ARTICLE IN PRESS
Continental Shelf Research 24 (2004) 2431–2454 www.elsevier.com/locate/csr
Sediment transport in distributary channels and its export to the pro-deltaic environment in a tidally dominated delta: Fly River, Papua New Guinea
Peter T. Harrisa,Ã, Michael G. Hughesb, Elaine K. Bakerb, Robert W. Dalrymplec, Jock B. Keeneb
aMarine and Coastal Environment Group Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia bSchool of Geosciences, University of Sydney, Sydney, NSW 2006, Australia cDepartment of Geological Sciences and Geological Engineering, Queens University, Kingston, Ontario, Canada K7L 3N6
Available online 11 November 2004
Abstract
Current metre deployments, suspended sediment measurements and surface sediment samples were collected from three locations within distributary channels of the tidally dominated Fly River delta in southern Papua New Guinea. Net bedload transport vectors and the occurrence of elongate tidal bars indicate that mutually evasive ebb-and flood- dominant transport zones occur in each of the distributary channels. Suspended sediment experiments at two locations show a phase relationship between tidal velocity and sediment concentration such that the net suspended sediment flux is directed seaward. Processes that control the export of fluid muds with concentrations up to 10 g l 1 from the distributary channels across the delta front and onto the pro-delta are assessed in relation to the available data. Peak spring tidal current speeds (measured at 100 cm above the bed) drop off from around 100 cm s 1 within the distributary channels to o50 cm s 1 on the delta front. Gravity-driven, 2-m thick, fluid mud layers generated in the distributary channels are estimated to require at least 35 h to traverse the 20-km-wide, low-gradient (2 10 3 degrees) delta front. The velocities of such currents are well below those required for autosuspension. A 1-month time series of suspended sediment concentration and current velocity from the delta front indicates that tidal currents alone are unable to cause significant cross-delta mud transport. Wave-induced resuspension together with tides, storm surge and barotropic return-flow may play a role in maintaining the transport of fine sediment across the delta front, but insufficient data are available at present to make any reliable estimates. r 2004 Published by Elsevier Ltd.
Keywords: Bedload; Sediment transport; Suspended load; Autosuspension; Hyperpycnal flow; Turbidity; Mutually evasive; Deltaic; Tide-dominated
ÃCorresponding author. Tel.: +61-2-6249-9611; fax: +61-2-6249-9920. E-mail address: [email protected] (P.T. Harris).
0278-4343/$ - see front matter r 2004 Published by Elsevier Ltd. doi:10.1016/j.csr.2004.07.017 ARTICLE IN PRESS
2432 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454
1. Introduction: Fly River delta workers, including Taylor (1973), Spencer (1978), Alongi et al. (1991, 1992), Harris et al. (1993), The Fly River delta is situated in southwestern Baker et al. (1995), Baker (1999) and most recently Papua New Guinea, along the western margin of by Dalrymple et al. (2003). The delta contains the Gulf of Papua (Fig. 1). The Fly River and its three main distributary channels (the Southern, main tributary, the Strickland River, drain the Northern and Far Northern Entrances) that mountains (peak elevation of 4000 m) of the island branch from a common point (the ‘‘apex’’; see of New Guinea. The Fly ranks as the 17th largest Fig. 1). The distributary channels are 5–15 m in river in the world based on its pre-industrial depth, separated by elongate, sand-mud islands sediment discharge of 85 million tonnes a 1, due to that are stabilised by lush mangrove vegetation abundant rainfall in the highlands which exceeds (Robertson et al., 1993). The islands are eroded 10 m a 1 (Harris et al., 1993). This sediment and rebuilt rapidly in the apex area, where they discharge is notable given the modest size of the have lateral migration rates of up to 150 m a 1, catchment area (79,000 km2), and is characteristic with slower rates for the more seaward islands of the wet, mountainous islands in the south- (Baker, 1999). The comparison of bathymetric western Pacific (Milliman and Syvitski, 1992). charts and remotely sensed imagery from the last The Fly delta exhibits a distinctive funnel shape 50 years suggests that, while the number of islands in plan view, attesting to its tidal dominance (e.g. has changed and islands have moved, the overall Galloway, 1975). Mean spring tidal ranges are area of the islands and distributaries has remained amplified within the delta, from around 3.5 m at constant (Baker, 1999). The implications of this the seaward entrance of the distributary channels, observation are that the overall shape of the reaching a peak of about 5 m at the delta apex. Fly distributary channels is in dynamic equilibrium delta sediments have been described by several with the tidal regime, and that seaward prograda-
Fig. 1. Location of the Fly River delta in southern Papua New Guinea. The delta plain, delta front and pro-delta geomorphic zones are indicated. The three survey areas shown in Fig. 3 are also indicated. ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2433 tion of the delta front is balanced by the seaward inland, where the river flows along the low- translation of the mainland coast. gradient axis of the foreland basin before reaching The mean annual discharge of the Fly is about the sea (see also Dietrich et al., 1999). 6500 m3 s 1, with a seasonal variation of 725% Distributary channels are floored by a thin layer (Wolanski et al., 1997). Extreme events occur in of fine to very fine, cross-bedded sand and mud- the Fly River catchment. In 1997, for example, an pebble conglomerate (Baker et al., 1995). These El Nino-associated drought occurred in southern are typically overlain abruptly by fluid-mud Papua New Guinea and the discharge of the Fly deposits that form in the channel bottoms just was reduced to well below normal levels for a after spring tides. The mud layers in this facies are period of over 3 months (Moi, 2001). Salinity commonly41-cm thick and channel-floor deposits measurements taken during non-drought condi- are characterised by interbedding of the coarsest tions show that the limit of saltwater intrusion and finest sediments. Above these mud layers, the differs between the Southern and Far Northern sediments are pervasively heterolithic and show a Entrances. In the Southern Entrance, saltwater net upward coarsening to about mid-depth level intrusion is generally restricted to the seaward on the tidal bars because of the thinning of the portion of the channel, reaching landward only mud layers. The bars may contain 50% (or more) about halfway between the channel opening and mud and display lateral-accretion bedding with the delta apex. In contrast, saltwater penetrates rare bioturbation. The sediments then fine upward nearly to the delta apex in the Far Northern into the intertidal zone (Dalrymple et al., 2003). Entrance (Wolanski and Eagle, 1991; Wolanski et Unequivocal indications of a tidal origin for the al., 1997). Such observations suggest that 60–80% heterolithic stratification are relatively uncommon, of the present-day river outflow reaches the Gulf although tidal rhythmites are present locally with- of Papua by way of the Southern distributary, with in active channels (Baker, 1999; Dalrymple et al., less than 10% of the discharge exiting via the Far 2003). The distributary mouth bar deposits are Northern Entrance (Wolanski et al., 1997). predominantly fine sand and also contain lateral- accretion bedding (Dalrymple et al., 2003). 