Encyclopedia of Sediments and Sedimentary Rocks

Soil characteristics and landcover relationships on soil hydraulic conductivity at a hillslope scale: a view towards local flood management

The streambed constitutes the physical interface between the surface and the subsurface of a stream. Across all spatial scales, surface water-groundwater interactions are controlled by the physical properties of the streambed. Streambed properties such as topography or hydraulic conductivity are continuously altered through erosion and sedimentation processes. Recent studies from the fields of ecology, hydrogeology and sedimentology provide field evidence that sedimentological processes themselves can be heavily influenced by surface water-groundwater interactions, giving rise to complex feedback mechanisms between sedimentology, hydrology and hydrogeology. More explicitly, surface water-groundwater exchanges play a significant role in the deposition of fine sediments, which in turn modify the hydraulic properties of the streambed. We explore these feedback mechanisms and critically review the extent of current interaction between the different disciplines. We identify opportunities to improve current modeling practices. For example, hydrogeological models treat the streambed as a static rather than a dynamic entity, while sedimentological models do not account for critical catchment processes such as surface water-groundwater exchange. A blueprint for a new modeling framework is proposed that bridges the conceptual gaps between sedimentology, hydrogeology and hydrology. Specifically, this blueprint (1) fully integrates surface-subsurface flows with erosion, transport and deposition of sediments, and (2) accounts for the dynamic changes in surface elevation and hydraulic conductivity of the streambed. The opportunities for new research within the coupled framework are discussed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016RG000530

Blueprint for a coupled model of sedimentology, hydrology, and hydrogeology in streambeds

Relating in situ hydraulic conductivity, particle size and relative density of superficial deposits in a heterogeneous catchment

Here we study differential stress equation models for the subgrid scale SGS stress tensor in large eddy simulations of urbulent incompressible flow. A study of the SGS stress equation is performed using the principle of frame indifference and the concept of realizability. Closure models are proposed that satisfy these constraints together with the additional requirement that the modeled balance equation must degenerate into the ordinary balance equation for SGS kinetic energy when contracted. The SGS stress equation model is applied to forced homogeneous isotropic turbulence and fully developed turbulent channel flow. For low Re numbers the differential stress equation model behaves in a similar manner to the linear combination model. At higher Re numbers it behaves increasingly like an eddy-viscosity model, but is better able to handle flow and grid anisotropy than traditional SGS models.

Differential subgrid stress models in large eddy simulations

In mixed sediment beds, erosion resistance can change relative to that of beds composed of a uniform sediment because of varying textural and/or other grain-size parameters, with effects on pore water flow that are difficult to quantify by means of analogue techniques To overcome this difficulty, a three-dimensional numerical model was developed using a finite difference method (FDM) flow model coupled with a distinct element method (DEM) particle model The main aim was to investigate, at a high spatial resolution, the physical processes occurring during the initiation of motion of single grains at the sediment–water interface and in the shallow subsurface of simplified sediment beds under different flow velocities Increasing proportions of very fine sand (D50=008 mm) were mixed into a coarse sand matrix (D50=06 mm) to simulate mixed sediment beds, starting with a pure coarse sand bed in experiment 1 (0 wt% fines), and proceeding through experiment 2 (65 wt% fines), experiment 3 (105 wt% fines), and experiment 4 (287 wt% fines) All mixed beds were tested for their erosion behavior at predefined flow velocities varying in the range of U
 1-5=10–30 cm/s The experiments show that, with increasing fine content, the smaller particles increasingly fill the spaces between the larger particles As a consequence, pore water inflow into the sediment is increasingly blocked, ie, there is a decrease in pore water flow velocity and, hence, in the flow momentum available to entrain particles These findings are portrayed in a new conceptual model of enhanced sediment bed stabilization

A conceptual model of pore-space blockage in mixed sediments using a new numerical approach, with implications for sediment bed stabilization

