CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS BASED UPON
A QUANTITATIVE ANALYSIS OF WAVE, TIDAL, AND RIVER POWER
P.T. HARRIS1, A.D. HEAP 2, S.M. BRYCE3, R. PORTER-SMITH1, D.A. RYAN3, AND D.T. HEGGIE3
2
1 Geoscience Australia and Antarctic CRC, University of Tasmania, GPO Box 252-80, Hobart TAS 7001, Australia
Geoscience Australia, School of Geography and Environmental Studies, University of Tasmania, GPO Box 252-78, Hobart TAS 7001, Australia
3 Geoscience Australia, GPO Box 378, Canberra ACT 2601, Australia
e-mail: [email protected]
ABSTRACT: A statistical assessment of wave, tide, and river power was
carried out using a database of 721 Australian clastic coastal depositional environments to test whether their geomorphology could be predicted from numerical values. The geomorphic classification of each
environment (wave- and tide-dominated deltas, wave- and tide-dominated estuaries, lagoons, strand plains, and tidal flats) was established
independently from remotely sensed imagery. To our knowledge, such
a systematic numerical analysis has not been previously attempted for
any region on earth.
The results of our analysis indicate that a relationship exists between
the ratio of annual mean wave power to mean tidal power and the
geomorphic development of clastic coastal depositional environments.
Deltas and estuaries are associated with statistically significant differences in mean wave and tidal power. Statistically significant distinctions between populations of deltas, estuaries, strand plains, and tidal
flats are also associated with river discharge and river flow rate (defined as discharge divided by open water area). Our results support
the hypotheses of previous workers that wave, tide, and river power
exert the principal control over the gross geomorphology and facies
distribution patterns in clastic coastal depositional environments. Mean
values and confidence limits of wave power, tide power, and river flow
for the coastal depositional environments targeted in this study may
provide a basis for the comparison of modern environments as well as
constraints for paleo-reconstructions.
INTRODUCTION
The geomorphic evolution of different clastic coastal depositional environments (including deltas, estuaries, lagoons, strand plains, and tidal flats) is
controlled by the relative importance of three main factors: sediment supply,
physical processes, and sea level. In their reviews of this subject, Boyd et al.
(1992) and Dalrymple et al. (1992) provided a synthesis and a ternary classification, emphasizing the geomorphic relationships that exist between different
clastic coastal depositional environments over space and time (Fig. 1A–C). The
conceptual wave–river–tide ternary classification, originally proposed by Coleman and Wright (1975) and Galloway (1975) for deltas, was adopted by
Boyd et al. (1992) and applied to clastic coastal depositional environments
(Fig. 1B, C). This new classification uses relative wave, tide, and river power
to position each environment within the ternary diagram. Boyd et al. (1992)
and Dalrymple et al. (1992) expanded this simple two-dimensional ternary
diagram into a three-dimensional prism to incorporate the evolutionary relationships between the spectrum of clastic coastal depositional environments,
where the z axis represents relative time, with reference to changes in relative
sea level and sediment supply (Fig. 1A).
Boyd et al. (1992) and Dalrymple et al. (1992) proposed the ternary
prism (Fig. 1A, B) mainly as a means to portray the relative importance
of the three processes. These workers plotted individual systems within the
diagram on the basis of their perceived wave, tide, and river power, rather
than based on measured values. To improve our understanding of the relative importance of these three key physical processes in governing the
geomorphology of coastal depositional environments, the next logical step
is to quantify the relative power (i.e., sediment transporting capacity) of
JOURNAL OF SEDIMENTARY RESEARCH, VOL. 72, NO. 6, NOVEMBER, 2002, P. 858–870
Copyright q 2002, SEPM (Society for Sedimentary Geology) 1527-1404/02/072-858/$03.00
waves, tides, and rivers for a statistically significant number of cases.
Therefore, the aim of this study was to test the hypothesis of Boyd et al.
(1992) and Dalrymple et al. (1992) that wave, tide, and river power are
the major controls on the geomorphology of modern clastic coastal depositional environments. This test was conducted by plotting a population of
coastal depositional environments having a known geomorphology within
a ternary diagram, with axes based on numerical estimates of wave, tide,
and river power. The results provide not only the first calibration of the
Boyd et al. (1992) and Dalrymple et al. (1992) model but also the first
published continental-scale approach to the quantitative classification of
clastic coastal depositional environments. In undertaking this work, we anticipate determining boundary conditions characteristic to each environment
in the context of the existing ternary classification (Fig. 1A, B) and establishing the first quantitative approach for comparing coastal depositional
environments of the same classification. As such, our approach provides
the basis for identifying clastic coastal depositional environments most susceptible to changes in global wave climates as a result of global warming,
and a mechanism for constraining paleo-environmental conditions for ancient coastal systems discovered in the rock record.
Quantification of Wave, Tide, and River Power
In the coastal zone, sediment erosion, transport, and deposition are physical
processes that are closely related to the power of waves and tides (e.g., Swift
and Thorne 1991). The geomorphology of clastic coastal depositional environments is strongly influenced by the relative contribution of waves and tides in
governing the transport and deposition of available sediment (e.g., Davis and
Hayes 1984). The ratio of wave power to tide power is thus used to classify
each environment along the base of the ternary diagram (Fig. 1A).
