial
long term C sequestration rates)2.


Blue carbon sinks are globally valuable for their capacity to store carbon, as well as the
local ecosystem services which they provide, however these ecosystems are under threat
worldwide due to human pressures with 1/3 of such ecosystems estimated to have been
lost over the past few decades.
Page 43 of 88
Figure 16: Average long term rates of carbon sequestration in soils of terrestrial and coastal (blue
carbon) ecosystems, modified from Mcleod et al. (2011). The significantly increased ability of
coastal ecosystems to sequester carbon over the long term (i.e. millennia) is due to their ability to
vertically accrete sediment (i.e. build soil profiles upwards) in line with rising sea levels, as well as
having an abundant external supply of carbon rich sediment in the form of calcium carbonate
skeletal sediments (e.g. broken shells).
3.2.2
Blue carbon in seagrasses meadows
Seagrasses are globally distributed flowering submarine plants that can form extensive
meadows in the intertidal and shallow subtidal coastal zone. Seagrasses are a blue
carbon ecosystem of high significance, being globally important for their ability to
sequester carbon, and locally important for the range of ‘good and services’ they provide
to the surrounding marine environment. Examples of the local goods and services
include stabilising sediments, fixating nutrients and provide the basis for their
surrounding coastal food web (Mount et al., 2010a); Globally, seagrasses are responsible
for storing 15% of the oceans total carbon storage (Kennedy and Björk, 2009).
The seagrass Posidonia oceanica is endemic to the Mediterranean and is currently
recognised as the most efficient seagrass at fixating carbon. Posidonia australis has a
similar structure to its relative Mediterranean species, with strap like leaves and dense
rhizome mat and lives in temperate southern Australian sub-tidal waters, including in far
northwest Tasmania. Like P. oceanica, P. australis develops high below ground biomass
in the form of a dense matte of roots and rhizomes which stores large amounts of carbon
which can persist for millennia (Kennedy and Björk, 2009). An overview of seagrasses,
including Posidonia sp, and their carbon fixating potential, is summarised in Table 19.
Page 44 of 88
Table 19: Blue carbon and seagrass overview, modified from FSC (2009)1, Kennedy and Björk,
(2009)2 and Mcleod et al. (2011)3.
Fast facts: Seagrass meadows and blue carbon

Seagrasses are flowering submarine plants that can form extensive meadows in the
intertidal and shallow subtidal coastal waters.

They are globally distributed, covering approximately 1% of the oceans, and have
important environmental functions, including providing ecosystem services and playing a
significant role in the global carbon cycle2.

Seagrass meadows store ~15% of the global oceans total carbon storage 2, 3.

Posidonia oceanica is a Mediterranean endemic seagrass which is thought to be the most
effective carbon sequestering seagrass2.

Similar in structure to P. oceanica, Posidonia australis lives in southern Australia’s shallow
subtidal temperate waters, and hence our regional Posidonia seagrass meadows may
potentially house huge carbon stocks.

P. australis form a dense rhizome mat buried in sand and/or mud which produce vertical
shoots that emerge through the sediment with 2-4 strap-like leaves; plants are slow to
develop (+ 10 years) and their meadows are slow to expand1.

Large seagrass species like P. australis encourage sediment deposition, hence long term
carbon deposition can occur through seagrass sediments accreting (building) vertically
and/or prograding (building) seawards.

Globally seagrass ecosystems are in decline, due to human related activities leading to
nutrient runoff (and coastal eutrophication), increased sedimentation and physical
disturbances2.

