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Within a wave‐exposed mangrove forest, novel field observations are presented, comparing millimeter‐scale turbulent water velocity fluctuations with contemporaneous subtidal bed elevation changes. High‐resolution velocity and bed level measurements were collected from the unvegetated mudflat, at the mangrove forest fringe, and within the forest interior over multiple tidal cycles (flood–ebb) during a 2‐week period. Measurements demonstrated that the spatial variability in vegetation density is a control on sediment transport at sub‐meter scales. Scour around single and dense clusters of pneumatophores was predicted by a standard hydraulic engineering equation for wave‐induced scour around regular cylinders, when the cylinder diameter in the equations was replaced with the representative diameter of the dense pneumatophore clusters. Waves were dissipated as they propagated into the forest, but dissipation at infragravity periods (> 30 s) was observed to be less than dissipation at shorter periods (< 30 s), consistent with the predictions of a simple model. Cross‐wavelet analysis revealed that infragravity‐frequency fluctuations in the bed level were occasionally coherent with velocity, possibly indicating scour upstream of dense pneumatophore patches when infragravity waves reinforced tidal currents. Consequently, infragravity waves were a likely driver of sediment transport within the mangrove forest. Near‐bed turbulent kinetic energy, estimated from the turbulent dissipation rate, was also correlated with bed level changes. Specifically, within the mangrove forest and over the unvegetated mudflat, high‐energy events were associated with erosion or near‐zero bed level change, whereas low‐energy events were associated with accretion. In contrast, no single relationship between bed level changes and mean current velocity was applicable across both vegetated and unvegetated regions. These observations support the theory that sediment mobilization scales with turbulent energy, rather than mean velocity, a distinction that becomes important when vegetation controls the development of turbulence. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: Mangrove forests comprise the dominant plant communities in tropical and subtropical coasts and provide many important physical and biological functions. However, mangrove forests are largely in decline worldwide, and hence, there is interest in rehabilitating or replanting damaged forests. The success rate of these efforts may be improved by understanding the physical forces that shape mangrove ecosystems. In this study, we deployed several high‐resolution velocity sensors within the pneumatophore roots of a coastal mangrove forest in the lower Mekong Delta, Vietnam. From velocity measurements, we assessed the spatial distribution of turbulence that formed in the wake of the roots when they were submerged during the rising or falling tide. We found that enhanced turbulence was associated with denser vegetation, particularly near the bed. This result indicates that pneumatophore roots do not necessarily shelter the bed from erosional forces and instead may enhance sediment transport occurring within the forest. Key Points: Turbulent dissipation, Reynolds stress, and eddy viscosity are enhanced near the bed within clusters of mangrove pneumatophore rootsEddy viscosities up to 1.9 x 10‐3 m2/s were estimated within the vegetationPneumatophore roots may inhibit sediment deposition, which contradicts the expectation that dense vegetation promotes sediment retention [ABSTRACT FROM AUTHOR]

We study water flows and wave dissipation within near-bed pneumatophore canopies at the wave-exposed fringe of a mangrove forest on Cù Lao Dung Island, in the Mekong Delta. To evaluate canopy drag, the three-dimensional geometry of pneumatophore stems growing upward from the buried lateral roots of Sonneratia caseolaris mangroves was reconstructed from photogrammetric surveys. In cases where hydrodynamic measurements were obtained, up to 84 stems per square meter were observed, with stem heights < 0.6 m, and basal diameters 0.01–0.02 m. The parameter a = (frontal area of pneumatophores blocking the flow)/(canopy volume) ranged from zero to 1.8 m −1 . Within-canopy water velocity displayed a phase lead and slight attenuation relative to above-canopy flows. The phase lead was frequency-dependent, ranging from 0 to 30 degrees at the frequencies of energetic waves ( > 0.1 Hz), and up to 90 degrees at lower frequencies. A model is developed for wave-induced flows within the vertically variable canopy. Scaling suggests that acceleration-induced forces and vertical mixing were negligible at wave frequencies. Consistent with theory, drag-induced vertical variability in velocity scaled with Λ = T w / ( 2 π T f ) , where T w = wave period, T f = 2 / ( C D a | u | ) is the frictional time scale, C D ≈ 2 is the drag coefficient, and | u | is a typical flow speed. For fixed wave conditions ( | u | and T w ), theory predicts increasing dissipation with increasing vegetation density (i.e. increasing a ), until a maximum is reached for order-one Λ . For larger Λ , within-canopy flow is so inhibited by drag that further increases in a reduce within-canopy dissipation. For observed cases, Λ ⩽ 0.38 at energetic wave frequencies, so wave dissipation near the forest edge is expected to increase with increasing pneumatophore canopy density. However, under different wave conditions, the most dense canopies may occasionally approach the dissipation maximum ( Λ ≈ 1 ). Predicted dissipation by the pneumatophore canopy was sufficient to attenuate most wave energy over distances slightly less (more) than 100 m into the marsh in 1 m (2 m) water depth. [ABSTRACT FROM AUTHOR]

Aquatic vegetation can shelter coastlines from energetic waves and tidal currents, sometimes enabling accretion of fine sediments. Simulation of flow and sediment transport within submerged canopies requires quantification of vegetation geometry. However, field surveys used to determine vegetation geometry can be limited by the time required to obtain conventional caliper and ruler measurements. Building on recent progress in photogrammetry and computer vision, we present a method for reconstructing three-dimensional canopy geometry. The method was used to survey a dense canopy of aerial mangrove roots, called pneumatophores, in Vietnam’s Mekong River Delta. Photogrammetric estimation of geometry required 1) taking numerous photographs at low tide from multiple viewpoints around 1 m 2 quadrats, 2) computing relative camera locations and orientations by triangulation of key features present in multiple images and reconstructing a dense 3D point cloud, and 3) extracting pneumatophore locations and diameters from the point cloud data. Step 3) was accomplished by a new ‘sector-slice’ algorithm, yielding geometric parameters every 5 mm along a vertical profile. Photogrammetric analysis was compared with manual caliper measurements. In all 5 quadrats considered, agreement was found between manual and photogrammetric estimates of stem number, and of number × mean diameter, which is a key parameter appearing in hydrodynamic models. In two quadrats, pneumatophores were encrusted with numerous barnacles, generating a complex geometry not resolved by hand measurements. In remaining cases, moderate agreement between manual and photogrammetric estimates of stem diameter and solid volume fraction was found. By substantially reducing measurement time in the field while capturing in greater detail the 3D structure, photogrammetry has potential to improve input to hydrodynamic models, particularly for simulations of flow through large-scale, heterogenous canopies. [ABSTRACT FROM AUTHOR]