The energy of a gravity wave (E, J m22) in the ocean is:
E 5 1/8(rgH 2)
(1)
where r is the density (kg m23) of the sea water, g is acceleration due to
gravity (m s22), and H is the wave height (m) (Soulsby 1997). The wave
power (units J m21 s21) is the energy times the wave group speed (Cg 5
L/T), where L is the wavelength and T is wave period. We have estimated
wave ‘‘power’’ as the energy per wave period (E/T, units J m22 s21) and
ignored wavelength, which is complex to calculate for shallow-water waves
(i.e., ocean tides). However, our ‘‘power’’ estimate accounts for the increase in near bed orbital velocity that occurs for decreasing period in
waves of a given height (e.g., Soulsby 1997). Because ocean gravity swell
waves and tides are both wave phenomena, the ratio of wave power to tide
power can be estimated by:
Power ratio 5 wave power/tidal power 5 K[H 2 /T]wave /[H 2 /T]tide
(2)
where K is a dimensionless coefficient that corrects for the greater power
of tidal waves over ocean gravity waves.
Calculation of the vertical axis (river power) of the ternary diagram (Fig.
1B) is a function of the fluid power (P, N m21 s22) available to transport
sediment (Swift and Thorne 1991):
P 5 Ut
(3)
CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS
859
FIG. 1.—A) Evolutionary classification of
clastic coastal depositional environments (after
Dalrymple et al. 1992 and Boyd et al. 1992).
The long axis of the three-dimensional prism
represents relative time, with reference to
relative sea level (transgression) and sediment
supply (progradation). B) The triangular sections
of the prism reflect relative wave/tide power (the
x axis) versus the rate at which the river delivers
sediments to the coast (y axis). Deltas occupy
the uppermost area of the ternary diagram.
Estuaries are located in the intermediate
trapezoidal area and coastal strand plains and
tidal flats occupy the base of the ternary
diagram. C) Plan-view maps for idealized coastal
depositional environments, showing the
relationships between wave and tidal power,
prograding and transgressive environments, and
different geomorphic types.
where U is the mean river current (m s21) and t is the bed shear stress (N
m22). Although U is proportional to the river discharge through the crosssectional area of the river channel, the estimation of t requires information
on the shape of the velocity profile and bottom roughness characteristics
(e.g., Sternberg 1972).
Australian Clastic Coastal Depositional Environments
Our study encompasses the clastic coastal depositional environments of
the entire Australian continent to ensure that a wide range of environments
is included in the analysis. Because the impetus for this study came about
as an investigation into the ‘‘health’’ of Australia’s coastal waterways
(Heap et al. 2001), the clastic coastal depositional environments considered
in this paper are restricted to those associated primarily with Holocene
terrigenous sediment sources. As such, extensive regions of the coastline,
including most of southern and parts of western Australia, where transgressive wave–dune systems predominate, have not been not considered.
Information on the wave, tide, and river regimes around the Australian
coast have been discussed in relation to the evolution of depositional environments in numerous previous case studies. Examples include studies
of macrotidal estuaries (e.g., Wright et al. 1973; Cook and Mayo 1978;
Harris 1988; Woodroffe 1996); wave-dominated estuaries (e.g., Roy 1984);
strand plains and cheniers (e.g., Roy et al. 1980; Rhodes 1982; MurrayWallace et al. 1999); tidal flats (e.g., Semeniuk 1981); and deltas (e.g.,
Johnson 1982; Lees 1992; Jones et al. 1993; Woodroffe and Chappell
1993). However, to ensure that only consistently collected and comparable
data are used in the analysis, we have recalculated the wave, tide, and river
power for all Australian clastic coastal depositional environments considered in this study.
Previous systematic collations of information have been used to classify Australian clastic coastal depositional environments on the basis
mainly of climate, freshwater discharge, and tidal range (e.g., Bucher
and Saenger 1991, 1994; Eyre 1998; Digby et al. 1998). However, none
of these studies included waves as a classification criterion. The distribution of continental shelf zones dominated by swell waves, tides, cyclone-induced currents, and intruding ocean currents was described by
Harris (1995) on the basis of available published data, but this classification was not extended to the coastal zone. From these studies it is
clear that Australia’s coastal environment spans a range of climatic
zones (tropical to temperate) and contains examples from the complete
spectrum of clastic coastal depositional environments of varying wave,
tide, and river influence, and is therefore an appropriate location to
carry out this study. We acknowledge, however, that by restricting our
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HARRIS ET AL.
TABLE 1.—Mean and standard deviation of the tide power, wave power, (dimensionless) wave/tide power ratio, mean annual river discharge, and river flow rates
calculated for seven different clastic coastal sedimentary environments around Australia.
Type of
System
Tide Power
(J m22 s21)
Wave Power
(J m22 s21)
Wave/Tide
Power Ratio
River Discharge
(m3 s21)
River Flow
(3 1026 m s21)
n
Wave-Delta
Tide-Delta
Wave-Estuary
Tide-Estuary
Strandplain
Tidal Flat
Lagoon
Other
398 6 437
828 6 1190
177 6 280
2244 6 2220
696 6 1260
1730 6 2120
416 6 1000
—
181 6 192
42.8 6 63.4
318 6 247
41.7 6 55.6
127 6 181
60.6 6 74.1
266 6 202
—
4.10 6 10.5
0.406 6 0.794
27.3 6 52.9
0.179 6 0.338
2.686 5.81
0.450 6 0.819
14.1 6 23.2
—
19.2 6 26.9
33.4 6 67.6
26.66 170
35.5 6 71.0
1.68 6 2.33
1.49 6 2.25
0.248 6 0.259
—
11.4 6 18.6
8.45 6 16.2
3.46 6 10.5
1.38 6 5.11
6.09 6 11.8
2.84 6 8.26
0.503 6 1.63
—
81
69
145
99
43
273
11
59
study to the Australian continent some clastic coastal depositional environments, such as those influenced by glacial processes and those
associated with major river systems, are not represented, and our results
may not be directly applicable in these situations.
