Structure of turbulence and sediment stratification in wave‐supported mud layers

Abstract

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 Re D ' 450, where Re D is the Stokes Reynolds number. In the low-velocity regime (Re D < 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 (Re D > 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 Re D 5450 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.

Wave Generation of Gravity‐Driven Sediment Flows on a Predominantly Sandy Seabed

“…Sediment concentration profiles at both sites show a high concentration layer with a thickness ranging between 5 and 20 cm and high concentration gradients (Figures g–i). The concentration profiles show an exponential decay with maximum concentrations near the bed (Figures g–i), similar to other field and laboratory experiments (Hooshmand et al, ; Lamb & Parsons, ; Souza et al, ; Traykovski et al, ).…”

Observations of Near‐Bed Shear Stress in a Shallow, Wave‐ and Current‐Driven Flow

“…We found that the velocity decayed to zero at the bottom boundary, unlike our field measurements in which the velocity decayed to zero at the top of the canopy. These results mirrored those found in a recent laboratory study using a Vectrino in a sediment flume (Hooshmand et al, 2015). This is further evidence that our reported velocity profiles are showing physical flow features rather than measurement errors.…”

In situ observations of wave‐supported fluid‐mud generation and deposition on an active continental margin

“…ABS instruments have been calibrated in a similar manner to this study, using a maximum SSC of 33 g L −1 , based on the sound absorption parameters of their specific suspended‐sediment distribution [ Traykovski et al , ]. In flume experiments with a similar grain‐size distribution to this study, observed SSC maxima were 50–70 g L −1 [ Hooshmand et al ., ]. Because SSC exerts a first‐order control on U g in equation , this study imposes a maximum SSC (SSC max ) of 50 g L −1 to prevent artificially over‐predicting the relative importance of WSFM in off‐shelf sediment transport.…”

“…Furthermore, using sensitive instrumentation capable of detecting thin, near‐bed layers in a harsh ocean environment is difficult. To improve our understanding of the controls of WSFM dynamics, several studies have successfully generated high‐density conditions in laboratory flumes [e.g., Lamb and Parson , ; Hooshmand et al ., ] and numerical simulations [e.g., Traykovski et al , ; Ozdemir et al , , ]. These studies accommodate control of the individual WSFM drivers (e.g., U w , SSC, and α ), and allow for detailed observation of their behavior.…”