Near‐bed turbulence dissipation measurements in the inner surf and swash zone

Lanckriet, Thijs; Puleo, Jack A.

doi:10.1002/2013jc009251

Cited by 19 publications

(22 citation statements)

References 63 publications

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“…The TKE dissipation rate was estimated from velocity data using the structure function method (Wiles et al, ). The structure function has been employed to study turbulence in the ocean (Mohrholz et al, ; Thomson, ), in estuaries (MacDonald & Mullarney, ; Mullarney & Henderson, ), in surf zones (Lanckriet & Puleo, ), and, recently, in mangrove forests (Norris et al, ). The structure function uses differenced adjacent along‐beam locations (“bins”) up to a number of lags (bin distances) within a velocity profile to estimate dissipation rates.…”

Section: Field Experiments and Methodsmentioning

confidence: 99%

Turbulence Within Natural Mangrove Pneumatophore Canopies

et al. 2019

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High‐resolution velocity measurements were collected within and above two dense canopies of mangrove pneumatophore roots in a wave‐exposed mangrove forest. In both canopies, root density decreased steadily with height above bed owing to the variability in root heights and the tapered shape of the roots. Within the canopies, we consider turbulence within three zones: near the bed above the wave boundary layer, around the mean canopy height, and above the canopy. The near‐bed turbulence was particularly intense (up to 6.5 × 10−4 W/kg), likely owing to oscillatory wave‐driven currents flowing past dense vegetation. Near the bed and around the mean canopy height, peaks in horizontal velocity power spectra at frequencies corresponding to Strouhal numbers of ~0.2 may indicate Von Kármán wake shedding in the lee of the pneumatophores. Furthermore, a recirculation zone was observed immediately behind a cluster of pneumatophores at intermediate heights. These coherent flow structures were associated with zones of enhanced Reynolds stresses (up to 5.3 × 10−3 m2/s2) and eddy viscosities (up to 1.9 × 10−3 m2/s). Large near‐bed stresses were associated with near‐bed drag coefficients that are up to an order of magnitude larger than those expected in the absence of vegetation. Observed eddy viscosities are consistent with theoretical expectations, derived from scaling arguments using a standard mixing‐length model. These results suggest that pneumatophore roots can contribute greatly to turbulent mixing (e.g., eddy viscosities were on average O(10−4–10−3 m2/s) and therefore may enhance the sediment transport occurring in mangrove forest fringes.

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Section: Field Experiments and Methodsmentioning

confidence: 99%

Turbulence Within Natural Mangrove Pneumatophore Canopies

et al. 2019

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“…Injection of bore related turbulence contributes to the cross-shore advection of particles into the swash zone that, together with the local mobilization, are effective in transporting sediment during swash cycle initiation [Butt and Russell, 1999;Puleo et al, 2000;Petti and Longo, 2001;Jackson et al, 2004;Masselink et al, 2005;Aagaard and Hughes, 2006]. During the remainder of the decelerating uprush motion, the turbulence decays similar to grid turbulence [Cowen et al, 2003;Sou and Yeh, 2011;Lanckriet and Puleo, 2013] and more homogeneously [Zhang and Liu, 2008] as the flow reaches the maximum inundation limit toward land (e.g., run up distance). Hence, sediment particles are more likely to settle during the flow reversal phase.…”

Section: Introductionmentioning

confidence: 99%

“…Shortly afterward, the flow evolves from subcritical to supercritical as the favorable pressure gradient becomes increasingly stronger during the backwash phase [Baldock and Hughes, 2006]. Sediment transport is then dominated by bed-generated turbulence [Longo et al, 2002;Cowen et al, 2003;Zhang and Liu, 2008;Lanckriet and Puleo, 2013] that may also mobilize sediment as sheet flow [Hughes et al, 2007;Lanckriet et al, 2014]. The relative dominance of the different flow phases (i.e., uprush versus backwash asymmetry) during a swash cycle largely depends on the beach face slope and incident wave conditions [Masselink and Puleo, 2006] and is known to have a large influence controlling the overall morphological response of the beach [Osborne and Rooker, 1999;Blenkinsopp et al, 2011;Puleo et al, 2014a;Masselink et al, 2009].…”

