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Structure of turbulence and sediment stratification in wave‐supported mud layers
Author(s) -
Hooshmand Abbas,
HornerDevine Alexander R.,
Lamb Michael P.
Publication year - 2015
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2014jc010231
Subject(s) - settling , stratification (seeds) , boundary layer , turbulence , flume , geology , reynolds stress , sediment , mechanics , sediment transport , geomorphology , flow (mathematics) , physics , seed dormancy , germination , botany , dormancy , biology , thermodynamics
We present results from laboratory experiments in a wave flume with and without a sediment bed to investigate the turbulent structure and sediment dynamics of wave‐supported mud layers. The presence of sediment on the bed significantly alters the structure of the wave boundary layer relative to that observed in the absence of sediment, increasing the TKE by more than a factor of 3 at low wave orbital velocities and suppressing it at the highest velocities. The transition between the low and high‐velocity regimes occurs when R e Δ ≃ 450 , where R e Δis the Stokes Reynolds number. In the low‐velocity regime ( R e Δ < 450 ) the flow is significantly influenced by the formation of ripples, which enhances the TKE and Reynolds stress and increases the wave boundary layer thickness. In the high‐velocity regime ( R e Δ > 450 ) the ripples are significantly smaller, the near‐bed sediment concentrations are significantly higher and density stratification due to sediment becomes important. In this regime the TKE and Reynolds stress are lower in the sediment bed runs than in comparable runs with no sediment. The regime transition at R e Δ = 450 appears to result from washout of the ripples and increased concentrations of fine sand suspended in the boundary layer, which increases the settling flux and the stratification near the bed. The increased stratification damps turbulence, especially near the top of the high‐concentration layer, reducing the layer thickness. We anticipate that these effects will influence the transport capacity of wave‐supported gravity currents on the continental shelf.