Turbulence Within Natural Mangrove Pneumatophore Canopies

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 6.5x10(-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 similar to 0.2 may indicate Von Karman 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 5.3x10(-3)m(2)/s(2)) and eddy viscosities (up to 1.9x10(-3)m(2)/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)m(2)/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.