METHODS
Australian Database of Clastic Coastal Depositional Environments
The database used in this study is the Australian Estuarine Database
(AED; http://www.ozestuaries.org), which contains physical information
for 780 Australian coastal waterways (Bucher and Saenger 1991; Digby et
al. 1998). All of these waterways were identified as ‘‘estuaries’’ based on
a definition similar to that proposed by Pritchard (1967), as: ‘‘semi-enclosed bodies of water and adjacent wetlands which have input both from
marine inundation and terrestrial runoff’’ (Digby et al. 1998, p. 15). For a
waterway to be contained in the AED, it also must be large enough to be
shown on a 1:100,000 scale topographic map and have a catchment area
of at least 15 km 2 (Digby et al. 1998).
These 780 coastal waterways span a range of climatic and oceanographic
settings and display a range of geomorphic types. They include a range of
clastic coastal depositional environments, together with other nondepositional environments such as rocky embayments and channels between islands and the coast that are not included in the Boyd et al. (1992) classification (Fig. 1C). Because in the present study we are concerned with the
geomorphology of clastic coastal depositional environments, we have
adopted the definitions of deltas, strand plains, lagoons, and tidal flats as
summarized by Boyd et al. (1992) and for estuaries we have adopted the
definition of Dalrymple et al. (1992).
An independent assessment of the geomorphology of the 780 waterways
contained in the AED (Bucher and Saenger 1991; Digby et al. 1998) was
undertaken in this study by a visual inspection of maps, nautical charts,
LANDSAT-TM images, and aerial photographs (see acknowledgments).
Each waterway was classified, using the principles of Boyd et al. (1992)
and Dalrymple et al. (1992), as wave- or tide-dominated estuaries, waveor tide-dominated deltas, lagoons, strand plains, or tidal flats (Fig. 1C).
Only the geomorphology visible in the images was used to determine the
classification. In cases where the waterway was judged to be an embayment, or a channel between an island and the coast, or where the diagnostic
geomorphic features were ambiguous or unclear, a ‘‘mixed/other’’ category
was used. This resulted in a total of 721 clastic coastal depositional environments targeted for further analysis.
Quantitative Wave, Tide, and River Data
To quantify the wave power around the Australian coast, heights and
periods of surface gravity waves were estimated using the Australian Bureau of Meteorology’s regional atmospheric model. Surface wind speed
estimates generated by this model provided input to the Wave Model,
WAM (Hasselmann et al. 1988; Komen et al. 1994) to yield estimates of
mean wave height and period. These data are 6-hourly predictions of sig-
nificant wave height (Hsig.) and mean wave period, gridded at a spatial
resolution of 0.18 (11 km), for the period March 1997 to February 1998,
inclusive. The gridding procedure employed a weighted inverse distance
(weighting 5 4) algorithm, which produces representative interpolations
in situations where the original data are infrequent and irregularly distributed. The mean annual Hsig. and periods were then calculated for each
coastal waterway by assigning the model grid value closest to the location
of the waterway mouth.
To quantify the tidal power along the Australian coast, the maximum
spring tidal range for 423 tide gauges located around Australia (Australian
National Tide Tables 2000) was gridded at a spatial resolution of 0.18 (11
km) using an inverse distance algorithm with a weighting of 4. The maximum spring tidal range for each coastal waterway was then determined
by assigning the model grid value closest to the mouth. Two tidal periods
were used for the calculation of tidal power: diurnal and semidiurnal. The
difference between diurnal and semi-diurnal tides is defined in the Australian National Tide Tables (2000) on the basis of the ratio of four major
tidal constituents as
semidiurnal 5 (K1 1 O1)/(M2 1 S2) , 0.5, and
diurnal 5 (K1 1 O1)/(M2 1 S2) . 0.5,
where K1 is the lunisolar diurnal, O1 is the principal lunar diurnal, M2 is
the principal lunar, and S2 the principal solar tidal component. The wave
and tide power, derived here, are considered to best represent the conditions
at the coast (i.e., at the mouth), and in most cases they probably do not
reflect conditions inside the waterway.
We were not able to derive a reliable estimate of river power because
in situ measurements of river currents and boundary shear stress are not
available for most Australian waterways. However, Digby et al. (1998)
estimated the mean annual river discharge for the 780 Australian waterways
contained in the AED as
mean annual discharge (m3 s21 )
5 [catchment area (m2 ) 3 mean annual rainfall (m)
3 runoff coefficient].
(4)
The mean annual discharge was then divided by the open water area of
each waterway, which was measured from LANDSAT TM images and,
where available, from aerial photographs, to calculate river flow (m s21).
River flow is proportional (but not equivalent) to river power (Equation 3).
Although we have calculated mean annual values for wave and tide
power and river flow, it is important to note that the discharge of many
Australian river systems is event-driven (Neil and Yu 1995; Wasson et al.
