Session G34: Geophysical Fluid Dynamics: Sediment Transport

The Effect of Dune-Dune Interaction on Migration Speed of Subaqueous Bedfoms

Interfaces between granular matter and fluids are commonplace in nature, ranging from sand deserts to river beds. It is well known that sufficiently energetic fluid flow mobilises and transports grains. Moreover, dynamical coupling between topography and flow fields may lead to the formation of ripples and dunes. The bedforms migrate downstream at a rate controlled by their own shape and the fluid flow. It is generally observed that under fixed flow conditions small dunes migrate faster, but there is no consensus about the exact scaling law. In order to test the competing theories, we investigate experimentally the migration speed of subaqueous bedforms under turbulent forcing in a custom-built experimental set-up. The annular geometry provides a unique opportunity to track bedforms in a steady flow over long periods of time. Here we show that apart from the size, the streamwise separation length between neighbouring bedforms can significantly affect their migration speed. Our results reveal strong dune-dune interactions and challenge the universality of local scaling laws relating dune sizes to migration speed.   

On the role of mangrove root flexibility and porosity in sediment deposition and erosion control.

Mangrove forests are dynamic ecosystems that provide many ecological services in coastal areas. Mangrove decreases erosion through biophysical interactions of aerial roots (pneumatophores) comprised of rigid and flexible branches oscillating in the water. Precise prediction of the morphological evolution for these changes requires an understanding of interactions between root porosity, water flows, and sediment transport. We present rigid cylinders as well as an elastically mounted rigid patch of circular cylinders as simplified flexible mangrove roots to understand the role of porosity and flexibility on the sediment deposition and erosion control. We carried out the force and PIV measurements for the flexible cylinders limited to a transverse oscillation inside a water tunnel at constant velocities. We investigate the effect of the porosity and flexibility on VIV of the patch over a wide range of Reynolds numbers (1000≤Re≤10000). Our preliminary results indicate that the amplitude of response depends on several parameters including Reynolds number, mass ratio, damping ratio, and natural frequency of the system. We discuss the effect of these parameters on the range of synchronization and their implications for the enhancement of sediment deposition and erosion control.

  

Flow over and inside Mars-like impact craters: an experimental study

The morphological processes that control the evolution of impact craters are yet to be fully understood. Recent studies have hypothesized a specific mechanism based upon long-term aeolian deflation of the original intracrater sediment deposit. In this work we perform physical modeling of Martian craters, including idealized craters based used in previous work. This study used a refractive index matching (RIM) approach. Transparent models of craters were fabricated by casting acrylic resin material into 3D printed molds. PIV velocity measurements were performed to reveal the instantaneous 3D flow structures generated by the crater ridge and mound, as well as to observe their evolution. The flow scenario revealed by our measurements is consistent with previous numerical work and supports the exhumation hypothesis previously proposed. Two streamwise oriented high-stress regions suggest that the wake is populated by counter-rotating eddies which are shed by the mound, convect downstream and eventually impinge onto the rim. 3D measurements reveal the shedding of complex alternating counter-rotating vortices caused by the mound.   

Impacts of Sediment-Induced Stratification on Turbulent Momentum and Sediment Fluxes In Wave- And Current-Driven Flows

Sediment entrainment and mixing processes in shallow-water environments are complicated by gravitational settling of suspended sediment because it inherently stratifies the water column, suppressing vertical transport. In this work we apply DNS to investigate the effects of sediment-induced stratification on wave and current driven estuarine flows. Through conditional statistics, we show stratification modifies the flow by suppressing the exchange of high- and low-momentum fluid by intense sweeps and ejections.This suppression manifests as a reduction in the vertical Reynolds stress, reducing shear production of TKE near the bed. However, higher in the water column shear production increases because of increased mean shear, acting to restore the vertical component of the Reynolds stress to its unstratified state. Similar dynamics occur for the vertical turbulent transport of sediment. However, no restoration mechanism exists for the sediment phase. As a result, near-bed stratification has a larger impact on the sediment dynamics than it does on the fluid dynamics.
  

A mass-conserving, cut-cell moving-bed model for scour simulation

We propose a moving-bed, cut-cell model to simulate scour with an existing unstructured-grid Navier-Stokes solver. The computational mesh includes cells in the water and within the bed, and the interface between them is set as the top of the sediment layer and moves due to erosion and deposition. Although the horizontal grid is unstructured, a Cartesian coordinate system is used in the vertical to avoid problems related to steep slopes when using curvilinear, bottom-following grids. To better represent the bathymetry, the cut-cell method is used in which finite-volume cells near the bottom are cut to ensure that the faces are aligned with the bed. To eliminate discontinuous shear caused by small cut cells at the bed, the re-mapping method is used. The Exner sediment mass balance equation is solved at the vertices of the unstructured grid cells while bedload tranpsort is computed tangential to cell faces and erosion and deposition are computed at cell centers. These components of sediment transport are incorporated into the model in a way that ensures conservation of total (suspended + bedload) sediment mass. We demonstrate that the model reproduces scour patterns for lab test cases and conserves sediment mass to machine precision.