Hydraulic Jumps in Turbidity Currents and Their Deposits

George Postma and Matthieu Cartigny
Earth Sciences, Utrecht University, Utrecht, Netherlands

The depositional characteristics that are ascribed to hydraulic jump settings are all based on inferences. Most commonly associated features are structures known as backset bedding, i.e., upslope dipping cross-stratification reported from steep slope settings. Field evidence suggests upslope migrating bedforms, dm to several meters high and (tens of) meters long as products associated with hydraulic jumps. The main problem is that there are no data to link direct observation of the formation of these sedimentary structures to flow parameters. As a matter of fact, the types of bed form development (e.g. the backset bedding), being well documented for free surface water flows (antidunes) have never been studied experimentally in turbidity currents.
In an attempt to gather insight in the relation between jump process in turbidity currents and their products, a combined numerical and experimental approach is chosen. The experiments must help to evaluate deterministically the essential processes at work. Since direct application of these processes to ancient, real world analogues is hampered by scaling issues, we plan to use a numerical model in a next step. A numerical model, if validated by a real world analogue, will bridge the gap between the small-scale laboratory model and the real world prototype. The recent Var canyon system (France) and the Monterey canyon system (USA) are such potential prototypes that have been described in sufficient detail to allow validation of numerics.

The results combine conductivity concentration measurements, electromagnetic velocity measurements and high speed camera images of the flow. The movies show in detail the flow structure and the flow-bed interaction before, during and after the jump. Second, we aim to compare the flume results with field analogues.

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