Management aimed at preserving the health of seagrass ecosystems, such as those of the
Circular Head coastal region, is necessary to maintain their important carbon and
ecosystem services2.
3.2.3
Palaeo-environmental investigations using soft
sediment cores
Coring is a common geological technique that is used to investigate past softunconsolidated sedimentary environments. In its most simple form this method includes
a three stage process: (a) pushing a length of tube into the ground; (b) retrieving the
tube and the subsurface sediments contained within it (i.e. the sediment core); and (c)
examining the cored sediments for past (palaeo-) environmental information.
The sedimentary record (i.e. the stratigraphy) of soft sediment cores provide geological
information on how the local-regional environmental conditions have evolved through
time, where each distinctive sedimentary unit (or suite of related units known as a
sedimentary facies) intersected in the core is representative of a past environment that
was experienced regionally during its time of deposition. Sedimentary units increase in
age down the core. Past environmental conditions are determined through the
sedimentary characteristics contained within each unit, or facies (including grain size,
sorting, rounding, mineralogy, structures and fossils). Periods of past erosion are
recorded as breaks in the sedimentary record, often represented by sharpunconformable boundaries between adjacent sedimentary units.
Selecting optimised sample sites is an important consideration to be made for any
sampling program. The ideal coring sites for a particular coastal investigation may vary
depending on the research objective. For example, programs aimed at reconstructing
past sedimentary environments usually sample from a transect perpendicular to the
shore located in an area representative of the greater region. This methodology has the
advantage of minimising the land based edge effects on the sedimentary record (e.g.
Page 45 of 88
mobile tidal channels) and optimises the record of regional influences on their
depositional environment (e.g. sea level change; Ellison, 2008). However coastal coring
programs aimed specifically at investigating the local variation of a particular
sedimentary environment across a region, such as documenting the variability in blue
carbon stored beneath a seagrass meadow, may design a sampling program which
targets the influences of a number of local variables on the sediments of interest (e.g.
geographic location, habitat, surface depth). This latter approach has been applied to
seagrass blue carbon studies at Shark Bay, WA and Florida Bay, Florida (see Fourqurean
et al., 2011).
The aim of our study was to both investigate the extent of sequestered blue carbon
beneath the RP-BB seagrass meadows as well as the palaeo-environmental evolution of
the region, and hence we applied a combination of these two approaches.
3.3 Methods
3.3.1
Site selection
The Boullanger Bay coring program focussed on investigating the sedimentology and
depositional history of the Posidonia australis dominated subtidal seagrass beds. This
target habitat was chosen as Mount et al. (2010a) hypothesised that northwest
Tasmania’s Posidonia seagrass meadows are highly effectively at accumulating carbon
rich deposits within their sub surface sediments.
Six sites were initially chosen across the dense subtidal seagrass meadows at Boullanger
Bay, based on detailed regional coastal habitat mapping. These sites included a three
core north-south transect aimed at investigate the depositional evolution of the
Boullanger Bay seagrass beds, and three additional cores spread east to west across the
bay to investigate the regional variability in sequestered carbon. However time
restrictions and weather conditions limited our Boullanger Bay sampling program to four
cores only. An additional two cores were subsequently sampled from the more protected
and accessible seagrass meadows in the far east of Robbins Passage (Figure 17).
Figure 17: Seagrass sediment core location from the Robbins Passage - Boullanger Bay.
Page 46 of 88
3.3.2
Soft sediment coring
The coring equipment, sampling platform and personal were transported to the remote
RP-BB sampling sites by a chartered fishing vessel (Figure 18). Cores were collected
using a custom made semi-automated double tube percussion corer. The corer was
operated from a purpose designed shallow marine sampling platform which included a
twin hull dingy with a central well installed for water based coring purposes. Continuous
cores were collected, penetrating depths of approximately 1.8 - 3 m. Extracted cored
sediments were spit lengthways, with half being archived and the other half
incrementally sampled and frozen for future analysis.
Figure 18: We used a 24 foot Shark Cat (a) to tow the sampling platform and equipment to the
remote sites in Boullanger Bay (b) and eastern Robbins Passage.
Design and operation of the corer
Our semi-automated double tube percussion corer was constructed by David Shaw. This
sediment coring rig was initially adapted from Tratt and Burne (1980) for a palaeotsunami project (Cochran and Wilson, 2007). Additional design modifications were made
during this project to enable its use in shallow marine conditions to increase its
penetration depth and improve its efficiency of operation. The primary advantage of the
double tube coring method is that the outer tube, or sleeve, reduces the friction on the
sampled tube (i.e. the coring tube) during the withdrawal process, and hence facilitates
the successful recovery of moderate length (>1 m) continuous cores. All of our coring
was undertaken from a small twin hull aluminium dinghy through a central well in the
boat (Figure 19). The boat was secured with three anchors while sampling.
The components of our corer are shown in Figure 20 and listed in Table 20. The corer
consists of two 3 m lengths of PVC tube; an inner 50 mm PVC sampling tube and an
outer 65 mm PVC sleeve. The inner tube sits snugly within the outer while the corer is
driven into the sediment. Steel leads with bevelled edges are attached to the leading end
of both tubes for reinforcement and improve the cores ability to penetrate hard
substrates (e.g. shell rich layers). An internal core catcher is attached to the inside of
the sampling tube to maximise core recovery.
A pipe adapter is temporarily secured to the inner sampling tube with a removable pin
prior to sampling. The drive anvil is placed over the top of the corer during the sampling
process, and the corer is driven into the sediments with an air compressor powered
Page 47 of 88
Figure 19: Operation of the semi-automated double tube percussion corer, showing the sampling
(insert a, b, c) and withdrawal (insert d) process.
pneumatic post driver. Once the top of the corer approaches the water level, extension
‘push’ rods are attached to the sampling tube with the pipe adapter to allow the entire
sampling tube to be driven into the submarine sediments.
A tripod mounted winch is used to first withdraw the inner sampling tube, by hooking
onto the lift plug chain. The lift plug is attached to the top of push tube, which is in turn
attached to the inner tube (and then to the pipe adapter once the inner core is lifted
above the water level). Following the withdrawal of the sampled inner tube, the outer
tube is then retrieved (where possible) with the winch, or else cut at ground level.
Figure 20: Semi-automated double tube percussion corer hardware (see Table 20 for description
of individual components)
Page 48 of 88
Table 20: Individual components of the semi-automated double tube percussion corer.
Figure 15
label #
1
Core component
Details
1
Lift plug
(with chain)
Steel lift plug with machined thread; screws onto pipe
adapter post sampling to withdrawal the inner tube with a
tripod and winch; the winch hooks onto the chain.
2
Drive anvil
Rubber anvil; sits on top of tubes for protection when corer
is being driven into the sediment by the post driver.
3
Pipe adapter
Steel machined adapter; attaches inner tube with
attachment pin to the lift plug and chain for core
withdrawal.
4
Attachment pin
10 mm steel rod; fixes the inner tube to the pipe adapter
during withdrawal by slotting through appropriate holes
drilled into each.
5
Core catcher
(flata and rolledb)
Thin brass core catcher, cut with tin snips; flat (5a) and
rolled (5b); joined to inner tube and steel inner lead with
araldite; teeth face up the tube to allows sampled sediment
to move past with ease and inhibits sediment loss during
core withdrawal.
6
Inner lead
50 mm steel bevelled leading edge; joined to inside of inner
tube with araldite.
7
Outer lead
65 mm steel bevelled leading edge; joined to outside of
outer tube with araldite.
8
Attachment pin hole
template
Steel template for drilling pin holes in inner tube.
9
Inner tuber
1.5 m length of 50 mm PVC pipe (Class 12)1; inner leading
edge of tube widened to allow flush attachment of inner
lead and core catcher; attachment pin holes drills near top.
10
Outer tube (sleeve)
1.5 m length of 65 mm PVC pipe (DWC)1
NA
Post driver (and air
compressor)
Pneumatic post driver; drives the corer into the sediments;
powered by an air compressor.
NA
Push tubes
1.5 m lengths of steel pipe; attaches to inner and outer tube
for to push whole length of tubes beneath the water level
NA
Tripod and winch
Tripod with winch used for extracting cores; winch wire
hooks onto lift plug.
Three metre length tubes were used for this study, but 1.5 m lengths are shown in Figure 20.
Core extraction, handling, splitting and sampling
The cores were extracted from the subsurface with a tripod and winch mounted on the
sampling platform. Once extracted, each core was sealed at both ends with sample bags
and tape to contain the saturated sediments within the tube. The tubes were stored
horizontally on the sampling platform (Figure 19). Some mixing of the upper (~10-20
cm) saturated sediments occurred.
The cores were cut into 1 m lengths and then split lengthwise with an angle grinder.
Once the cores were visually logged and photographed, one half of the core was sub
sampled at 10 cm increments and stored in a freezer to stop oxidisation of the sediments
and allow future chemical analysis to be conducted on the samples. The other half was
wrapped in plastic and archived.
Page 49 of 88
Figure 21: The cores were split lengthways in the field with an angle grinder, prior to being
visually logged and photographed and systematically sampled.
3.3.3
Visual logging of sediment core
Sediment cores were visually logged and photographed back at the field camp
documenting their sedimentological characteristics, including: unit (and sub-unit)
thickness and their boundary relationships, lithology, texture (e.g. grainsize),
sedimentary structures (where apparent), colour, and blue carbon content including the
presence of organic and/or inorganic carbon (e.g. seagrass material and/or shelly
material).
Lithology, texture and sedimentary structures were visually recorded following standard
sedimentological classification schemes (e.g. see Tucker, 2003). Wet sediment colour
was recorded using a Munsell Colour Chart (e.g. see Munsell, 2009). Presence and
relative abundance of blue carbon were recorded from visual observation of organic and
inorganic carbon contents greater than silt in size. Potential blue carbon content in the
silt to clay fraction could not be determined from visual analysis of the cores.
Unit thickness was measured and recorded from the extracted cores, however these
thicknesses are less than the true thicknesses of the in situ sediments due to core
shortening7 (also known as core compaction) occurring in all cores. On average, each
core was shortened by 24.4%, however the degree of shortening which occurred within
the core was likely non linear and thus no thickness corrections were estimated for each
unit8.
7
Core shortening is the sampling artefact of the coring processes resulting in the length of the
extracted sediments being less than the depth of tube penetration. This occurs due to the
progressively increasing pressure which forms ahead of the sampling tube which laterally displaces
the underlying in situ sediments prior to them being sampled (Glew et al., 2001).
8
The degree of the shortening which occurred within the cores is not likely linear due to the
progressive thinning which takes place, and thus no thickness corrections were made. However,
total core lengths were significantly shortened for all cores, where: core shortening (%) = 100 [core penetration depth (m) / core recovery] x 100. Core 1 experienced the least shortening of
17.6% and Core 3 experienced the greatest shortening of 29.6%. Core penetration depth was not
recorded in the field for Core 5.
Page 50 of 88
3.4 Results
3.4.1
Site location and description
Boullanger Bay
Boullanger Bay is a large swell sheltered, crescent shaped embayment located off the far
western end of the Tasmania’s north coast, bound by Robbins Passage, Robbins Island
and Walker Island in the east; a string of islands connecting Woolnorth Point to Hunter
Island in the west; and Bass Strait in the north (Figure 17). The bay comprises an area
of approximately 160 km2, including vast intertidal sand and seagrass flats and extensive
subtidal seagrass inhabited platform, both of which are both dissected by a complex
network of mobile tidal channels. The surrounding shorelines are dominated by beaches
and saltmarsh, and include estuarine inlets (Shoal Inlet, Welcome Inlet and Swan Bay)
and sections of rocky coasts. Sandy and rocky islands, and off shore rocky reefs are also
located throughout the bay.
Our initial plan was to extract a total of 6 cores spread throughout the subtidal P.
australis beds of Boullanger Bay; three from a south to north transect line perpendicular
to the shore in the middle of the bay (to investigate the depositional evolution of the
seagrass beds); and three cores spread across the bay from west to east targeting the
outer deep edge of the beds where the carbon accumulation is thought to be the
greatest (to investigate the regional variability in sequestered carbon across the
seagrass beds). Time and weather restrictions limited our Boullanger Bay field sampling
program to retrieving four (BB1 – BB4) out of the six planned cores,
