Wave influences on river-mouth depositional process: Examples from Australia and Papua New Guinea

Estuarine and Coastal z~Iarine Science (I98o) xI, 263-277

Wave Influences on River-mouth Depositional Process: Examples from Australia and Papua New Guinea

L. D. Wright Coastal Studies Unit, Department of Geography, University of Sydney, Sydney, New South lVales zoo6, Australia

B. G. Thorn Department of Geography, University of New South Wales, R.llLC. Duntroon, Canberra, A.C.T., Australia

and

R. J. Higgins* Bougainville Copper Ltd, Pauguna, Bougalnville, Papua New Guinea Received 7 February I979 and in revlsed form 17 October I979

Keywords: river deposition coast; deltas; river plumes; sediment transport; waves; bar; Australia east coast; New Guinea Field observations of river-mouth effluent dynamics and resulting patterns of sediment transport have been replicated in and seawards of the mouths of the Shoalhaven River (New South Wales, Australia) and Jaba River (Bougainville, Papua New Guinea). T h e mouths of both rivers are strongly influenced by wave processes. At both river mouths, breaking waves cause intense mixing between river and sea waters while wave-induced momentum flux and setup oppose outflow. Rapid deceleration and lateral expansion result, creating broad crescent-shaped bars near the outlets. Subaqueous levees assume the form of broad shoals surmounted by shoreward-migrating swash bars. River water with high suspended load remains trapped by waves along the beach on either side of the river mouths. In the case of the Jaba, fines accumulate in the trough and wave reworking of bar sands leads to a succession of low beach ridges separated by mud-filled swales. T h e seawardprotruding accumulations at both river mouths cause wide, dissipative surf zones and pronounced shore-normal and shore-parallel gradients in radiation stress and setup. This results in a flow of water away from the locus of maximum deposition toward adjacent regions of lower setup; flow then turns seaward as large-scale rips. The rips create pronounced delta-margin erosion and result in arcuate embayments flanking the river-mouth bulge. U n d e r low stage conditions the low-gradient lower course of the Shoalhaven behaves as a partially-mixed estuary. Breaking waves enhance flood-tide currents and inhibit ebb currents. Bar sands migrate shoreward and enter the estuary producing an elongate shore-normal sand body in the form of channel fill and estuarine shoals. *Present address: School of Civil Engineering, University of New South Wales, Kensington, New South '~Vales, Australia.

263 o3o2-3524[8o[o9o263+x5 $oz.oo[o

~ x98o Academic Press Inc. (London)Ltd.

z64

L. D. Wright, B. G. Thorn & R. J. Higgins

Introduction Deposition at river mouths involves interactions between riverine and marine processes. River-mouth deposltional patterns are controlled by at least three primary effluent processes on which modifications by tides or waves are superimposed to different degrees (Wright, x977a ). The three primary processes are turbulent jet diffusion, turbulent bed friction, and bouyant expansion. In low-energy environments the primary processes dominate (e.g. Mississippi: see Wright & Coleman, i974). However, many river mouths experience either strong tidal influences or direct attack by powerful waves, or both. The depositional products of tlde-dominated river mouth processes have been discussed by Wright et al. (x973, x975) and Wright (x977a), while Wright & Coleman (I972, x973) have discussed larger-scale delta morphologies in relation to the magnitude of river versus wave forces. In this paper we discuss wave and river-mouth interactions with reference to two wave-dominated but othe~'~.ise dissimilar river mouths: (i) the mouth of the Shoalhaven on the tectonically stable, high-energy coast of southeastern Australia; and (ii) the mouth of the Jaba River on the tectonically active, moderate energy coast of Bougainville, Papua New Guinea. Outflow behaviour, estuarine circulation, wave and tide characteristics, and resultant morphologlc and sedimentologic patterns, were observed at the mouth of the Shoalhaven over the period August x974 through January I976; observations of sediments and morphology continued to x978. Effluent processes and morphologie patterns at the mouth of the Jaba were observed in August x977. These data are supplemented by 6 years of daily observations of water and sediment discharge and periodic delta surveys by Bougainville Copper Ltd. In the Shoalhaven, bathymetrie surveys were made with a Raytheon Model DE73t echo sounder. Autolab salinity-temperature meters were used for salinity-temperature profiling and Toho-Dentan direct reading current speed and direction meters were used for current profiling in both deltas. Larger-scale circulation patterns in the Shoalhaven were determined using drogues and dye and by repeated aerial photo reconnaissance. Tidal and river stage data from the lower Shoalhaven were recorded by means of five Bristol tide gauges. Wave characteristics from the Shoalhaven were determined from data collected by the ~,iaritime Services Board of New South Wales (see Lawson & Abernethy, I975). Wave statistics for the Jaba are based largely on summaries of shipboard observations and hindcasting (Morgan, I973).