1996). The mean annual river flow does not reflect extreme river power
values associated with flood events, when most, if not all, sediment transport occurs. Because of this, it is probable that the rate and degree of
infilling of Australian coastal depositional systems by river-derived sediment are more closely associated with the frequency and duration of storms
rather than with average annual sediment yield from the catchments. Al-
CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS
861
FIG. 2.—Distribution of 721 different clastic coastal depositional environments around Australia based on the interpretation of aerial photographs and LANDSAT TM
imagery. The 59 ‘‘mixed/other’’ coastal environments are not shown. Five regions are identified, on the basis of the combination and distribution of coastal depositional
environments (see also Table 2).
though sediment discharge data are available for about 20 waterways (e.g.,
Harris 1995; Wasson et al. 1996) they have not been considered in our
analysis.
RESULTS
Spatial Distribution of Coastal Depositional Environments
The visual inspection of the waterways resulted in the geomorphic classification of 721 clastic coastal depositional environments using the criteria
of Boyd et al. (1992) and Dalrymple et al. (1992), and 59 cases of ‘‘mixed/
other’’ types (Table 1). The data indicate that tidal flats are the most common coastal depositional environment in Australia (n 5 273), followed by
wave-dominated estuaries (n 5 145), tide-dominated estuaries (n 5 99),
wave-dominated deltas (n 5 81), tide-dominated deltas (n 5 69), strand
plains (n 5 43), and lagoons (n 5 11).
The spatial distribution of these environments around the coast exhibits
a distinct zonation, such that five major coastal regions can be identified:
southeast coast; southwest coast; northwest coast; Gulf of Carpentaria
coast; and northeast coast (Figs. 2, 3; Table 2). The southeast and southwest
coasts are wave-dominated environments, whereas the northern coastal areas (northwest, Carpentaria, and northeast) are mainly tide-dominated
(Figs. 2, 3). Although wave-dominated estuaries occur almost exclusively
in the southeast and southwest, wave-dominated deltas and strand plains
are widely scattered across the southeast, northeast, Gulf of Carpentaria,
and to a lesser extent in the northwest (Fig. 3). This pattern conforms to
the general distribution of wave- and tide-dominated shelf environments
described by Harris (1995), such that tide-dominated estuaries, deltas, and
862
HARRIS ET AL.
FIG. 3.—Maps showing the distribution of the
seven different clastic coastal depositional
environments around Australia based on the
interpretation of aerial photographs and
LANDSAT TM imagery. Note that the
depositional environments targeted in this study
are restricted to those influenced by Holocene
terrigenous sediment; these environments are
absent across most of southern and parts of
western Australia.
CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS
TABLE 2.—Number (and percentage) of Australian clastic coastal depositional environments listed by geographical area.
Wave-Delta
Tide-Delta
Wave-Estuary
Tide-Estuary
Strandplain
Tidal Flats
Lagoon
n
Southeast
Coast
Southwest
Coast
Northwest
Coast
Gulf of
Carpentaria
Coast
Northeast
Coast
81
69
145
99
43
273
11
27 (33%)
0
114 (79%)
2 (2%)
9 (21%)
12 (4%)
8 (73%)
1 (1%)
0
18 (12%)
0
0
0
1 (9%)
4 (5%)
11 (16%)
0
51 (52%)
4 (9%)
95 (35%)
1 (9%)
14 (17%)
24 (35%)
7 (5%)
17 (17%)
21 (49%)
80 (29%)
1 (9%)
35 (43%)
34 (49%)
6 (4%)
29 (29%)
9 (21%)
86 (32%)
0
tidal flats occur mostly adjacent to tide-dominated shelves, wave-dominated
estuaries, lagoons, and deltas, and strand plains occur mostly adjacent to
wave-dominated shelves (Fig. 2). However, each of the five coastal regions
contains a distinct mixture of coastal depositional environments.
The southeast coast contains 79% of Australia’s wave-dominated estuaries but also significant numbers of wave-dominated deltas, strand plains,
and lagoons plus some tide-dominated environments (Table 2). This mixture of environments distinguishes this region from the southwest coast,
where there are also mainly wave-dominated estuaries, but only one wavedominated delta and no strand plains; the southwest coast lacks progradational coastal depositional environments. There are no tide-dominated
coastal depositional environments on the southwest coast.
The northwest coast is characterized by the largest number of tide-dominated estuaries (n 5 51). These are supplemented with significant numbers
of tidal flats and some tide-dominated deltas. Wave-dominated environments are rare, with only 9 out of 166 environments being wave-dominated
(Table 2).
The Gulf of Carpentaria coast is characterized by the largest number of
strandplains (n 5 21). This region also has significant numbers of tidal
flats, tide-dominated estuaries, and tide-dominated deltas. However, the
greater influence of waves in this region relative to the northwest coast is
manifest by the occurrence of wave-dominated estuaries and wave-dominated deltas (Fig. 2; Table 2).
The northeast coast is differentiated from the Gulf of Carpentaria coast
by its abundance of both wave- and tide-dominated deltas (n 5 69) but
fewer strand plains (Fig. 3; Table 2). The northeast and Gulf of Carpentaria
coasts are similar in that the total number of tide-dominated environments
863
(tide-dominated estuaries, deltas, and tidal flats) is about three times as
large as the total number of wave-dominated environments (wave-dominated estuaries, deltas, strand plains, and lagoons). Hence, although these
coastal regions are classed as being mainly tide-dominated, there is a greater mixture of wave- and tide-dominated environments on the northeast and
Gulf of Carpentaria coasts than in any of the other three regions (northwest,
southwest, and southeast; Figs. 2, 3; Table 2).
