Ocean & Coastal Management 46 (2003) 457–478
Environmental management of clastic coastal depositional environments: inferences from an Australian geomorphic database Peter T. Harris*, Andrew D. Heap Geoscience Australia, Petroleum and Marine Division, GPO Box 378, Canberra ACT 2601, Australia
Abstract Simple, conceptual geomorphic models can assist environmental managers in making informed decisions regarding management of the coast at continental and regional scales. This basic information, detected from aerial photographs and/or satellite images, can be used to ascertain the relative significance of several common environmental issues, including: sediment trapping efficiency, turbidity, water circulation, and habitat change due to sedimentation for different types of clastic coastal depositional environments. The classification of 780 Australian clastic coastal depositional environments based on their geomorphology is used to derive a coastal regionalisation, comprised of a distinctive suite of environments for each region. Because of the close link between the relative influence of waves and tides and the geomorphology of clastic coastal depositional environments, a basic understanding of the broad geomorphic and sedimentary characteristics by environmental managers will assist them in ascertaining the relative significance of environmental issues in each region. The benefit of this approach is that it provides guidance in tailoring management schemes differently for each region, resulting in more effective and efficient treatment of these issues. Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Our coastlines are supporting an increasing number and variety of human activities. The coast is significant in terms of its economic, social and environmental values, including: fisheries production, port activities, recreation, pollution cycling, residential/commercial development and agriculture. In many countries, the coast *Corresponding author. Tel.: +61-2-6249-9111; fax: +61-3-6249-9915. E-mail address:
[email protected] (P.T. Harris). 0964-5691/03/$ - see front matter Crown Copyright r 2003 Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S0964-5691(03)00018-8
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has become a site for multiple, often competing, uses due to its ‘‘common property’’ status, and the effects of human activities have become preferentially located in coastal depositional environments. In many cases, coastal management practises targeting these activities, are reactive, ad hoc and non-integrated across all levels of government and stakeholders (e.g. [1]), resulting in fragmented management responses, particularly at regional and continental scales (e.g. [2]). Successful management of clastic coastal depositional environments requires that decisions are made based on an understanding of their characteristic physical and biological processes and features. However, in the case of large coastal regions containing numerous types of coastal environments, environmental managers are typically faced with the dilemma of formulating policies and management practices where environmental data are available for only a small number of systems. This is particularly true in situations where continental-scale, national management policies are required. In such cases, data that is readily obtained for different types of clastic coastal depositional environments from easily accessible and cost-efficient methods can provide essential basic information required to make informed management decisions for a range of environmental issues. The aim of this paper is to demonstrate the value of an understanding of the geomorphology of coastal depositional environments when formulating regionalscale environmental policies and management practices based on physical and biological processes and attributes. It is shown that a concise classification scheme may be applied to a suite of clastic coastal depositional environments, based only upon their gross geomorphology detected from aerial photographs and/or satellite images. Specifically, it is demonstrated that this classification permits basic deductions to be drawn that target particular management issues such as sediment trapping efficiency, turbidity and water circulation, together with insights into the susceptibility of an environment to significant habitat changes due to sedimentation. A database of 780 Australian clastic coastal depositional environments is then used to show that it is possible to derive an assessment of these management issues for a suite of environments, and ascertain their relative significance at a regional scale.
2. Clastic coastal depositional environments Clastic coastal depositional environments are defined as areas of the coast where sediments supplied from terrestrial and/or marine sources are accumulating. Coastlines characterised by erosional cliffs or rocky shores are not included in this classification and they are beyond the scope of this paper. Because this study originated as an assessment of the condition of Australian waterways [3], the clastic coastal depositional environments considered in this paper are restricted to those associated primarily with Holocene terrigenous sediment sources. As such, beachdune systems are not considered in this paper. Two broad types of clastic coastal depositional environment are recognised: (i) those that receive a large sediment supply and are actively prograding seawards
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(e.g., deltas, strand plains and tidal flats), and (ii) those that receive a small sediment supply and which exhibit geomorphic features associated with the Holocene sea level rise and have yet to completely fill their paleo-valleys (e.g. [4]). Under conditions of stable sea level, the existence of these types of clastic coastal depositional environments depends on the relative quantities of terrestrial and/or marine sediment supplied in relation to the size of the receiving basin. The gross geomorphology of clastic coastal depositional environments is also affected by the relative importance of waves and tides in controlling the amount, nature, distribution and transport of sediment along the coast (Figs. 1 and 2). Large swell waves generate significant alongshore sediment transport that produces coastparallel sedimentary features such as spits, barriers, sand bars and barrier islands. In contrast, large tidal ranges (>4 m) and strong tidal currents generally produce coastnormal sedimentary features, including: elongate tidal sand banks, wide-mouthed estuaries, funnel-shaped (in plan view) deltaic distributary channels, and broad intertidal flats (Fig. 1). Because of the close link between the geomorphology of clastic coastal depositional environments and the relative influence of waves and tides at the coast, it is possible to distinguish between wave-dominated coasts (characterised by wavedominated deltas, wave-dominated estuaries, strand plains and lagoons) and tidedominated coasts (characterised by tide-dominated deltas, tide-dominated estuaries and prograding tidal flats, Fig. 1). This ‘‘geomorphic’’ approach makes it feasible to identify and classify each clastic coastal depositional environment from aerial photographs or satellite images (e.g. [3,5]). The gross geomorphology and combination and arrangement of diagnostic sedimentary environments in clastic coastal depositional environments are summarised by idealised facies models [6].
