Environmental Earth Sciences
River Effluent and Hypercynal Turbidity Currents
Kathryn Amos, Jan Alexander and Mike Leeder
When river effluent has elevated suspended sediment concentration, to the extent that its density is greater than that of the receiving marine water, a hyperpycnal (underflowing) turbidity current may form.
Turbidity currents are a type of gravity current, whereby flow results from density differences between two fluids. In a turbidity current, the density contrast results from suspended particulate matter within the fluid, held in turbulent suspension. Empirical work by Mulder and Syvitski (1995) estimated that 66% of the 230 world rivers that they studied are capable of producing underflows into marine basins because of elevated suspended sediment concentrations during floods with return periods of 1000 years or less. Hyperpycnal flows may be sustained for periods of hours to weeks or months, and thus are likely to be an important mechanism of sediment transport to the oceans. The deposits of such flows are likely to be common in the world's marine basins and the rock record - although they are not very commonly described in the published literature.
Hyperpycnal turbidity currents are difficult to observe and record in the natural environment. The deposits of both modern and ancient flows can be studied in an attempt to infer flow characteristics. However, there are many problems with inferring flow behaviour from deposits, and analogue, numerical and scaled experimental models are used to further understanding of the sediment transport processes and depositional mechanisms of these currents.
Scaled experimental hyperpycnal turbidity current flow in the laboratory can be used as an effective tool to observe and record flow behaviour and deposit characteristics. Individual controlling factors can be isolated and their effects on the experimental current quantified.
Research on hyperpycnal turbidity currents is being carried out at UEA by Kathryn Amos, Dr Jan Alexander and Prof. Mike Leeder. Kathryn's PhD project title is 'Quasi-steady turbidity currents and their deposits', investigating flow characteristics and deposits of quasi-steady turbidity currents, through scaled laboratory experiments, theoretical models and case-studies (literature and field-based, modern and ancient).
Kathryn's lab work involves analysis of experimentally generated quasi-steady hyperpycnal turbidity currents. In these experiments the density excess of the current is produced by mixing glass Ballotini (45 - 90 micron) with tap water in a mixer tank, which is then pumped into a flume tank filled with standing ambient water (flume dimensions: 6 m x 0.33 m x 0.5 m). The experimental set-up allows for a time span of input of the dense fluid into the flume tank in the order of 30 minutes to one hour. Analysis of experimental results will establish the quasi-steady nature of the flows.
The deposits of these laboratory experiments are examined and general patterns of flow behaviour observed. Use of ultrasonic Doppler velocity probes loaned to us from the Turbidites Research Group at the University of Leeds has allowed non-intrusive measurement of within-flow velocity structures. These scaled-experiments will give us insights into the behaviour of flow patterns and depositional characteristics of quasi-steady hyperpycnal turbidity currents. These data will then allow modelling and prediction of the behaviour of natural flows.
The Burdekin River in North Queensland, Australia, acts as a modern analogue and focus for Kathryn's theoretical work. The Burdekin River is one of the largest rivers draining Australia (Fielding et al., 1999). It has a mean annual discharge of 9.8 x 109 m3 yr-1, with about 90% of the annual discharge occurring between January - April (Fielding et al., 1999). The highest recorded discharge of the Burdekin River (near to the mouth) was 40 393 m3s-1 on 4th March 1946 (Fielding et al., 1999). Several million tonnes of sediment can be transported to the Burdekin River mouth in 24hr at peak flow (Alexander et al., 1999; Belperio, 1979), which may mean that the river transports high enough concentrations of suspended sediment for hyperpycnal currents to form at the river mouth. The aim of this part of Kathryn's studies is to investigate this possibility and to use the Burdekin River as a case study for comparison with published theoretical models of quasi-steady hyperpycnal turbidity current flow. Following her experimental work, Kathryn plans to examine possible field examples of ancient quasi-steady turbidity current deposits. These sites will act as case studies for comparison with laboratory results and theoretical ideas.
References
Alexander, J., Fielding, C.R., and Pocock, G.D., 1999, Flood behaviour of the Burdekin River, tropical north Queensland, Australia, in Marriott, S.B., and Alexander, J., (Eds) Floodplains: Interdisciplinary Approaches, Geological Society London Special Publication 163, 27 - 40.
Belperio, A.P., 1979, The combined use of wash load and bed material load rating curves for the calculation of total load: an example from the Burdekin River, Australia, Catena, 6, 317 - 329.
Fielding, C.R., Alexander, J., and McDonald, R., 1999, Sedimentary Facies from ground-penetrating radar surveys of the modern, upper Burdekin River of north Queensland, Australia: consequences of extreme discharge fluctuations, Special Publication of the International Association of Sedimentologists, 28, 347 - 362.
Mulder, T., and Syvitski, J.P.M., 1995, Turbidity Currents Generated at River Mouths during Exceptional Discharges to the World Oceans, Journal of Geology, 103, 285 - 299.
