Model Versus Nature: Hydrodynamics in Mangrove Pneumatophores

Abstract

Water flows through submerged and emergent vegetation control the transport and deposition of sediment in coastal wetlands. Many past studies into the hydrodynamics of vegetation fields have used idealized vegetation mimics, mostly rigid dowels of uniform height. In this study, a canopy of real mangrove pneumatophores was reconstructed in a flume to quantify flow and turbulence within and above this canopy. At a constant flow forcing, an increase in pneumatophore density, from 71 m -2 to 268 m -2 , was found to cause a reduction of the within-canopy flow velocities, whereas the over-canopy flows increased. Within-canopy velocities reduced to 46% and 27% of the free-stream velocities for the lowest and highest pneumatophore densities, respectively, resulting in stronger vertical shear and hence greater turbulence production around the top of the denser pneumatophore canopies. The maximum Reynolds stress was observed at 1.5 times the average pneumatophore height, in contrast to uniform-height canopies, in which the maximum occurs at approximately the height of the vegetation. The ratios of the within-canopy velocity to the free-stream velocity for the pneumatophores were found to be similar to previous observations with uniform-height vegetation mimics for the same vegetation densities. However, maxima of the scaled friction velocity were two times smaller over the real pneumatophore canopies than for idealized dowel canopies, due to the reduced velocity gradients over the variable-height pneumatophores compared to uniform-height dowels. These findings imply that results from previous studies with idealized and uniform vegetation mimics may have limited application when considering sediment transport and deposition in real vegetation, as the observed turbulence characteristics in nonuniform canopies deviate significantly from those in dowel canopies.

Are flow-vegetation interactions well represented by mimics? A case study of mangrove pneumatophores

“…In a pilot study, we demonstrated that real pneumatophores may have a different effect on the flow dynamics when compared to published studies using dowel mimics [39]. However, the range of experimental conditions used in the published studies made it difficult to isolate and quantify differences.…”

The Dynamics of Expanding Mangroves in New Zealand

“…In some cases, for the smaller mangroves in New Zealand, the branches and leaves of the trees themselves are submerged during high tides, and therefore, once water levels reach the tree canopy, will likely enhance drag (as observed in similar mangroves in Japan and Vietnam (Chen et al 2016;Mazda et al 1997;Quartel et al 2007)) and possibly also generate additional turbulence. Whether these differences suggest that other factors such as sediment supply may play a stronger role in setting sediment deposition patterns than local turbulence conditions is an active area of research (see for example Horstman et al 2016;Horstman et al 2017). (Horstman et al 2017).…”

Threats to Mangrove Forests

“…In some cases, for the smaller mangroves in New Zealand, the branches and leaves of the trees themselves are submerged during high tides, and therefore, once water levels reach the tree canopy, will likely enhance drag (as observed in similar mangroves in Japan and Vietnam (Chen et al 2016;Mazda et al 1997;Quartel et al 2007)) and possibly also generate additional turbulence. Whether these differences suggest that other factors such as sediment supply may play a stronger role in setting sediment deposition patterns than local turbulence conditions is an active area of research (see for example Horstman et al 2016;Horstman et al 2017). Figure 1) along (B) a transect covering tidal flat, fringe and mangrove forest (Horstman et al 2017).…”