Of the four cores sampled, one was located from the western beds and three from a
middle transect in the bay (Figure 22). The western core (BB1) was sampled from
marginal subtidal seagrass beds, which formed dense P. australis dominated meadows
approximately 1.5 km of the east of the rocky Murkay Islets and 0.5 km northeast of the
wide sandy tidal channel. Here the surface sediments felt muddy underfoot. Bare
patches and mixed seagrass beds were observed in the greater area.
The three cores collected from the south to north transect (BB4, BB2, BB3) sampled the
shallower subtidal beds across the middle of the bay. The cores were spaced at
approximately 650 m intervals and sampled single species P. australis subtidal
meadows, including: the shallowest, landwards core (BB4) which sampled dense
marginal subtidal Posidonia beds, located approximately 100 m north from a tidal
channel; the middle core (BB2) which sampled dense Posidonia beds; and the outer core
(BB3) that was extracted from a region of locally dense Posidonia beds which was within
an area mapped as patchy seagrass (Figure 22).
Page 51 of 88
Figure 22: Boullanger Bay core location map, with subtidal seagrass habitat shown (adapted from
Mount et al., 2010a).
Robbins Passage - east
Robbins Passage forms an extensive network of tidal channel and flats which separate
Robbins Island from the mainland. The passage spans an area of approximately 60 km2,
which can be broadly separated into two geographic regions, Robbins Passage – east,
and Robbins Passage – west. Each region is characterised by a two main tidal channels
that are separated by a low tide - exposed drainage divide at Robbins Crossing. Robbins
Passage – east forms a shallow marine environment comprising easterly draining
network of tidal channels, rocky reefs, sand flats and seagrass meadows.
Two Robbins Passage cores were extracted from seagrass meadows near the far
southeast coast or Robbins Island, a few hundred metres off shore from the Robbins
Creek Inlet (Figure 23). The aim of coring these alternate locations was to investigate
the sedimentology of various seagrass habitats. The first core (RP1) was sampled from
dense P. australis dominated subtidal meadows, and the second core (RP2) was sampled
from marginal mixed intertidal seagrass meadows located adjacent sand flats that
extended to the shore.
Page 52 of 88
Figure 23: Robbins Passage – east core location map, with subtidal seagrass habitat shown
(adapted from Mount et al., 2010a).
3.4.2
Core stratigraphy
Boullanger Bay: Core BB1 – BB4
Similarities exist between the sedimentary record of all four Boullanger Bay cores, with
the lower sediments from three of the four cores comprising well sorted clean quartzcarbonate sands, and the upper surficial sediments comprising moderately sorted,
fibrous silty quartz-carbonate sands. A third silt rich fibrous layer was also found at the
base of one of the cores. The Boullanger Bay stratigraphy is summarised in Table 21 –
24 and Figure 24 – 27).
Robbins Passage: Core RP1 – RP2
Similarities exist within the sampled sedimentary record from Robbins Passage, and
between the Robbins Passage and Boullanger Bay cores. The lower sediments extracted
from eastern Robbins Passage comprise a cohesive clay rich layer, which is
unconformably overlain by moderate to well sorted fibrous quartz carbonate sands. The
Robbins Passage stratigraphy is summarised in Table 25 – 26 and Figure 28 – 29.
Page 53 of 88
Boullanger Bay: Core BB1 (Core 1)
Table 21: Sedimentology of Core BB1, western Boullanger Bay.
BB1 (Core 1) – Western Bay, Boullanger Bay
Coords (GDA94)
0310575 mE
Site Description
East of Murkay Islands, in patchy-mixed Posidonia dominated meadows, near inter- to sub-tidal boundary, 500 m
northeast of the primary tidal channel which joins to the Welcome Inlet.
Unit
5495920 mN
From
(cm)
To
(cm)
Munsell
colour
(wet)
Description
Sedimentary
1.1
000
005
5Y 4/1
Olive grey
Olive grey organic rich muddy quartz-carbonate fine sands, with
abundant fibrous cellulose material and leaf sheaths; minor shell grit;
loosely packed - very high water content; gradational boundary.
1.2
005
027
As above
Olive grey fine quartz-carbonate sands in a silty organic rich matrix;
shell grit common throughout, with minor whole shells; minor cellulose
fibrous material; moderately sorted; high water content; gradational
boundary.
1.3
027
126
As above
Olive grey silty fine quartz-carbonate sand with abundant fibrous
cellulose material, locally enriched; shell grit common with minor whole
shells; moderately sorted; sharp boundary.
1.4
126
169
5Y 6/1
Light olive
grey
Light olive grey silty fine quartz-carbonate sand; fibrous seagrass
material common; abundant shell grit throughout with minor whole
shells; moderately sorted; silty fraction decreases with depth;
gradational boundary.
1.5
169
180
As above
Light grey fine quartz-carbonate sands; shell grit abundant throughout
with minor broken shells; minor silt enriched laminae; no seagrass
material present; moderately-well sorted; 180 cm end of hole.
Facies
Page 54 of 88
Palaeoenvironment
SF3a
Subtidal
Posidonia
australis
seagrass
platform.
SF2
Intertidal or
subtidal sand
flats
Figure 24: Stratigraphy of Core BB1, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note
the 1 cm scale bar).
Page 55 of 88
Table 22: Sedimentology of Core BB2, (middle) central Boullanger Bay.
BB2 (Core 2) – Centre bay (middle transect), Boullanger Bay
Coords (GDA94)
0312906 mE
Site description
Subtidal seagrass meadow in middle of the bay, with abundant P. australis.
Unit
5494432 mN
From
(cm)
To
(cm)
Munsell
colour
(wet)
Description
Sedimentary
2.1
000
005
5Y 4/1
Olive grey
Olive grey organic rich silty fine quartz-carbonate sands; abundant
fibrous cellulose material and leaf sheaths; minor shell grit and broken
shell pieces; moderately sorted; loosely packed - very high water
content; gradational boundary.
2.2
005
036
As above
Olive grey fibrous-rich silty very fine-carbonate quartz sands; fibrous
cellulose material abundant throughout; common shell grit and irregular
whole and broken shells; moderate sorting; gradational boundary.
2.3
036
069
As above
Olive grey organic silty fine quartz-carbonate sands; moderate fibrous
cellulose material dispersed throughout; moderate shell grit and
irregular whole and broken shells; moderate sorting; gradational
boundary.
2.4
069
168
As above
Olive grey fibrous rich silty fine quartz-carbonate sands, with common
shell grit; abundant fibrous cellulose material, locally enriched (154 –
168 cm); broken and whole shells dispersed throughout; moderate
sorting; gradational boundary with whole shells at base.
2.5
168
199
5B 7/1
Light bluish
grey
Clean light bluish grey fine quartz-carbonate sand; minor seagrass
fibres and silt rich laminae at upper boundary only (170 -175 cm), no
fibrous material in remainder of unit; burrows infilled with silty sand
present; well to very well sorted; 199 cm end of hole.
Facies
Page 56 of 88
Palaeoenvironment
SF3a
Subtidal
Posidonia
australis
seagrass
platform.
SF2
Intertidal or
subtidal sand
flats
Page 57 of 88
Table 23: Sedimentology of Core BB3, (outer) central Boullanger Bay.
BB3 (Core 3) – Centre bay (outer transect), Boullanger Bay
Coords (GDA94)
0312971 mE
5495114 mN
Site description
Dense Posidonia meadow in the middle of the bay; on outer shallow subtidal flats.
Unit
From
(cm)
To
(cm)
Munsell
colour
(wet)
Description
Sedimentary
Facies
Palaeoenvironment
3.1
000
005
5Y 4/1
Olive grey
Olive grey organic rich silty fine quartz sands, with abundant shell grit;
abundant fibrous seagrass material and leaf sheaths; moderately
sorted; loosely packed - very high water content; gradational boundary.
SF3a
3.2
005
036
As above
Olive grey fibrous rich, silty fine quartz sands, with abundant shell grit;
abundant fibrous seagrass material throughout; broken shells sporadic;
moderately sorted; gradational boundary
Subtidal
Posidonia
australis
seagrass
platform.
3.3
036
045
As above
Olive grey organic, silty fine quartz sand, with abundant shell grit;
moderate fibrous seagrass material throughout; broken shells sporadic;
moderately sorted; gradational boundary
3.4
045
132
As above
Olive grey fibrous rich, silty fine quartz sands, with abundant shell grit;
abundant- highly abundant organic fibrous material, increasing with
depth (112 – 132); broken shells sporadic and minor whole shells;
moderately sorted; peat clast present near base; gradational boundary.
3.5
132
198
5B 6/1
Lightmedium
bluish grey
Clean light-medium bluish grey medium quartz sand; massive; whole
shells and silty sands present in upper gradational boundary only (132 135 cm); no fibrous material present; burrows infilled with clean quartz
sand present; well to very well sorted; 198 cm end of hole.
SF2
Intertidal or
subtidal sand
flats
Page 58 of 88
Page 59 of 88
Table 24: Sedimentology of Core BB4, (inner) central Boullanger Bay.
BB4 (Core 4) – Centre bay (inner transect), Boullanger Bay
Coords (GDA94)
0312921 mE
5493806 mN
Site description
Dense Posidonia beds in middle of the bay, close to tidal channel.
Unit
From
(cm)
To
(cm)
Munsell
colour
(wet)
Description
Sedimentary
Facies
Palaeoenvironment
4.1
000
010
5Y 4/1
Olive grey
Olive grey organic rich silty fine quartz-carbonate sand; common leaf
sheaths and fibrous cellulose material; shell grit and small shells
present; moderately sorted; loosely packed (sloped around in core) very high water content; gradational boundary.
SF3a
4.2
010
056
As above
Olive grey fibrous rich, silty quartz fine sands, with minor shell grit;
moderately abundant to abundant fibrous cellulose material; some
whole shell present; moderately-poorly sorted; gradational boundary.
Subtidal
Posidonia
australis
seagrass
platform.
4.3
056
178
Mottled N2
Dark grey
Mottled dark grey silty fine quartz-carbonate sands, with regular dark
grey organic rich silty laminae; minor fibrous cellulose material,
increasing in abundance at base (from 174 -178 cm – fibrous
plug/coring artefact?); regular broken shells and minor whole shells;
moderately sorted; 178 end of hole.
SF3b
Palaeo-tidal
channel infill.
Page 60 of 88
Page 61 of 88
Robbins Passage: Core RP1 (Core 5)
Table 25: Sedimentology of Core RP1, (inner) far eastern Robbins Passage
RP1 (Core 5) – SE Robbins Island, Robbins Passage – east
Coords (GDA94)
0334278 mE
5490375 mN
Site description
Shallow subtidal Posidonia beds adjacent Robbins Island, in Robbins Passage (east), in area of most locally continuous
Posidonia beds.
Unit
From
(cm)
To
(cm)
Munsell
colour
(wet)
Description
5.1
000
008
5Y 4/1
Olive grey
Olive grey organic rich quartz fine sand, with minor shell grit; abundant
seagrass fibres and leaf sheaths; whole shell present; moderately
sorted; gradational boundary
5.2
008
108
5Y 6/1
Light olive
grey
Light olive grey fibrous rich fine quartz sand, with moderate shell grit;
seagrass fibres abundant, locally enriched; whole and broken shells
infrequently dispersed throughout; massive; moderately sorted; sharp
basal boundary.
5.3
108
134
Mottled
Mottled greenish black and brownish black clayey very fine quartz sand;
massive; cohesive; no fibrous seagrass or shelly material present; 134
cm end of hole.
5GY 2/1
Greenish
black &
5YR 2/1
Brownish
black
Page 62 of 88
Sedimentary
Facies
Palaeoenvironment
SF3c
Subtidal
Posidonia
australis
seagrass sand
flats.
SF1
Terrestrial
alluvial/swamp
deposits.
Figure 28: Stratigraphy of Core RP1, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-c; note
Page 63 of 88
Robbins Passage: Core RP2 (Core 6)
Table 26: Sedimentology of Core RP2, (outer) far eastern Robbins Passage.
RP2 (Core 6) – SE Robbins Island, Robbins Passage - east
Coords (GDA94)
0335199 mE
Site description
5490490 mN
Boundary of mixed seagrass meadows and sand flats, in marginal sub-intertidal zone; ~200 m off Robbins Island shoreline
- Robbins Passage (east). Multiple seagrasses present.
Unit
From
(cm)
To
(cm)
Munsell
colour
(wet)
Description
Sedimentary
Facies
Palaeoenvironment
6.1
000
016
N6
Light grey
Light grey massive quartz fine sands, with subordinate dark organic
fibres and minor shell grit; moderately well sorted; gradational
boundary.
SF3d
Intertidal mixed
species seagrass
flats.
6.2
016
100
Mottled
5Y 6/1
Light olive
grey &
5Y 4/1
Olive grey
Mottled light olive grey and olive grey clean fine quartz sands with some
shell grit and whole shells; minor to moderate dark organic fibres;
moderately well sorted; gradational boundary.
6.2
100
126
As above
Mottled light olive grey and olive grey clean fine quartz sands with shell
grit and whole shells; moderate cellulose fibres, locally abundant
massive; moderately well sorted; sharp boundary.
SF3c
Subtidal
Posidonia
australis
seagrass sand
flats.
6.3
126
154
5GY 2/1
Greenish
black
Greenish black silty-clay with very fine sand; massive; cohesive; no
seagrass fibres or shelly material present; gradational boundary.
SF1
Terrestrial
alluvial/swamp
deposits.
6.4
154
164
Mottled
As above &
10YR 4/2
Dark yellow
brown
Mottled greenish grey and dark yellow brown clay; cohesive; no
seagrass fibres or shell material present; 164 cm end of hole.
Page 64 of 88
Figure 29: Stratigraphy of Core RP2, including a whole of core photo (left), stratigraphic core log (centre) and selected detailed photos (inset a-d; note
Page 65 of 88
3.4.3
Sedimentary facies
To assist with interpreting the depositional environments and associated sub
environments of the cored sediments, we have differentiated comparable sediment
bodies by their geological characteristics such as litho logy (grain size, sorting, and
mineralogy), presence/absence of fossil biota (seagrass fibres, shells) and sedimentary
structures. These differentiated sediment bodies are referred to as sedimentary facies.
We identified three unique sedimentary facies from the six RP-BB cores, with each facies
containing an important suite of features indicative of its depositional environment.
These three facies include: a clay rich facies, comprising terrestrial sediments only; a
silty sand to sandy facies represented by the presence of organic fibres; and a well
sorted sandy facies largely devoid of organic fibres. We have further divided the
seagrass associated sedimentary facies (i.e. the facies containing organic fibres) into
four sub-facies, based on subtle, yet important sediment logical variations which are
indicative their unique depositional sub environments. These are summarised in Table
