Stephen M. Simmons

Monitoring Suspended Sediment Dynamics Using MBES

This technical note describes use of a multibeam echosounder (MBES) to quantify the dynamics of suspended sediment in a large open channel. A methodology is detailed that uses the backscatter magnitude from the MBES water-column data to adjust the magnitude of sonar returns for the various sonar settings, spatially and temporally average the data to account for the random nature of acoustic backscatter from the suspended sediment, and calibrate the processed data with direct samples. A case study of flow at the confluence of the Rio Parana and Rio Paraguay, Argentina, where there is a distinct turbidity difference along the mixing interface of the two flows, is used to demonstrate the unique capabilities of MBES to quantify sediment concentrations and dynamics within the water column.

Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons

Runaway turbidity currents stretch into the deep ocean to form the largest sediment accumulations on Earth. Seabed-hugging flows called turbidity currents are the volumetrically most important process transporting sediment across our planet and form its largest sediment accumulations. We seek to understand the internal structure and behavior of turbidity currents by reanalyzing the most detailed direct measurements yet of velocities and densities within oceanic turbidity currents, obtained from weeklong flows in the Congo Canyon. We provide a new model for turbidity current structure that can explain why these are far more prolonged than all previously monitored oceanic turbidity currents, which lasted for only hours or minutes at other locations. The observed Congo Canyon flows consist of a short-lived zone of fast and dense fluid at their front, which outruns the slower moving body of the flow. We propose that the sustained duration of these turbidity currents results from flow stretching and that this stretching is characteristic of mud-rich turbidity current systems. The lack of stretching in previously monitored flows is attributed to coarser sediment that settles out from the body more rapidly. These prolonged seafloor flows rival the discharge of the Congo River and carry ~2% of the terrestrial organic carbon buried globally in the oceans each year through a single submarine canyon. Thus, this new structure explains sustained flushing of globally important amounts of sediment, organic carbon, nutrients, and fresh water into the deep ocean.

Imaging of Large‐Scale Sediment Transport Dynamics with Multibeam Sonar

Multibeam Echo‐Sounder (MBES) systems have developed rapidly over recent decades and are routinely deployed to provide high‐resolution bathymetric imaging. Modern data handling and storage technologies have facilitated the logging of the back‐scatter information previously discarded by these systems. This paper develops a novel methodology to exploit this logging capability to quantify the concentration and dynamics of suspended sediment within the water column. This development provides a multi‐purpose tool for the holistic surveying of sediment transport by imaging suspended sediment concentration, associated flow structures and providing concurrent high‐resolution bathymetry. This paper presents the results obtained from the deployment a RESON 7125 MBES in the field, with the aim of examining the dynamics of suspended sediment transport over dune bedforms and in the region of flow mixing between large rivers of significantly different suspended sediment concentration. Results from controlled tank tests are also presented, obtained using different types and mixes of sediment. The results demonstrate the capability of MBES systems to successfully resolve the contrast in suspended sediment concentrations. The large sets of data recorded in the two‐dimensional MBES swath enables the real‐time monitoring of suspended sediment transport and related flow processes on a scale previously unrealisable with single‐beam acoustic systems.

Measuring suspended sediment concentrations and flow velocities using multibeam sonar.

Modern data handling and storage technologies facilitate the logging of the large quantity of water‐column backscatter information received by multibeam sonars. Methods of using these data to derive estimates of the mass concentration and flow velocities of suspended sediment flow structures have been developed. The results obtained by the application of these methodologies to data collected at the confluence of the Parana and Paraguay rivers in Argentina and the confluence of the Mississippi and Missouri rivers in the United States will be presented. An analysis of those data in conjunction with a set of experimental data collected in a large‐scale test facility will be also given. The applicability and limitations of the use of multibeam sonar for deriving suspended sediment concentrations will be discussed. By enabling the simultaneous measurements of suspended sediment concentration, flow velocities, and bathymetric data, multibeam echo‐sounders are demonstrated to be a versatile tool for the surveyin...