Mangroves grow in the coastal and intertidal zones at tropical and subtropical latitudes. It is widely accepted that the establishment, growth and survival of mangrove seedling depend on the environmental conditions such as temperature, tidal regime and hydrodynamics. To date we know that, in cohesionless sediment, the higher the flow velocity the greater the eroded volume and, thus, the stronger the scour around the mangrove seedling which can lead to its uprooting or death. However, how a mangrove seedling (cm-scale) alters the flow pattern and the sediment transport is, to date, still poorly understood. Four suits of numerical experiments were setup with the same mangrove seedling and subjected to flow speeds of 5 cm·s−1, 10 cm·s−1 and 15 cm·s−1 as well as a rarely occurring extreme scenario 50 cm·s−1. The hydro- and sediment dynamics were simulated using a coupled sediment-hydrodynamic continuum approach: the Finite Volume Method utilizing the software package OpenFOAM. The two phases, silty sediment and water, constitute a mixture. The distribution of the sediment phase in the water phase was estimated with the drift-flux approximation. A natural mangrove seedling (Rhizophora mucronata) was digitized employing photogrammetry and discretized into the model domain. This numerical model was validated against experimental observations and is able to capture the main features of the flow and sediment transport around a mangrove seedling. The numerical simulations showed that a downward flow associated to a horseshoe vortex enhances scour in front of the mangrove seedling and a vortex shedding keeps the sediment in suspension or re-suspends the sediment in the rear of the mangrove seedling. Thus, a mangrove seedling has a significant influence on the flow pattern and sediment transport: the higher the flow speed, the less stable the sediment bed. Additionally, these findings could help to better understand how the settlement of mangrove seedlings and sediment dynamics affect mangrove establishment and why the colonization or restoration of tidal flats is successful or not. This paper is an initial study focusing on a single mangrove seedling on a cm-scale. It is anticipated to extend the interactions between several seedlings as well as mangrove roots. With this successfully utilized approach, it will be possible in the future to investigate the impact of several mangrove seedlings forming a matrix on a m-scale on sediment dynamics. So, this work creates the basis to study more complex sediment transport processes such as erosion and sediment trapping in mangrove forests.

Numerical modelling of hydraulics and sediment dynamics around mangrove seedlings: Implications for mangrove establishment and reforestation

Layered deposits of relatively light and heavy minerals can be found in many aquatic environments. Quantification of the physical processes which lead to the fine-scale layering of these deposits is often limited with flumes or in situ field experiments. Therefore, the following research questions were addressed: (i) how can selective grain entrainment be numerically simulated and quantified; (ii) how does a mixed bed turn into a fully layered bed; and (iii) is there any relation between heavy mineral content and bed stability? Herein, a three-dimensional numerical model was used as an alternative measurement to study the fine-scale process of density segregation during transport. The three-dimensional model simulates particle transport in water by combining a turbulence-resolving large eddy simulation with a discrete element model prescribing the motion of individual grains. The granular bed of 0.004 [m] in height consisted of 200,000 spherical particles (D50 = 500 μm). Five suites of experiments were designed in which the concentration ratio of heavy (5000 [kg/m³]) to light particles (i.e. 2560 [kg/m³]) was increased from 6%, 15%, 35%, 60% to 80%. All beds were tested for 10 s at a predefined flow speed of 0.3 [m/s]. Analysis of the particle behaviour in the interior of the beds showed that the lighter particles segregated from the heavy particles with increasing time. The latter accumulated at the bottom of the domain, forming a layer, whereas the lighter particles were transported over the layer forming sweeps. Particles below the heavy particle layers indicated that the layer was able to armour the particles below. Consequentially, enrichment of heavy minerals in a layer is controlled by the segregation of a heavy mineral fraction from the light counterpart, which enhances current understanding of heavy mineral placer formation.

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Understanding heavy mineral enrichment using a three-dimensional numerical model

Hydraulic conductivity measurements

Framework-forming cold-water corals provide a refuge for numerous organisms and, consequently, the ecosystems formed by these corals can be considered as impressive deep-sea biodiversity hotspots. If suitable environmental conditions for coral growth persist over sufficiently long periods of time in equilibrium with continuous sediment input, substantial accumulations of coral mound deposits consisting of coral fragments and baffled sediments can form. Although this conceptual approach is widely accepted, little is known about the prevailing hydrodynamics in their close proximity, which potentially affect sedimentation patterns. In order to refine the current understanding about the hydrodynamic mechanisms in the direct vicinity of a model cold-water coral colony, a twofold approach of a laboratory flume experiment and a numerical model was set up. In both approaches the flow dynamics around a simplified cold-water coral colony used as current obstacle were investigated. The flow measurements of the flume provided a dataset that served as the basis for validation of the numerical model. The numerical model revealed data from the vicinity of the simplified cold-water coral such as the pressure field, velocity field, or the turbulent kinetic energy in high resolution. Features of the flow like the turbulent wake and streamlines were also processed to provide a more complete picture of the flow that passes the simplified cold-water coral colony. The results show that a cold-water coral colony strongly affects the flow field and eventually the sediment dynamics. The observed decrease in flow velocities around the cold water-coral hints to a decrease in the sediment carrying potential of the flowing water with consequences for sediment deposition.

/pdf/investigating-the-prevailing-hydrodynamics-around-a-cold-5c04guung6.pdf

Gerhard Bartzke

Papers

A conceptual model of pore-space blockage in mixed sediments using a new numerical approach, with implications for sediment bed stabilization

Numerical modelling of hydraulics and sediment dynamics around mangrove seedlings: Implications for mangrove establishment and reforestation

Understanding heavy mineral enrichment using a three-dimensional numerical model

Hydraulic conductivity measurements

Investigating the Prevailing Hydrodynamics Around a Cold-Water Coral Colony Using a Physical and a Numerical Approach