1.1. Deltaic sedimentation and dynamics The delta front to pro-delta facies are hetero- lithic, with millimetre-to decimetre-thick sand/ Seismic profiles and radiometrically dated core mud alternations occurring with a limited degree samples were interpreted by Harris et al. (1993) as of bioturbation (Harris et al., 1993). Surface swell indicating that the delta is prograding seawards at waves propagate from the Coral Sea and across an average rate of about 6 m a 1. Progradation is the open Gulf of Papua shelf and cause sediment indicated also by the occurrence of beach ridges reworking of the delta front sediments in depths of aligned parallel to the coastline of the Southern 5–17 m throughout the SE Trade Wind season Entrance (Baker, 1999) and by seismic data that (April to October). During the summer monsoon show pro-delta muds downlapping progradation- (December to March), winds are predominantly ally onto the modern shelf carbonates that mark from the northwest and the wave power at the the maximum flooding surface (Harris et al., 1993, coast is much reduced (Thom and Wright, 1983). 1996). Thus, the deltaic system is prograding Current and turbidity observations taken from the seawards and actively exporting significant deltaic distributary channels demonstrate that amounts of river-supplied sediment to the inner tidal currents are the primary agent controlling shelf (Harris et al., 1993, 1996; Robertson and sediment dynamics in these locations (Wolanski et Alongi, 1995). al., 1995, 1998). Hence, the delta is characterised Dalrymple et al. (2003) examined the deposits of by longitudinal energy gradients, in which tidal the Fly delta and presented descriptions of cores energy reaches a maximum in the distributary and geophysical data collected in 1994. Deltaic channels and decreases seawards across the delta sediments are dominated by mud, because deposi- front (Harris, 1995), whilst wave energy reaches a tion of sediment coarser than fine sand occurs maximum on the delta front and decreases in ARTICLE IN PRESS
2434 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 relative importance landwards into the distribu- Northern and Far Northern Entrances (Stations 5 tary channels. and 6; Fig. 2). A sediment budget based on the 210Pb dating of the pro-deltaic sand/mud alter- fluxes extrapolated from these stations to the nations indicates that sedimentation rates are entire width of the distributary channels could 1–2 cm a 1 on the delta front and on the order of not be reconciled by these workers with observa- 4cma 1 on the pro-delta (Harris et al., 1993; tions of sediment accumulation rate in the delta. Walsh et al., 2001). The delta front sand/mud Wolanski et al. (1998) conclude that the net alternations have been interpreted by Harris et al. sediment export from the distributary channels (1993) to be varves, associated with the seasonal during the SE Trade season is zero, and suggest variation in wave power: wave reworking during that sediment may be exported during the mon- the SE Trades winnows the fine fraction and leaves soon season. a sandy lag deposit which is overlain by a mud Dalrymple et al. (2003) noted that many pro- drape during the summer monsoon. This pattern delta mud layers are not bioturbated, which they of delta front sedimentation is consistent with attribute to rapid accumulation resulting from satellite and ship-based observations of suspended fluid mud deposition (defined as suspension sediment concentration, which suggest that turbid concentrations410 g l 1). Further, Walsh et al. water is usually located landward of the 20 m (2001) presented detailed 210Pb dating of pro-delta isobath. However, it presents a paradox, since it cores which show that mud layers within the pro- does not explain how sediment is delivered to the delta were deposited rapidly, possibly by fluid mud rapidly accreting pro-delta, which is located in density currents (see also Harris et al., 1993, p. 18–40 m water depth. 467). Hence, a possible explanation is that pro- Current and turbidity data collected by Wo- delta accumulation is event driven and associated lanski et al. (1995, 1998) during the SE Trade with fluid mud export from the deltaic distributary season suggest that the net flux of sediment varies channels or delta front environments (or both). In from mainly seawards in the Southern Entrance addition, seaward diffusion of suspended sediment (Stations 1–4; Fig. 2) to mainly landwards in the would also result in a net flux of material to the
Fig. 2. Locations of current metre deployment sites of this study and of Wolanski et al. (1998). Wave model data in Fig. 10 refer to position ‘‘W’’. ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2435 pro-delta. The aim of this paper is to present new depth was recorded using a Raytheon precision data on sediment transport processes observed in depth recorder, corrected for tidal elevation and the deltaic distributary channels and on the delta reduced to lowest astronomical tide (LAT) chart front and to consider the mechanisms that are datum (Baker, 1999). likely to control pro-delta mud accumulation. Aanderaa model RCM-7 self-recording current metres were deployed to examine the current regime in each of the three areas. The metres had conductivity cells and temperature probes attached 2. Methods and were deployed with rotors located 100 cm above the seabed. Instrument details and deploy- Data were collected on cruises carried out in ment intervals are listed in Table 1. September 1991, February to April 1993 and in Bedload transport rates were estimated using a January 1994. The 1991 cruise was a brief (1-week) variation of Bagnold’s bedload equation proposed sortie into the South Entrance of the Fly delta. by Hardisty (1983), given as One current metre was deployed at station 7 (Fig. 2 2) for 2.58 days (Table 1). The cruises in 1993 q ¼ k1ðU 100 U 100crÞ U 100; (1) deployed and recovered the current metre at where U is the current speed measured 100 cm station 8 on the delta front. The 1994 cruise lasted 100 above the bed, U is the threshold speed at for 3 weeks, during which data were collected from 100cr which bedload transport commences and k is an three different areas, selected to provide informa- 1 empirically derived coefficient (modified by Wang tion on processes operating at the seaward as well and Gao, 2001), dependent only upon grain size: as the landward ends of deltaic distributary channels and to provide two cross-channel trans- 4 2 k1 ¼ 0:10 expð0:17=DÞ gcm s (2) ects of current speed information. The three areas were (1) the apex of the Fly delta, (2) the seaward in which D is the mean sediment grain size in end of the Far Northern Entrance (the northern- millimetres. In order to provide a standard most distributary channel), and (3) the mouth of comparison of relative transport between the the South Entrance (Fig. 2). stations, a threshold speed U100cr was selected as A Shipek grab sampler was used to obtain 32 cm s 1 based on a 0.1-mm mean grain size samples of surficial sediment at stations located on derived using the empirical curve of Miller et al. an 2 km grid in each of the three areas. Water (1977).
