4th International Submarine Canyon Symposium (INCISE2018)
5-7 November 2018, Shenzhen, CHINA
This is the detail of Going with the flow: Tracking turbidity currents along the seafloor with man-made motion-recording boulders.
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Charles Paull, Brian Kieft, Dave Caress, Peter Talling, Krystle Anderson, Eve Lundsten, Matthieu Cartigny
Going with the flow: Tracking turbidity currents along the seafloor with man-made motion-recording boulders.
During the course of 18 months the Coordinated Canyon Experiment (CCE) documented the passage of 15 sediment gravity flows along the axis of Monterey Canyon, offshore California, USA, utilizing a comprehensive array of moored sensors and instruments placed between 200m to 1,850m water depths (mwd). Boulder-like devices capable of recording their own motion were placed between 200- and 400- mwd to document the arrival, velocity and flow evolution of sediment density flows. These novel instruments called Benthic Event Detectors (BED) were housed inside spheres or cubes of 50 cm size made of metal, plastic and syntactic foam ballasted to a density of 2.1 gr/cc and half buried in the seafloor during deployment. One BED was also placed atop an 800 kg ~2m-tall metal tripod (density > 7 gr/cc). BEDS contain accelerometers along three orthogonal axes, a time recorder and a pressure sensor inside a pressure case rated to 500 mwd. Changes in orientation trigger data collection at a recording rate of 50 Hz until motion stops. Built-in acoustic beacons and modems allow for BEDs to be relocated and data to be downloaded, even when BEDs are buried in sediment to depths of >1 m. Over the course of the study twenty-four BED motions were recorded as a result of nine sediment density flows sweeping though the upper Canyon (<500 mwd). During a sediment gravity flow on the 24th of November 2016, round and square BEDs and the tripod-mounted BED all travelled at nearly the transit speed of the event, reaching 4 m/s. The simultaneous movement of 3 BEDS of different densities and shapes at nearly the same velocity at the front of the flow, and the simultaneous movement of 5 BEDs separated by a 3-km distance along the channel during this event, suggest the entire seabed along the canyon axis was in motion. This observation supports the proposition that sediment gravity flows consist of fast and dense near-bed layers of remobilized seafloor. The longest BED motion lasted half an hour and covered a distance of 4.5 Km. The detrended trajectory of this BED shows that the down-canyon movement of the BED includes multiple vertical deviation of ~ 1.7 m from the general down-canyon trajectory. In order to evaluate whether this variability reflects transport along crescent shaped bedforms the detrended BED trajectory is compared with the detrended bathymetry of the thalweg mapped with an autonomous underwater vehicle 2 weeks after the event. The detrended bathymetry shows vertical deviations of ~1.3 m. Significantly, the number of vertical deviations along the detrended BED record (n = 131) is close to the number of vertical deviation from the detrended bathymetry along the same distance (n =135). These observations are consistent with the BEDS riding within a basal layer that follows the crescent shaped bedforms morphology. The detailed record of BEDs motions during sediment gravity flows highlights the promise of this new tool for the study of marine sediment transport processes
Session 2: New ways to study submarine canyons: integrated programmes, new technologies and coordinated monitoring efforts