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 lÀ1 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 sÀ1 within the distributary channels to o50 cm sÀ1 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 Â 10À3 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 aÀ1, 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 aÀ1 (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 aÀ1, 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 sÀ1, 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 aÀ1. 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.