Table 1 Fly Delta current metre deployment locations and recording times
Serial no. Latitude Longitude Depth Date deployed Observation time Stn no. (Fig. 2)
10402 8125.8 14318.2 7.6 m 10-1-94 10.41 days 1 11058 8116.4 143137.1 6.0 11-1-94 9.75 2 10545 8117.6 143137.0 7.7 11-1-94 9.88 3 11061 8119.5 143137.1 13.8 11-1-94 5.90 4 10401 8138.6 143130.1 7.4 10-1-94 14.0 5 11062 8141.3 143128.5 7.2 10-1-94 13.81 6 10159 8141.54 143126.67 5 22-9-91 2.58 7 9305 9100.2 143147.5 20 17-1-93 69.60 8 10402* 9100.2 143147.4 20 28-3-93 30.14 8
The 1991 deployment of 1 m in the Southern Entrance was set at a 1 min sampling frequency and the delta front metres deployed in 1993 were set at 20 min sampling frequency. All metres in the deployments were set at 5 min sampling frequency. Metre 10402* had a Seatech Transmissometre attached in the 1993 deployment (see Harris, 1995). ARTICLE IN PRESS
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2.1. Suspended sediment measurements of fine particles and flocs (eg. Maa et al., 1992). An in situ calibration of the OBS using the siphon Two deployments using instrumented frames, samples was not possible, because there were anchored to the seafloor at sites 50 and 189 (Fig. insufficient samples obtained at each OBS eleva- 2), investigated sediment resuspension by tidal tion, particularly those at higher elevations. processes. The deployment at Site 50 occurred on 16–17 January 1994, 4 days after the maximum (perigee) spring tide, whereas the deployment at 3. Results Site 189 occurred on 23–24 January 1994, 3 days before the following (apogee) spring tide. Each 3.1. Surficial sediments and channel morphology deployment included a full diurnal tide cycle (25 h). Bathymetric maps and distribution of mean Three-dimensional velocity was measured at 1 m surficial sediment grain size for the three study above the bed using two co-located electromag- areas are shown in Figs. 3A–C. Since disaggre- netic current metres (Valeport Series 800). Water gated samples were analysed for grain size in the turbidity was measured at 0.06, 0.20, 0.40, 0.60 laboratory, there is potential for pebble-sized mud- and 0.95 m above the bed using five optical clasts to be (erroneously) recorded as a finer grain backscatter sensors (OBS; D & A Instruments). size. Further, due to the shoal water depths of the Samples of suspended sediment adjacent to each distributary channels, there is no statistically OBS were obtained using self-siphoning suspended significant relationship between water depth and sediment samplers (Nielsen, 1984). Due to opera- mean grain size. Nevertheless, in all three areas, tional problems, reliable siphon samples were only sediments having the smallest mean grain size of obtained at 0.06, 0.40 and 0.95 m at Site 50, and o16 mm appear to be located in channel bottoms, 0.06 m at Site 189. Instruments were logged at 4 Hz generally in depths exceeding 6 m. Also, in all the for 10-min bursts every hour (Site 50) or every three areas, sediments having the largest mean 45 min (Site 189). At each site, a surface grab- grain size of463 mm appear to be associated with sample and a vibrocore of the bed sediment were low-relief shoals, generally in depths of less than also obtained. The core data for these sites are 6m (Figs. 3A–C). We infer that this pattern of discussed in Dalrymple et al. (2003). mean grain size distribution reflects the develop- The particle size of representative surface ment of ebb-and flood-dominated tidal channels, sediment samples and suspended sediment samples separated by tidal sand bars. It appears, further- were analysed in the laboratory using a Malvern more, that pebble-sized mud-clasts are unable to 2600 laser-diffraction particle sizer. The particle withstand prolonged bedload transport (over the sizes obtained are dispersed sizes, not floc sizes. length scales of the sandy shoals; Fig. 3), and The optical backscatter sensors were calibrated for hence the grain size distribution integrated over suspended sediment concentration in the labora- many tidal cycles is represented by the (disaggre- tory using bed sediment obtained from the gated) sample measurements. experiment site. Due to the large range in The relative areas of sandy shoals versus muddy suspended sediment concentrations encountered channels vary between the three areas. In the apex, at Site 50, the OBS output was not linear over the sandy shoals are restricted to about 20% of the full range, so a piecewise calibration equation was surface area. In contrast, about 40% of the employed. This meant that large output voltages Southern Entrance and more than 50% of the from the OBS could correspond to one of two surface area of the Far Northern Entrance are suspended sediment concentrations. In these cases, characterised by sandy shoals (Figs. 3A–C). The the siphon samples were used to indicate the shoals are generally elongate, aligned parallel to correct concentration. The laboratory calibration the axis of the main channels, and are commonly of the OBS does not account for the difference in covered with medium-scale dunes (nomenclature optical backscatter intensity between a dispersion of Ashley, 1990; see also Dalrymple et al., 2003). ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2437
Fig. 3. Bathymetry, surface sediment grain size and surface sample positions for (A) the delta apex; (B) the Far Northern Entrance and (C) the Southern Entrance. See Fig. 1 for locations in the delta. ARTICLE IN PRESS
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3.2. Currents and bedload transport speed was 95 cm s 1. A scatter plot of the data (Fig. 5A) shows the reversing tidal flow is slightly 3.2.1. Apex of the Fly delta non-rectilinear (especially the peak ebb currents), Currents at the apex had a mean speed of possibly caused by steering of the currents around 44 cm s 1 and a maximum of 124 cm s 1. A scatter sandbanks in the vicinity. The data clearly indicate plot of the data (Fig. 4A) shows the reversing tidal flood dominance of the site (Fig. 5A), with flow is slightly non-rectilinear, possibly caused by strongest flows in the flood (west) direction. There steering of the currents around islands and shoals is a pronounced diurnal inequality, with one flood in the vicinity. The strongest current is in the ebb current attaining much higher speeds than the (east) direction and the progressive vector plot other three tidal stages. The progressive vector (Fig. 4A) indicates a residual current towards the plot (Fig. 5A) indicates that over the first 5 days, east (i.e. towards the Southern Entrance; Fig. 3A). corresponding to the spring tides, there was These observations are consistent with a unidirec- virtually no net current, with a nearly equal tional (eastward) river current of up to about displacement occurring with each tidal oscillation. 20 cm s 1 superimposed upon the oscillatory tidal However, over the last 4 days of recorded data motion, exiting the apex area via the Southern (neap tides) the plot suggests a residual current of Entrance (as described by Wolanski et al., 1998). about 9 cm s 1 towards the north. A westward Net bedload transport is towards the east (ebb- residual flow at this site (referenced to 100 cm dominated) at this location, although sand is above the bed) is consistent with the establishment transported in both directions over a tidal cycle of estuarine circulation during neaps (i.e. salty (i.e. the flood current is also competent in bottom water flowing landward into the channel transporting sand). with freshwater flowing seawards at the surface; see below). Net bedload transport is towards the 3.2.2. Far Northern Entrance west (flood-dominated) at this location. Bedload In this area, three current metres were moored, transport goes to zero during the neap tides when aligned in a north–south transect: CM2 in the the selected threshold speed (32 cm s 1) is not north positioned approximately on a sand bar exceeded. crest, CM3 positioned mid-channel and on the At the mid-channel station (CM3), the mean southern flank of a tidal bar, and CM4 positioned current speed was 35 cm s 1 and the maximum within the deepest section of the southern channel speed was 98 cm s 1 (i.e. somewhat higher speeds margin (Fig. 3B). than in the north at CM2). The scatter plot (Fig. At the northern station (CM2), the mean 5B) shows the reversing tidal flow is essentially current speed was 26 cm s 1 and the maximum rectilinear (except for peak flood currents which