Wave, Tide, and River Power
A plot of wave and tidal power shows that Australia’s clastic coastal
depositional environments are partitioned into two populations (Fig. 4) that
are distinguished by environments that display wave-dominated geomorphology (wave-dominated deltas and estuaries, strand plains, and lagoons)
and environments that display a tide-dominated geomorphology (tide-dominated deltas and estuaries, tidal flats). A line of best fit separating these
two populations can be drawn with a slope of 3.2. This slope was used as
the coefficient K in Equation 2 to center the distribution of coastal depositional environments at a wave/tide power ratio of 1 (i.e., Log (W/T ) 5
0).
Coastal depositional environments that display wave-dominated geomorphology number 280, with a mean annual wave power of between 1.56
3 1023 and 1,190 J m22 s21 (average 5 246 6 235 J m22 s21). Tidal
power for these environments ranges from 7.9 to 6000 J m22 s21 with an
average of 327 6 621 J m22 s21. Coastal depositional environments that
display tide-dominated geomorphology number 441, with a mean annual
wave power of between 2.47 3 1023 and 497 J m22 s21 (average 5 53.6
6 69.2 J m22 s21). Tidal power for these environments ranges from 31.6
to 10,900 J m22 s21 with an average of 1,700 6 2,070 J m22 s21.
A statistical t-test analysis of the data demonstrates that the average wave
and tide power of wave-dominated environments are significantly different
(at 95% confidence limits) from the average wave and tide power of tidedominated environments (Fig. 4). The differences between the average
wave/tide power ratios for wave-dominated environments (5.23 6 12.9, n
5 280) are also statistically significant compared with tide-dominated environments (0.119 6 0.231, n 5 441).
In order to investigate whether there are statistically significant differences in the mean values of tidal power, wave power, river flow, and river
discharge between the seven clastic coastal depositional environments (Fig.
FIG. 4.—Stability diagram for clastic coastal
depositional environments around Australia.
Tide-dominated systems plot on the upper lefthand side of the diagram whilst wave-dominated
systems plot on the lower right-hand side. The x
and y axes are log of wave and tidal power
estimated for 721 coastal depositional
environments. The line dividing the two groups
was drawn by eye and has a slope of 3.2.
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HARRIS ET AL.
FIG. 5.—Histograms arranged in order of decreasing values (from top to bottom) for tide power, wave power, river discharge, and river flow in relation to the occurrence
of difference Australian clastic coastal depositional environments. The occurrence of statistically significant differences between populations of environments in each column
is illustrated in Fig. 6. The range in values specific to each type of environment provides constraints that may be applied to paleoenvironmental reconstructions and
interpretations (see text for discussion).
1B), a one-way analysis of variance (ANOVA) was calculated using the
statistical software package STATISTICAy (Fig. 5; Table 1). The ANOVA indicated that there are significant differences between category means
for tidal power, wave power and river flow. The Scheffe’s test was then
applied as a post-hoc comparison to determine which combinations of
means in the ANOVA were significantly different from each other at the
95% confidence limit (highlighted cells in Fig. 6). These tests provide a
statistical check on the significance of the relationships between the geomorphology of each of the seven clastic coastal depositional environments
and wave power, tide power, river discharge, and river flow. The significant
differences also confirm situations where a positive correlation exists between the independently derived geomorphology and wave, tide, and river
values. Interestingly, no significant differences were identified between the
mean river discharges of the seven coastal depositional environments.
Plotting the data on ternary diagrams (Figs. 7A, B) illustrates a strong
separation of coastal depositional environments along the x axis (wave/tide
power ratio). Tide-dominated environments plot almost without exception
on the right side of the diagram and wave-dominated estuaries and lagoons
clearly plot on the left side. However, strand plains and wave-dominated
deltas exhibit much scatter around the center of the ternary diagram, indicating that they occur where the wave/tide power ratio is close to 1.
There is generally wide scatter for all environments along the vertical
(river discharge/river flow) axes (Figs. 7A, B). The separation of the different populations varies, depending upon which parameter (river discharge
or river flow) is used. For river discharge, deltas and estuaries plot as
overlapping and widely scattered groups, but strand plains, lagoons, and
tidal flats all have a mean annual discharge , 10 m3 s21 (Fig. 7A). As
noted above, no significant differences were identified between the mean
annual river discharges of the seven coastal depositional environments
based on Scheffe’s test (Fig. 6).
Nearly all deltas have a mean annual river flow of . 1 3 1027 m s21
(Fig. 7A), which corresponds to deltas being significantly different from
estuaries and tidal flats based on Scheffe’s test (Fig. 6). Estuaries, strand
plains, and tidal flats exhibit a wide scatter against mean annual river flow
(Fig. 7B), whilst lagoons have a mean annual river flow of , 1 3 1027
m s21 (Fig. 7B).
DISCUSSION
The hypothesis of Boyd et al. (1992) and Dalrymple et al. (1992), that
wave, tide, and river power controls the geomorphology of clastic coastal
depositional environments is generally supported by the results of our
study. A clear relationship exists between the ratio of mean annual wave
power to mean annual tidal power (i.e., the wave/tide power ratio) and
the geomorphology of coastal depositional environments (Fig. 4). There
are also statistically significant differences in the mean annual wave power, mean tidal power, and mean annual river flow between the seven types
of clastic coastal depositional environments targeted in our study (Figs.
5, 6).
Our results show that most clastic coastal depositional environments plot
on either the wave- or tide-dominated side of the ternary diagrams (Fig.
