A dimensionless framework for predicting submarine fan morphology

Limited observations of active turbidity currents at field scales challenges the development of theory that links flow dynamics to the morphology of submarine fans. Here we offer a framework for predicting submarine fan morphologies by simplifying critical environmental forcings such as regional slopes and properties of sediments, through densimetric Froude (ratio of inertial to gravitational forces) and Rouse numbers (ratio of settling velocity of sediments to shear velocity) of turbidity currents. We leverage a depth-average process-based numerical model to simulate an array of submarine fans and measure rugosity as a proxy for their morphological complexity. We show a systematic increase in rugosity by either increasing the densimetric Froude number or decreasing the Rouse number of turbidity currents. These trends reflect gradients in the dynamics of channel migration on the fan surface and help discriminate submarine fans that effectively sequester organic carbon rich mud in deep ocean strata.

A dimensionless framework for predicting submarine fan morphology

A remarkable diversity exists in the morphology and dynamics of submarine fans, which influence the transport of microplastics, burial of organic carbon, subsea geo-hazards, and their potential to house geofluids and high-resolution paleo-environmental records. Like river deltas, submarine fan morphology is a product of evolving fluid and sediment transport fields, but unlike their terrestrial counterparts, we lack a unifying framework to predict their morphology. Here, we simplify critical environmental forcings, like regional slopes and sediment properties, through a dimensionless framework defined by the densimetric Froude number (ratio of inertial to gravitational forces) and Rouse number (ratio of settling velocity of sediments to shear velocity) of turbidity currents. We explore this framework by leveraging a depth-averaged numerical model and measure fan rugosity as a proxy for their morphological complexity. We show a systematic increase in rugosity by either increasing the densimetric Froude number or decreasing the Rouse number of the simulated flows. These changes reflect observed gradients in the dynamics of channel migration and help discriminate submarine fans that have the potential to impact global climate through sequestration of organic carbon.

Experimental Analysis of Scour Features at Chevrons in Straight Channel

Eco-friendly river restoration structures are used to create localized scour pools which serve as fish nurseries and promote biodiversity. In this category, chevrons are relatively new structures designed to maintain navigability in rivers. The scour hole formed in the wake region of chevrons can either act as a disposal site for dredged material or as a resting spot for different fish species. However, only few studies are present in the literature dealing with the scour mechanism due to chevrons. Therefore, this work aims to analyze the scour features at equilibrium, under different hydraulic conditions and transversal locations in a straight channel. Tests were conducted with both isolated and multiple chevrons in series arrangement. Scour morphology types were classified and their fields of existence were established as well. A detailed dimensional analysis was conducted, allowing us to identify the main parameters governing the scour phenomenon and derive a novel equivalent densimetric Froude number. Finally, empirical equations were developed to predict the maximum scour depth and length as well as the maximum dune height.

Hydrodynamic aspects of large river confluence with different water densities

<p>River confluences are characterized by complex 3D changes in flow hydrodynamics and bed morphology and provide important ecological functions. The current literature on river confluences suggests that their hydrodynamics and morphodynamics are controlled by three aspects: (1) the geometry (planform and junction angle) of the confluence, (2) the momentum flux ratio of the tributaries and (3) the level of concordance between channel beds at the confluence entrance. However, the difference in water densities between the tributaries, and the associated stratification, potentially may impact on hydrodynamics and mixing as well, but such aspects has received less attention by far, and has not yet been subject to systematic investigation.</p><p>The objective of this study is to investigate hydrodynamics and mixing within the confluence zone of the Kama and Vishera rivers (Russia). During the warm period, the water densities in these rivers are similar due to the peculiarities of their hydrological basins. Hence density effects are negligible. However, in winter, the mineralization level of waters in the Vishera river significantly exceeds that in the Kama river. Even due to a significant decrease in the discharge of these rivers, the densimetric Froude number Fr is of the order of unity. This condition provided the motivation for investigating the effects of density differences on hydrodynamic and mixing at such river confluence.</p><p>The study of these effects was carried out on the basis of full-scale field measurements and numerical experiments in a full 3D formulation (i.e. with no hydrostatic approximation). Both the field measurements and the numerical results suggest that hydrodynamics processes at the confluence in the absence and in the presence of density stratification are fundamentally different.. At large densimetric Froude numbers (at small density differences) the waters of the Vishera and Kama rivers flow, practically without mixing, for several kilometers in the form of two parallel streams and at Fr of the order of unity, the more mineralized (more dense) waters of the Vishera river flow under the less dense waters of the Kama river leading to much more rapid mixing.</p><p>The reported study was funded by Russian Foundation for Basic Research (RFBR) and Perm Krai (grant 20-45-596028) and by RFBR (grant 19-41-590013).</p>