Fig. 4. Scatter plot of current speed/direction data and progressive current vector plot for CM1 at the delta apex. See Fig. 2 for locations. The dashed circle in the scatter plot refers to the assumed threshold speed of 32 cm s 1. ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2439
Fig. 5. Scatter plot of current speed/direction data and progressive current vector plot for (A) CM2 in the northern part of the Far Northern Entrance, (B) CM3 in the central part of the Far Northern Entrance, and (C) CM4 in the southern part of the Far Northern Entrance. See Fig. 2 for locations. The dashed circle in the scatter plot refers to the assumed threshold speed of 32 cm s 1. ARTICLE IN PRESS
2440 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 are offset to the southwest), with strongest north positioned on a bar crest, CM6 positioned in currents flowing towards the west (flood domi- a relatively deep, mid-channel area, and CM7 nant). The diurnal inequality is evident but less positioned on the southern flank of a bar on the pronounced than the site immediately to the north southern channel margin (Fig. 3C). (CM2), with one flood current attaining higher At the northern station (CM5) the mean current speeds than the other three tidal stages. The speed was 35 cm s 1 and the maximum speed was progressive vector plot (Fig. 5B) indicates a similar 118 cm s 1. The scatter plot (Fig. 6A) shows the pattern to the northern site. Over the first 3 days reversing tidal flow is rectilinear, with flow in the (spring tides), there was only a weak residual flood (northwest) direction exceeding that in the current. Over the last 3 days of recorded data ebb direction by about 10 cm s 1. The diurnal (neap tides), the plot suggests a weak residual inequality manifests itself by one flood current current of about 7 cm s 1 towards the northwest. attaining much higher speeds than the other three Net bedload transport is towards the west (flood- tidal stages. The progressive vector plot (Fig. 6A) dominated) and is about six times greater than at indicates that over the first 5 days, corresponding the northern location (CM2). The threshold is to the spring tides, there was virtually no net exceeded during the neap tides but bedload current, with a nearly equal displacement occur- transport is much reduced. ring with each tidal oscillation. However, over the At the station located on the south side of the middle 4–6 days of recorded data (neap tides), the Far Northern Entrance (CM4), the mean current plot suggests a residual current of about 9 cm s 1 speed was 49 cm s 1 and the maximum speed was towards the northwest. During the second set of 130 cm s 1. Hence, both mean and maximum spring tides (last 3 days of data) the progressive current speeds are much stronger along this vector plot shows a return to the no net motion southern margin than they are at the northern situation. Net bedload transport is towards the stations. The scatter plot (Fig. 5C) shows the west (flood-dominated) at CM5. During neap reversing tidal flow is slightly non-rectilinear, with tides, the instantaneous bedload transport rate is strongest currents flowing towards the east (ebb reduced, but still significant, in comparison with dominant). The diurnal inequality is evident but transport rates during spring tides. different from the sites immediately to the north, At the mid-channel station (CM6) the mean such that ebb-directed flows are always strong but current speed was 41 cm s 1 and the maximum only one flood current attains high speeds and the speed was 135 cm s 1; this was the largest current other flood current is much weaker. The progres- speed measured at any station. The scatter plot sive vector plot indicates a reversed pattern to the (Fig. 6B) shows the reversing tidal flow is northern sites, in which over the first 3 days (spring essentially rectilinear, with strongest currents tides) there was 12 cm s 1 eastward residual flowing towards the west (flood dominant) but current whereas over the last 3 days of recorded only exceeding maximum ebb currents by about data (neap tides) the plot suggests no significant 5cms 1. The diurnal inequality at this site (CM6) residual flow was present. Net bedload transport is is manifested by one flood current being signifi- towards the east (ebb-dominated) and attained its cantly weaker than the other flood; in contrast highest rate at this station (20 times higher than both of the ebb flows attain peak speeds not much CM2 and four times higher than CM3). During less than the stronger flood flow. The consequence neap tides, current speeds exceed the threshold of this is that, in terms of bedload transport, the velocity, but the transport rate is much reduced site is ebb dominated. As in the case of the other from its peak spring values and becomes flood ebb-dominated sites in the Far Northern Entrance dominant (west-directed). (CM4), instantaneous bedload transport is flood dominated during the neap tides (although much 3.2.3. Southern Entrance reduced from the spring transport rates). How- In this area, three current metres were moored, ever, averaged over a neap–spring cycle, the two aligned in a north–south transect: CM5 in the strong ebb currents overcome the single strong ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2441
Fig. 6. Scatter plot of current speed/direction data and progressive current vector plot for (A) CM5 in the northern part of the Southern Entrance, (B) CM6 in the central part of the Southern Entrance, and (C) CM7 in the southern part of the Southern Entrance. See Fig. 2 for locations. The dashed circle in the scatter plot refers to the assumed threshold speed of 32 cm s 1.
flood current and net bedload transport is in the ward net current of about 9 cm s 1. During the ebb direction. The progressive vector plot (Fig. middle 4–6 days of recorded data (neap tides) the 6B) indicates that over the first 5 days, corre- plot suggests a residual current of about 15 cm s 1 sponding to the spring tides, there was a south- towards the west. During the second set of spring ARTICLE IN PRESS
2442 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 tides (last 3 days of data) the progressive vector plot shows a return to the net southward flow. Hence, the data are consistent with the establish- ment of estuarine circulation during neap tides. The southward residual flow occurring during spring tides may be a residual tidal current, generated by sandbanks or other local bathymetric features. At site CM7, located on the southern margin of the channel, 62 h of data were obtained during the 1991 cruise. The results show that currents are highly rectilinear (little scatter in direction) and asymmetrical in speed/direction (Fig. 6C). The strongest flood (northwesterly) currents achieved 115 cm s 1, whereas the weaker ebb flow (south- easterly) currents rarely exceeded 80 cm s 1. Cur- rents plotted in time series show very nearly equal duration of ebb and flood flows, but higher peak speeds are achieved by the flood currents. Residual currents were towards the northwest over the short duration of the data collection period (Fig. 6C). The net transport is towards the northwest; a similar landward movement at this same location was inferred by Wolanski and Eagle (1991) for fine-grained suspended sediments.