27, and detailed below.
Table 27: Sedimentary facies summary description.
Sedimentary
facies &
sub-facies
Sedimentary facies &
sub-facies summary
description
Present
in cores:
Overlies
facies:
Underlies
facies:
Relative
‘blue
carbon’
abundance:
SF1
Mottled silty clayey sand to
clay, cohesive, massive,
no organic fibres of shell
material present.
RB1, RB2
?
SF3c
None
SF2
Well sorted grey quartzcarbonate sands. Sands
fine to medium.
BB1, BB2,
BB3
?
SF3a
Low
SF3a
Olive grey organic silty
quartz-carbonate sand,
variably rich in cellulose
fibres. Moderately sorted.
Sands fine. Some broken
and whole shells.
BB1, BB2,
BB3, BB4
SF2,
SF3b
-
High
SF3b
Dark grey organic rich silty
sand with regular silt rich
laminae, minor cellulose
fibres. Moderately sorted.
Sands fine. Broken and
whole shells present.
BB4
?
SF3a
Moderate
(to high?)
SF3c
Olive grey organic quartzcarbonate sand, variably
rich in cellulose fibres.
Moderately sorted. Sands
fine. Some broken and
whole shells.
RP1, RP2
SF1
SF3d
High
SF3d
Olive grey quartzcarbonate sand, with dark
organic fibres. Moderatewell sorted. Sands fine.
Some broken and whole
shells.
RP2
SF3c
Page 66 of 88
Moderate
The basal sediments from the Robbins Passage cores RP1 and RP2 forms the first
sedimentary facies (SF1), represented by cohesive and structure less clay rich sediments
which vary from clayey sand to clay. Where present, the sands are very fine and
terrigenous in nature, with no marine-sourced calcium carbonate sediments and
seagrass sourced organic fibres present. Its colouring is variable, but often mottled and
dark.
The second sedimentary facies (SF2) found at the base of Boullanger Bay cores BB1,
BB2 and BB3 consists of well to very well sorted quartz-carbonate sands. Sands are
quartz dominated, range from fine to medium and are greyish in colour. Infilled burrows
and comminuted marine shells are present. This facies is found underlying the seagrassassociated sub-facies (SFa) where present, with a gradational boundary (2-5 cm) often
consisting of silty sand laminae and whole and broken shells, and minor seagrass fibres.
With the exception of its gradational upper boundary, no seagrass material is present in
this facies.
The third sedimentary facies is associated with the colonisation of seagrass, as indicated
through the presence of organic cellulose fibres. This facies has been split into four subfacies based on discernible changes in type and abundance of organic fibres, sediment
sorting, the presence/absence of silt and sedimentary structures. The most common
seagrass sediments found in Boullanger Bay is sub-facies SF3a, which consists of
moderately sorted olive grey silty quartz-carbonate sands which are variably rich in pale
cellulose fibres. The cellulose fibres bind the SFa sediments and are often matted
together in dense clumps. The sands are dominated by quartz and mostly fine, with
comminuted shell common and whole shells also present. Silt content is variable. This
surficial facies is present in the upper section of all four Boullanger Bay cores which were
universally extracted from subtidal P. australis meadows. Shortened thickness’ of the
sub-facies SF3a ranges from 0.56 – 1.68 m.
The sub-facies SF3b forms a unique sedimentary layer found at the base of Boullanger
Bay core BB4 only, consisting of moderately sorted organic rich silty quartz-carbonate
sands with dispersed cellulose fibres and regular silt rich laminae. This sub-facies is
distinguished from SF3a by its higher silt content which is increasingly expressed as silt
rich laminae towards the base; relatively minor abundance and dispersed nature of
cellulose fibres; and mottled dark grey colouring. Sands are mostly fine, dominated by
quartz and include comminuted shells. Broken shells are common, with minor whole
shells present. This sub-facies is overlain by sub-facies SF3a, with a gradational
boundary.
Sub-facies SF3c was sampled in both Robbins Passage cores, consisting of olive grey
quartz-carbonate sands, with variably abundant and locally enriched cellulose fibres and
organics. This sub-facies is comparable with the Boullanger Bay sub-facies SF3c, but silt
is absent. Sediments are moderately-moderately well sorted; sands are fine and
dominated by quartz. Broken and whole shells are common. This sub-facies comprises
the upper-most sediments in core RP1, which was extracted from a subtidal
environment, and forms the middle sub-facies in Core RP2 which was extracted from the
intertidal zone.
Finally, sub-facies SF3d is represented by olive grey quartz-carbonate sands with
dispersed dark organic fibres. Sands are mostly fine, dominated by quartz and
moderately well sorted. Articulated carbonate sands, and broken and whole shells are
present. The pale, cellulose fibres common in all of the other seagrass associated subfacies is absent. This sub-facies is found in core RP2 only which was extracted from the
Page 67 of 88
intertidal zone, in a marginal seagrass sand flat habitat, and forms the surficial
sedimentary layer in this core. Its lower boundary with sub-facies SF3d is gradational. It
has a gradational lower boundary with sub-facies SF3c.
3.4.4
Sedimentary facies interpretation
Terrestrial alluvial/lacustrine deposit (Robbins Passage)
The sedimentary facies, SF1, found in the basal sediments of both Robbins Passage
cores are represented by cohesive and structureless clay-rich sediments devoid of any
seagrass fibres, marine shells or skeletal sediments. As such, facies SF1 is interpreted to
be terrestrial in origin and therefore Pleistocene in age, deposited under lower sea levels
and colder climatic conditions. The fine nature of facies SF1’s sediments indicates
deposition occurred in a low to very low energy environment, by alluvial or lacustrine
processes in a freshwater swamp or lake environment. Such palaeo-environments have
been previously interpreted to exist in this region. For example, Sharples (in Mount et
al., 2010a) hypothesised that under the last glacial climatic phase (when sea levels were
below present, and much of the Bass Strait was sub-aerially exposed) the Circular Head
coastal landscape primarily formed an extensive dunefield comprising mobile linear sand
dunes with interspersed widespread swamps and lakes forming in dune swales and in
deflation hollows. Alternatively, this basal clayey sedimentary facies may have formed a
distal floodplain deposit of the palaeo-Montagu River which likely crossed modern
eastern Robbins Passage, following a similar path to the present main tidal channel.
Intertidal or subtidal sand flats (Boullanger Bay)
The well sorted quartz-carbonate sands of the lower Boullanger Bay facies SF2 is marine
in origin, as indicated by the presence of comminuted marine shell. The well sorted
nature of this deposit, and absence of fines indicates that deposition occurred under
relatively high hydrodynamic energy indicating deposition in a marginal marine
environment where wave energy and/or tidal currents favoured the winnowing of silts
and muds, and concentration of sands. Based on this evidence, facies SF2 is interpreted
to form either an intertidal sand flat/sandy beach, or else a shallow subtidal sand
flat/sand wave. The subtidal bathymetric range of this facies would indicate deposition
to have occurred in either the mid Holocene (circa 6-8 ka BP) under rising sea levels to
form an intertidal deposit, or else during the Mid-Late Holocene (<6 ka BP) to form a
subtidal deposit under stable sea levels comparable to modern day conditions. Infilled
burrows also occur in the basal sands of cores BB2 and BB3, indicating the presence of
burrowing fauna during and/or after their deposition. Such trace fossils are also common
in marginal marine environments. This sandy marginal marine deposit grades up into the
seagrass sub-facies SF3a indicating that the conditions which promoted deposition
persisted during the time when seagrasses colonised the bay.
Subtidal Posidonia australis seagrass platform (Boullanger Bay)
The modern depositional environment of the surficial seagrass sub-facies SF3a at
Boullanger Bay comprises a shallow subtidal seagrass platform dominated by a dense P.
australis dominated meadows. The sediments of this sub-facies are characterised by
moderately sorted, fibrous rich silty quartz-carbonate sands with common broken shells,
indicating that deposition occurred in an environment analogous to the modern subtidal
setting. Pale cellulose fibres analogous to those within this facies have been previously
documented in P. australis sediments from the Spencer Gulf in South Australia and were
identified as forming the decay resistant residue of P. australis leaf sheath fibres (Kuo,
1978 in Belpario et al., 1984). The moderately sorted nature of the sub-facies SF3a
sediments are representative of the sedimentary processes associated with P. australis
Page 68 of 88
habitat; the high density foliage of these meadows increases the seafloor roughness and
locally reduces the hydrodynamic (wave/tide current) energy near which encourages
sediments to become trapped and accumulate. Additionally, direct deposition of epiphytic
derived carbonate sediments associated with P. australis meadows also occurs, forming
the broken shelly fraction that is associated with this facies. The source of the silt is
unknown, however it may originate from either decaying seagrass plant material, fine
alluvial sediments sourced from terrestrial inputs (e.g. Welcome Inlet), or else a
combination of these sources. If so, it is possibly rich in organic carbon.
Palaeo-tidal channel infill (Boullanger Bay)
Tidal channels are dynamic features with lateral migration, cutting and abandonment of
channels occurring episodically (Bridge and Demicco, 2008). The basal sedimentary subfacies SF3b of Boullanger Bay Core BB4 is interpreted to form a palaeo-tidal channel infill
deposit based on its geographic location, the presence of regular silt rich laminae and
the comparably lower abundance and more dispersed nature of cellulose fibres of this
seagrass sub-facies. Core BB4 was extracted from the margins of a subtidal seagrass
meadow some 50-100 m south of a modern tidal channel. This location falls within the
area which may have been previously traversed by the adjacent modern channel. The
silt rich laminae of this sub-facies indicate that periodic deposition occurred in a low
energy environment, like that which would occur in a tidal channel cut-off. The cyclic
nature of these laminae forms a sedimentary record of a relict depositional process
which was cyclic in nature, possibly including those produced by tides9 or climatic events
(e.g. periodic high rainfall)10. Finally, the dispersed nature of the seagrass sediments of
this sub-facies indicates that seagrasses did not densely colonise the seafloor during its
deposition, as the sedimentary signature of in situ seagrass debris deposition includes
densely matted fibres like that described in the seagrass sub-facies SF3a and SF3c.
Rather, we suggest that these dispersed fibres represent the periodically accumulation of
sheaths which were transported into the infilling cut-off channel. Possibly the reasons
that seagrasses did not grow here is because once the cut-off was formed, sediments
infilled this palaeo-channel depression at a rate faster than the seagrasses could
establish. This sub-facies grades into a subtidal seagrass platform sediments (sub-facies
SF3a), indicating that P. australis habitat eventually colonised this location. This
probably occurred once the palaeo-channel became infilled.
Subtidal Posidonia australis seagrass sand flats (Robbins Passage)
Robbins Passage seagrass sub-facies are characterised by moderately sorted fibrous
quartz-carbonate shelly sands which are largely devoid of fines. The sub-facies SF3c is
interpreted as a subtidal Posidonia australis seagrass flat deposit, based on the abundant
presence of cellulose fibres, and environmental location with which both the Robbins
Passage cores were extracted. This sedimentary facies is comparable to the Boullanger
Bay subtidal P. australis seagrass platform sediments (see Subtidal Posidonia australis
seagrass platform (Boullanger Bay)), however an absence of silt indicates deposition
occurred under conditions with higher hydrodynamic energy which allowed these fines to
be winnowed. In Core RP2, this subtidal sub-facies grades up to an intertidal sub-facies
at depths of about 1 m below the cores surface. This change in depositional environment
is consistent with the bathymetric range from which those sediments were extracted.
9
The cyclic silt rich laminae may have been a tidal deposit, where the silty layer were laid down
during slack water conditions at high and low tide, or else during ~fortnightly occurrence of neap
tides when the bays tidal energy is at its lowest.
10
High rainfall events associated with cyclic low pressure systems may also be responsible for the
silty laminae, with increased terrestrial runoff increasing the influx of alluvial silts entering the bay.
Page 69 of 88
Intertidal mixed species seagrass flats (Boullanger Bay)
The upper sediments of Core RP2 form a seagrass sub-facies comprising moderately-well
sorted olive grey quartz-carbonate sands with dispersed dark organic fibres. These
sediments are interpreted to have deposited in intertidal (mixed species) seagrass flats,
like those of the modern seafloor environment where the core was extracted from. The
organic fibres in this sub-facies differ from the cellulose fibres common to the other
seagrass associated sub-facies, being dark in colour, less abundant and more dispersed.
The sandy nature and moderately-well sorted nature of the sandy sediments are
indicative of the relatively higher hydrodynamic energy of an intertidal flats
environment.
3.5 Carbon sequestration potential of Robbins Passage –
Boullanger Bay seagrass meadows
Visual analysis of the cored RP-BB sedimentology has shown the seagrass-associated
facies (SF3a-d) to have significant blue carbon content in their sediments, with the
subfacies SF3a and SF3c being notably rich in organic carbon. This indicates that the
subtidal Posidonia seagrass platform in Boullanger Bay and the subtidal Posidonia
seagrass flats in Robbins Passage, have a high capacity to preserve blue carbon. As
such, two key findings can be deduced from these observations:


