Jasim Imran

Distinguishing sediment waves from slope failure deposits: Field examples, including the 'humboldt slide', and modelling results

Abstract Migrating sediment waves have been reported in a variety of marine settings, including submarine levee–fan systems, floors of fjords, and other basin or continental slope environments. Examination of such wave fields reveals nine diagnostic characteristics. When these characteristics are applied to several features previously attributed to submarine landslide deformation, they suggest that the features should most likely be reinterpreted as migrating sediment-wave fields. Sites that have been reinterpreted include the ‘Humboldt slide’ on the Eel River margin in northern California, the continental slope in the Gulf of Cadiz, the continental shelf off the Malaspina Glacier in the Gulf of Alaska, and the Adriatic shelf. A reassessment of all four features strongly suggests that numerous turbidity currents, separated by intervals of ambient hemipelagic sedimentation, deposited the wave fields over thousands of years. A numerical model of hyperpycnal discharge from the Eel River, for example, shows that under certain alongshore-current conditions, such events can produce turbidity currents that flow across the ‘Humboldt slide’, serving as the mechanism for the development of migrating sediment waves. Numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor (i.e. numerical steps), sediment waves can form and migrate upslope. Hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves. Physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms. The morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field, including those that were previously described as submarine landslides.

A numerical model of submarine debris flow with graphical user interface

Abstract A 1-D numerical model of the downslope spreading of a finite-source subaqueous debris flow is presented. The model incorporates the Bingham, Herschel–Bulkley, and bilinear rheologies of viscoplastic fluids. Any of these rheologies can be selected by the user. The layer-integrated conservation equations of mass and momentum balance are solved in a Lagrangian framework using an explicit finite difference scheme. The flow is assumed to remain laminar throughout the computation. Starting from an initial parabolic shape, the debris mass is allowed to collapse and propagate on a given topography. The code is written in the visual basic programming language and has a graphical user interface. The required input parameters can be specified interactively, and the propagation of the debris flow can be viewed as the solution proceeds. The user interface for the software is described in detail. Simulated results from different rheological models are compared.

A nonlinear model of flow in meandering submarine and subaerial channels

A generalized model of flow in meandering subaqueous and subaerial channels is developed. The conservation equations of mass and momentum are depth/layer integrated, normalized, and represented as deviations from a straight base state. This allows the determination of integrable forms which can be solved at both linear and nonlinear levels. The effects of various flow and geometric parameters on the flow dynamics are studied. Although the model is not limited to any specific planform, this study focuses on sine-generated curves. In analysing the flow patterns, the turbidity current of the subaqueous case is simplified to a conservative density flow with water entrainment from above neglected. The subaqueous model thus formally corresponds to a subcritical or only mildly supercritical mud-rich turbidity current. By extension, however the analysis can be applied to a depositional or erosional current carrying sand that is changing only slowly in the streamwise direction. By bringing the subaqueous and subaerial cases into a common form, flow behaviour in the two environments can be compared under similar geometric and boundary conditions. A major difference between the two cases is the degree of superelevation of channel flow around bends, which is modest in the subaerial case but substantial in the subaqueous case. Another difference concerns Coriolis effects: some of the largest subaqueous meandering systems are so large that Coriolis effects can become important. The model is applied to meander bends on the youngest channel in the mid-fan region of the Amazon Fan and a mildly sinuous bend of the North-West Atlantic Mid-Ocean Channel. In the absence of specific data on the turbid flows that created the channel, the model can be used to make inferences about the flow, and in particular the range of values of flow velocity and sediment concentration that would allow the growth and downfan migration of meander bends.

Three-dimensional modeling of density current. I. Flow in straight confined and unconfined channels

The lateral development of density-driven flow in a subaqueous channel is studied using a three-dimensional numerical model. The model is applied to four different cases of confined and unconfined flow in straight channels. The density current is generated by injecting a heavier fluid as a plug flow at the upstream boundary of the channel. The inflow transforms into a density current within a short distance. The current increases in thickness by entraining the ambient lighter fluid as it travels in the downstream direction. In the unconfined cases, the flow begins to spill in the lateral direction as soon as its thickness exceeds the bank height. The spilling results in a distinct lateral flow in the overbank area and limits the growth of the current in the vertical direction. The vertical structure of the overbank flow in the lateral direction is found to be similar to the primary flow structure of a density current. The magnitude of the lateral flow is found to depend strongly on the lateral slope of the floodplain. In the confined case, high sidewalls keep the flow inside the channel and the current continues to grow in the vertical direction. Two weak circulation cells are observed near the sidewalls in all cases. These circulation cells may act to redistribute fluid density across a channel cross-section. Although the model is applied here to a conservative density current, the analysis is valid in general for turbidity current driven by fine sediment.

Numerical analysis of the mobility of the Palos Verdes debris avalanche, California, and its implication for the generation of tsunamis

Abstract Analysis of morphology, failure and post-failure stages of the Palos Verdes debris avalanche reveals that it may have triggered a significant tsunami wave. Our analysis of the failure itself indicates that the slope is stable under aseismic conditions but that a major earthquake (with a magnitude around 7) could have triggered the slide. A post-failure analysis, considering the debris avalanche as a bi-linear flow, shows that peak velocities of up to 45 m/s could have been reached and that the initial movement involved a mass of rock less than 10 km wide, 1 km long and about 50–80 m thick. Initial wave height estimates vary from 10 to 50 m. Tsunami waves propagating to the local shoreline would be significantly smaller. Such a range demonstrates our lack of proper knowledge of the transition from failure to post-failure behavior related to mass movements. Further investigations and analyses of terrestrial and submarine evidence are required for a proper hazard assessment related to tsunami generation in the Los Angeles area.