River-mouth processes: general principles Primary processes and tidal effects Diffusion of a river-mouth effluent into sea-water and its consequent patterns of deceleration are influenced by one or more of the three primary processes (Wright, x977a). In most cases turbulent jet diffusion is operative. Eddies generated at the free boundaries of the turbulent jets are responsible for fluid and momentum exchange between the effluent and basin waters and for expansion, mixing and deceleration of the effluent (e.g. Bates, x953; Wright & Coleman, x974). In the second process, outflow velocities and bed shear stress are high relative to water depth and turbulent bed friction considerably enhances effluent deceleration and lateral expansion rates (e.g. Borichansky & Mikhailov, x966; Wright & Coleman, x974; Wright, x977a ). Friction is normally accompanied by lateral turbulent jet diffusion. Third, in cases where low tidal ranges and deep outlets favour strong density stratification within the lower reaches of the channel in the form of salt-wedge intrusion, buoyant forces may dominate effluent behaviour (Wright & Coleman, x97x, x974). The

Wave influences on rlver.mouth depositional processes

265

pronounced vertical density stratification associated with buoyant effluents eliminates friction between the effluent and the bed and inhibits the generation of turbulent eddies. Lateral effluent spreading and consequent vertical thinning result largely from the pressure gradients created by the relative superelevation of the lighter fresh effluent water; deceleration is gradual. Buoyancy also influences the gradual spreading and dispersion of slow-moving partially mixed river water in regions offshore from the river mouth following initial mixing and deceleration at the outlet. Where tidal range is significant relative to river discharge, turbulent mixing due to tidal currents wholly or partially reduces vertical density gradients subduing the effects of buoyancy; in macrotidal rivers, intense tidal mixing causes vertical homogeneity Wright et aL, r973, 1975). When tides account for a greater fraction of sediment-transporting fluid power than does the river in the lower river course and at the mouth, bidirectional transport results, occasionally with a net upstream component. Processes of reave-river mouth interaction Significant wave attack causes appreciable modification of effluent processes as well as postdepositional redistribution of the river-mouth sediments. In addition, waves are altered by the effluent and wave-current interaction processes are generated. These include modification of wave behaviour by the effluent and by the resulting river-mouth accumulations. River-mouth outflow contra to wave incidence shortens wave length and reduces absolute wave phase speed (see Jonsson et aL, I97o). The effect is to steepen incident waves, increase the shoaling coefficients and cause breaking in water depths greater than would otherwise be the case. Phase speed reduction by the outflow combined with the effects of shallowing around the convex-seaward river-mouth deposits enhances refraction so as to concentrate wave energy flux in the effluent region. High turbulent viscosity which results from the steep breaking waves promotes intense vertical and lateral mixing and momentum exch,'mge between effluent and ambient waters within the surf zone. Wave-induced shoreward radiation stress (excess momentum flux due to the waves) (Longuet-Higgins & Stewart, 1962, 1964) and shoreward mass transport oppose and may locally impound outflow (Wright, x977b). Increased wave-energy dissipation over protruding river-mouth deposits causes pronounced shore normal and shore parallel gradients in radiation stress and results in pronounced setup over the rlver-mouth bulge. Finally, wave reworking of ovexflattened river-mouth deposits causes shoreward return of sand over shallow subaqueous shoals (Wright, 1977b). Explicit mathematical treatment of the theory of wave-current interactions is offered by Jonsson et aL (197o) and Noda (1974). Bruun's (x978) detailed discussion of tidal inlet processes is also relevant.

Environments o f wave-domlnated river mouths Wave-dominated river mouths may be found in many regions of the world. The most obvious requirement is for significant shore-incident deepwater wave energy and hence, for comparatively long fetches and direct exposure of the river mouth to the open ocean. Equally important, however, are the slope and width of the inner continental shelf and nearshore zone. Very flat offshore slopes dissipate the energy of waves which reach the river mouth; river mouths fronted by steep nearshore profiles receive the full, relatively unattenuated effects of waves and are most likely to exhibit wave-dominated river mouths (Wright & Coleman, 1973). The wave energy flux which reaches the river mouth must also be considered relative to the power of the river outflow and its ability to transport sediment to the coast. Wave energy flux necessary to maintain a wave-dominated condition must increase with increasing

266

L. D. Wright, B. G. Tltom & R . J . Higgins

river discharge. Wave-dominated river mouths are thus most commonly associated with coastal and shelf regimes where the rate of sediment reworking by marine processes substantially exceeds the rate of sediment supply by rivers. The Shoalhaven The Shoalhaven River delta and estuary (Figure I) are situated on the tectonically stable coast of New South Wales, Australia, approximately xSO km south of Sydney. Although the

