Modeling sediment entrainment and transport processes limited by bed armoring

Sediment gravity flows, particularly those in the marine environment, are dynamically interesting because of the non-linear interaction of mixing, sediment entrainment/suspension and water-column stratification. Turbidity currents, which are strongly controlled by mixing at their fronts, are the best understood mode of sediment gravity flows. The type of mixing not only controls flow and deposition near the front, but also changes the dynamics of turbidity currents flowing in self-formed channels. Debris flows, on the other hand, mix little with ambient fluid. In fact, they have been shown to hydroplane, i.e. glide on a thin film of water. Hydroplaning enables marine debris flows to runout much farther than their subaerial equivalents. Some sediment gravity flows require external energy, from sources such as surface waves. When these flows are considered as stratification-limited turbidity currents, models are able to predict observed downslope sediment fluxes. Most marine sediment gravity flows are supercritical and thus controlled by sediment supply to the water column. Therefore, the genesis of the flows is the key to their understanding and prediction. Virtually every subaqueous failure produces a turbidity current, but they engage only a small percentage of the initially failed material. Wave-induced resuspension can produce and sustain sediment gravity flows. Flooding rivers can also do this, but the complex interactions of settling and turbulence need to be better understood and measured to quantify this effect and document its occurrence. Ultimately, only integrative numerical models can connect these related phenomena, and supply realistic predictions of the marine record.

Section: Box 4 Calculation Of the Total Flux In A Wave-supported Sedimentioning

confidence: 99%

The Mechanics of Marine Sediment Gravity Flows

Parsons

Friedrichs

Traykovski

et al. 2007

“…This winnowing of fine sediment out of the bed surface layer leaves a coarsened, armoured layer at the bed surface that must be mobilized before additional fine sediment in the bed can become available for suspension (Kachel & Smith, 1986;Reed et al, 1999). Therefore, for non-cohesive beds, the grain-size distribution and active-layer thickness of the bed surface control sediment availability for resuspension and transport, and thus the volume of sediment in suspension.…”

Section: Critical Shear Stress and Sediment Erodibilitymentioning

confidence: 99%

“…14a). This surface-active layer, typically in the order of a few centimetres in thickness, controls the availability of sediment for resuspension in sandy beds (Kachel & Smith, 1986;Reed et al, 1999). Larger active bedforms have the potential to rework the bed to greater depths as they migrate, but the time required for complete reworking of the surface layer (the time required for a bedform to migrate one complete wavelength) depends on the bedform migration rate, which can be slow for large bedforms and bedforms in oscillatory flows with little net transport.…”

Section: Bedforms and Controls On Sediment Availabilitymentioning

confidence: 99%

Post‐Depositional Alteration and Preservation of Sedimentary Strata

Wheatcroft

Wiberg

Alexander

et al. 2007

All sediment that is delivered to the seabed is subject to post-depositional alteration before it becomes part of the preserved stratigraphic record. Physical and biological processes occurring in the upper few decimetres of the seabed alter newly deposited sediment, thereby creating the fine-scale sedimentary record. The geographical region of focus is the Eel margin of northern California, where a combination of high precipitation, frequent winter storms and intense benthic biological processes exert fundamental controls on the fine-scale stratigraphy. Major findings are that:1 although the wave conditions on the Eel shelf are highly energetic, only a few millimetres to one or two centimetres of bed material are typically put in suspension during storms; 2 despite a high initial porosity (i.e. low strength), flood beds quickly become resistant to erosion; 3 shelf macrofauna show considerable resilience to deposition of flood sediment; 4 storm deposits have a low preservation potential on open continental margins; 5 episodic sedimentation is key to preservation of strata and signals produced by flood and storm events.Future insight into post-depositional alteration will benefit from focused observations and theoretical modelling that explicitly couple physical and biological processes on multiple time-scales.

“…First, a number of efforts have been made to improve the representations of near-bed transport processes and model parameterizations. These studies address the parameterization of bottom roughness (Xu & Wright, 1995;Li & Amos, 1998), reference concentration (Li et al, 1996), effects of flocculation (Carr, 2002), near-bed stratification due to suspended sediment (Styles & Glenn, 2000), and bed armouring (Wiberg et al, 1994;Reed et al, 1999). Second, twodimensional, time-dependent models can resolve effects of advection as well as patterns of net erosion or deposition (Zhang et al, 1999;Harris & Wiberg, 2001).…”

Section: Sedimentmentioning

confidence: 99%

Prediction of Margin Stratigraphy

Syvitski

Pratson

Wiberg

et al. 2007

A new generation of predictive, process-response models provides insight about how sedimenttransport processes work to form and destroy strata, and to influence the developing architecture along continental margins. The spectrum of models considered in this paper includes short-term sedimentary processes (river discharge, surface plumes, hyperpycnal plumes, wave-current interactions, subaqueous debris flows, turbidity currents), the filling of geological basins where tectonics and subsidence are important controls on sediment dispersal (slope stability, compaction, tectonics, sea-level fluctuations, subsidence), and acoustic models for comparison to seismic images. Recent efforts have coordinated individual modelling studies and catalysed Earth-surface research by:1 empowering scientists with computing tools and knowledge from interlinked fields; 2 streamlining the process of hypothesis testing through linked surface dynamics models; 3 creating models tailored to specific settings, scientific problems and time-scales.The extreme ranges of space-and time-scales that define Earth history demand an array of approaches, including model nesting, rather than a single monolithic modelling structure. Numerical models that simulate the development of landscapes and sedimentary architecture are the repositories of our understanding about basic physics and thermodynamics underlying the field of sedimentology.