The Posidonia australis dominated subtidal seagrass meadows at RP-BB are
highly effective at sequestering carbon, and
Large carbon stocks exist beneath the subtidal seagrass meadows of RP-BB.
These finding provide additional evidence to show that Australian P. australis meadows
form valuable coastal carbon sinks, supporting previous studies from Spencer Gulf,
South Australia (Belperio et al., 1984) and Shark Bay, Western Australia (Forqurean et
al., 2011). Additionally, our research demonstrates the carbon sequestration capacity of
Tasmanian P. australis meadows, which unlike the seagrass habitats of the Spencer Gulf
and Shark Bay experience a cool-temperate climate.
The volume of sequestered carbon at RP-BB is likely significant. Habitat mapping of RPBB shows subtidal seagrass meadows to cover an area of approximately 61 km2, with P.
australis and/or Amphibolis antarctica dominated meadows occupying much of this area
(Mount et al., 2010a). Thus the subsurface extent of the carbon rich sediments
associated with the P. australis subfacies (SF3a and SF3c) is likely to be comparable to,
or even greater than this area11. Assuming this is the case and based on the depths of
the carbon rich deposits found in our 6 cores (which were commonly between 1 – 1.5,
see
Table 28), a crude estimation of the volume of carbon rich sediments (61 km 2 x 1.25 m)
is approximately 76,250 m3 (note: this should be considered as an indicative minimum
volume only, as the carbon rich seagrass deposits likely deepen seawards towards
Walker Channel in Boullanger Bay). Further investigations are required to calculate the
total carbon pool stored beneath the RP-BB meadows and/or infer the rate of seagrass
sediment deposition/carbon sequestrations (see Section 3
‎ .8: Future work).
11
As shown with the Core RP2, subtidal seagrass sub-facies stratigraphically underlie the intertidal
seagrass sub-facies, and therefore the older subtidal seagrass sediments likely comprise a carbon
rich wedge that extends and thins shoreward’s beneath the more modern intertidal deposits. As
such, the aerial extent of carbon rich sediments is most likely greater than the mapped 61km 2 of
modern seagrass sediments.
Page 70 of 88
Table 28: Depth of carbon rich sediments.
Core #
1
Site
(and geographic location;
relative to shoreline and site)
Carbon rich
(P. australis) sub-facies
Thickness1
(cm)
BB1
B. Bay (mid-west)
SF3a
169
BB4
B. Bay (inner-centre)
SF3a
56
BB2
B. Bay (inner mid-centre)
SF3a
168
BB3
B. Bay (mid-centre)
SF3a
132
RP1
R. Passage (inner mid-east)
SF3c
108
RP2
R. Passage (inner-east)
SF3c
26
Compressed thickness (cores were shortened on average by 25%).
3.6 Management implications
Seagrass meadows have the capacity to accrete vertically in response to sea level rise
and thus the seagrass habitat of RP-BB will continue to perform as a carbon sink into the
future as long as their health is maintained. However, seagrass ecosystems are
inherently susceptible to degradation by human disturbances, including eutrophication
and siltation of coastal waters. Increasing human pressures on the coastal environment
is leading to continued significant decline in the global distribution of seagrasses
(Kennedy and Björk, 2009). To ensure that RP-BB seagrass beds effectively sequester
carbon into the future, the terrestrial wetland habitats (importantly riparian and
shoreline vegetation), their associated waterways and the physical foreshore
environment of this region must be properly managed.
Environmental risks associated with the mismanagement of the RP-BB wetlands are
high. Large scale loss of seagrass habitat would not only lead to the loss of their
important carbon sink service but would also expose their underlying carbon-rich
sediments to erosion. Such an occurrence would have long term consequences for global
atmospheric carbon concentrations (Mount et al., 2010a). Management recommendation
for the RP-BB seagrass meadows are provided below.
3.7 Management recommendations
Management efforts should aim to preserve the general health of RP-BB seagrass
meadows through:

