A Question of Scale: How Turbulence Around Aerial Roots Shapes the Seabed Morphology in Mangrove Forests of the Mekong Delta

Mullarney, Julia C.; Henderson, Stephen M.; Norris, Benjamin K.; Bryan, Karin R.; Fricke, Aaron T.; Sandwell, Dean R.; Culling, D.

doi:10.5670/oceanog.2017.312

Cited by 33 publications

(35 citation statements)

References 35 publications

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“…In the NE, the mean wave direction was also generally onshore, while mean currents were oriented in the along‐shore direction. Previous studies have discovered that flood tides combined with along‐shore oriented winds generate strong along‐shore currents (reaching up to 0.3 m/s) that flow obliquely to the coastline of Cù Lao Dung (Mullarney, Henderson, Norris, et al, ). Although current velocities are rapidly reduced over ~100 m of mangrove forest, the fringe environment is subject to the currents generated along the unvegetated mudflat.…”

Section: Resultsmentioning

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: Resultsmentioning

confidence: 99%

Turbulence Within Natural Mangrove Pneumatophore Canopies

et al. 2019

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“…However, the production of TKE in the wakes of roots and trunks enhanced the TKE within the model forest, relative to the bare bed (blue symbols in Figures and ). Elevated turbulence within the pneumatophores of a mangrove forest has also been observed in the field and attributed to root‐wake generation (Mullarney et al, ). Mullarney at al.…”

Section: Resultsmentioning

confidence: 99%

Velocity and Drag Evolution From the Leading Edge of a Model Mangrove Forest

et al. 2017

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An experimental study of unidirectional flow through a model mangrove forest measured both velocity and forces on individual trees. The individual trees were 1/12th scale models of mature Rhizophora, including 24 prop roots distributed in a three‐dimensional layout. Thirty‐two model trees were distributed in a staggered array producing a 2.5 m long forest. The velocity evolved from a boundary layer profile at the forest leading edge to a vertical profile determined by the vertical distribution of frontal area, with significantly higher velocity above the prop roots. Fully developed conditions were reached at the fifth tree row from the leading edge. Within the root zone the velocity was reduced by up to 50% and the TKE was increased by as much as fivefold, relative to the upstream conditions. TKE in the root zone was mainly produced by root and trunk wakes, and it agreed in magnitude with the estimation obtained using the Tanino and Nepf (2008) formulation. Maximum TKE occurred at the top of the roots, where a strong shear region was associated with the change in frontal area. The drag measured on individual trees decreased from the leading edge and reached a constant value at the fifth row and beyond, i.e., in the fully developed region. The drag exhibited a quadratic dependence on velocity, which justified the definition of a quadratic drag coefficient. Once the correct drag length‐scale was defined, the measured drag coefficients collapsed to a single function of Reynolds number.

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“…These vegetated regions represent a small but relatively stable part of the total intertidal area of tidal basins (Hunt et al, 2016). Mangroves have the ability to influence intertidal profile shapes by enhancing sedimentation and promoting hypsometric profile convexity (Van Santen et al, 2007;Bryan et al, 2017;Mullarney et al, 2017). Intertidal vegetation can also alter current velocities and directions and attenuate waves Horstman et al, 2018).…”

Section: Study Limitationsmentioning

confidence: 99%

The links between entrance geometry, hypsometry and hydrodynamics in shallow tidally dominated basins

Ruiter

Mullarney

Bryan

2019

Earth Surf Processes Landf

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Geomorphological characteristics of tidal basins control hydrodynamics and sediment transport potential within such basins, for example, by adjusting the balance in tidal asymmetry. In this study we examine the effects of entrance geometry on tidal velocity asymmetry, slack water asymmetry, bed shear stress patterns and hypsometric profile shapes by comparison of six shallow meso‐tidal basins of Tauranga Harbour, New Zealand. Numerical model results show how tidal distortion increases with distance from a basin entrance. A simple ratio between basin width and entrance width defines levels of basin dilation. Sub‐basins with a constricted geometry and deep entrance channels are associated with small bed shear stress values and high rates of flood‐directed tidal velocity asymmetry in the sheltered basin centres, indicating a large potential for sediment deposition of larger particles. Moreover, slack water asymmetry within these basins is weakly ebb‐directed, indicating a small potential for transport of fine sediments out of the basins. The constricted depositional basins are characterized by convex hypsometric profiles with elevated intertidal regions. Unconstricted geometries are associated with larger bed shear stress values and more ebb‐directed tidal velocity asymmetry within basin centres, suggesting limited potential for overall sediment deposition. The slack tide duration asymmetry is weakly flood‐dominant indicating that limited input of fine sediment into the basins is possible. The comparatively high‐energy conditions within these exposed basins are associated with a less convex hypsometric intertidal profile. The ability to estimate tidal asymmetries is advantageous when developing management strategies related to ecosystem functioning, navigability or coastal protection in specific geomorphic settings. © 2019 John Wiley & Sons, Ltd.

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A Question of Scale: How Turbulence Around Aerial Roots Shapes the Seabed Morphology in Mangrove Forests of the Mekong Delta

Cited by 33 publications

References 35 publications

Turbulence Within Natural Mangrove Pneumatophore Canopies

Turbulence Within Natural Mangrove Pneumatophore Canopies

Velocity and Drag Evolution From the Leading Edge of a Model Mangrove Forest

The links between entrance geometry, hypsometry and hydrodynamics in shallow tidally dominated basins

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