Transgressive
Increasing tidal power Tide Dominated Estuary
Increasing wave power
Wave Dom. Est.
Lagoon strand plain barrier
Linear coasts mar. sed supply
Embayed coasts Lobate coasts
Linear coasts with marine sediment supply
Prograding
Delta
Tidal Flats
Strand Plain
Marsh
Mud
Sand
Fig. 1. Boyd et al. [4] diagram of coastal depositional environments.
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s lta De W-
Deltas
De ltas
R-Deltas
River
Galloway's (1975) ternary delta classification scheme
T-
460
Tide Wave Dominated Dominated Estuaries Estuaries Lagoons Strand Plains
Wave
Tidal Flats
Tide
Fig. 2. Triangular diagram representing the occurrence of coastal depositional environments in relation to relative wave, tide and river power. The upper triangle contains the ternary delta classification scheme originally proposed by Galloway [10].
3. Facies models for clastic coastal depositional environments Facies are a suite of geomorphic and sedimentary attributes that are diagnostic of a sedimentary environment, or physical, chemical and biological processes. By themselves, individual facies are of little interpretative value [6]. However, when used in combination as facies models, facies successions highlight lateral and vertical variations between different sedimentary environments. Facies models represent a generalisation of the physical attributes for a certain type of depositional environment, where the local variations from numerous modern and ancient examples have been ‘‘distilled away’’ to leave only the common features [6]. Because each clastic coastal depositional environment has a diagnostic geomorphology (represented by the unique combination of facies), a precise classification can be established for each system (Fig. 1). From a management perspective, it is important to emphasise that ‘‘facies’’ as used here are analogous to biological ‘‘habitats’’ in most clastic coastal depositional environments (e.g. [7]), and the two terms are used interchangeably throughout this paper. This study is concerned with seven different types of clastic coastal depositional environments: wave- and tide-dominated deltas, wave- and tide-dominated estuaries, strand plains, tidal flats, and lagoons (Fig. 1). 3.1. Deltas Deltas are defined as a coastal sedimentary protuberance (including offshore subaqueous features) where a river empties into the sea. The formation of a delta relies on the river supplying sediment to the coast more rapidly than can be redistributed by waves and tides, causing seaward progradation [8]. The river is connected to the sea by distributary channels, in which fresh water mixes with salt
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water. The distributary channels are comprised wholly of modern sediment, derived almost entirely from the catchment. Hence, the geomorphology of deltas owes nothing to the antecedent bedrock geometry [8,9]. Galloway [10] formulated a conceptual classification scheme that included three basic types of delta represented on a ternary diagram (top of Fig. 2). The gross geomorphology of each delta varies as a function of relative wave/tide power (the X axis) compared with the river power (Y -axis). At the top of the ternary diagram, where the river power and sediment supply are very large compared with the wave and tide energy, the delta progrades seawards forming elongate levee banks which look like a ‘‘bird’s foot’’ in plan view (Fig. 3A). The Mississippi River Delta located in the low-wave energy, microtidal Gulf of Mexico is the classic bird’s foot delta (e.g. [8,11]). The facies most strongly associated with bird’s-foot deltas are levee banks colonised by halophytic vegetation (i.e., mangroves and salt marshes, Fig. 4). With increasing tidal power, the ‘‘bird’s foot’’ shape changes to include funnelshaped distributary channels and tidal sand banks which are detached from, and trend normal to the coast (Fig. 3B). The Fly River Delta in Papua New Guinea is a good example of a tide-dominated delta [12,13]. In tide-dominated deltas, the maximum tidal range and tidal current speeds occur within the distributary channels (Fig. 3B). River power steadily decreases in a seaward direction from some maximum value due to the reduction in hydraulic gradient, and wave power decreases landward of the mouth because of the shallow water depths and the blocking effect of tidal sand banks (Fig. 3B). Facies typically associated with tidedominated deltas are intertidal flats with halophytic vegetation (i.e., mangroves and salt marshes, e.g. Woodroffe [14]) and tidal sand banks (Fig. 4). In the case of wave-dominated deltas, sediment is arranged into coast-parallel beach ridges, barrier bars, barrier islands and spits. The Niger delta in Africa is a good example of this type [15,16]. The river channel is often deflected parallel to the coast by these features and the river power remains high along the length of the river channel until just landward of the mouth (Fig. 3C). Wave power drops off landward of the mouth and tidal power is negligible outside of the channel mouth (Fig. 3C). Facies typically associated with wave-dominated deltas are intertidal flats with halophytic vegetation (i.e., mangroves and salt marshes) and wave-built beach ridges and sandy barriers (Fig. 4). 3.2. Estuaries From a geological perspective, estuaries are a particular category of clastic coastal depositional environment best defined as ‘‘the seaward portion of a drowned valley system which receives sediment from both fluvial (i.e., river) and marine sources and which contains facies influenced by tide, wave and fluvial processes’’ ([17, p. 1132]). This definition of an estuary is significantly different from that used by oceanographers, which is based on the dilution and mixing of freshwater with seawater [18]. Using this oceanographic definition, estuaries and deltas cannot be distinguished from each other, although they behave very differently with respect to their physical processes (see below). The literature is replete with alternative