Reducing nutrient loads in the coastal waters (Kennedy and Björk, 2009).

Preserving water clarity through conserving, managing and improving the regions
coastal and riparian vegetation (Kennedy and Björk, 2009).

Avoid physical disturbance of the RP-BB coastal sediments (e.g. from vehicular
driving and foreshore engineering), as these sediments were found to have high
nutrient levels (Mount et al., 2010a).
Page 71 of 88
We also recommend that this report be provided to appropriate research bodies actively
working in the area blue carbon (e.g. UNESCO Blue Carbon International Scientific
Working Group), to:

Assist in the dissemination of information attained through this study. This will
increase the likelihood of further research being undertaken on RP-BB’s significant
blue carbon stock

Contribute to the development global blue carbon inventories (e.g. Forqurean et
al., 2012)
3.8 Future work
The results obtained in this research provide a very preliminary assessment of the RP-BB
capacity to sequester and store carbon. Future work is required to adequately estimate
the total carbon pool beneath the RP-BB seagrass beds, as well as to calculate the rate
at which these meadows are sequestering carbon. Thus to fully assess the carbon
sequestration potential of the RP-BB seagrass beds, we recommend that the following
work be done:
Measure the carbon stocks in RP-BB sediments.



Measure carbon content of the seagrass sediments by undertaking geochemical
analysis of their sediments. This analysis could be completed on our stored core
samples.
Measure the dry bulk density of the RP-BB seagrass sediments. This requires
additional cores to be sampled using a piston corer to collect uncompressed
cores. The mass of the RP-BB seagrass sediments per volume can then be
accurately measured from these uncompressed cores.
These data (i.e. carbon content and dry bulk density), along with our data on the
depth of seagrass sediments could be used to produce a crude approximation of
the total carbon pool. However, additional data on the three dimensional nature
of the carbon rich sediments is required to conduct a more rigorous assessment
of the total RP-BB carbon stocks (see below).
Define the three dimensional body of RP-BB’s carbon rich sediments.


Undertake a comprehensive coring program to determine the varying depth of
seagrass sediments across the region, notably towards the outer limits of the
seagrass meadows. A vibracore rig may be suitable to extract long deep cores for
this purpose.
Alternatively, undertake high resolution seismic survey, coupled with a small
number of additional cores to determine the three dimensional body of the carbon
rich sediments (see Lo Iacono et al., 2008, for methods regarding this approach).
Calculate the carbon sequestration rate at RP-BB

Measure the rate of sediment accumulation on the RP-BB seagrass sediments by
undertaking multiple radio carbon dating of the buried seagrass fibres. Dating
various depths within the cored sedimentary record will allow the annual
sequestration rate to be calculated, as well as improving understanding of the
Mid- to Late Holocene geomorphic history of the region (which will additionally
contribute to understanding the carbon sequestration processes).
Page 72 of 88
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Page 75 of 88
Appendix: Metadata for sea level observations and tide
predictions data files, Robbins Passage and Boullanger Bay,
Tasmania
A.1 Introduction
These data were collected and produced by the Blue Wren Group, School of Geography
and Environmental Studies, University of Tasmania for the Cradle Coast Natural
Research Management initiated 2011-2012 research project to investigate the tides
characteristics in far northwest Tasmania.
The sea level observations were made from November/December 2011 to May 2012
using four HOBO U20 water level loggers (pressure transducers); three deployed in near
shore waters across Robbins Passage-Boullanger Bay to measure total (submarine)
pressure; and one positioned onshore to measure regional barometric (subaerial)
pressure, near sea level. The water level loggers were deployed in remote stilling wells
to mechanically dampen the influence of high frequency fluctuations in water level (i.e.
waves) on the observational data. The Howie Island and Kangaroo Island stilling wells
extended above the highest water mark and were most effective at filtering out sea level
fluctuations due to waves, the Welcome Inlet stilling well extended to approximate half
tide level and the some noise from waves can be seen in this data.
The total (submarine) pressure data were converted to water levels (i.e. water depth
with respect to the loggers pressure sensor) with the HOBOware Pro Barometric
Compensation Assistant, using the regional barometric data and applying the softwares
default fluid density input for salt water (1,025 kg/m3). Water levels were corrected to
the Australian Height Datum based on a differential GPS levelling of the loggers’ sensors.
Tidal analysis was conducted on the sea level observations using T-TIDE (version 1.3) in
MATLAB, to solve 21-23 tidal constituents for each site. These data were subsequently
used to compute the astronomical tidal predictions.
The final Robbins Passage-Boullanger Bay datasets detailed here include:

Raw HOBO water level logger (total and barometric pressure) observations, for
the period of November/December 2011 – May 2012 (see Section A.2).

Water level logger observations and corrected sea levels, for the period of
November/December 2011 – May 2012 (see Section A.3).

Observed sea levels, predicted astronomical tides and non tidal residuals, for the
period of November/December 2011 – May 2012 (see Section A.4).

Predicted astronomical tidal cycle, for the period of June 2012 – June 2022 (see
Section A.5).