Figure x. The $hoalhaven Delta. river is small by world standards, it is the largest on the south coast of New South Wales. The catchment area is 725 o km 2 and average annual rainfall is76o mm. The mean discharge rate at the Nowra Bridge near the head of the delta is only 57"3 m3 s-X; however, flows of over 5ooo m 3 s - x occur at a return interval of xo years. Flows exceeded this magnitude in x974 and x975. The bed load of the river consists of fine to medium sands of mostly angular quartz, feldspar and rock fragments. During floods the river also transports high concentrations of suspended silts and clays. The Shoalhaven has built a Holocene deltaic plain 85 km ~-in area, extending from Nowra to the coast. At the present time the discharge of the Shoalhaven enters the sea via two outlets: the Crookhaven entrance, which is situated in a relatively protected environment in the lee of Crookhaven Heads, and the Shoalhaven entrance to the north which is exposed to the full spectrum of wave conditions (Figure I). The river debouches into Shoalhaven Bight, a broad, arcuate embayment bordering the Tasman Sea. The nearshore bed is relatively steep, with

Wave influences on flyer-mouth depositional processes

267

an average gradient of o°45 ' and a concave-upwards profile. Water depths of 2o m occur within x. 5 km of the present shoreline. Tides are semidiurnal with a mean range of I-z m and a spring range of 1.8 m. Under low stage conditions the low gradient lower reaches of channel behave as a partially mixed estuary. Tidal influences extend approximately 2o km upstream. The combined tidal prism of the low-gradient Shoalhaven and Crookhaven estuaries is about 23 × Io ~ m a during spring tides. This exceeds the base flow by x8 times, but is only one-fifth the volume of extreme • flood discharge. River mouth and inshore processes are dominated by a relatively highenergy wave regime with a highly variable wind-wave climate superimposed on persistent long southerly and southeasterly swell. Significant wave heights of x.5, 2. 5 and 4 m are respectively exceeded for 5o%, io% and I % of the time; storm wave heights exceed xo m. The ffaba The Jaba River drains the central portion of the island of Bougainville at the northern end of the Solomon chain (Figure 2). The island lies squarely on the cireum-Pacific zone of crustal instability near the convergence of the Indian (Australian) and Pacific plates.

Figure 2. Bougainville Island and the Jaba River

268

L.D. Wright, B. G. Thorn ~ R.J. Higgins

Seismicity and vulcanism are very active (Brooks, i965). The Jaba catchment has a total area of 460 km 2 and receives an average annual rainfall of over 4oo0 mm. Discharge is relatively aseasonal and averages 4 ° m a s-1 at the head of the delta. Art elevation difference of 225o m exists between the highest point of the catchment and the delta, along a trunk stream length of 5o km. Relative to its catchment size and discharge, the Jaba has a high sediment load consisting largely of tailings and overburden from the Bougainville Copper Ltd mine at Panguna (Figure 2). Daily monitoring of water and sediment discharge and of accumulation rates reveals art average sediment discharge rate of 26 × xo6 tonnes per year reaching the coast. The river is shallow and braided all the way to its mouth. Similar situations often occur where volcanic eruptions cause rapid sediment input to steep gradient streams (e.g. the nearby Torokina River, the headwaters of which include the active volcano ~it Bagana). Jaba sediments have produced a pronounced deltaic bulge extending into Empress Augusta Bay on the western margin of the island. As in the Shoalhaven example the nearshore profile is steep and narrow. Nearshore wave-power attenuation is minimal and most of the incident wave energy is expended within the surf zone and on the beach face. Although wave data are much less reliable than for the Shoalhaven, data are sufficient to indicate that incident waves are generally less than x m; the modal wave height is o. 5 m (Morgan, I973). However, despite the lower energy, the effectiveness of waves relative to river discharge remains high for most of the year. Occasional tsunamis are an additional process not experienced by the Shoalhaven. The most recent major tsunami accompanied an earthquake in July i975 . These events have the effect of trimming back the delta and 'swamping' the beach ridges. Tides affecting the Jaba are somewhat lower than for the Shoalhaven; the nearest tide station (Anewa Bay) shows a maximum spring range of x.5 m. In contrast to the Shoalhaven, the steep stream gradient and shallow bed of the Jaba River excludes salt water and tidal influences from the mouth and lower channel. Therefore, there is no tidal prism in the lower Jaba, and undiluted fresh water is discharged directly into the sea.