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100%
TOTAL
River
Relative Power
Waves
Tides
0
100%
Relative Power 0
100%
Riv
er
Tides
W
TOTAL s e v a
(F) Tidal Flats
0
TOTAL
es Wav
Tides
1 2 3
100%
(G) Strand Plains
Tides
TOTAL Wav es
100%
Relative Power
es
straight
meandering
straight
es es Tid Wav
(E) Wavedominated Estuary
r
TOTAL
0
TOTAL ve
es
Wav
Tides
Relative Power
Ri
TOTAL
er
0
0
100%
Riv
Wav
100%
100%
TOTAL
(C) Wavedominated Delta
Relative Power
Relative Power
Tides
0
(B) Tidedominated Delta
(D) Tidedominated Estuary
River
Relative Power
Wav es
(A) Bird'sfoot Delta
Relative Power 0
Tides
(H) Lagoon
Fig. 3. Idealised drawings of coastal depositional environments and their relative river-wave-tide power distribution along the axis of waterways: (A) bird’s-foot delta, (B) tide-dominated delta, (C) wavedominated delta, (D) tide-dominated estuary (after Dalrymple et al. [17]), (E) wave-dominated estuary (after Dalrymple et al. [17]), (F) tidal flats, (G) strand plain, and (H) lagoon. In ‘‘E’’, numerals refer to (1) the bay head delta facies, (2) central muddy basin and (3) ebb-flood-tidal delta complex. The original (transgressed) shoreline is shown in black with Holocene deposits and infilling sediments depicted by shading. The dashed lines represent indicative bathymetric contours along the coast. Arrows indicate evolutionary paths, as infilling of estuaries leads to the formation of a delta, and the infilling of a lagoon leads to the formation of a strand plain. Dalrymple et al. [17] proposed that the straight-meanderingstraight morphology exhibited by fluvial channels distinguishes tide-dominated estuaries from tidedominated deltas (which have straight-only fluvial channels).
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Reefs
Rocky shores
Salt Flats
Barrier System
Ebb/Flood Tidal Delta
Bayhead Delta
Central Muddy Basin
ENVIRONMENT
Tidal Sand Banks
= Weak association
Intertidal Flats
= Some association
Mangroves/Salt Marsh
FACIES - HABITATS = Strong association
Bird’s-foot Delta Wave-Dominated Delta Tide-Dominated Delta Wave-Dominated Estuary Tide-Dominated Estuary Strand Plain Tidal Flat Lagoon
Fig. 4. Matrix diagram showing the 7 types of coastal depositional environment in relation to different sedimentary facies recognised.
definitions of an ‘‘estuary’’ (e.g. [19]), but in the present study the above geological definition of Dalrymple et al. [17] will be used. A key distinction is that estuaries form in situations where the sediment supplied by rivers and the sea has not yet infilled the original valley. Consequently, rocky shores and rocky reefs exposed along the unfilled portions of the valley are common features of both wave- and tidedominated estuaries (Fig. 4). There are only two geomorphic estuarine end members: wave-dominated and tide-dominated (Figs. 3D and E). There is no ‘‘riverdominated’’ end member because ‘‘the relative influence of the river primarily determines the rate at which the estuary fills and does not alter the fundamental morphology of the system’’ ([17, p. 1132]).
3.2.1. Tide-dominated estuaries Good examples of tide-dominated estuaries include the Bay of Fundy in Canada [20], the Bristol Channel and Thames estuaries in the UK [21] and the Gironde estuary in France [22,23]. In tide-dominated estuaries, tidal energy reaches a peak inside the estuary, and waves have less effect on sediment transport than tidal currents (Fig. 3D). Total energy rises to a maximum at some point along the estuary because the shoaling depth and converging sides of the funnel-shaped valley amplify the advancing tidal wave [22]. Further landward, frictional dissipation of the tidal power overcomes amplification and total energy falls to a minimum (Fig. 3D). Still further landward total energy rises again in the relatively narrow, river-dominated zone due to the influence of the freshwater flow. The facies distribution in tidedominated estuaries is organised into tidal sand banks [24], migrating dune fields and upper flow regime (sand) flats [21,25], intertidal mud flats, and river sand and mud colonised by halophytes and/or containing salt marshes ([26,27], Fig. 4).
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3.2.2. Wave-dominated estuaries Some better known examples of wave dominated estuaries are from studies in eastern Canada [28], eastern United States [29], Australia [30] and South Africa [31]. Roy et al. [30] described the tripartite zonation of facies that is common to all wavedominated estuaries: (1) Barrier—tidal inlet—flood/ebb tidal deltas, (2) Central basin—low energy zone, and (3) Bay head delta (Fig. 4). The relative spatial extent of each facies varies between estuaries, depending on the relative wave/tide power ratio, the relative marine versus river sediment supply, the inherited valley shape and depth, and its degree of infilling. The barrier is formed by waves supplying sediment to the coast via longshore drift. Tidal currents form ebb and flood tidal deltas with respect to their position (seaward or landward, respectively) to the tidal inlet [29]. A key process affecting the form of the barrier, tidal inlet, and flood/ebb tidal deltas is the relative balance between the tidal prism (i.e., the volume of water that enters or exits an estuary between consecutive high tides) and wave energy [32]. Littoral drift delivers sediment into the tidal inlet, which is dispersed by tidal currents. In some cases, the tidal prism is so small in relation to the littoral drift rate that no inlet can be maintained and the barrier is continuous across the estuary entrance (i.e., a blind estuary). The central basin, located landward of the barrier, is protected from swell waves and inhibits amplification of the tide landward of the inlet, giving rise to a low energy zone (Fig. 3E). Due to the low energy fine-grained sediment is able to accumulate in the central basin. The bay head delta is formed almost exclusively of river-derived sediment, deposited at the point where the river currents decelerate and lose energy (Fig. 3E).