7). An exception to this are the strand plains, which have the widest scatter
in terms of wave/tide power ratio, and they tend to plot towards the center
rather than on the wave-dominated half of the ternary diagram (Fig. 7), as
expected (Fig. 1B). We have not separated chenier-ridge complexes from
strand plains, because of the difficulty in discriminating between the two
in the remotely sensed images used. Hence these depositional environments
are plotted together, even though they may be associated with different
wave/tide power regimes. Wave-formed chenier ridges are separated by
fine-grained, tidal-flat deposits and are generally considered to be more
‘‘mixed’’ wave/tide environments (e.g., Dalrymple 1992), which probably
explains their scattered distribution in the ternary diagram (Fig. 7).
An interesting observation is that the matrix of statistically significant
differences between clastic coastal depositional environments (Fig. 6)
shows that not all environments are differentiated by the same independent
variables, either separately or in combination, and that some environments
are not differentiated by any of the variables. For example, the differences
between wave-dominated deltas, tide-dominated estuaries, and tidal flats
CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS
865
FIG. 6.—Matrices of significant differences
between populations of clastic coastal
depositional environments around Australia.
Highlighted cells indicate statistically significant
combinations based on the Scheffe’s test for the
difference in means at the 95% confidence level,
applied using the statistical software package
STATISTCAy. Blank cells indicate that no
significant difference was detected.
were significant for all three of the independent variables (wave power,
tide power, and river flow; Figs. 5, 6). On the other hand, there was no
statistically significant difference between strand plains and tide-dominated
deltas, wave-dominated deltas, or lagoons identified by any of the independent variables (Fig. 6). For the other three environments, however, it
appears that there may be some other factor (or factors) that controls the
geomorphology.
One variable that is not accounted for in the Boyd et al. (1992) and
Dalrymple et al. (1992) classification (Fig. 1) is the initial volume of the
incised valley. It is implied in the diagram (Fig. 1A) that all valley systems start out having a comparable volume (accommodation space),
which is then filled with river and marine sediment under a steady sea
level. However, some valleys are larger than others, depending on a wide
range of geological parameters, and hence they do not infill at the same
rate. Valleys having identical wave, tide, and river power may have already infilled and become deltas (where the accommodation space was
relatively small), or they may still be estuaries (where the accommodation
space was relatively large). Hence variations in the initial volume of
incised valleys may explain some of the scatter exhibited by the data
(along the vertical axes) in Figs. 7A and 6B. Nevertheless, the results of
the present study suggest that there are statistically significant differences
between groups of clastic coastal depositional environments based on
river discharge and river flow.
River Discharge and Flow
In Australia, river discharge is larger for deltas and estuaries than for
strand plains and tidal flats but does not vary significantly between deltas
and estuaries (Fig. 5; Table 1). The mean annual river discharge is not
significantly different between the seven clastic coastal depositional environments (Fig. 6). However, a statistical t-test demonstrates that the
mean annual river discharge for deltas and estuaries combined (28.5 6
113 m3 s21, n 5 394) is significantly different (at the 95% confidence
level) from the mean annual river discharge of strand plains and tidal
flats combined (1.51 6 2.26 m3 s21, n 5 316; Fig. 6A). Furthermore,
lagoons have a significantly lower river discharge rate than any other
environment (Fig. 5; Table 1). This result is consistent with the view of
Boyd et al. (1992), that strand plains, lagoons, and tidal flats occur along
coastlines that do not have any significant river input (Fig. 1C). In our
study, these environments were found not to have incised valleys, but
contained small drainage areas that extended only as far as the immediately adjacent hinterland.
Despite significant overlap in the populations, statistically significant
differences do occur between mean annual river flow values for some of
the different environments (Figs. 5, 6). Most significantly, wave- and tidedominated deltas can be discriminated from wave- and tide-dominated
estuaries (Figs. 6, 7B). Although the difference in mean annual river flow
rate between the seven coastal depositional environments is scattered
866
HARRIS ET AL.
FIG. 7.—Triangular classification diagrams of
Australian clastic coastal depositional
environments plotting log wave/tide power ratio
versus: A) log mean annual river discharge, and
B) log mean annual river flow. The slope of the
line in Fig. 4 was used as a correction
coefficient (K ) in Equation 2 for plotting the
data so that at the data in the ternary plot was
centered on a wave/tide power ratio 5 1. Note
the Murray–Darling River (Fig. 2) has a mean
annual discharge of 2,032 m3 s21 and plots
beyond the limit of the ternary diagrams.
CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS
867
FIG. 7.—Continued.
(Figs. 5, 7B), a t-test indicates that the combined mean annual river flow
for wave- and tide-dominated deltas (1.0 3 1025 6 1.8 3 1025 m s21,
n 5 150) is significantly different from the combined mean annual river
flow rate of estuaries, strand plans and tidal flats (2.9 3 1026 6 8.8 3
1026 m s21, n 5 571) at the 90% confidence level. The infilling of
estuaries to form deltas results in a smaller open water area, and it follows
that deltas should have a larger river flow rate than estuaries (given a
similar river discharge). However, tidal creeks and lagoons associated
with tidal flats and strand plains also have small open water areas and
they exhibit a wide range of river flow rates (i.e., large standard deviations), depending on the size of their catchment areas (Table 1). Some
strand plains, with small open water areas, have river flow rates similar
868
HARRIS ET AL.
to some deltaic systems (Fig. 6). Hence, the type of depositional environment and their stage of development may explain some of the scatter
in the river-flow data.