$hoalhaven rlver-mouth processes Flood-stage processes A low stream gradient, relatively large tidal prism, and low base flow, result ha seaward discharge of Shoalhaven River sediments only when the river is ha flood and sea-water is flushed from the estuary. During extreme flood conditions in August x974 and in Jdne x975 peak discharge rates were respectively 7400 m 3 s - t and 69oo m 3 s - l , both of which substantially exceeded the' xo year' flood discharge rate. A comprehensive data set was obtained during the I975 flood. A flood peak of 5.o9 m at Nowra was accompanied by a peak at the mouth (Shoalhaven Heads) of 2.i 5 m. Salt water was completely flushed from both the exposed and sheltered outlets although tidal rises and falls persisted within the extreme lower reaches of the channel causing variations in outflow speeds with tidal phase (Wright, 197'73). Surface outflow velocities of over 2 m s - I together with negliglble density stratification produced a very high densimetric Froude number (>20) at the outlet, so that outflow was fully turbulent. Bed shear stresses were large, and pronounced seaward transport of bed load was evident from the presence of seaward-migrating dunes with amplitudes of 5o-xoo cm. As is common on the southeast coast of Australia, the same atmospheric low pressure cell which brought the flood also generated powerful waves; the significant deepwater wave height (recorded off Botany Bay) just prior to the arrival of the flood crest was 7 m with a peak period of x3 s. Wave-induced setup and increased effluent diffusion and deceleration

269

Wave influences on river-mouth depositional processes

at the exposed Shoalhaven Heads outlet were among the most obvious effects of high-wave energy on flood-stage outflow. During the 7 m significant wave height peak, excess setup at Shoalhaven Heads was at least 60 cm (Wright, z977b ). With more moderate wave conditions wave influences are still important. Figure 3 shows the seaward current deceleration, density structure and wave behaviour across the Shoalhaven Heads bar on 28 June x975, associated with deepwater significant wave heights of about 2 m. Maximum velocities decreased from I"5 m s-a at the entrance throat to 2o cm s-1 over a distance of two channel widths (25o m). This deceleration rate must be attributed to wave effects since it is appreciably greater than that predictable from the theory of turbulent or buoyant jets. The role of the waves in impeding outflow is most readily apparent from the plan view surface outflow pattern shown in Figure 4. Waves breaking over the outer crest of the bar abruptly blocked the effluent and resulted in the emergence of low-turbidity salt water over the bar front. The effluent was forced to diverge laterally alongshore. Steep velocity gradients occurred lateral to the effluent centerline. Seaward of the breaker zone turbid, partially mixed river water was dispersed northward as a thin ( < I m thick) buoyant surface layer.

I

DEPTH IN METRES •

o

:::.... :

I ....

-0.5

Zone of

ropld

m i x i n g and

I

wove InduCed

momentum f l u x / ~ Breoker {rodiotion sfressl#.~=

iiii

iiiii

-

setup

outflow velocity decreasing

iiiiiiiiiiiiii:

. .......

= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

KMS UPSTREAM FROM 1 -.25

THROAT ! 0

i.-

N

.......

+.25

Figure 3. Flood-stage effluent/wave interaction and resultant density and flow structure and cross-sectional morphology at the mouth of the Shoalhaven River (Shoalhaven Heads) as observed on 28 June I975. Low-stage processes During normal and low river stages tidal influences become more dominant and both entrances function primarily as tidal inlets. Salt water intrudes into the lower reaches of the channel and tidal mixing results in a well-to partially-mixed estuary. Marine salinities prevail at and immediately upstream of the Shoalhaven Heads entrance; salinities decrease progressively upstream, with surface salinities normally becoming fresh at xz-r 5 km upstream of the mouth. After prolonged low-stage periods, it is common for a brackish salt wedge to intrude upstream about x5 km. Flows through both outlets are largely tide-domlnated and bidirectional during low stage. However, processes of wave-induced radiation stress and setup play important roles in impeding ebb outflow and augmenting flood-tide inflow at the exposed mouth bar. The impounded water is forced to spread alongshore within the highly dissipative surf zone created by the river-mouth bulge [see Wright et aL (x979a, b) for a discussion of dissipative surf zones]. The combination of shoreward wave transport and dissipation and impoundment

270

L. D. Wright, B. G. T/tom & R. J. Higglns

Wave incidence

\

r , - - * -,, / L

Breo

~--"

N

...........

~!~ii'~l~ rlp~! "'E$;" ~ - ~ "• r "~ l ' ! "i

.~'~- ",a* :~

--~'~:

. . . . . . . . . . .

..........

\

-:*"

\e~!

-

.