3.2.3. Tidal flats Low-gradient wedges of tidal flat sediment are common along prograding coasts characterised by mean spring tidal ranges >B4 m (Fig. 3F). Good examples are known from The Wash, UK [33] and from San Sebastian Bay, Argentina [34]. They are usually comprised of fine-grained marine sediment that has been transported towards the coast by strong currents associated with the large tides. Waves are less effective at transporting sediment along these coasts because the low-gradient and shallow depth of the tidal flats dissipates wave energy and the large excursion between high and low tide precludes waves breaking on any part of the tidal flat for extended periods (Fig. 3F). During the falling tide, drainage of seawater from the intertidal flats causes the development of tidal creeks [33,34]. The banks of these creeks may be stabilised by halophytes and/or salt marshes [35]. Large tidal creeks often contain tidal sand banks and dunes (Fig. 4) but are distinguished from tide-dominated estuaries by the absence of a river channel entering from the hinterland (i.e., rivers do not supply sediment to tidal creeks) and by the fact that they are incised into wholly marine, Holocene sediments (i.e., they contain no rocky shorelines or reefs, Figs. 3F and 4).
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3.3. Strand plains Strand plains form where wave-induced sediment transport (littoral drift) results in the formation of a series of coast-parallel depositional features. These may include elongate, coast-parallel beach ridges, cheniers or barrier-islands (Fig. 3G). 3.3.1. Beach ridges Beach ridges are semi-continuous, generally linear mounds of shelly sand and gravel, deposited above the high tide line [36]. A sandy beach is always present in front of the beach ridge. As marine-derived sediment accumulates along the coast, the sequence progrades seawards leaving the coarser-grained ridges ‘‘stranded’’ within the finer-grained coastal plain (e.g. [37]). Beach ridges are formed by the combination of large swell waves and high tides associated with storms. Depressions between beach ridges may be connected and form a salt flat or shallow lagoon (Fig. 3G), joined to the sea by tidal inlets that punctuate the seaward ridges. 3.3.2. Cheniers Cheniers are comprised of coarse-grained sediment deposited as a narrow linear ridge above the level of high tide but separated from the shoreline by a marshy area comprised of fine-grained sediment [38,39]. Cheniers form by reworking and erosion of the shoreface by storm waves followed by a depositional phase that leaves a ridge fronted by fine-grained sediment. Cyclical erosion and progradation of tidal flats (e.g., from successive storm events associated with varying rates of sediment supply) produces a series of parallel cheniers. Thus, grain size is a major factor differentiating cheniers from beach ridges. However, beach ridges with wide swales infilled by fine-grained sediment have been mistaken for cheniers, and hence knowledge of the subsurface stratigraphy of the coastal sequence may be required for definitive classification in many cases [40]. Due to this constraint, cheniers have not been differentiated from beach ridges in the present study. 3.3.3. Barrier islands Barrier islands are particularly abundant on wave-dominated coasts characterised by relatively large ocean swell waves, abundant marine sediment (sand) supply, lowtidal ranges, and a relatively low-gradient shelf. Barrier islands form low relief, coast-parallel offshore sediment bodies separated by narrow tidal inlets that are usually backed by a relatively shallow low-energy lagoon. There are a number of alternative theories regarding the formation of barrier islands (e.g. [41–43]). However, once the barrier island becomes sub-aerial, storm waves and aeolian processes are responsible for their continued development. 3.4. Lagoons Lagoons are formed along wave-dominated coasts by flooding of beach ridges (Fig. 3G) or by the partial closure of a coastal embayment by a subaerial barrier or
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bar (Fig. 3H). Lagoons are differentiated from wave-dominated estuaries by a lack of any significant river input [4]. The complete enclosure of the embayment by the barrier or bar may result in the formation of a (often brackish) lake. Facies in lagoons are the same as those found in wave-dominated estuaries, with the exception that there is no fluvial bay head delta (Fig. 4).