3.2.4. Delta front Fig. 7. Plots of data from station CM8 on the delta front: (A) On the delta front, current metres were moored scatter plot of current speed/direction data; (B) progressive at station CM8 over a total time of nearly 100 current vector plot; (C) current velocity time series; and (D) turbidity time series (NTUs). The data collection time days, covering the late monsoon and early SE corresponds to the transition from the Monsoon to the SE Trade seasons. Over 30 days of that time (28-3- Trade wind season. See Fig. 2 for locations. The dashed circle in 93–27-4-93), a Seatech transmissometre was at- the scatter plot (b) refers to the assumed threshold speed of 1 tached to the current metre, calibrated to yield 32 cm s . nephelometric turbidity units (NTUs). Over the 100 days of deployment, the mean current speed was 19 cm s 1 and the maximum an order of magnitude compared with all the other speed was 47 cm s 1. The scatter plot (Fig. 7A) stations. During neap tides, the instantaneous shows the reversing tidal flow is rectilinear/rotary. bedload transport rate goes to zero, in comparison The speed/direction time series (not shown) with instantaneous (20 min averaged) transport indicates a mostly semidiurnal tidal regime, with rates of over 1 g 1cm s 1 during spring tides. the presence of a slight diurnal inequality. The The transmissometre data obtained at CM8 progressive vector plot indicates a tidal excursion suggest that mean turbidity is about 50 NTUs with of about 8 km during spring tides with a steady a maximum value of 196 NTUs (Fig. 7D). The northward component (Fig. 7B). During neaps, peak in turbidity lags behind the peak in spring there is virtually no net current, with a nearly tidal current velocity (Fig. 7C and D). This equal displacement occurring with each tidal ‘‘erosion lag’’ (Dyer, 1994) effect has been oscillation. Net bedload transport is towards explained in previous studies in relation to the 3051 (flood-dominated) but the rate is lower by consolidation of the bed during neaps, which then ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2443 requires peak-spring tidal flows before erosion of effectively the root-mean-square (RMS) of the U the bed releases the sediments into suspension (e.g. time series. Similar formulations exist for the Hughes et al., 1998). Combining current direction horizontal velocity that is orthogonal to the with turbidity data indicates a tendency for higher principal flow direction, V, and the vertical NTUs to correlate with northward flowing cur- velocity, W. The total turbulent kinetic energy of rents. The product of X and Y current components the flow (TKE) per unit mass is with NTUs similarly shows a net northward TKE ¼ 1ðu02 þ v02 þ w02Þ (4) sediment flux; the monthly averaged vector is 2 1 0.138 NTUs cm s towards 3581. Hence, the (Mathieu and Scott, 2000). Similarly, time series of monthly averaged suspended sediment transport instantaneous suspended sediment concentration, vector is oriented 601 more towards the north than C, can be written as the bedload transport vector measured over this ¼ þ 0 same time period. C c¯ c ; (5) where c¯ is the time-averaged concentration and c0 3.3. Salinity time series and residual water is the fluctuating component. Of particular rele- circulation vance to suspension dynamics is the vertical flux rate across a given level (plane) in the flow, F, The progressive vector plots at stations CM2, which can be written as CM3, CM5 and CM6 (Figs. 5 and 6) suggest a F ¼ w0c0 cW¯ ; (6) near-bed landward flow of water (which is also s more saline; Fig. 8) on neap tides within the Far where Ws is the sediment fall velocity. The Northern and Southern Entrances. The establish- fluctuating (w0c0) term provides a record of ment of estuarine circulation is favoured on neap instantaneous mixing of sediment across the tides because mixing during springs inhibits sensor level (Lapointe, 1996). stratification and dampens the estuarine circula- Results from our instrument deployments at tion (Wolanski and Eagle, 1991). The overall Sites 50 and 189 are shown in Figs. 9 and 10, salinity intrusion is more pronounced in the Far respectively. At Site 50, the tidal velocities were Northern Entrance (Wolanski et al., 1997), where ebb dominant with respect to magnitude asymme- neap-tidal salinities reached 34.7 ppt (20 Janu- try (Fig. 9A). The two ebb maxima in the mean ary; see Fig. 8). The intrusion of saline water is horizontal velocity were 87 and 102 cm s 1, also detected at the apex, where salinities rise from whereas the flood maximum was 71 cm s 1. Ebb- background levels of 0–4 ppt on springs up to 8 ppt directed flows persisted for 1.3–2.1 h longer than on neaps (Fig. 8). The overall range in salinity is flood-directed flows, which is attributed to a fluvial greatest in the Southern Entrance, where semi- contribution at the apex. Peaks in the TKE (Eq. diurnal oscillations are up to 17 ppt on springs (4)) at Site 50 are well correlated with maximum and the spring–neap salinity range is from around horizontal velocity, with the largest peaks occur- 6 ppt on springs (lowest values) to over 32 ppt on ring during the ebb tide (Fig. 9B). For most of the neaps (Fig. 8). time the vertical turbulent flux of suspended 0 sediment, w0c (Eq. (6)), was positive and thus 3.4. Suspended sediment transport directed up away from the bed (Fig. 8c). This is consistent with sediment diffusing upwards from The time series of instantaneous horizontal the bed. It was not possible to calculate the vertical velocities, U, in the streamwise (principal flow) sediment flux rate here, because we do not know direction can be written as the sediment fall velocity (being cohesive sediment, it is expected to be a function of both suspension ¼ þ 0 U u¯ u ; (3) concentration and salinity). Nevertheless, the where u¯ is the steady time-averaged velocity and u0 w0c0.record in Fig. 9C does provide some insight is the fluctuating turbulent velocity. The latter is into the suspension’s behaviour. When the turbu- ARTICLE IN PRESS
2444 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454
Fig. 8. Time series of salinity recorded at 100 cm above the bed at CM1 (delta apex), CM3 (middle of the Far Northern Entrance) and CM5 (middle of the Southern Entrance). Salinity is generally highest in the Far Northern Entrance and lowest at the apex. Lower but highly variable salinity ranges during spring tides (longer tidal excursion length) give way to higher, less variable salinity ranges during neaps (shorter tidal excursion lengths). See text for explanation and discussion. lent sediment flux at the current metre elevation is of the water column. A lutocline occurs close to large, suspended sediment is well mixed through- the bed at 3 and 22 h (Fig. 9D). These times out the lowest metre of the water column (e.g. at 6, coincide with a zero or negative (downward- 12, 18 h; Fig. 9D). In contrast, when the turbulent directed) turbulent vertical sediment flux at 1 m flux is small, strong vertical gradients in suspended above the bed. The combined effect of low TKE, sediment concentration emerge in the lowest metre low or negative turbulent vertical sediment flux, ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2445
Fig. 9. Observational data from Site 50. 0 h corresponds to 0130 h 16 January 1994. (A) Mean horizontal velocity in principal direction of flow. Positive and negative indicate flood-and ebb-directedflow, respectively. (B) Mean turbulent kinetic energy (TKE) calculated using all three turbulent velocity components (Eq. (4)). (C) Mean vertical turbulent sediment flux (Eq. (6)) at 1 m above the bed. Positive and negative indicates up-and downward directed fluxes, respectively. (D) Greyscale contours (logarithmic scale) of suspended sediment concentration (g l 1) in the lowest metre of the water column.
and enhanced sediment fall velocities are hypothe- occasions that a lutocline occurred, the flow was sised as necessary conditions to enable sediment to ebb-directed. settle quickly towards the bed during slack water The mean velocities at Site 189 are similar in periods. Hindered settling of the near-bed suspen- magnitude to those measured at Site 50 (Fig. 10A). sion, due to high concentrations (up to 5 g l 1; Fig. In contrast, the TKE and consequently the 9D), then results in the formation of a lutocline. turbulent sediment flux at Site 189 were both The rapid formation of turbid bottom water near larger (Fig. 10B and C). Moreover, the sediment slack water may also be explained by the ‘‘con- suspension is persistently well mixed with respect vective settling’’ mechanism described recently by to concentration in the lowest 1 m of the water Parsons et al. (2001), in which fingers of turbid column (Fig. 10D). The consistently elevated TKE water flow vertically downwards to form a and positive (upward) turbulent sediment flux hyperpycnal bottom layer. Important to our later seem to have precluded the formation of a discussion is the phase coupling of the tide and the lutocline at Site 189. When both the TKE and sediment suspension magnitudes. On both the the turbulent sediment flux approach zero, ARTICLE IN PRESS
2446 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454
Fig. 10. (A–D) Observational data from Site 189. 0 h corresponds to 1245 h 23 January 1994. All other details as for Fig. 9.
