Gareth M. Keevil

Global (latitudinal) variation in submarine channel sinuosity

Current classifications of submarine channels and fans link channel sinuosity to gradient, and in turn to sediment caliber, with end members being high-sinuosity, low-gradient, fine-grained systems and low-sinuosity, high-gradient, coarse-grained systems. However, the most sinuous modern submarine channels, such as the Amazon, Bengal, Indus, and Zaire, along with ancient sinuous submarine channels, are located in equatorial regions. Here we quantitatively compare slope versus latitude controls on submarine channel sinuosity and show that the latitudinal control is strong, while that of slope is weak. Variation in sinuosity with latitude is shown to occur uniquely in submarine channels; no comparable relationship is observed for terrestrial river channels. Possible causal mechanisms for this latitudinal variation are explored, focusing on the influence of the Coriolis force, flow type, and sediment type. Although climate does not vary straightforwardly with latitude, climatic controls on flow and sediment type may explain some of the latitudinal variation; Coriolis force, however, varies with latitude alone and produces an excellent fit to the observed sinuosity-latitude distribution. Regardless of which control predominates, latitudinal global variation in channel sinuosity should have changed over geologic time. Since deposit architecture and facies are linked directly with sinuosity, submarine channel deposits should also systematically vary in space and time.

Sub-aqueous sand extrusion dynamics

Extruded sandstone mounds and sandstone sheets are increasingly recognized in the geological record and seismic data, yet the underlying mechanisms that control these features, and determine whether deposits form discrete accumulations or areally extensive sheets, remain poorly understood. Here we address these issues through study of a well-exposed ancient sand sheet in the Shannon Basin, Ireland. Sand volcanoes are shown to have erupted contemporaneously following liquefaction and fluidization of delta-front mouth-bar sands and silts, the expelled sediment forming an extruded sheet of sand. The sand sheet formed during a single prolonged eruption event with the interaction of radial gravity currents from the hundreds of vents exerting control on the internal architecture of the extrudite. Based on our observations, here we develop a process-based model that demonstrates that sand extrusions can form sheets only if (1) multiple vents are extruding coevally, causing gravity currents to interact, or (2) topographic forcing, such as channelling, redirects the otherwise radial gravity currents, resulting in sheet-like deposition of extruded material away from the vent site. This study provides a new model of sand extrudite formation, and examines the potential for identifying extrudites from core and bed-scale studies and differentiating them from liquefied beds.

Near‐bed and surface flow division patterns in experimental river bifurcations

Understanding channel bifurcation mechanics is of great importance for predicting and managing multichannel river processes and avulsion in distributary river deltas. To date, research on river channel bifurcations has focused on factors determining the stability and evolution of bifurcations. It has recently been shown that, theoretically, the nonlinearity of the relation between sediment transport and flow discharge causes one of the two distributaries of a (slightly) asymmetrical bifurcation to grow and the other to shrink. The positive feedback introduced by this effect results in highly asymmetrical bifurcations. However, there is a lack of detailed insight into flow dynamics within river bifurcations, the consequent effect on bed load flux through bifurcating channels, and thus the impact on bifurcation stability over time. In this paper, three key parameters (discharge ratio, width-to-depth ratio, and bed roughness) were varied in order to examine the secondary flow field and its effect on flow partitioning, particularly near-bed and surface flow, at an experimental bifurcation. Discharge ratio was controlled by varying downstream water levels. Flow fields were quantified using both particle image velocimetry and ultrasonic Doppler velocity profiling. Results show that a bifurcation induces secondary flow cells upstream of the bifurcation. In the case of unequal discharge ratio, a strong increase in the secondary flow near the bed causes a larger volume of near-bed flow to enter the dominant channel compared to surface and depth-averaged flow. However, this effect diminishes with larger width-to-depth ratio and with increased bed roughness. The flow structure and division pattern will likely have a stabilizing effect on river channel bifurcations. The magnitude of this effect in relation to previously identified destabilizing effects is addressed by proposing an adjustment to a widely used empirical bed load nodal-point partition equation. Our finding implies that river bifurcations can be stable under a wider range of conditions than previously thought.

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.

Scaling Analysis of Multipulsed Turbidity Current Evolution With Application to Turbidite Interpretation

Deposits of submarine turbidity currents, turbidites, commonly exhibit upward‐fining grain size profiles reflecting deposition under waning flow conditions. However, more complex grading patterns such as multiple cycles of inverse‐to‐normal grading are also seen and interpreted as recording deposition under cycles of waxing and waning flow. Such flows are termed multipulsed turbidity currents, and their deposits pulsed or multipulsed turbidites. Pulsing may arise at flow initiation, or following downstream flow combination. Prior work has shown that individual pulses within multipulsed flows are advected forward and merge, such that complex longitudinal velocity profiles eventually become monotonically varying, although transition length scales in natural settings could not be predicted. Here we detail the first high frequency spatial (vertical, streamwise) and temporal measurements of flow velocity and density distribution in multipulsed gravity current experiments. The data support both a process explanation of pulse merging and a phase‐space analysis of transition length scales; in prototype systems, the point of merging corresponds to the transition in any deposit from multipulsed to normally graded turbidites. The scaling analysis is limited to quasi‐horizontal natural settings in which multipulsed flows are generated by sequences of relatively short sediment failures ( 10 km) sequences of breaches or where pulsing arises from combination at confluences of single‐pulsed flows, such flows may be responsible for the pulsing signatures seen in some distal turbidites, >100 km from source.

MEMS-Integrated Load Cell for Measuring Pressure, Erosion, and Deposition in Dynamic Environmental Flows

A microelectromechanical system-based load cell is integrated with signal conditioning circuitry, temperature, and tilt sensors for measurement of sediment-fluid interaction and flow under turbulent conditions. Such an instrument has potential value for quantifying geophysical flows and sediment dynamics within a range of environments. Sensor sensitivity is tested to a mass of 0.5 g but can be shown theoretically to extend to 50 mg. The sensor is found to have no attenuation of frequencies up to 2.5 Hz and would therefore be suitable for monitoring turbulent flow. Laboratory flume experiments, simulating a dam burst, demonstrate the applicability of the sensor for measuring highly dynamic and transient flow phenomena in unprecedented detail.

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...