4. Geologic evolution of clastic coastal depositional environments 4.1. Estuaries and Lagoons The geological evolution of transgressive clastic coastal depositional environments is a fundamental distinguishing factor that separates them from prograding clastic coastal depositional environments (Fig. 1). With sufficient time and under a stable sea level and continuous sediment supply, estuaries will infill their palaeovalleys to form deltas (indicated by the arrows in Fig. 3). Lagoons may also infill their basins and become incorporated into prograding strand plains. Strand plains and tidal flats are formed in association with prograding coasts, at some distance from a river (i.e., they receive no direct river sediment input, see Fig. 1). 4.1.1. Wave-dominated estuaries and lagoons Wave-dominated estuaries evolve (mature) by the seaward progradation of the bay head delta and/or landward progradation of the flood tidal delta, infilling the central basin (Fig. 3C and E). Major controls on the rate of infilling are the rate of sediment supplied by the river and rate of sediment supplied by waves and tides, which is directly related to the wave climate and tidal range at the estuary mouth [32,44]. As the central basin fills its surface area is reduced, thus allowing the river channel to establish a more direct connection to the tidal inlet. As the system evolves (infills) sediment transported by the river increasingly bypasses the basin and is transported directly to the sea. Ultimately, when the central basin is completely filled, the river discharges directly through the tidal inlet to the sea, and the system becomes a delta. At this stage a coastal protuberance may form if the waves and tides are unable to redistribute the sediment at a greater rate than it is deposited at the coast. In situations where the wave energy at the mouth is relatively high, the entrance may be blocked or greatly restricted by a barrier or bar. Throughout the life of a lagoon, marine sediment may be transported through the tidal inlet by tidal currents, and across the barrier by wave and wind currents, and is deposited in the central basin. Infilling of the lagoon with marine sediment transforms the lagoon to a prograding strand plain. 4.1.2. Tide-dominated estuaries Evolution of tide-dominated estuaries is characterised by the expansion in surface area of the tidal sand banks and their merging and interdigitation with the marginal intertidal flats [21]. In macrotidal systems (i.e., those with tidal ranges >4 m), the overall funnel-shape of the estuary is preserved during all stages of evolution, and
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persists to the deltaic stage (Fig. 3B and D), as in the case of the Fly River delta in Papua New Guinea [12]. Recognising the point at which a tide-dominated estuary evolves into a delta is not trivial. Diagnostic of all deltas is the existence of a coastal protuberance at the point of river input. As the system progrades seawards, the tidal sand ridges may extend offshore from the overall trend of the coast. However, in the case of systems that are located in a large embayment or along an irregular coast, where it is difficult to determine any linear coastal trend, the protuberance may not be easy to distinguish. The existence of prodeltaic mud overlying coarse-grained shelly marine sand is diagnostic of the estuary-delta transition, but requires seismic and sediment core information that is often not available. Dalrymple et al. [17] proposed a diagnostic geomorphic criterion associated with the profile of the river channel that distinguishes estuaries from deltas. Estuaries contain a straight-meandering-straight river channel profile, which becomes straight with the onset of the deltaic stage (Fig. 3D). The more complex river channel profile found in estuaries is attributed to the convergence of the seaward-moving, rivertransported sediment and landward-moving, tidally transported sediment ([17], see Fig. 3B).
5. Environmental indicators inferred from facies models On a regional scale, the facies models for clastic coastal depositional environments provide environmental managers with important information about the functioning of individual systems. Specifically, the different geomorphic classes of clastic coastal depositional environment behave generally in a predictable manner with respect to sedimentation and oceanographic processes such as water circulation and turbidity. 5.1. Sediment trapping efficiency Sediment delivered into clastic coastal depositional environments may have different fates in terms of its retention or export, depending on whether a particular environment is progradational or transgressive (Fig. 1). This phenomenon is known as the ‘‘trapping efficiency’’ (TE), and is calculated from the combined volume of river and marine sediment delivered to the waterway (Qr+Qm) minus the volume of sediment exported offshore (Qe): TE ¼ ðQr þ Qm QeÞ=ðQr þ QmÞ 100%: Considering that the fate of all particle-associated contaminants is linked to the dispersal and deposition of fine-grained material [45], the management implications of the trapping efficiency are immediately obvious. Sediment delivered to deltas is exported from the distributary channels to the adjacent marine environment (i.e., prodeltaic deposition) and hence the trapping efficiency of the distributary channels is low (Fig. 5). This is essential for the channel
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Type of Coastal Environment
Sediment Trapping Efficiency
Turbidity
Circulation
Habitat Change due to Sedimentation
Tidedominated Delta
Low
Naturally High
Well Mixed
Low Risk
Wavedominated Delta
Low
Naturally Low
Salt Wedge/ Partially Mixed
Low Risk
Tidedominated Estuary
Moderate
Naturally High
Well Mixed
Some Risk
Wavedominated Estuary
High
Naturally Low
Salt Wedge/ Partially Mixed
High Risk
Tidal Flats
Low
Naturally High
Well Mixed
Low Risk
Strand Plains
Low
Naturally Low
Negative/ Salt Wedge/ Partially Mixed
Low Risk
High
Naturally Low
Negative/ Well Mixed
High Risk
Lagoon
Fig. 5. Diagram showing the 7 types of coastal depositional environment in relation to the 4 management issues discussed in this paper: sediment trapping efficiency, turbidity, water circulation and habitat change due to sedimentation.