suggesting that the total vertical sediment flux rate be expected at Site 50 (falling phase) than at Site may be directed towards the bed, the lower part of 189 (rising phase), even for tides of similar range the water column becomes relatively clear (e.g. at 9 and peak current speed. and 21 h; Fig. 9D). Under similar circumstances at Site 50, a lutocline would develop (Fig. 8D). The difference in behaviour between the two sites may 4. Discussion be related to a difference in the sediment fall velocity. Mean grain size of the bed sediments at 4.1. Mutually evasive sand transport patterns in Site 50 and Site 189 are 9 and 87 mm, respectively. distributary channels Alternatively, it is known from previous studies that there is commonly a time lag between the Flood and ebb tidal currents, as measured at a current speeds and the concentration of suspended point, are rarely equal in strength and/or duration, sediments, and that over a neap–spring cycle the because of interaction between the tidal flow and concentration on the rising (spring–neap) phase is the local or regional bottom topography (e.g. smaller than that on the falling (neap–spring) Ludwick, 1975; Pattiaratchi and Collins, 1987). phase (i.e. ‘‘erosion lag’’; Dyer, 1994). Because of These inequalities produce a residual transport of this hysteresis effect, higher concentrations would bed material, which is spatially organised into a ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2447 system of mutually evasive, ebb-and flood- butes of mutually evasive, tidal transport patterns. dominated sediment-transport pathways separated This has implications for understanding the by elongate, tidal bars. The best known and most mixing and dispersal of introduced contaminants thoroughly studied examples occur in estuaries within the deltaic environment and also for the such as the Thames and Humber River estuaries construction of sediment budgets. (Robinson, 1960), Chesapeake Bay entrance (Lud- An interesting complication is the existence of wick, 1975), Bristol Channel (Harris, 1988), cohesive, pebble-sized mud clasts in the distribu- Moreton Bay entrance (Harris and Jones, 1988), tary channels, which blurs the differences between Bay of Fundy (Dalrymple et al., 1990), and on suspended load and bedload processes. A grain of sandy continental shelves such as the southern silt, incorporated in a mud clast could be North Sea (Houbolt, 1968; Kenyon et al., 1981). transported as bedload in one direction. Disinte- All of these settings are transgressive in nature and gration of the clast releases the silt grain and are characterised by an abundance of sandy makes it available for suspended load transport, sediment. Estuaries, in particular, are typified by possibly in a different (opposite?) direction. the interdigitation of ebb-and flood-dominant Individual grains may undergo many cycles of channels, perhaps because they experience a net deposition and erosion on the distributary channel input of bed material from both the landward and floors. seaward directions (Dalrymple et al., 1992). In attempting to derive a sediment budget for By comparison, relatively little is known about the Fly delta distributary channels, Wolanski et al. bedload-transport patterns in progradational, (1998) assumed that the suspended sediment flux tide-dominated (river) deltas, where the net sedi- recorded at individual mooring sites could be ment transport is directed seawards (Dalrymple et extrapolated over the whole channel width. Our al., 2003) and especially those with large amounts estimates of bedload fluxes suggest that there are of fine-grained (muddy) sediment. Our results reversals in net transport direction across these from the Fly River suggest that mutually evasive, channels and hence any extrapolations to sur- ebb-and flood-dominated transport patterns also rounding areas should be done with extreme characterise muddy, progradational, tide-domi- caution. Further, the reversal in direction of nated deltas. This is demonstrated by the reversal residual currents observed in our data from the in net bedload transport vectors, from ebb-to Far Northern Entrance (Fig. 5) suggests that flood-dominated, in both of the cross-channel reversals in the direction of net suspended trans- deployments of current metres in the present study port may also occur in this channel. In the Far (Figs. 5 and 6). The mutually evasive pattern is Northern Entrance, the low rate of net westward also suggested by the deposition of elongate, low- (flood-dominated) bedload transport at the two relief tidal bars that are particularly evident in the northern stations (CM2 and CM3) is offset by a Southern Entrance, as indicated by the grain size much larger net eastward (ebb-dominated) trans- and channel morphology information (Fig. 3C). port rate at the southern station (CM4). Such a A further piece of evidence that supports the pattern is similar to that observed in the macro- occurrence of mutually evasive transport patterns tidal Bristol Channel in Great Britain. In that in the Fly delta was provided by Cole et al. (1995), estuary, Harris and Collins (1988) described how who described planktonic foraminifera found large bedload transport rates along the estuary deposited in the Southern Entrance deltaic sedi- margins could more than balance the smaller, but ments. This observation points to the marine oppositely directed bedload transport rates occur- origin of some (albeit minimal) marine-sourced ring in much wider mid-channel areas. The sediments being transported landward into the existence of mutually evasive, ebb-and flood- Fly’s distributary channels. We conclude that, transport pathways in the Fly delta distributary despite its progradational and muddy character, channels (as indicated by our data) provides a the tidally dominated Fly River delta shares with likely solution for the unsustainable sediment tide-dominated estuaries some important attri- budget proposed by Wolanski et al. (1998),in ARTICLE IN PRESS
2448 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 which the Far Northern Entrance is supposed to they occur throughout the distributary channels. be importing sediment from the sea at a rate of Moreover, suspensions with a concentration ap- 30–40 tonnes s 1 (95–126 million tonnes a 1). proaching fluid mud occur at least as far landward In deltas, distributary channels are considered as the delta apex (Site 50 of this study; Fig. 9). to act mainly as conduits for sediment transport Spring tide current velocities consistently reach and dispersal from the river mouth to the pro- 100 cm s 1 throughout the distributary channels deltaic depositional environments on the inner (this study Figs. 5 and 6; Wolanski et al., 1995, continental shelf (e.g. Wright, 1985; Elliott, 1986). 