............ :--::,.%

t

-"-'):;='~:J| L~

.5~:~i~/I~v'4-~

ko. ,one

.~

,::~::ii~i

~. ~

~t " ~ ~ ~ -

~

/~. ~ / SHOALHAVEN / " ~ RIVER //

• .~. • -Beach • - : • •

......~\\\ repreienls zone of ,~soltwater ernerg=nce

F i g u r e 4. Horizontal surface outflow patterns a n d p l a n - v i e w m o r p h o l o g y at the m o u t h o f t h e S h o a l h a v e n R i v e r d u r i n g flood stage, 28 J u n e x975.

of ebb outflow makes the river-mouth bulge a region of water accumulation and anomalously high setup must be inferred. The suggestion of high setup is supported by a comparison of tide-gauge records from Shoalhaven Heads and Crookhaven Heads (Wright, z977b ). As at high stage, sediment laden efftux spreads alongshore within the surf zone fronting the adjacent Seven Mile Beach and ultimately escapes seaward by way of large-scale rips which flank the river-mouth accumulations. These rips occupy regions of net seaward transport which occur on either side of the river-mouth deposits although the one on the (northern) downdrift side is the best developed. This region is situated about x-5-2.o km north of Shoalhaven Heads; it is an area of chronic shoreline erosion where a large delta-margin embayment has been created. Similar delta-margin embayments occur adjacent to most tidal inlets and river mouths on the New South Wales coast (Wright, z98o). Waves breaking at the mouth also cause asymmetry between flood and ebb transport through Shoalhaven Heads• This asymmetry results from the fact that ebb discharge is impeded while upstream flood transport is increased by waves. Continuity is maintained in the estuary by increased ebb discharge through the Crookhaven outlet which also accommodates most of the river's base flow. The lower Shoalhaven below the junction with the Crookhaven arm, is thus partially isolated from the base flow and is a region of net upstream (flood-tide dominated) transport during the low stage.

Shoaihaven river-mouth and estuarlne depositional patterns Ri~'er-mouth deposition and morphology River-mouth morphology produced by flood-stage processes is illustrated in profile in Figure 3 and in plan in Figure 4. A major feature is the broad crescent-shaped river-mouth bar with a crest situated at about two channel widths seaward of the entrance throat. Bar

Wave influences on river-mouth depositional processes

z7x

form and position are related to rapid wave-induced deceleration and to post-depositlonal redistribution of sediment. Surveys in June, August and December x975 revealed that the bar consistently had two crests. The outer crest was asymmetrical with a steeper landward face suggesting shoreward migration under shoaling waves. In addition to the highly regular bar, other distinguishing features include a constricted outlet and broad subaqueous levees which widen shoreward and are surmounted by swash bars. The levees are the most extensive single depositlonal unit in terms of both area and volume. They are apparently related to the combination of steep lateral velocity gradients and to wave-reworking of sands. Wave-reworking is responsible for the well-developed swash bars which migrate shoreward and accumulate near the entrance, constricting the outlet until a balance is achieved between wave forces and concentrated outflow. The crescentic bar and wide subaqueous levees are respectively similar in form and function to the terminal lobes and ramp-margin shoals of tidal inlets (e.g. Hayes et al., i97o; Oertel, i97z ). Adjacent to the river-mouth protrusion pronounced delta-margin erosion and consequent embayment occurs, particularly on the downdrift (northern) flank. Figure 5 shows the morphology of the outlet as it appeared in January z976, after 5½ months of low-stage conditions. In the region of the entrance throat large swash bars migrate shoreward under low-stage conditions and enter the mouth as a succession of recurred ridges which substantially reduce the width of the outlet. When conditions of low river flow and wave dominance persist for prolonged periods, bar sands continue to migrate shoreward in an attempt by the waves to eliminate the overflattened river-mouth bulge. This narrows the throat and chokes the outlet, causing outflow to be increasingly diminished. Occasionally, shoreward migration of bar sands completely seals off the Shoalhaven Heads entrance. Once sealed the mouth remains closed until breached by the next flood. Deposition in the lower Shoalhaven estuary Pronounced lithologic and textural differences permit the sands of the open coast and inner continental shelf to be distinguished from sands derived directly from the Shoalhaven River.

T

"

,

A

S

M

A N

Wave Incidence

$ E A

:"

,..,........-

|i~~

~

,."~:,

-

------~

Figure 5. Horizontal surface outflow patterns and plan-view morphology at the mouth of the Shoalhaven River following a prolonged period of low-stage conditions, January x976.