to maintain a cross-sectional area that is in hydraulic equilibrium with the combined river and tidal flows. Drainage channels of tidal flats and strand plains act only to redistribute sediment within the coastal zone. They are, therefore, also in hydraulic equilibrium with the channels carrying a flow volume determined by water-tide drainage, and hence they do not act as a net sink for coastal sediment (Fig. 5). By contrast, sediment supplied to immature lagoons and estuaries is largely trapped in the central basin, and very little escapes offshore (i.e., Qe is small or negligible). For lagoons, the river sediment input (Qr) is negligible, and the only source is from the adjacent marine environment (Qm). Blind estuaries will trap 100% of all river sediment, until such time as a flood event cuts a new inlet through the barrier and flushes the central basin. Even these flood events may not dislodge contaminants trapped within the accumulated and buried fine-grained sediment in the central basin (e.g. [46]). Examples of the environmental problems associated with poorly flushed, wave-dominated and blind estuaries are more fully discussed by Day [31], Hodgkin and Hesp [47] and Heggie and Skyring [48]. Generally, wave-dominated estuaries have greater trapping efficiencies than tidedominated estuaries (Fig. 5). This is because the low-energy environment of the
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central basin provides suitable conditions for sediment deposition and retention (Fig. 3E). An important management consideration is that as wave-dominated estuaries mature and the central basin fills with sediment, the trapping efficiency is reduced due to the increased connectivity between the river channel and tidal channel(s). Shoaling in the central basin also makes it easier for internal windgenerated waves to resuspend material from the bed (e.g. [49]). During high flows and flood events, fine-grained sediment may bypass the central basin entirely and be transported offshore, carried in buoyant, turbid, freshwater plumes. Gradually, with continued infilling and increased connectivity, the estuary loses a greater percentage of its river sediment load to the adjacent shelf, and the trapping efficiency decreases even further. Eventually the river channel merges with the tidal inlet and all of the river derived sediment exits the system at which point the estuary evolves into a delta with low trapping efficiency (e.g. [50,51]). By contrast, tide-dominated estuaries are typically highly energetic throughout their length (Fig. 3D). Continual reworking by strong flood and ebb tidal currents prevents significant amounts of fine-grained sediment accumulating in the channel(s). Instead, fine-grained sediment is deposited as intertidal flats that are spread out along the margins of the channel(s). This marginal deposition prevails throughout the evolution of tide-dominated estuaries and does not vary significantly with maturation. Storm events and wind-driven circulation can cause advection of turbid estuarine water onto the adjacent shelf, causing sediment to be exported seawards. Thus, the sediment trapping efficiency of tide-dominated estuaries is moderate (as compared to a wave-dominated estuary of similar Qr+Qm). An important consequence of the high-energy conditions in tide-dominated systems is the significant recycling of large volumes of fine-grained sediment and intermixing of river- and marine-derived sediment, resulting in the dilution of anthropogenic contaminants (e.g. [52]). 5.2. Turbidity The presence of strong tidal currents in tide-dominated estuaries, tidal flats and deltas means that these systems are naturally turbid, with total suspended solids attaining several grams per litre in some macrotidal systems (e.g. [53], Fig. 5). In contrast, turbidity levels in wave-dominated estuaries and lagoons are naturally low because the central basin is relatively protected from vigorous wave action and tidal currents by the barrier. Elevated turbidity levels may be present in wave-dominated estuaries and lagoons where wind-generated waves inside the estuary resuspend finegrained sediment from the bed of shallow central basins (e.g. [49]). Although turbidity is naturally lower in wave-dominated deltas than in tide-dominated deltas, elevated turbidity will obviously occur in both systems during peak river discharge events. A common feature of most tidally influenced estuaries and deltas (e.g. [22]) and some wave-dominated estuaries (e.g. [54]) is a ‘‘turbidity maximum’’. This is a naturally occurring phenomenon and should not be confused for elevated turbidity levels associated with anthropogenic increases in sediment runoff. In systems that
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contain turbidity maxima (e.g. [55]), deformation of the tidal wave as it propagates landward causes a shorter, but stronger flood tide current that achieves a greater competency in transporting sediment than the weaker, but longer ebb tide current, and hence there is a net landward transport of sediment. This process persists landwards from the mouth until a location is reached where the seaward-flowing river flow counterbalances the tidal asymmetry. This is the location of the estuary turbidity maximum [22]. Increased levels of turbidity in clastic coastal depositional environments are an important water quality issue for environmental managers because of the limitations they present for normal photosynthesis and the impacts this has on seagrass habitat and phytoplankton viability (e.g. [56]). The turbid water is also aesthetically displeasing. Whereas turbidity can be used as a water quality indicator in coastal waters that are normally ‘‘clear’’, it is not a useful measure in systems that have naturally high turbidity levels. With this knowledge, established from the geomorphic classification, a decision as to the appropriateness of turbidity as a health status criterion for any clastic coastal depositional environment can be quickly ascertained. For example, persistent, relatively high turbidity levels in a deep-water, wave-dominated estuary might be an indicator of anthropogenic impact such as catchment clearing (e.g. [57,58]). 5.3. Water circulation Each type of clastic coastal depositional environment has a characteristic water circulation pattern that is produced by the mixing of freshwater by wave and tidal processes. Wave-dominated estuaries and wave-dominated deltas are typically characterised by stratified, partially mixed or salt-wedge circulation because the barrier-tidal inlet system restricts significant mixing of the fresh and saltwater masses [59]. However, coastal waterways located in hot, arid regions where evaporation rates exceed precipitation and runoff may possess negative (reverse) circulation patterns, in which dense, saltier bottom water flows seawards and is replaced by fresher, surface ocean water (e.g. [60,48]). Because of high-energy conditions, tidedominated estuaries and deltas tend to be well mixed. Tidal flats, strand plains and lagoons do not receive any significant river input, hence tidal creeks and lagoons are also prone to negative circulation in hot, dry climate zones. The circulation pattern is a crucial factor governing water quality in clastic coastal depositional environments. More importantly, the mixing rate and flushing efficiency are important management considerations, because they are related to a system’s susceptibility to contaminants received from the catchment (e.g. [48]). The capacity of a system to reduce the detrimental effects of introduced contaminants is directly related to the energy available to disperse and dilute them. Hence, more energetic, well-mixed, tide-dominated systems are better able to disperse and dilute introduced contaminants than low energy, stratified, wave-dominated systems. Since many contaminants are associated with fine-grained sediment [45], the principal sinks for the contaminants will be the central basin in wave-dominated estuaries and lagoons, and the intertidal flats in tide-dominated estuaries and deltas.