Predicted astronomical high and low tides, for the period of June 2012 – June
2022 (see Section A.6).
Page 76 of 88
Appendix
A.1.1
Period of sea level observation
The period of data collection is detailed in Table 29, below.
Table 29: Period of water level logger data collection.
Survey Id
Logger
type
Howie Island
UTC (UTC + 0 hrs)
EST (UTC + 10 hrs)
From
(Date, Time)
To
(Date, Time)
From
(Date, Time)
To
(Date, Time)
Water level
logger
26/11/2011
23:00:00
8/05/2012
04:55:00
27/11/2011
09:00:00
8/05/2012
14:55:00
Kangaroo
Island
Water level
logger
26/11/2011
23:00:00
07/05/2012
07:10:00
27/11/2011
09:00:00
7/05/2012
17:10:00
Welcome Inlet
Water level
logger
23/12/2011
05:25:00
07/05/2012
14:50:00
23/12/2011
15:25:00
8/05/2012
00:50:00
Stony Point
Barometric
logger
26/11/2011
23:00:00
08/05/2012
06:05:00
27/11/2011
09:00:00
08/05/2012
16:05:00
A.1.2
Sea level logger location and heights
The water level logger’s elevation and geographic location is detailed below. The loggers
were surveyed using differential geodetic grade Leica Viva GPS receivers and Leica AS10
antennas. The measured elevation data were processed with Leica GeoOffice version 7
(LGO) using the AUSGeoid09 to calculate Australian Height Datum (AHD) heights.
Table 30: Logger survey details.
Survey Id
Logger type
Easting (AHD)
Northing (AHD)
Height (m, AHD)
Howie Island
Water level logger
328821.1012
5488136.0736
-1.377
Kangaroo Island
Water level logger
318,580.208
5,492,735.900
-1.213
Welcome Inlet
Water level logger
312,250.475
5,490,913.792
-0.907
Stony Point
Barometric logger
328,696.421
5,487,139.538
NA
Potential sources of error in surveying the logger heights are attributed to systematic
and random errors associated with AHD height datum’s and the GPS equipment. A error
margin of ±0.138 m for is estimated
Page 77 of 88
A.2 Raw HOBO water level logger (total and barometric
pressure) observations (Nov/Dec 2011 – May 2012).
A.2.1
Files
Data model
Howie Island HOBO U20 water level logger observations
26 Nov 2011 – 08 May 2012
(howie_water_level_logger_1_1_GMT+10.hobo;
howie_water_level_logger_1_2_GMT+10.hobo;
howie_water_level_logger_1_3_GMT+11.hobo)
Kangaroo Island HOBO U20 water level logger observations
26 Nov 2011 – 07 May 2012
(kangaroo _water_level_logger_2_1_GMT+10.hobo;
kangaroo_water_level_logger_2_2_GMT+10.hobo;
kangaroo _water_level_logger_2_3_GMT+11.hobo)
Welcome Inlet HOBO U20 water level logger observations
23 Dec 2011 – 07 May 2012
(welcome_water_level_logger_3_1_GMT+10.hobo;
welcome_water_level_logger_3_2_GMT+10.hobo;
welcome _water_level_logger_3_3_GMT+11.hobo)
Stony Point HOBO U20 barometric logger observations
26 Nov 2011 – 07 May 2012
(stonypoint_barometric_logger_3_1_GMT+10.hobo;
stonypoint_barometric_logger_3_2_GMT+10.hobo;
stonypoint_barometric_logger_3_3_GMT+11.hobo)
Type
Onset HOBO U20 (.hobo) data file
Description Raw observed barometric and total pressure downloaded from data
loggers. Datasets span time period of 26 Nov 2011 to 08 May 2012 for
Howie Island and Kangaroo Island, and 23 Dec 2011 to 07 May 2012 and
are divided into separate blocks according to periodic field download. All
data is recorded at 5 minute intervals.
Notes
There are errors with the timing within the HOBO dataset. The times in the
first and second (only) data downloads for all loggers are in UTC + 10
hours (EST), and thus their header files stating “Launch GMT offset: 11 Hr
00 Min” is incorrect. To plot data in EST (UTC + 10hrs) in HOBOware Pro,
set the “offset from GMT” to "+11" hours in the plot setup.
The times in the third data download for all loggers in UTC + 11. To plot
data in EST (UTC + 10hrs) in HOBOware Pro, set the “offset from GMT” to
"+10" hours in the plot setup.
Page 78 of 88
Appendix
Table 31: Howie Island HOBO data files summary information.
File info
Howie Island HOBO data files
File name
howie_water_level_logger_1_1_G
MT+10.hobo
MT+10.hobo
MT+11.hobo
UTC time offset
UTC + 10 hours
UTC + 10 hours
UTC + 11 hours
File start
27/11/2011 9:00:00 AM
21/12/2011 2:45:00 PM
23/02/2012 8:20:00 AM
File end
21/12/2011 2:40:00 PM
Data series


23/02/2012 7:15:00 AM
 Date time (UTC + 10)
 Absolute (water) pressure
(kPa)
 Temperate (°C)
8/05/2012 3:55:00 PM
(kPa)
 Temperate (°C)
10011600
10011600
Logger serial number
Date & time (UTC + 10)
Absolute (water) pressure
(kPa)
 Temperate (°C)
10011600
Launch description
water level logger_1
Deployment number
11
12
13
Notes
To plot data in EST (UTC + 10hrs)
in HOBOware Pro, set “offset from
GMT” to "+11" hrs in plot setup;
Deployment info in file header is
incorrect!: “Launch GMT offset” IS
“10 Hr 00 Min” (and NOT “11 Hr
00 Min” as recorded in file).
correct: “Launch GMT offset” IS
“11 Hr 00 Min”.
Page 79 of 88
Table 32: Kangaroo Island HOBO data file summary information.
File info
Kangaroo Island HOBO data files
File name
kangaroo_water_level_logger_2_
1_GMT+10.hobo
2_GMT+10.hobo
2_GMT+11.hobo
UTC time offset
UTC + 10 hours
UTC + 10 hours
UTC + 11 hours
File start
27/11/2011 9:00:00 AM
21/12/2011 5:50:00 PM
22/02/2012 10:35:00 AM
File end
21/12/2011 5:45:00 PM
Data series


22/02/2012 9:30:00 AM
(kPa)
 Temperate (°C)
7/05/2012 6:10:00 PM
(kPa)
 Temperate (°C)
10011601
10011601
Absolute (water) pressure
(kPa)
 Temperate (°C)
10011601
Launch description
Deployment number
8
9
10
Notes
“11 Hr 00 Min”.
Page 80 of 88
Appendix
Table 33: Welcome Inlet HOBO data files summary information.
File info
Welcome Island HOBO data files
File name
welcome_water_level_logger_1_3
_GMT+10.hobo
_GMT+10.hobo
_GMT+11.hobo
UTC time offset
UTC + 10 hours
UTC + 10 hours
UTC + 11 hours
File start
NA
23/12/2011 3:25:00 PM
22/02/2012 6:15:00 PM
File end
NA
Data series