272

L. D. IVright, B. G. Thorn & R. J. Itiggins

Whereas river sands are coarse to fine, relatively poorly sorted and angular consisting of high percentages of lithie rock fragments, feldspars and quartz, marine-reworked sands consist of well-rounded quartz grains in addition to abundant marine shell fragments (Roy, x977). These contrasts in sediment properties readily facilitate description of sediment exchange within the estuary. Above x3 km upstream from the mouth, sands are totally of riverine origin. This position is near the normal limit of marine water penetration. The large mid-channel island (Pig Island) situated xo km above the mouth (Figure x) appears to be the position of convergence of riverine and estuarine sediment. Below this point the relative contribution of nearshore sands increases. %~Iarine' sands become generally dominant within the lower estuary between Shoalhaven Heads and the Crookhaven junction indicating a significant influx of sediment through the exposed entrance. This mixed riverine/marine sediment remains stored in the lower estuary during low stage; it is largely this mixed material which is flushed seaward during high stage and subsequently returned by waves at low stage. The upstream migration of the estuarine sediments under low stage conditions is evidenced from bedform asymmetry patterns. In addition to channel fill, large estuarine shoals and islands capped by fine muds have accumulated in the lower estuary where convergence of upstream and downstream transport occurs. This has produced a significant shore-normal sand body occupying the lower xo km of the river. The sand body is distinguished from the open-coastal or purely riverine deposits by its mixed sediment suite and bidirectional cross-bedding.

River-mouth processes and depositional patterns in the Jaba Delta River-mouth processes Most of the wave--effluent interactions as described for the Shoalhaven during high stage are operative at the mouth of the Jaba year round. However, the Jaba contrasts with the Shoalhaven with respect to its saturated sediment load, shallow channel and river-mouth depths, and high gradient. Tidal effects and salt water are completely excluded from the mouth and lower channel at all times. The Jaba outlet is completely dominated by seaward river flow to the river-mouth bar. Water depths in the outlet average x-x'5 m whereas depths over the bar are less than I m at low tide. Hence frictional effects are pronounced. Although the Jaba experiences considerably less wave energy than the Shoalhaven, the shallower river mouth promotes a comparable degree of wave-induced turbulence and dissipation. Effluent river-water concentrations in and seaward of the main Jaba motith as observed in August x977 are shown in Figure 6. Owing to rapid and considerable day-to-day changes in bar morphology, specific river-mouth morphologic details are not shown. Convergence and mixing of riverine and marine waters takes place immediately within the breaker zone over the bar crest. The breaker zone is the region of rapid deceleration and longshore divergence of flow and of consequent rapid deposition of bed load. As at the Shoalhaven mouth, ar deposition takes place within a very short distance seaward of the outlet throat and produces a broad, arcuate bar morphology. The shallowness of the Jaba bar and outlet throat creates extreme turbulent bed friction. Frictional effects increase the lateral spreading of the effluent; this causes partial bifurcation of the outflow into two weakly developed jets separated by a triangular high on the bar surface (Figure 7)- This high is analogous to the middle ground shoals associated with friction-dominated plane jets in lower energy environments (e.g. Wright, I977a), but is highly ephemeral in position owing to wave effects. Thus the bar and the position of the main outlet are unstable. Effluent channels are subject to appreciable lateral migration and vary in number on a daily basis.

Wave bzfluenees on river-mouth depositional processes

273

Figure 6. River-water concentrations in and around the mouth of the Jaba River, t6 August x977. Note high concentrations inshore of breaker zone corresponding to wave-trapped, mud-laden fresh water.

/nefro$

LEGEND mud fiats

~ ' ~ delta-margln erosion

Figure 7. Depositional morphology and net sediment drift patterns of the Jaba Delta, August %977.

274

L. D. Wright, B. G. Thom& 12. J. Higgins

The effects of buoyancy are more pronounced than at the mouth of the Shoalhaven despite the intense mixing which occurs over the bar. This is due to the comparative narrowness of the mixing region. Seaward of the bar crest a thin surface layer of brackish water less than i m in vertical thickness spreads as a buoyant plume dispersing suspended fines over the delta front. Surface plumes of suspended sediment vary in position and extent depending on tidal phase and wind direction. The highly dissipative surf zone which prevails off the river mouth and surrounds the shoreline of the active delta causes partially mixed, sediment-laden river water to remain trapped by waves along the beach on either side of the river mouth. This tendency is evident from aerial photographs and from salinity-temperature data which reveal the presence of a high concentration of river water within the trough between the delta shoreline and the welldeveloped longshore bar 3o-5 ° m offshore (Figure 6). Salinities increase abruptly under the break point on the bar crest suggesting that the bar crest represents a discontinuity between marine and river water. This trapping process results in longshore spreading of fine-grained suspended sediment between the bar and the beach. As at the Shoalhaven Heads outlet, wave power convergence by refraction and wave dissipation and consequent setup centre on the river mouth and diminish away from the locus of maximum deposition. Water trapped inshore at and adjacent to the river mouth ultimately escapes offshore in regions of lower radiation stress marginal to the protruding deltaic lobe. Deltaic deposltional patterns Figure 7 shows the morphology of the active Jaba Delta as it appeared in I977. The delta is almost entirely the product of deposition occurring since x972 when the disposal of mine tailings began. The classic regular, areuate sediment lobe reflects the combination of frictioninduced channel switching and wave reworking and redistribution of sediments. Surveys by Bougainville Copper Ltd suggest a progradational delta front which slopes steeply to a depth of about 2o m within xooo m of the river mouth. Beyond the 25 m contour the sea floor slopes gently to a depth of 4 ° m at a distance of 6 km. XX~ajorfeatures of the deltaic plain include crescent-shaped river-mouth bars which are subject to rapid reworking into longshore bars or low beach ridges; numerous abandoned channels filled with fine sediments; and low shore-parallel beach ridges separated by mud-filled swales. ~Iuds in the swales are derived from the river water trapped shoreward of the longshore bars. On a larger scale four dynamic environments can be recognized within the area influenced, directly and indirectly, by river-mouth deposition: (i)