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5.4. Habitat change due to sedimentation Sedimentation in clastic coastal depositional environments may result in two quite different changes in habitats, depending on whether a particular environment is prograding or transgressive in nature (Fig. 1). Sedimentation along prograding coasts merely results in the seaward translation of facies as the entire succession advances seawards into the marine basin. However, along transgressive coasts, continued sedimentation results in a change in the configuration of habitats in clastic coastal depositional environments as sediment is deposited in the receiving basin (Fig. 3). Since the end of the Holocene sea level rise, when incised valleys and embayments had their maximum volume, all wave- and tide-dominated estuaries and lagoons have infilled to some extent. As the estuary matures, the configuration of the habitats will alter, with attendant reduction in the overall diversity of habitat types [7]. Rocky shores and offshore reefs become buried as the estuary infills. The infilling of wave-dominated estuaries and lagoons eventually results in the loss of the central basin (Fig. 3E). Hence, these systems are the most vulnerable to infilling and have the greatest risk of habitat changes due to sedimentation (Fig. 5). An important management consequence of maturation of these systems is that, as the configuration of habitats alters towards the deltaic stage, there is a concomitant reduction in the overall species diversity [7]. Recent mapping of habitats in tide-dominated estuaries around Australia [3] indicates that the coarse-grained facies (e.g., tidal sand banks) typically move offshore during late stage maturity. Thus, it is inferred that these systems have some risk to habitat change due to sedimentation.
6. Application: case study from Australia A database containing physical information for 780 Australian coastal waterways was created by Bucher and Saenger [61,62] and later updated and modified by Digby et al. [63]. The most recent version of this database (now known as the Australian Estuarine Database, AED) was produced by Geoscience Australia and includes information on over 1000 coastal waterways that may be accessed over the Internet (http://www.ozestuaries.org). The AED includes an independent assessment of the geomorphology of 780 clastic coastal depositional environments undertaken by Heap et al. [3] via a visual inspection of aerial photographs, LANDSAT TM images, maps, and nautical charts (see acknowledgements). Each waterway was classified, using the principles of Boyd et al. [4] and Dalrymple et al. [17], as wave- or tidedominated estuaries, wave- or tide-dominated deltas, lagoons, strand plains, tidal flats or lagoons (Fig. 1). Only the visible geomorphology was used to determine the classification [3]. In total, 721 clastic coastal depositional environments were identified with 59 ‘‘mixed/other’’ classes (e.g., embayments). Of the 721 clastic coastal depositional environments, tidal flats are the most common (n ¼ 273), followed by
Wave-estuary 145 Wave-delta 81 Strand plain 43 Tide-estuary 99 Tide-delta 69 Tidal flats 273 Lagoon 11 Trapping efficiency Turbidity Circulation Habitat change
114 (79%) 27 (33%) 9 (21%) 2 (2%) 0 12 (4%) 8 (73%) High Naturally low Stratified (negative) High risk
Total number Southeast Coast 18 (12%) 1 (1%) 0 0 0 0 1 (9%) High Naturally low Stratified (negative) High risk
Southwest Coast 0 4 (5%) 4 (9%) 51 (52%) 11 (16%) 95 (35%) 1 (9%) Low to moderate Naturally high Well mixed Moderate to low risk
Northwest Coast
Table 1 Number (and percentage) of coastal depositional environments listed by geographical area
7 (5%) 14 (17%) 21 (49%) 17 (17%) 24 (35%) 80 (29%) 1 (9%) Low Low to high Both mixed and stratified Generally low risk
6 (4%) 35 (43%) 9 (21%) 29 (29%) 34 (49%) 86 (32%) 0 Low Low to high Both mixed and stratified Generally low risk
Gulf of Carpentaria Coast Northeast Coast
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t
as
o tC
rt
No
st ea rth No
Gulf of Carpentaria
s
e hw
Co t
as
Desert or no drainage to the sea
t as Co st
t
ea
ut
Tide-dominated deltas Tide-dominated estuaries Tidal flats Wave-dominated deltas Wave-dominated estuaries Strand Plains Lagoons
he
as
uth
as
Co
So
tC
oa
So
st
we
uth
So MurrayDarling
Great Australian Bight
st
Fig. 6. Distribution of 721 different coastal depositional environments around Australia, based on the interpretation of air photographs and LANDSAT imagery. The 59 ‘‘mixed/other’’ coastal environments are not shown. Five regions are identified, based on the distribution of coastal depositional environments, each having a unique set of basic management guidelines (see also Table 1).
wave-dominated estuaries (n ¼ 145), tide-dominated estuaries (n ¼ 99), wavedominated deltas (n ¼ 81), tide-dominated deltas (n ¼ 69), strand plains (n ¼ 43) and lagoons (n ¼ 11; see Table 1). The spatial distribution of these 721 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 (Fig. 6, Table 1). The southeast and southwest coasts are wave-dominated environments, whereas the northern coastal areas (northwest, Carpentaria and northeast) are mainly tide-dominated (Figs. 2 and 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. 6).