22/02/2012 5:10:00 PM
 Absolute (barometric)
pressure (kPa)
 Temperate (°C)
8/05/2012 1:50:00 AM
 Absolute (barometric)
pressure (kPa)
 Temperate (°C)
10011602
10011602
Absolute (barometric)
pressure (kPa)
 Temperate (°C)
10011602
Launch description
Deployment number
11
12
13
Notes
Launch failed, data series blank.
“11 Hr 00 Min”.
Page 81 of 88
Table 34: Stony Point HOBO data files summary information.
File info
Stony Point HOBO data files
Data file
stonypoint_barometric_logger_1_
1_UTC+10.hobo
2_ UTC +10.hobo
2_ UTC +11.hobo
UTC time offset
UTC + 10 hours
UTC + 10 hours
UTC + 11 hours
File start
27/11/2011 9:00:00 AM
21/12/2011 1:35:00 PM
23/02/2012 9:25:00 AM
File end
21/12/2011 1:30:00 PM
Data series
8/05/2012 5:05:00 PM
 Barometric pressure (kPa)
 Temperate (°C)
 Date & time (UTC + 10)
 Temperate (°C)
9991888
23/02/2012 8:20:00 AM
 Date & time (UTC + 10)
 Temperate (°C)
9991888
9991888
Launch description
barometric_logger_1
barometric_logger_1
barometric_logger_1
Deployment number
8
9
10
Notes
“11 Hr 00 Min”.
Page 82 of 88
Appendix
A.3 Water level logger observations and corrected sea levels
(Nov/Dec 2011 – May 2012).
A.3.1
Files
Data model
Howie Island sea level observations 26 Nov 2011 – 08 May 2012
(howie_sea-level_observations_nov2011_may2012.csv)
Kangaroo Island sea level observational data 26 Nov 2011 - 07 May 2012
(kangaroo_ sea-level_observations_nov2011_may2012.csv)
Welcome Inlet sea level observational data 23 Dec 2011 – 07 May 2012
(welcome_sea-level_observations_dec2012_may2012.csv)
Type
Comma-separated value (CSV) files
Description Observed barometric and total pressure, corrected water depths and sea
levels. Datasets span time period of 26 Nov 2011 to 08 May 2012 for
Howie Island and Kangaroo Island, and 23 Dec 2011 to 07 May 2012. All
data is recorded at 5 minute intervals.
Table 35: Data model for the water level logger observations and corrected sea levels data files.
Colum header
Data attributes
Comments
Date Time UTC (UTC + 0)
Date and time of data in
Coordinated Universal Time
(UTC).
Refers to date and time of
observed data. This date
coincides with the time data
in UTC.
Stated in 24 hour format
as:
DD/MM/YYYY HH:MM
Date Time EST (UTC+10)
SP Pressure, Barometric
(kPa)
Australian Eastern Standard
Time (UTC + 10).
observed data in EST (UTC
+ 10 hours; above).
as:
DD/MM/YYYY HH:MM
Note that day light saving
time is disregarded.
Barometric pressure in
kilopascals (kPa)
Barometric pressure
observations was observed
from Stony Point (Montagu)
only, from near sea level.
This site was located some
1 km from Howie Island,
11.5 km from Kangaroo
Island and 17 km from
Welcome Inlet Island sites.
This data shows the
variation in barometric
pressure observed over
time at Stony Point
This regional barometric
pressure data was used to
correct all three absolute
(water) pressure
observational datasets to
water level
Page 83 of 88
HI/KI/WI Pressure, Total
(kPa)
Total (water) pressure in
kilopascals (kPa)
This data shows the
variation in total (water +
air) submerged pressure
observed over time at
Howie Island (HI),
Kangaroo Island (KI) and
Welcome Inlet (WI).
Total pressure observations
were observed from the
near shore offshore remote
pressure sensors, deployed
in remote stilling wells at
Howie Island (HI),
Welcome Inlet (WI).
Each site specific total
pressure dataset was
corrected to water level
with the regional barometric
pressure data from to
Stony Point.
Note that the total pressure
equals the barometric
pressure when the loggers’
sensor periodically became
exposed to the air during
notably low tides.
HI/KI/WI Sensor Depth (m)
Depth of the water level
logger pressure sensor in
metres (m)
This data shows the
variation in water level
above the submerged
pressure sensor over time
at Howie Island (HI),
Welcome Inlet (WI).
HI/KI/WI Observed Sea
Level (m, AHD)
Elevation of the sea level in
relation to the Australian
Height Datum (AHD).
This data shows the
variation in sea level over
time at Howie Island (HI),
Welcome Inlet (WI).
Page 84 of 88
Depth of the sensor has
been converted from each
sites total pressure data
and the regional barometric
pressure data.
Note that the water level
equals 0 m (±0.005 m)
equals when the loggers’
sensor periodically became
exposed to the air during
notably low tides.
Sea level has been
calculated from the
surveyed loggers’ sensor
elevation, derived from a
Differential GPS survey.
Note that a null value
(“NaN”) has been given to
those times when the
loggers’ pressure sensor
became exposed to the air.
As sea level were not
recorded for these times (as
they had receded beneath
the loggers’ sensor).
Appendix
A.4 Observed sea levels, predicted astronomical tides and nontidal residuals (Nov/Dec 2011 – May 2012).
A.4.1
Files
Data model
Howie Island observed and predicted sea levels, and non-tidal residual
26 Nov 2011 – 08 May 2012
(howie_observed_predicted_sea-levels_nov2011_may2012.csv)
Kangaroo Island sea level observational data 26 Nov 2011 – 07 May 2012
(kangaroo_observed_predicted_sea-levels_nov2011_may2012.csv)
Welcome Inlet sea level observational data 23 Dec 2011 - 07 May 2012
(welcome_observed_predicted_sea-levels_dec2011_may2012.csv)
Type
Description Sea level observations, astronomical tide predictions and non tidal residual
computed for the three survey locations. Datasets span time period of 26
Nov 2011 to 08 May 2012 for Howie Island and Kangaroo Island, and 23
Dec 2011 to 07 May 2012. All data is recorded at 5 minute intervals. Tide
predictions produced from tidal analysis of observed data using T-TIDE
V1.3.
Table 33: Data model for the observed sea levels, predicted astronomical tides and non-tidal
residuals data files.
Colum header
Data attributes
Comments
(UTC).
observed data in UTC.
as:
DD/MM/YYYY HH:MM
Date Time EST (UTC + 10)
HI/KI/WI Observed Sea
Level (m, AHD)
Time (UTC + 10).
+ 10 hours; above).
as:
DD/MM/YYYY HH:MM
time is disregarded.
Observed elevation of the
sea level in relation to the
Australian Height Datum
(AHD).
Sea level has been
calculated from the site
specific total (subaqueous)
pressure and regional
barometric pressure
observations.
This data shows the
variation in sea level over
time at Howie Island (HI),
Kangaroo Island (KI) or
Welcome Inlet (WI).
Page 85 of 88
Note that a null value
(“NaN”) has been given to
those times when the
loggers’ pressure sensor
became exposed to the air
during periods of notably
low tides.
HI/KI/WI Predicted Sea
Level (m, AHD)
Predicted elevation of the
astronomical tides in metres
above the Australian Height
Datum (AHD).
This data shows the
predicted variation in tides
over time at Howie Island
(HI), Kangaroo Island (KI)
or Welcome Inlet (WI).
The astronomical tides have
been predicted in MATLAB
based T-TIDE (version 1.3),
from summing the 21-23
tidal constituents which
were solved for each site
from undertaking a tidal
analysis on their
corresponding observational
data.
The predicted tides account
for 97% of the observed
data.
HI/KI/WI Non-Tidal
Residual (m)
Non-tidal residual of the
predicted tides in metres.
This data shows the
variation in water level
heights between the
observed and predicted sea
level data for Howie Island
Page 86 of 88
Non-tidal residual is
calculated by subtracting
the predicted tides from the
observed tides.
This difference between the
observed (total) and
predicted (astronomical)
sea levels are largely due to
environmental effects e.g.
weather tides, shallow
water conditions).
Appendix
A.5 Predicted astronomical tide cycle (June 2012– June 2022).
A.5.1
Files
Data model
Howie Island predicted astronomical tides 01 Jun 2012 – 01 Jun 2022
(howie_predicted_sea-levels_jun2012_jun2022.csv)
Kangaroo Island predicted astronomical tides 01 Jun 2012 – 01 Jun 2022
(kangaroo_predicted_sea-levels_jun2012_jun2022.csv)
Welcome Inlet predicted astronomical tides 01 Jun 2012 - 01 Jun 2022
(welcome_predicted_sea-levels_jun2012_jun2022.csv)
Type
Description Astronomical tide predictions computed for the three survey locations. All
tide predictions are computed from 01 June, 2012 to 01 June, 2022 at 30
minute intervals, produced from tidal analysis of observed data using TTIDE V1.3.
Table 34: Data model for the predicted astronomical tide cycle data files.
Colum header
Data attributes
Comments
(UTC).
as:
DD/MM/YYYY HH:MM
Level (m, AHD)
Time (UTC + 10).
+ 10 hours; above).
as:
DD/MM/YYYY HH:MM
times are disregarded.
Datum (AHD).
based T-TIDE (version 1.3)
analysis on their
data.
This data shows the
data.
Page 87 of 88
A.6 Predicted astronomical high and low tides (June 2012– June
2022).
A.6.1
Files
Data model
Howie Island predicted high and low astronomical tides
01 June 2012 – 01 June 2022
(howie_predicted_high-low_tides_jun2012_jun2022.csv)
Kangaroo Island predicted high and low astronomical tides
01 June 2012 - 01 June 2022
(kangaroo_predicted_high-low_tides_jun2012_jun2022.csv)
Welcome Inlet predicted high and low astronomical tides
01 June2012 – 01 June 2022
(welcome_predicted_high-low_tides_jun2012_jun2022.csv)
Type
Description High and low tide predictions for the three survey locations. All tide
predictions are computed from 01June, 2012 to 01June, 2022 to the
closest 5 minute interval, produced from tidal analysis of observed data
using T-TIDE V1.3.
Table 35: Data model for the predicted astronomical high and low tides data files.
Colum header
Data attributes
Comments
(UTC).
as:
DD/MM/YYYY HH:MM
Level (m, AHD)
Time (UTC + 10).
+ 10 hours; above).
as:
DD/MM/YYYY HH:MM
times are disregarded.
Datum (AHD).
based T-TIDE (version 1.3),
analysis on their
data.
This data shows the
data.
Page 88 of 88

Project Supporting Document

Transcription

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