Flávio Norberto de Almeida Junior

Hyperpycnal flows: are they sediment-gravity flows?

Authors: Flávio N. de Almeida Júnior & Ronald Steel

Hyperpycnal flow is a process involving the underflow of denser material through sediment-laden river currents that enter lighter ambient water, e.g., lake or seawater. Relatively high suspended sediment concentration in the river flow is often the main cause of the high flow density forcing the river-discharge to plunge in the area of the river mouth. Sediment particles make hyperpycnal flows denser than the water they displace, otherwise fresh water would be lighter than seawater, so that many authors ubiquitously consider hyperpycnal flow as a type of sediment-gravity flow. Hyperpycnal flow models also idealize a hydraulic continuum between the fluvial current and the plunging of hyperpycnal flow, in the notion that hyperpycnal flow discharges beyond the river mouth into ambient water would have velocity profiles and duration approximate to the parental river currents.

However, recent studies show that : (i) the initiation of underflows may occur from extremely dilute river plumes so that any defined value of threshold sediment-concentration is not enough to explain the plunging of river discharges – thus, the notion of denser does not directly equate to excess of sediment-mass (volume x density) or sediment gravity transport in the flow; (ii) not all energy fluctuations in hyperpycnal flows are necessarily a function of river hydrography; and (iii) large-magnitude river floods do not necessarily imply high sediment concentration or hyperpycnal flows, and not all sediment-gravity currents generated during river-flood periods must necessarily be of hyperpycnal origin (e.g., slope collapse at Squamish Delta during river flood periods). All the above constraints are allusive: fluid mechanics of hyperpycnal flows are mired in controversy, and there has been a fuzzy use of the term ‘hyperpycnal flow’ since its early definition – in particular regarding the connection between river dynamics and hyperpycnal flows. Hence, there is still uncertainty on how to recognize the deposits of hyperpycnal flows in the sedimentary record.

This presentation aims to bridge between hydrodynamic processes and the deposits of riverine hyperpycnal flows by critically analyzing what is known-to-date from experiments, modern environments and the empirical evidence. We therefore propose a novel classification for hyperpycnal-flows based on the concept of discharge momentum, in which initial conditions of discharge momentum at river flow unconfinement and its subsequent dissipation into ambient water would primarily constrain hyperpycnal flow parameters. This approach allowed distinction of three hyperpycnal-flow types, which not all actually represent sediment-gravity flows.  For the first time, we provide a simple resolution that links river-discharge hydrodynamics to hyperpycnal flows that can be used as diagnostic criterion for the identification of hyperpycnal-flow deposits (or hypepycnites) on strata. We support our arguments with published literature and new diagnostic field data from the Early-Middle Jurassic Lajas and Los Molles formations in the southernmost Neuquén Basin, Argentina.