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From this analysis, it is possible to make some generalisations about the sediment trapping efficiency, turbidity, water circulation, and habitat change due to sedimentation related to the seven types of clastic coastal depositional environments (Fig. 5). Since the southeast and southwest coasts of Australia are characterised by mainly wave-dominated estuaries ([5,47,64], Table 1), it is implied that these coasts have relatively high sediment trapping efficiencies with naturally low turbidity levels. The conceptual facies model implies that water circulation within the coastal waterways of these regions is mainly stratified, with negative (saline water) circulation occurring locally during times of very low precipitation and high evaporation. It is further implied that coastal waterways along these coasts are at a high risk of habitat change due to increased sedimentation. The northwest coast of Australia (Fig. 6) is characterised mainly by tide-dominated estuaries [65] and prograding tidal flats, with only a few deltas ([66], Table 1). Therefore, the overall trapping efficiency is moderate to low in this region. Since the clastic coastal depositional environments along this coast are all strongly tidally dominated, it is inferred that turbidity levels are naturally high and that water circulation is generally well mixed (Fig. 5). The risk of habitat change due to sedimentation is moderate to low, reflecting the generally stable geomorphic configurations of tide-dominated estuaries and tidal flats, respectively. The Gulf of Carpentaria coast contains a broad mixture of clastic coastal depositional environments and hence any generalisations of their behaviour must take into account this high degree of natural variability. It is evident that prograding depositional environments (deltas, strand plains and tidal flats) dominate [40,67,68]), and hence the overall sediment trapping efficiency of the coastal waterways is inferred to be low. The progradational nature of the coast implies that the risk of habitat changes due to sedimentation is low. Tide-dominated environments are slightly more abundant than wave-dominated environments and hence it is inferred that the overall turbidity of coastal waterways might be expected to be relatively high, but occurring over a wide range. Similarly, the coastal waterways in this region are usually generally well mixed, with some systems exhibiting stratification and possibly negative (saline) circulation, particularly wave-dominated estuaries during periods of very low river runoff and high evaporation. The northeast coast of Australia also contains a broad mixture of clastic coastal depositional environments. Consequently, their response to sedimentation and evolutionary behaviour on a regional scale will exhibit a high degree of variability. It is evident that prograding depositional environments (deltas, strand plains and tidal flats) dominate, but there are also a large number of tide-dominated estuaries (n ¼ 29; e.g. [69]) and hence the overall sediment trapping efficiency of the coastal waterways is inferred to be low to moderate (see also [70]). The progradational nature of this region implies that the risk of habitat changes due to sedimentation is generally low. Tide-dominated environments are slightly more abundant than wavedominated environments and hence it is inferred that the overall turbidity of coastal waterways might be expected to be relatively high. The high proportion of wave-dominated deltas in this region (43%), however, would be expected to result in
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a region consisting of many systems with naturally low turbidity levels. Due to the strong tidal influence in the region, it is inferred that the coastal waterways are generally well mixed, although the wave-dominated deltas are likely to be stratified during periods of very low river runoff.
7. Summary and conclusions Clastic coastal depositional environments, defined on a geomorphic basis, are associated with important environmental processes that are relevant for environmental management. Prograding coastlines characterised by deltas, strand plains and tidal flats, export most of their sediment loads to the sea and have a generally low sediment trapping efficiency. They contain a suite of habitats that will not be significantly affected by sedimentation. In contrast, transgressive coastlines characterised by estuaries and lagoons have a high sediment trapping efficiency. They are, therefore, more susceptible to the accumulation of particle-associated contaminants such as heavy metals. They also contain a suite of habitats that will change (evolve) as they infill with sediments, and are therefore more susceptible to catchment perturbations that affect river sediment loads. The water contained in tide-dominated deltaic distributary channels, tidedominated estuaries and creeks that drain intertidal flats is naturally turbid and generally well mixed. In contrast, water contained in wave-dominated deltaic distributary channels, wave-dominated estuaries and lagoons is naturally clear (low turbidity) and exhibits mainly stratified (estuarine) circulation patterns. In Australia, where river run off is generally low by global standards, this circulation is often of the inverse (negative) type, driven by high evaporation rates. From a management perspective, therefore, human activities that give rise to higher turbidity levels are likely to have a greater impact in wave-dominated systems (that are naturally clear) than in tide-dominated systems (that are naturally turbid). An assessment of 780 Australian clastic coastal depositional environments allows for a coastal regionalisation, with management guidelines being identified for each region. The same approach (of using a database of coastal environments) could be applied to any region on earth where clastic coastal depositional environments may be identified from remotely sensed imagery.
Acknowledgements The authors are grateful for the financial support of the National Land and Water Resources Audit for this work. We thank Dr. David Heggie, Dr. Brendon Brooke and David Ryan of Geoscience Australia for their insightful comments on an earlier version of the manuscript. Line drawing maps of all 780 Australian waterways [61] can be viewed at: www.ea.gov.au/coasts/information/reports/estuaries/index.html. This manuscript is a contribution of the Cooperative Research Centre for Coastal
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Zone Management and is published with permission of the Executive Director, Geoscience Australia.
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