Dalrymple et al. (1992) suggested that the river sediment load (and not
necessarily the water discharge) exerts the most control over the geomorphology of clastic coastal depositional environments. If this is true, then
river power plays only a secondary role in controlling the geomorphology
of coastal depositional environments. Testing of this hypothesis must await
the development of a reliable database containing sediment yields for Australia’s river catchments, which is currently not available (Harris 1995;
Wasson et al. 1996).
Flux of River Sediment to the Coast
The distribution of different geomorphic classes of clastic coastal depositional environments provides insight into the flux of river sediment to
the coast during the Holocene. In Australia estuaries are more common
than deltas, which is also true globally and is attributed to the relatively
brief time available in the Holocene for estuaries to infill under a steady
sea level (Nichols and Biggs 1985). The abundance of estuaries over deltas
in Australia also probably reflects the continent’s relatively arid climate,
low relief, and hence relatively low sediment yield (Milliman and Syvitski
1992).
Two-thirds of all of Australia’s deltas and nearly half of the prograding
strand plains and tidal flats are located on the northeast and Gulf of Carpentaria coasts. The abundance of these progradational coastal depositional
environments (Fig. 1C) indicates that the hinterland of this region has a
relatively high sediment yield in comparison with other regions of the continent. Geological investigations of the postglacial evolution of the northeast margin of Australia (e.g., Harris et al. 1990; Woolfe et al. 1999; Dunbar et al. 2000) have revealed that this margin has been characterized by
significant input of fluvioclastic sediment during most of the Holocene
associated with the dominant quasi-monsoonal climate regime in the region
(e.g., Suppiah 1992).
Wave-dominated estuaries that occur on the southeast coast are only
partially filled with postglacial sediment, most of which is marine-derived
siliciclastic sediment (e.g., Roy 1984; Nichol and Murray-Wallace 1992;
Nichol et al. 1994; Nichol et al. 1997). The part of the southeast region
that borders the Great Australian Bight (Fig. 2) is drought-dominated and
receives little or no runoff (e.g., Erskine and Warner 1988). Clastic coastal
depositional environments in the southeast regions of Australia are thus
characterized mostly by partially filled lagoons, also indicating the dominance of marine sediment deposition in these systems. In contrast, coastal
systems throughout southern and western Australia are dominated by marine-derived sediment from carbonate sources, including the adjacent shelf
and coral reefs (e.g., Short in press). Similarly, published case studies from
clastic coastal depositional environments of the northwest coast (e.g.,
Woodroffe et al. 1989; Woodroffe and Chappell 1993; Woodroffe 1996)
show that the (mainly) tide-dominated estuaries found here are also only
partially filled with marine-derived Holocene sediment. The abundance of
estuaries and lagoons containing ample accommodation space and marinederived sediment along the southwest, southeast, and northwest coasts of
Australia implies that these margins have been characterized by moderate
to low flux of terrigenous sediment during the Holocene.
The Relative Influence of Wave and Tide Power on
Coastal Geomorphology
An important factor that plays a role in determining the geomorphology
of clastic coastal depositional environments is the relative power of waves
and tides, acting separately and in combination. As noted by Davis and
Hayes (1984), it is possible, for example, for a coastal environment to be
wave-dominated even along macrotidal coasts, if the wave power is great
enough. An interesting result of our analysis is that both wave-dominated
and tide-dominated deltas have significantly lower mean wave and tidal
power than wave-dominated and tide-dominated estuaries (Figs. 5, 6). In
other words, our results suggest that deltas are formed in lower-energy
environments than estuaries. This may be because the higher-energy systems tend to lose more sediment to the adjacent shelf and coastal areas,
thus inhibiting delta development. Delta sediment may be smeared out
along the coast where the receiving basin is characterized by relatively high
wave or tide power.
The average tidal power for the 780 coastal environments (1,200 J m22
s21) is greater than the average wave power (130 J m22 s21; see Fig. 4)
because power is a function of the wave height squared (Equation 2). Hence
the absolute value of fluid power expended around Australia is highest on
the macrotidal northwest coast (Fig. 2), which is characterized by transgressive tide-dominated estuaries and tidal flats, with strong tidal currents
and continuously turbid water (e.g., Wright et al. 1973; Semeniuk 1981).
Tide-dominated deltas are most abundant and best developed in central and
north Queensland (Fig. 2), where the coast is characterized by upper-micro
to lower-meso ranges and a broad, shallow-gradient, progradational coastal
plain (Hopley 1982; Belperio 1983). Wave power along this coast is attenuated by the Great Barrier Reef, which acts to block out long-period swell
waves generated in the Coral Sea (Hopley 1982; Hopley 1984). The distribution of tide-dominated deltas suggests that they are confined to coasts
where tidal power is relatively weak, allowing sediment supplied by rivers
to be deposited close to the river mouth, and thus permitting the delta to
prograde seaward (e.g., Galloway 1975).
Wave-dominated deltas are also generally found along relatively lowenergy coasts. The formation of deltas on the northeast Queensland coast
is probably facilitated by the sheltering effect of the Great Barrier Reef as
mentioned above (Hopley 1982; Orpin et al. 1999). In contrast, the highenergy exposed southeast coast has relatively few deltas.