Section: Introductionmentioning

confidence: 99%

On the role of infiltration and exfiltration in swash zone boundary layer dynamics

et al. 2015

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Boundary layer dynamics are investigated using a 2‐D numerical model that solves the Volume‐Averaged Reynolds‐Averaged Navier‐Stokes equations, with a VOF‐tracking scheme and a k ‐ ϵ turbulence closure. The model is validated with highly resolved data of dam break driven swash flows over gravel impermeable and permeable beds. The spatial gradients of the velocity, bed shear stress, and turbulence intensity terms are investigated with reference to bottom boundary layer (BL) dynamics. Numerical results show that the mean vorticity responds to flow divergence/convergence at the surface that result from accelerating/decelerating portions of the flow, bed shear stress, and sinking/injection of turbulence due to infiltration/exfiltration. Hence, the zero up‐crossing of the vorticity is employed as a proxy of the BL thickness inside the shallow swash zone flows. During the uprush phase, the BL develops almost instantaneously with bore arrival and fluctuates below the surface due to flow instabilities and related horizontal straining. In contrast, during the backwash phase, the BL grows quasi‐linearly with less influence of surface‐induced forces. However, the infiltration produces a reduction of the maximum excursion and duration of the swash event. These effects have important implications for the BL development. The numerical results suggest that the BL growth rate deviates rapidly from a quasi‐linear trend if the infiltration is dominant during the initial backwash phase and the flat plate boundary layer theory may no longer be applicable under these conditions.

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“…In order to reach a representative measurement of the concentration, the latter must be large enough to contain a sufficient number of particles and small enough to minimize spatial smoothing effects (Lanckriet et al, ) in the vertical direction. The larger vertical extent of the measurement volume of CCP 2mm leads to increased vertical smoothing of the concentration profiles (Lanckriet & Puleo, ), hence the observed higher volumetric concentration threshold at which the 2‐mm probe becomes unreliable, as well as the overestimated sheet flow layer thickness.…”

Section: Discussionmentioning

confidence: 99%

On Bedload and Suspended Load Measurement Performances in Sheet Flows Using Acoustic and Conductivity Profilers

et al. 2018

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Intense sediment transport experiments were performed in a gravity‐driven open‐channel flow with two sizes of uniformly distributed nonspherical acrylic particles having median diameters of 1.0 and 3.0 mm and a maximum packing volumetric concentration (ϕ) of 0.55. The flow conditions were adapted to each particle size to ensure similar sediment transport flow regimes as sheet flow corresponding to Shields numbers slightly above unity and a suspension number, ratio of settling velocity to friction velocity, near unity for the two experiments. An acoustic scattering‐based system (Acoustic Concentration and Velocity Profiler) and conductivity probes (Conductivity Concentration Profiler [CCP]) with two different vertical resolutions, 1 mm (CCP1mm) and 2 mm (CCP2mm) were used to measure instantaneous concentration profiles across the bedload and suspension layers. Measured concentration profiles, bed interface position, and sheet flow layer thicknesses are compared between the two techniques. The capabilities and limitations of both technologies are outlined. Average volumetric sediment concentration profiles were overestimated by 10% with the Acoustic Concentration and Velocity Profiler in the dense sheet layer when ⟨ϕ(z)⟩≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨ϕ(z)⟩≲ 0.015 and ⟨ϕ(z)⟩≲ 0.20 for CCP1mm and CCP2mm, respectively. Good agreement is found between the three systems in terms of average and time‐resolved bed level position and sheet layer thickness, validating the different bed interface detection methods on data from the two sensors.

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Near‐bed turbulence dissipation measurements in the inner surf and swash zone

Cited by 19 publications

References 63 publications

Turbulence Within Natural Mangrove Pneumatophore Canopies

Turbulence Within Natural Mangrove Pneumatophore Canopies

On the role of infiltration and exfiltration in swash zone boundary layer dynamics

On Bedload and Suspended Load Measurement Performances in Sheet Flows Using Acoustic and Conductivity Profilers

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