Near-Field Dispersion of a Turbidity Current on a Slope

Abstract

The current supply of critical materials falls short of keeping up with the required pace to comply with the commitment governments worldwide made towards a decarbonised energy mix by 2050. To avoid dependency on large suppliers, lower the pressure on land mining expansion and still meet the metal demand of the rapidly growing green economy, more companies are turning their focus towards the deep-sea, where these critical metals are present in the form of nodules. However, sediment plumes from nodule collection can negatively impact marine ecosystems and hinder mining operations. This research investigates how slope angles and directions affect the near-field dispersion of turbidity currents discharged from a moving source during deep-sea mining (DSM). The study focuses on the Clarion-Clipperton Zone (CCZ), rich in the so-called polymetallic nodules.
Experiments were conducted at TU Delft's Offshore and Dredging Laboratory using a modular flume tank with an experimental setup scaled at 1:20. The configuration included a moving cart and diffuser system on a sloping seabed, with precise control over slope angles and velocity ratio. A homogeneous suspension of glass beads (used instead of CCZ sediment) with water was prepared in a mixing tank and discharged into the flume to simulate a turbidity current. Measurement techniques involved the acoustic Doppler velocimeter (ADV) and ultrasonic velocity profiler (UVP) sensors for mixture concentration and velocity data, complemented by multi-angle camera footage for visual analysis. Various slope angles (-5° to 5°) and velocity ratios (source velocity over discharge velocity, ζ = 1.00 and 1.25) were tested to observe their effects on the behaviour of a turbidity current, with device calibration and controlled experimental parameters ensuring accurate and comparable results.
A notable phenomenon was observed during the experiments, namely the formation of a bulge directed towards the diffuser. A higher velocity ratio and steeper slopes led to more significant bulges, with the bulge length and impingement angle increasing with slope steepness. Regarding velocity ratios, higher ratios resulted in larger dispersion angles and larger turbidity current heights in downhill driving experiments, while uphill conditions showed an inverse trend. The slope angles influence the dispersion by reducing the dispersion angle for steeper slopes in downhill driving experiments. Data analysis from the UVP, camera footage, and concentration measurements support these findings.
Future research should focus on influencing factors on bulge formation, assessing the influence of the concentration on the dispersion of a turbidity current, identifying parameter ranges for floating plumes, conducting experiments with reduced source velocities, scaling down experiments and using real CCZ sediment in saltwater.