A region of active deltaic accumulation with its migrating distributaries and acereting beach ridges. (ii) Embayed regions of delta-margin erosion which are undergoing rapid transgression due to wave erosion of both 'old delta' and pre-delta deposits. These regions occur on either side of the active delta. As in the Shoalhaven case, delta-margin erosion is most pronounced on the downdrift (northern) margin. (iii) Immediately downdrift of the delta-margin embayment is an abandoned deltaic complex (the 'old delta') which has assumed the form of a complex spit migrating northward under the influence of the dominant southwest waves. (v) Regions of deltaic deposition and marginal erosion give way both to the south of the active delta and to the north of the old delta to steep reflective beaches (e.g. Wright et al., 1979a,b ). Evolution of the deltaic shoreline over the period July 1975-December x977 is shown in

Wave influences on river-mouth depositlonal processes

JABA

z75

DE me~e$

KEY ....... 1o-12-77

8-11-76 12-3-76 [] - - . - - 27-8-75 [] 6-7-75 original shoreline

....

Figure 8. Evolution of the shoreline of the Jaba Delta over the period July x975December z977. Figure 8. Except for a short-lived event of shoreline recession in response to a tsunami in August z975, relatively rapid progradation of the active delta is evident. Deltaic progradation has been accompanied by northward (downdrift) migration of the zone of delta-margin erosion and by a progressive reduction in size of the associated embayment. The tendency for the shoreline to become smoother with time probably represents an equilibrium adjustment in which shore-parallel gradients in radiation stress are reduced and the intensity of delta-margin erosion is gradually reduced.

Conclusions T o varying degrees all river mouths are subject to some wave influences; however, the roles of wave processes are often subdued relative to other factors. I n this paper we have examined two river mouths which would be vastly dissimilar were it not for the common denominator

z76

L. D. Wright, B. G. Thorn 6Y R . J . Higgins

of pronounced wave-effluent interactions. It is apparent that between the two cases studied, wave processes are operative together with differing contributions from tides and from primary effluent processes such as turbulent friction and buoyancy. Consequently, the overall morphology and depositional patterns of the two systems differ in several respects. However, there are some important generalities, shared by both deltas, which can be attributed to wave domination. These may be summarized as follows: I. Breaking waves intensify mixing while wave-induced radiation stress opposes outflow. This causes abrupt deceleration and lateral spreading of the effluent. 2. The above process combined with immediate post-depositional reworking of rivermouth sands produces a broad arcuate river-mouth bar within a very short distance (N two channel widths) seaward of the outlet throat. 3. Wave-reworking combined with steep lateral effluent velocity gradients produces subaqueous levees in the form of broad shoals capped by shoreward migrating swash bars. 4- Shoreward return of sand by waves tends to constrict the outlet. However, the degree of constriction appears to depend on the relative importance of turbulent friction between the effluent and the bar; friction promotes more rapid effluent expansion and hence partially counters the constricting effects of the waves. 5. Breaking waves cause partial inshore trapping and consequent longshore spreading o f turbid effluent water. 6. The regular seaward protrusions are responsible for wide dissipative surf zones and for pronounced longshore gradients in radiation stress and setup. Acting in concert with continual rivermouth deposition these hlgher-order wave-induced processes create deltamargin erosion which, in turn, produces arcuate shoreline embayments flanking the river mouth or deltaic bulge. 7. When stream gradients along the lower river course are low as in the Shoalhaven case, and estuarine conditions are able to prevail under low and normal river stages, wave activity at the river mouth assists flood-tide currents in causing upstream transport of sands to produce flood-tidal delta accumulations within the estuaries. The processes and depositional morphologies of wave-dominated river mouths, such as those described in this paper, differ markedly from those of river-dominated or tidedominated river mouths. Other notable examples of wave-dominated river mouths include the mouths of the Sao Francisco and Jequitinhonha (Brazil), Senegal (West Africa), Magdalena (Colombia), Orange (South Africa), Burdekln (Queensland, Australia) and Purari (Papua New Guinea). Acknowledgements Observations at the mouth of the Shoalhaven River were supported by the Australian Research Grants Committee (ARGC). We are grateful to Bougainville Copper Ltd, Panguna, Papua New Guinea for providing full logistic support and field assistance for observations in the Jaba Delta and to A. C. Hartley of Bougainville Copper Ltd for assistance both in the field and in other facets. References Bates, C. C. I953 Rational theory of delta formation. American Association of Petroleum Geologists Bulletin 37~ zxx9-zx6x. Borichansky, L. S. 8: Mikhailo¢, V. N. x966 Interaction of river and sea water in the absence of tides. In ,Scientific Problems of the Humid Tropical Deltas and Their Implications, UNESCO, Geneva, pp. t75-xSo.