1998), but drop to ca. o50 cm s 1 15 km seaward This situation probably applies to the Fly delta of the mouth (Fig. 7). The question arises, how is (Harris et al., 1993). Dalrymple et al. (2003) fine sediment transported from the distributary interpreted the compositional uniformity of deltaic channel-mouth region a further 20–30 km sea- sediments as evidence that material from the river ward, to account for pro-delta sedimentation is reaching the delta front and pro-delta. Hughes presently occurring at a rate of 3–4 cm a 1 (Harris and Baker (1996) concluded that there has been no et al., 1993)? detectible change to the combined surface area of Possible mechanisms for delivering substantial islands or distributary channels in the Fly delta amounts of fine sediment to the pro-delta region over the past 50 years. Wolanski et al. (1998) include (1) buoyant plumes, (2) tidal resuspension concluded that deposition rates in the distributary and advection, (3) wind waves and wind-generated channel mangroves are not significant in compar- currents, and (4) hyperpycnal flows. ison with the sediment budget for the delta. It Transport of fresh-turbid water across the delta follows that, whatever landward sediment trans- front as a buoyant plume may deliver sediment to port may occur within individual channels, the the pro-delta. However, the available data suggest approximate sediment load of the river (currently that vertical stratification is weak due to strong 125 million tonnes a 1 for the Fly) should approxi- tidal mixing and shallow water depths. Salinity mately equal the net amount of sediment that exits stratification occurs only on neaps (Fig. 8) when the distributary channel mouths. The question is, suspended sediment concentrations are relatively what processes control the seaward transport and low. Our only available time series of turbidity on dispersal of this sediment? the delta front (Fig. 7C and D) shows that near- bed concentrations are o60 NTUs during neaps 4.2. Cross-delta sediment transport mechanisms and only attain 120 NTUs on springs. These data (Fig. 7C and D) were obtained during the Tidal processes are responsible for resuspending monsoon season when wave power is relatively large amounts of fine sediment within the dis- low, and so they reflect mainly the influence of tributary channels of the delta. The spatial pattern tides. in the tide-and depth-averaged concentrations The rapid decline in tidal energy across the delta indicates the presence of a turbidity maximum front region precludes tidal resuspension and zone during springs, which is situated immediately advection from being an important dual mechan- landward of the distributary channel mouths. ism, as indicated by our delta front current and Seaward of this zone, the average suspension turbidity time-series data (Fig. 7C and D). concentration diminishes markedly (Wolanski et Residual tidal currents in concert with wave al., 1998). In the turbidity maximum zone, near- resuspension may be important, although our bed fluid mud layers often occur during spring limited observations of residual currents on the tides (suspension concentrations ca.410 g l 1 and delta front show they are parabathic (to the north, thicknesses of 1–2 m), whereas depth-averaged Fig. 7B). concentrations 1–2 orders of magnitude smaller During the Trade Wind season, there is certainly occur during neap tides (Wolanski et al., 1998). sufficient wave energy at times to resuspend Fluid mud layers are not restricted to the turbidity sediments in the delta front region. Wolanski maximum zone near the distributary mouths, but et al. (1995) present data showing near-bed ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2449 suspension concentrations during a storm, reach- flow, g0,is ing levels twice that achieved by tides alone in the r r absence of the storm. The fact that the SE Trade g0 ¼ g f o ; (8) r Winds are oriented almost orthogonal to the delta o front may result in a wind-driven pressure gradient where g is the gravitational acceleration, ro is the force that will drive a barotropic current seaward ambient fluid density, and the flow density rf can across the delta front. The existence of such a be calculated using current in conjunction with wave resuspension could result in the seaward transport of fine rf ¼ rsC þð1 CÞro (9) sediment from the distributary mouths to the in which C is the fractional concentration of solids pro-delta. Without knowing the bulk or dry and rs is the sediment density. This is the approach density of the delta front sediments to assess their used by Wright et al. (1990) in their study of the shear strength, it is difficult to quantify wave Huanghe delta. A more sophisticated approach resuspension concentrations. that includes the Coriolis force and feedback In both the Huanghe and Amazon deltas, between sediment resuspension and turbulent hyperpycnal flows have been identified as a damping can be found in Kineke et al. (1996) critically important mechanism for delivering fine and Traykovski et al. (2000). However, we used sediment to the pro-delta region (Wright et al., the simpler approach of Wright et al. (1990) 1990, 2001; Kineke et al., 1996). In order to assess because of the limited data available for the Fly the importance of such flows in the Fly delta, we River and the similar results produced by the two have estimated the velocity of a hypothesised approaches (see Kineke et al., 1996). hyperpycnal flow, UH, by balancing the downslope Table 2 lists predicted hyperpycnal flow velo- weight force of a negatively buoyant suspension cities for representative conditions in the Fly against the resisting force of fluid and bed shear River. Suspension concentrations of 5 g l 1 are stress at the top and bottom of the flow. We make considered to be the minimum necessary to no assumptions regarding the processes responsi- support hyperpycnal flow behaviour and 10 g l 1 ble for producing such a suspension; we simply is the maximum sustained concentration reported assume an a priori suspension concentration of 4 1 to date. A seabed slope of 1 10 is representa- 10 g l , which is known to occur at the distribu- tive of the distributary channels and 2 10 3 is tary mouths. This approach yields representative of the delta front. A flow thickness sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi of 1–2 m seems representative of the suspension h0g0 sin b concentration profiles reported here and in Wo- U H ¼ ; (7) CdT þ CdB lanski et al. (1995, 1998). Under ideal conditions (i.e. a hyperpycnal flow thickness of 2 m with an where h0 is the thickness of the hyperpycnal flow, b average concentration of 10 g l 1), it would take is the bed slope, and CdT and CdB are the drag roughly 35 h for a hyperpycnal flow to travel the coefficients for the top and bottom of the flow, 20-km distance from the mouth of the Southern respectively. The buoyancy per unit volume of the Channel to the pro-delta. If the sediment is
Table 2 Estimated velocities for a hypothesised hyperpycnal flow in the Fly River delta for a range of representative conditions
Slope 1 10 4 Slope 1 10 4 Slope 2 10 3 Slope 2 10 3
d ¼ 1m d ¼ 2m d ¼ 1m d ¼ 2m
Ave conc. 5 g l 1 0.018 m s 1 0.025 m s 1 0.079 m s 1 0.11 m s 1 Ave conc. 10 g l 1 0.025 m s 1 0.035 m s 1 0.11 m s 1 0.16 m s 1 ARTICLE IN PRESS
2450 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 delivered to the pro-delta in this way, as individual If we wish to maintain a suspension concentra- events, as suggested by 210Pb data (Walsh et al., tion of 10 kg m 3 due to waves, then 2001), then large sediment suspensions need to be maintained for a day and a half across the delta meðÞtb tc X10W s: (13) front region. This assumes a stable concentration A value for me under waves has not been is maintained by some combination of autosus- published to date, but empirical data suggest that pension, tides and waves during this time (Wright the erosion rate under waves is about an order of et al., 2001). We assess the role of each in the magnitude greater than that under unidirectional context of the Fly River delta in that order. currents (Eisma, 1993), which would suggest a Hyperpycnal flows are autosuspending if the value of 0.005 is probably of the correct order following criterion is satisfied: (Mitchener and Torfs, 1996). If we again assume 1 W the sediment fall velocity is 0.1 cm s , then the U X s ; (10) H a tan b excess bed shear stress must be greater than 2Nm 2 in order for waves to maintain suspended where a is a constant whose value is between 0.01 sediment concentrations sufficient to drive a and 0.1 (Stacey and Bowen, 1990). Substituting the hyperpycnal flow down the delta slope. Again, it delta-front slope, a representative settling velocity is not possible to evaluate Eq. (13), because we for a suspension concentration of 10 g l 1 of ca. have no knowledge of the bed shear strength in the 0.1 cm s 1 (Wolanski et al., 1995), the minimum delta front region. As a guide, we have used linear hyperpycnal flow velocity necessary for autosus- wave theory to calculate the wave-averaged bed pension is 500 cm s 1. If hyperpycnal flows occur shear stress in 20 m water depth, representative of in the Fly River, it appears that autosuspension is the pro-delta region seaward of the distributary unlikely. Wright et al. (2001) explained that tidal mouths (Figs. 2 and 11; Table 3). The results or other currents can enhance or diminish the suggest that the wave-averaged bed shear stress negative buoyancy of the hyperpycnal flow does exceed 2 N m 2. However, it is important to through resuspension or turbulence damping. On the Fly River delta, tidal resuspension, as indicated by turbidity at Station CM8, is minimal (Fig. 7). This again implies that wave resuspension on the delta front plays an important role in sediment transport to the pro-delta. In order to maintain an equilibrium suspension concentration the erosion rate due to waves, E, must equal the deposition rate, D. For muddy substrates E (kg m 2 s 1) is usually expressed as