The mean values and bounding limits of tide power, wave power, river
discharge, and river flow predicted by our study (Table 1; Fig. 5) may
prove useful for the characterization of paleoenvironments. If an ancient
deposit is interpreted as a tide-dominated estuary, for example, then it
might be inferred (from Fig. 5) that the mean annual river discharge into
the estuary was at least ; 1 m3 s21, that its tidal power was at least
; 100 watts m22, whilst its wave power was probably less than ; 100
watts m22. Taken together with information from the fossil record, these
values might provide a means of testing the validity or otherwise of basic
environmental interpretations.
Climate Change and the Stability of Coastal
Depositional Environments
Our results show that many Australian coastal depositional environments
are poised along the wave–tide power boundary (Figs. 4, 7) and may be
likely to change from one geomorphic state to another with changes to the
wave/tide power ratio. Geomorphic changes resulting from an increase in
wave power are likely to be most obvious for tide-dominated environments.
The most significant change would be the replacement of existing shorenormal features (e.g., tidal sand banks) with shore-parallel features (e.g.,
barriers and beach ridges). Coastal depositional environments that presently
contain permanently open tidal inlets may become subject to intermittent
or complete closure, and prograding environments may begin to recede
because of increased alongshore sediment transport.
Significant attention has focused on the rise in global sea level induced
by anthropogenic greenhouse warming and the effects this may have on
the coastal zone (e.g., Daniels 1992; MacDonald and O’Connor 1996).
Another apparent response to the warming of the Earth’s atmosphere is a
change in the ocean wave climate, manifested as increased wave heights
associated with more intense storm events (e.g., Carter and Draper 1988;
WASA 1998; Gulev and Hasse 1999; Allan and Komar 2000; Grevemeyer
CLASSIFICATION OF AUSTRALIAN CLASTIC COASTAL DEPOSITIONAL ENVIRONMENTS
et al. 2000). Our approach of quantifying wave, tide, and river power provides a useful method for identifying coastal depositional environments
most susceptible to these changes.
Changes to the geomorphology of many clastic coastal depositional environments on the northeast coast of Australia would have occurred during
the Holocene because of the presence of a ‘‘high-energy’’ wave window
(Hopley 1984) that existed because reef growth lagged behind sea-level
rise. Clastic coastal depositional environments along the northeast coast
would have originally evolved as wave-dominated systems during the early
to mid-Holocene and then as lower-energy wave- and tide-dominated systems as the tops of the reefs eventually caught up with sea level. Similar
geomorphic changes have been described by Pendon et al. (1998), who
documented the transition from tide domination to wave domination of
estuaries in southwestern Spain, where tidal prisms have been reduced by
siltation at the mouth during the late Holocene. Although these estuaries
did not evolve because of a change in wave/tide power ratio along the
coast, this study demonstrates that coastal depositional environments can
evolve from one (wave- or tide-dominated) state to another under a constant
relative sea level. The process-based classification derived here could be
useful for identifying environments most susceptible to altering their geomorphology in response to changes in the coastal wave/tide power regime.
CONCLUSIONS
Our study has confirmed that the general geomorphology of clastic coastal depositional environments can be predicted from numerical estimates of
wave, tide, and river power. The conceptual ternary diagram of Boyd et
al. (1992) and Dalrymple et al (1992), showing the distribution of environments as being controlled by wave, tide, and river power, is supported
by our results. In particular, we conclude that there is a statistically significant difference between 721 wave- and tide-dominated clastic coastal
depositional environments in Australia. The large database used here to
derive mean wave, tide, and river power values (Table 1) enables quantitative comparisons to be made between modern coastal depositional systems, and provides useful constraints for paleo-reconstructions of ancient
coastal systems.
Although we have found differences between combined groups of environments and river discharge and river flow, there is significant scatter
in the data related to the seven categories of clastic coastal depositional
environments targeted in our study (Figs. 7A, B). Possible explanations for
this scatter include: (1) that not all incised valleys had a comparable accommodation space at the onset of Holocene highstand; (2) variable sealevel conditions throughout the Holocene; and (3) that river power is not
represented by river discharge or river flow. Further, sediment flux rather
than river power may exert a primary control on the overall geomorphology
of clastic coastal depositional environments, as hypothesized by Dalrymple
et al. (1990). Reliable estimates of pre-European river sediment loads in
Australia are required to test this hypothesis.
Our results indicate that deltas are generally found along coasts having
conditions of lower wave and tide power than coasts where estuaries are
predominant. A possible explanation is that higher wave and tide power
results in estuaries losing a greater percentage of sediment to the adjacent
shelf and coastal areas, thus inhibiting delta development.
The large database of clastic coastal depositional environments compiled
here provides insights into continental-scale sediment loads to the coast.
Two-thirds of deltas and nearly half of the prograding strand plains and
tidal flats are located on the northeast Queensland and Gulf of Carpentaria
coasts, indicating that the hinterland of this region has a relatively high
sediment yield in comparison with other regions of the continent.
ACKNOWLEDGMENTS
We thank the reviewers R. Boyd, A. Short, S. Nichol, R. Dalrymple, and Dr. V.
Passlow for their comments on an earlier version of this manuscript. Dr. D. Green-
869
slade of the Bureau of Meteorology Research Centre, Melbourne, is thanked for
providing the wave data used in this study. The authors are grateful for the financial
support of the National Land and Water Resources Audit in undertaking this study.
This manuscript is a contribution of the Cooperative Research Centre for Coastal
Zone Management and is published with permission of the Executive Director, Geoscience Australia. Line drawings of all 780 Australian waterways contained in the
AED (Bucher and Saenger 1991) can be viewed at: www.ea.gov.au/coasts/
information/reports/estuaries/index.html.
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