Wave influences on river-mouth depositional processes

277

Brooks, J. A. *965 Earthquake activity and seismic rish in Papua New Guinea. Australian Bureau of 2~Iineral Resources Report No. 74. Bruun, P. 1978 Stability of Tidal Inlets: Theory and Engineerbzg Amsterdam, Elsevier. Hayes, M. O., Goldsmith, V. & Hobbs, C. H. x97o Offset coastal inlets, In Proceedings of the xeth Conference on Coastal Engineering, Washington, pp. x187-x2oo. Jonsson, I. V., Skougaard, C. & Wang, J. D. 197o Interaction between waves and currents. In Proceedings of the xeth Conference Engineering, Washington, pp. 489-5o 7. Lawson, N. V. & Abernethy, C. L. 1975 Long term wave statistics off Botany Bay. 2nd Australian Conference on Coastal and Ocean Engineering, Gold Coast, Queensland. Institute of Engineers, Australia, pp. 167-I 76. Longuet-Higgins, M. S. & Stewart, R. W. x96z Radiation stress and mass transport in gravity waves, with application to surf beats, ffournal of Fluid ~Xechanics C x3, 48t-5o4. Longuet-Higgins, M. S. & Stewart, R. W. x964 Radiation stress in water waves, a physical introduction with applications. Deep Sea Research, xx, 529-56z. Morgan, E. A. 1973 Sea-state characteristics for the Australian region. Department of Supply, Weapons Research Establishment, Technical Note No. 764. Noda, E. K. 1974 Wave-induced nearshore circulation, ffournal of Geophysical Research 79~ PP. 40974xo6. Oertel, G. F. x97z Sediment transport of estuary entrance shoals and the formation of swash platforms. Journal of Sedimentary Petrology 42, 858-863. Roy, P. S. I977 Does the Hunter River supply sand to the New South Wales coast today?j%urnal and Proceedings of the Royal Society of New Sonth Wales Ilo, I7-24. Wright, L. D. 1977a Sediment transport and deposition at river mouths: a synthesis. Bulletin of the Geological Society of America 88, 857-868. Wright, L. D. x977b Morphodynamics of a wave-dominated river mouth. Proceedings of the zSth Coastal Engineering Conference, pp. 18zx-1737. Wright, L. D. 198o Modes of beach cut in relation to surf-zone morphodynamics. Abstracts of the z7th Coastal Engineering Conference, Sydney, pp. 126-127. Wright, L. D. & Coleman, J. M. 1971 Effluent expansion and interracial mixing in the presence of a salt wedge, Mississippi River Delta.ffournal of Geophysical Research 76, 8649-8661. "~Vright. L. D. & Coleman, J. M. x972 River delta morphology: wave climate and role of the suhaqueous profile. Science, New York x76, 282-284. Wright. L. D. & Coleman, J. M. 1973 Variation in the morphology of major river deltas as functions of the ocean wave and river discharge regimes, with seven examples. American Association of Petroleum Geologists Bulletin 52, 370-398. Wright, L. D. & Coleman, J. M. 1974 Mississippi river mouth processes: effluent dynamics and morphologic development, ffournal of Geology 82, 75I'--778. Wright, L. D., Coleman, J. M. & Thorn, B. G. x973 Processes of channel development in a high tide range environment: Cambridge Gulf-Ord River Delta, Western Australia.ffournal of Geology 8I~ 15-41. Wright, L. D., Coleman, J. M. & Thorn, B. G. 1975 Sediment transport and deposition in a macrotldal river channel: Ord River, Western Australia. In Estuarine Research, Vol. z (Cronin, ed.), Academic Press, New York, pp. 3o9-32I. Wright, L. D., Chappell, J., Thom, B. G., Bradshaw, M. P. & Cowell, P. x979a Morphodynamics of reflective and dissipative beach and inshore systems: southeastern Australia. 2~[arbze Geology 3z~ xo5-14o. Wright, L. D., Thorn, B. G. & Chappell, J. 1979b Morphodynamie variability of high energy beaches. In Proceedings of the z6th Coastal Engineering Conference, Hamburg, 1978, pp. xx8o--x194.