BEAST—A portable device for quantification of erosion in natural intact sediment cores

A portable Particle Erosion Simulator (PES) device, also referred to as the BEAST (Benthic Environmental Assessment Sediment Tool) (Walker et al., 2008) has been re-designed for quantifying erosion in natural intact sediment cores. The BEAST was re-configured from an older design (Tsai and Lick, 1986), which had uncalibrated flow characteristics and was limited to viewing resuspension. In addition to calibrating friction velocity at the sediment–water interface, we employ a combination of real-time turbidity monitoring (via measurement of % transmission which decreases proportionally to suspended solid concentration) to quantify erosion threshold and calculate erosion rate, as well as digital imaging to document sequences of erosion and particle size response of resuspended material. The BEAST consists of a clear acrylic Plexiglas™ core liner with a perforated disc oscillating vertically in a piston motion. Performance of the device was calibrated by (a) comparing predicted to observed friction velocity as a function of motor speed, (b) using a hot film anemometer in the chamber to measure shear velocity, (c) verifying the applicability of anemometric calibration by relating the power of the grid stroke to stress dissipation, and (d) comparing measured critical stress of foundry sand to predictions from a validated model. Measurements indicate the friction velocity is uniform over >50% of the radial distance from the core center. Bottom stress is highly sensitive to the final height of piston down-stroke, a variable that can be altered to control the range of friction velocities. A plot of piston motor RPM vs. predicted $u_{\ast }$ was identical to the regression fit through the observed data. We verified that the proportionality between power input and thermistor heat dissipation corresponds to the scaling of $u_{\ast }$ and RPM, consistent with our calibration using the stress sensor. An example of an erosion sequence is demonstrated from a field core obtained in the Beaufort Sea in which two erosion stages were clearly indicated in the combined results from measurements of % transmission (to determine turbidity), particle size, and erosion rate. Our studies confirm that the BEAST has predictable flow characteristics expected from first principles, and that applied shear stress causes erosion in a way quantitatively similar to horizontal shear. In addition, the predicted erosion threshold of sand-sized particles corresponds to within 3%–18% of measured values made using the device. These multiple sources of BEAST validation demonstrate its practical capability to provide quantitative field measurements of transport parameters from intact marine sediments if applied in a similar manner, and further contribute to predictive capability in modeling of benthic–pelagic coupling.