Helical flow couplets in submarine gravity underflows

Active and relic meandering channels are common on the seafloor adjacent to continental margins. These channels and their associated submarine fan deposits are products of the density-driven gravity flows known as turbidity currents. The tie between channel curvature and its effects on these gravity flows has been an enigma. This paper records the results of both large-scale laboratory measurements and a numerical simulation that captures the three-dimensional flow field of a gravity underflow at a channel bend. These findings reveal that channel curvature drives two helical flow cells, one stacked upon the other. The lower cell forms near the channel bed surface and has a circulation pattern similar to that observed in fluvial channels, i.e., with a near-bed flow directed inward. The other circulation cell forms in the upper part of the gravity flow and has a streamwise vorticity with the opposite sense of the lower cell.

Dam-break flows over mobile beds: Experiments and benchmark tests for numerical models

In this paper, the results of a benchmark test launched within the framework of the NSF–PIRE project “Modelling of Flood Hazards and Geomorphic Impacts of Levee Breach and Dam Failure” are presented. Experiments of two-dimensional dam-break flows over a sand bed were conducted at Université catholique de Louvain, Belgium. The water level evolution at eight gauging points was measured as well as the final bed topography. Intense scour occurred close to the failed dam, while significant deposition was observed further downstream. From these experiments, a benchmark was proposed to the scientific community, consisting of blind test simulations, that is, without any prior knowledge of the measurements. Twelve different teams of modellers from eight countries participated in the study. Here, the numerical models used in this test are briefly presented. The results are commented upon, in view of evaluating the modelling capabilities and identifying the challenges that may open pathways for further research.

Simulation of turbid underflows generated by the plunging of a river

When the density of sediment-laden river water exceeds that of the lake or ocean into which it discharges, the river plunges to the bottom of the receiving water body and continues to flow as a hyperpycnal flow. These particle-laden underflows, also known as turbidity currents, can travel remarkable distances and profoundly influence the seabed morphology from shoreline to abyss by depositing, eroding, and dispersing large quantities of sediment particles. Here we present a new approach to investigating the transformation of a plunging river flow into a turbidity current. Unlike previous workers using experimental and numerical treatments, we consider the evolution of a turbidity current from a river as different stages of a single flow process. From initial commotion to final stabilization, the transformation of a river (open channel flow) into a density-driven current (hyperpycnal flow) is captured in its entirety by a numerical model. Successful implementation of the model in laboratory and field cases has revealed the dynamics of a complex geophysical flow that is extremely difficult to observe in the field or model in the laboratory.

Role of fine-grained sediment in turbidity current flow dynamics and resulting deposits

Abstract A numerical model of a turbidity current driven by poorly-sorted sediment is developed. The model is applied to study the role that finer fractions of suspended sediment play in the evolution of a submarine fan due to the flow of a subsiding turbidity current. The depth-integrated model can treat multiple size fractions of suspended sediment, ranging from clay to coarse sand. The model is implemented in a sensitivity study by varying inflow fractions of suspended sediments. Computed results clearly indicate that a modest increase in fine sediment content in the inflow dramatically increases the sand carrying capacity of a turbidity current. It is also found that clay is more effective than fine or coarse silt in maintaining the momentum and the identity of a current. Downstream fining and gradual thinning of the simulated deposit is observed in computations performed with multiple grain sizes.

Numerical simulation of mud-rich subaqueous debris flows on the glacially active margins of the Svalbard-Barents Sea

Abstract Seismic images and sediment core data from the Bear Island and Isfjorden fans localized along the Svalbard–Barents Sea continental margin, reveal an interesting depositional system consisting of stacked debris flow lobes. The frequent release of debris flows was associated with large volumes of sediment rapidly delivered to the shelf break during periods of maximum glaciation. The compositions of the lobes for both fans are similar, consisting of mainly clay and silt. The data show, however, a dramatic difference in runout distances for the two areas. Isfjorden debris lobes are 10–30 km in length whereas Bear Island lobes are 100–200 km in length. Even more intriguing is the fact that the large runout distances on the Bear Island fan occurred on slopes less than 1° whereas the Isfjorden fan flows occurred on slopes greater than 4°. Depth-averaged non-linear one-dimensional equations for balance of mass and linear momentum are applied to simulate the subaqueous debris flow. The equations are solved by the numerical model BING, describing the flow as a visco-plastic Bingham fluid. The model is employed to study the effect yield strength, viscosity and bathymetry have on debris flow runout. The study shows that the large runout distances can be achieved on the Bear Island fan by visco-plastic flows with sufficiently low yield strength. High yield strength sediments require an additional mechanism, such as hydroplaning, to reach measured runout distances. Most importantly, this study shows the necessity of good rheological measurements for accurate numerical modeling of subaqueous debris flows.