The role of hydraulics in fine-grained sedimentation within river channels: an example from the Ovens River, Australia

River channels contain sediment deposits such as bars and benches which provide important ecological habitat for fish, macroinvertebrates and riparian and aquatic vegetation. Bars and benches influence flow to provide variations in water depth and velocity, as well as complex aquatic-terrestrial interactions. Benches in particular, a fine-grained bank-attached sediment deposit, larger than your average patch of mud, are ubiquitous within natural river channels of southeastern Australia. Yet, because fine-grained sedimentation within the channel is greatly influenced by a river's flow and sediment regime the size and abundance of benches is often reduced by river regulation, specifically the prolonged conveyance of irrigation flows. Historically, geomorphologists have been most interested in the gross channel morphology changes influenced by regulation, whereas, now with a focus on providing water for ecological habitat we need to better understand the relationships between within channel features and the flow regime. In this case study for example: How are sediment deposits such as benches created within the channel?

This vignette demonstrates the role of flow (also referred to as discharge) and hydraulics (the way the water moves through the channel) in fine-grained sedimentation within the channel with a case study from southeastern Australia at the aptly named 'Big Eddy' bench. In particular, this vignette introduces concepts such as sediment movement relative to velocity, the dynamics of hydraulics at abrupt meander bends including 'flow separation', and how low velocities within the channel result in fine-grained sediment deposits such as benches. Hopefully you are inspired to look into rivers with new eyes linking the geomorphic features you see with the flow over and around them.

The size of a sediment particle greatly influences its movement, or lack thereof, by flowing waters. A fine-grained sediment particle, such as a clay, silt or medium-grained sand (less than 1 mm in diameter), can be kept in suspension within the water column when the water is only just perceptibly flowing. For this reason the deposition of fine-grained suspended sediment is most commonly associated with floodplains with their very placid hydraulic environments (still or slow moving water). Yet, many meandering channels contain benches comprised of fine-grained sediment. Benches can occur at high elevations up the bank suggesting deposition during larger events - events commonly associated with high velocities. Bench deposits suggest that placid hydraulic environments are present at these locations even during the largest of events from 'bankfull' (a full channel) and greater.

Big Eddy bench is located on the Ovens River (mean annual discharge of 1,620 gigalitres per year) which exhibits a relatively high suspended sediment load, by Australian standards, transporting an estimated 80,000 tonnes per year to the receiving River Murray. Big Eddy bench is located on the concave bank (outer bank) of an abrupt bend, which is why these benches are commonly termed 'concave benches'. To investigate the hydraulics at the site, and relationship of the hydraulics to flow, a two-dimensional hydraulic model (where velocity is averaged with depth) was developed from site topography. Sediment deposition was measured using artificial grass mats collected after each event over a period of one year. Sediment was analysed for quantity, size and coarse particulate organic matter (CPOM).

As flow in the river increases, and the water level rises, the bench is inundated and flow separation occurs - a split between downstream and a smaller component of upstream flow. Flow separation results in most water flowing downstream but the remainder pushed upstream over the bench (Fig. 1). Flow separation is caused by the abruptness of the channel bend and the enlarged channel area immediately upstream of the apex of the bend. Between the flow separation line and the concave bank a large 'eddy' forms (Fig. 2).

The concave bank is more commonly associated with a vigorous hydraulic environment, yet, the energy in the eddy dissipates quickly. Velocities in the eddy are very low, commonly less than 0.2 m s^-1, even though velocities in the downstream flow (stem flow) are commonly 1.5 m s^-1 or greater (Fig. 3).

Shear stress, a measure of the force the flowing water applies to a sediment particle, can also be used to determine whether scour (sediment removal) is likely from the bench surface. Scouring of sediments is commonly associated with shear stress values greater than 0.6 to 1.4 N m^-2. For Big Eddy bench modelled maximum shear stresses are less than 0.4 N m^-2 for all flows to bankfull, so scour is highly unlikely and deposition is the dominant process.

Within the placid flow environment of the eddy suspended sediment (sediment carried within the water column) falls from suspension and deposits on the bench. This process of sediment deposition is referred to as vertical accretion. Sediment collected from the bench surface also suggests that the flow over the bench is very placid: more than 75 percent of the sediment is finer than silt (< 0.06 mm diameter). The organic matter (CPOM) such as leaves and twigs, is also retained on the bench and incorporated within the deposit. The retention of CPOM that has fallen onto the bench prior to inundation reveals the inability of the flow to scour even leaves and twigs from the bench surface throughout the rise and fall of the flow event.

By digging into the bench it is possible to see the laminations (sediment layers) revealed from each inundation event. The bench surface grows in height by about 60 mm per year until, for a concave bench such as Big Eddy, the bench is incorporated into the floodplain after some 50 to 100 years of formation. This process is transient, and, as the channel migrates laterally the process may be initiated once more.

Surprisingly, some of the largest deposits coincide with the largest flow events, some greater than bankfull. The volume of sediment deposited increases with both increasing duration and peak flow of the event. By increasing flow in the hydraulic model we can reveal how the eddy over the bench remains a placid environment with low velocities for the full range of flows from bench inundation to bankfull (Fig. 4). Even though we often consider large flow events as destructive for many within channel geomorphic features the concave bench tends to be well protected from the high energy flow by the hydraulics at the bend.

Without the hydraulic diversity experienced within river channels many of the interesting geomorphic features we observe would not be present. In particular fine-grained depositional features, such as benches, are formed in the low velocity environments that occur within channel expansions, behind obstructions, and in abrupt bends as presented here. These low velocity environments are conducive to vertical accretion of suspended sediment. The ability for rapid sedimentation of Big Eddy bench is the result of the low velocity eddy and particular the persistence of the eddy over the full range of flows from bench inundation to bankfull. Fig. 4 illustrates how flow in the channel can increase with little increase in the velocities over the bench. In essence the abrupt bend causes the flow separation (the flow can't 'bend' itself as much) and the bench is protected and created in the placid waters away from the vigorous stem flow. So the morphology of Big Eddy bench can be closely mapped to the hydraulic environment of the eddy and the flows at which this occurs. Understanding these relationships between in-channel features such as benches and the flow and sediment regimes assists us to interpret the anthropogenic impacts (human induced) e.g. regulation, and climatologic impacts e.g. increased rare events, on channel morphology. Consequently, we can better understand these impacts on ecological habitat and the health of our rivers.

Next time you visit a nearby river identify some interesting fine-grained depositional features, then, revisit the same site at a range of higher flows and see if you can describe the patterns in water movement in relation to the formation, or destruction, of the features.

• Hickin, E. J. (1979). Concave bank benches on the Squamish River, British Columbia, Canada. Canadian Journal of Earth Sciences, 16(1), 200-203.
• Page, K. and G. Nanson (1982). Concave-bank benches and associated floodplain formation. Earth Surface Processes and Landforms, 7, 529-543.
• Vietz, G.J., Stewardson, M.J., and B.L. Finlayson (2006). Flows that Form: The Hydromorphology of Concave-Bank Bench Formation in the Ovens River, Australia. In Sediment Dynamics and the Hydromorphology of Fluvial Systems, Rowan, J.S., Duck, R.W. and Werritty A. (eds.), IAHS Publication 306, International Association of Hydrological Sciences, Wallingford, pp 267-276.