E ¼ meðÞtb tc (11)
(e.g. Mitchener and Torfs, 1996) where me (kg N 1 s 1) is an erosion constant that depends on seabed properties, tb is the bed shear stress and tc is the critical bed shear stress for entrainment, or 2 1 Fig. 11. Time series of wave-averaged bed stress derived from the shear strength of the bed. D (kg m s )is wave height and period data produced by the Australian simply Bureau of Meteorology, calculated using the WAM wave model (see Harris et al. (2000) for details). The reference position for D ¼ CW s; (12) the wave data is shown in Fig. 2. Wave-averaged bed stress was 2 calculated using: tb ¼ 2=3p rFUmax using a density (r)of 3 3 where C is the concentration (kg m ) and Ws is 1000 kg m and a friction factor (F) of 0.05. The excess stress the fall velocity (m s 1). of 2 N m 2 is also indicated. ARTICLE IN PRESS
P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 2451
Table 3 residual currents available to redistribute them. Wave statistics based on 3 years of wave data (1997–2000) Moreover, measurements of suspension settling generated by the Australian Bureau of Meteorology using the WAM model for the Australian region (see Harris et al. (2000) velocity and bed shear strength in the delta front for details) are required to determine critical entrainment stresses and suspension dynamics. Height (m) Period (s)
SE trades (April–November) 5. Conclusions Maximum 3.5 12.8 Mean 1.22 6.08 Current metre deployments across the mouths NW Monsoon of two distributary channels in the Fly River delta (December–March) indicate that a reversal in the direction of net Maximum 2.9 11.7 bedload transport occurs in each transect. Sand Mean 0.79 6.70 deposits form elongate deposits, similar to the tidal See Fig. 2 for location of the references point. sand banks of macrotidal estuaries, but of lower relief. Such cross-channel reversals in net bedload transport directions, in conjunction with the note that an excess bed shear stress must be occurrence of linear tidal bars, are consistent with achieved to first erode the bed and resuspend the the existence of mutually evasive, ebb-and flood- sediments. Also, in these type of environments the dominated bedload transport zones in the Fly bed shear stress associated with wave orbital delta. Cross-channel reversals in residual flow velocities is probably only secondary in impor- direction have also been observed and it is inferred tance to pressure fluctuations in causing bed that suspended sediment transport paths may be erosion and the production of fluid mud layers similarly complex. Such mutually evasive trans- (Wright et al., 1990). Data for the bed conditions port patterns must be taken into account when that are present on the delta front are required and developing deltaic sediment budgets. the modifications to the wave climate that occur Suspended sediment experiments at two loca- due to shallow water effects must be taken into tions, at the delta apex and in the Southern account to progress this line of analysis. Entrance, show that the effect of erosion and Our study has clearly identified a considerable settling lags is to produce a phase relationship gap in our understanding of the processes control- between tidal velocity and sediment concentration ling sedimentation on the Fly River delta. At such that the net suspended sediment flux is present, we can identify some mechanisms that directed seaward. These observations are consis- could explain fine sediment transport from the tent with the modelling results of Wolanski et al. distributary mouths, across the delta front, to the (1998). pro-delta. These include combined tide/wave- The export of fluid muds from the distributary induced resuspension and storm surge with baro- channels across the delta front and onto the pro- tropic return-flow, but we have insufficient data at delta cannot be explained by tidal current trans- present to make any reliable estimates. Unfortu- port alone, because of the marked drop-off in tidal nately, there are no data to date that even establish current energy seawards of the distributary chan- the existence of barotropic or hyperpycnal flows nel mouths. Gravity-driven fluid mud flows, 1–2 m across the delta front. In order to resolve this thick, generated in the distributary channels are fundamental question concerning the prograda- estimated to require at least 35 h to traverse the 20- tion of the Fly River delta, new data are required. km-wide, low-gradient (2 10 3 degrees) delta Specifically, measurements of mean, tidal and front. The velocities of such currents are well wave oscillatory flows during the Trade Wind below those required for autosuspension of the season are required to establish the bed shear fluid mud. Combined tide-and wave-induced stresses available to resuspend sediment and the resuspension may play a role in maintaining fluid ARTICLE IN PRESS
2452 P.T. Harris et al. / Continental Shelf Research 24 (2004) 2431–2454 mud flows across the delta front, but we have complex Cobequid Bay-Salmon River Estuary (Bay of insufficient data at present to make any reliable Fundy). Sedimentology 37, 577–612. estimates. Storm surge with barotropic return-flow Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual basis and stratigraphic is another potentially important process that implications. Journal of Sedimentary Petrology 62 (6), future field studies should consider as playing a 1130–1146. role in deltaic sediment export. Dalrymple, R.W., Baker, E.K., Harris, P.T., Hughes, M.G., (2003) Sedimentology and stratigraphy of a tide-dominated, foreland-basin delta (Fly River, Papua New Guinea). In: Sidi, F.H., Posamentier, H.W., Darman, H., Nummedal, Acknowledgements D., Imbert, P. (Eds.), Tropical Deltas of Southeast Asia and Vicinity—Sedimentology, Stratigraphy, and Petroleum Geology. SEPM Special Publication 76, Tulsa, Oklahoma, The work described in this paper was made pp. 147–173. possible by a grant awarded to PTH from the Dietrich, W.E., Day, G.M., Parker, G., 1999. The Fly River, Australian Research Council (Ref. No. Papua New Guinea: inferences about river dynamics, A39131170). Ok Tedi Mining Ltd. provided ship floodplain sedimentation, and fate of sediment. In: Miller, time on R.V. Western Venture for this project. The A.J., Gupta, A. (Eds.), Varieties of Fluvial Form. Wiley, New York, pp. 345–376. paper benefited from critical reviews provided by Dyer, K.R., 1994. Estuarine sediment transport and deposition. Dr. Peter Traykovski (Woods Hole Oceano- In: Pye, K. (Ed.), Sediment Transport And Depositional graphic Institute, USA) and by three other Processes. Blackwell Scientific, Oxford, pp. 193–218. anonymous reviewers. Published with permission Eisma, D., 1993. Suspended Matter in the Aquatic Environ- of the Executive Director, Geoscience Australia. ment. 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