Waiapu River, NZ

Across-Shelf Sediment Dispersal Off a High-Yield River:
Observations and Modeling of Gravity-Driven Transport and Deposition
 
waiapu

Motivation

Much of our knowledge of fluvial sediment dispersal on continental shelves derives from consideration of low-density suspensions, such as those characteristic of many large rivers that issue buoyant plumes into the coastal ocean. Early conceptual models of shelf sedimentation focused on clastic shelves to explain the occurrence of modern fine sediments in areas that receive significant fluvial input (Curray, 1965; Swift, 1970; McCave, 1972). These models were refined through the study of numerous river systems, providing significant understanding of the variations of shelf sediment dispersal patterns and controlling physical processes (e.g., Nittrouer and Wright, 1994; Nittrouer et al., 1995; Wright and Nittrouer, 1995). The vast majority of these
earlier studies have been conducted off large rivers with moderate to high sediment discharge, but with low initial sediment concentrations (~mg/L).

The importance of small to moderate-sized rivers, particularly those draining high
mountainous areas along collision margins, has been historically underestimated (Milliman and Syvitski, 1992; Nittrouer et al., 1994). The combination of high rainfall and high-relief drainage basins along with folded and sheared sedimentary rocks leads to extremely high specific sediment yields and highly concentrated river effluent. In certain areas, changes in land- use patterns (e.g., deforestation and agriculture) have accelerated basin sediment erosion, and, in the case of the Waiapu River in New Zealand, have resulted in a specific sediment yield of about 22,520 tons/(km2 yr), among the highest on earth (Hicks et al., 2000). We contend that the prevailing view of sedimentary processes on shelves has been significantly biased by the focus on large rivers, but that recent study of high-concentration flows is beginning to reveal contrasting modes of sediment dispersal and strata formation that will significantly alter the way we view shelf sedimentation over space and time.

Recent studies have shown that the movement of material across high-yield margins from river plumes to the continental shelf may often be dominated by gravitational transport mechanisms that operate only on dense suspensions (e.g., Mulder and Syvitski, 1995; Kineke et al., 1996; Wright et al., 2001). Negatively buoyant (hyperpycnal) plumes have been observed to move large volumes of material across the margin in the near-bed region (Kineke at al., 1996; Ogston et al., 2000; Traykovski et al., 2000). As opposed to hypopycnal (positively buoyant), or
homopycnal (neutrally buoyant) flows that are primarily dispersed by local circulation patterns, hyperpycnal flows are driven by gravity. Such flows can be generated either through major river flood events or through other processes that concentrate sediment suspensions, including estuarine-like circulation and rapid settling (Kineke and Sternberg, 1995), and stratification at the top of the wave boundary layer (Traykovski et al., 2000).

Transport mechanisms that operate on hyperpycnal flows are likely to create a different depositional signature than dilute dispersal modes. For example, cross-shelf transport of dense turbid layers would be likely to leave fairly thick layers of poorly-sorted material, whereas nearly neutrally buoyant, dilute suspensions might result in a widely dispersed, thin drape of well-sorted sediment.

Gravity-driven sediment transport and deposition is relatively well understood for
continental slopes, where high slope gradients maintain turbid flows through autosuspension. The role of gravity-driven processes on more gently dipping continental shelves, however, has until recently been either neglected or assumed to be unimportant. Recent results from the ONR-sponsored STRATAFORM and other studies now suggest that highly turbid hyperpycnal layers, nourished by a combination of river supply and resuspension by waves and currents, move downslope across the shelf under the influence of gravity, but without becoming autosuspending (Ogston et al., 2000; Traykovski et al., 2000; Wright et al., 2001; in press; Scully et al., in press; submitted). In the cases examined to date, deposition from the turbid mud layers occurs when either wave energy or gradients in bed slope diminish. Our studies further suggest the possibility of an equilibrium cross sectional profile shape for shelves dominated by gravity-driven transport of mud that may offer a universal model for subaqueous clinoforms observed off many rivers (Wright et al., 2001; Scully et al., in press). To test our hypotheses, however, requires a field laboratory where the hyperpycnal signal and across shelf slope variations are unmistakably clear and the supply of high concentration suspensions is sustained and predictable. The Waiapu river-mouth/shelf system on the east coast of New Zealand’s North Island offers such a laboratory.