A Coupled CFD-DEM Simulation of Immersed Granular Collapse

<p>Natural disasters normally involve the flow of polydispersed granular materials with interstitial fluid which may change the flow dramatically. Here we focus on a typical small-scale case of fluid–particle mixture flows, i.e., the immersed granular collapse using computational fluid dynamics coupled with discrete element method (CFD-DEM). The simulation parameters are calibrated with laboratory experiments and the immersed granular collapse process is reproduced in terms of different aspect ratios. We present a deeper investigation of the collapse based on simulation results. The granular front evolves in three stages, i.e., acceleration, steady propagation, and deceleration. We found that the constant propagation stage is maintained by the transition of particles’ motion from vertical to horizontal and the drag of the fluid. The constant propagation velocity is proportional to the free-fall velocity with a Stokes-number-dependent coefficient and the normalized final runout is linearly correlated with the densimetric Froude number. These conclusions may find its significance in geophysical applications.</p>

Evolutionary optimization of neural network to predict sediment transport without sedimentation

Sedimentation in open channels occurs frequently and is relative to system inflow. The long-term retention of sediments on channel beds can increase the possibility of variations in deposits and their eventual consolidation. This study compares three hybrid artificial intelligence methods in estimating sediment transport without sedimentation (STWS). We employed the Particle Swarm Optimization (PSO), Imperialist Competitive Algorithm (ICA) and Genetic Algorithm (GA) methods in combination with the Artificial Neural Network (ANN) to overcome the weakness of ANN training with conventional algorithms. We used the ICA, GA and PSO methods to optimize the weights of the ANN layers. Using dimensional analysis, we placed the effective parameters in predicting sediment transport into five non-dimensional groups. Six models are proposed and run using three hybrid methods (18 models in total). As the comparisons demonstrate, the proposed combined models are more accurate than ANN and existing equations in estimating the densimetric Froude number (Fr). However, we found the ICA–ANN superior to GA–ANN and PSO–ANN, as it produces explicit solutions to the problem. The ICA–ANN has the lowest prediction uncertainty band for Fr of all developed models. Moreover, the variation trend of the Fr for all input variables (except overall friction factor of sediment) is a second-order polynomial.

Study of Scour Characteristics Downstream of Partially-Blocked Circular Culverts

Debris accumulations upstream and through crossing hydraulic structures such as culverts cause the upstream water level and the downstream scour depth to increase, which can lead to structure failure. This experimental study aimed to investigate the effects of various inlet blockage ratios on culvert efficiency and scour hole depth. In a non-blocked case, various submergence ratios (S = 1.06, 1.33, 1.60, and 1.90) were tested with different discharge rates. In a blocked case, the effects of inlet blockage with various blockage ratios (Ar = 10%, 20%, and 30%) were seen as sediment blockage on the pipe bed or floating debris upstream of the culvert. The results show that as the submergence ratio increases, the maximum scour depth decreases at the same discharge rate, and the relative energy loss also decreases in the non-blocked case. In the sediment blockage (Ar d) case, the relative maximum depth increases with increasing densimetric Froude number and with an increasing blockage ratio. An empirical equation was developed to predict the relative scour depth under the present study conditions.