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Small-scale turbulence and its influence on forest-scale morphodynamics within a coastal mangrove forest

Mangroves are coastal wetland ecosystems consisting of salt-tolerant trees and shrubs that inhabit the upper intertidal zone of estuaries, river banks and barrier islands. These trees grow in the intersection between the coastal ocean and land, and form a barrier against hazards such as waves, coastal flooding and erosion for many densely populated areas that often lack hard coastal defense structures. By damping tidal currents and waves, mangroves also facilitate sedimentation, and may contribute to coastal stability in the face of rising sea levels. The valuable ecosystem services provided by mangroves are the result of characteristic bio-physical feedbacks between the mangrove vegetation, hydrodynamics and sediment dynamics within intertidal zones (Chapter 1). This thesis focuses on understanding these bio-physical feedbacks, specifically: (i) how mangrove root density affects tidal current and wave dissipation, (ii) how mangrove root structures modify the flow field, and (iii) how mangrove roots facilitate sediment transport within forested areas. Linking Mangrove Root Density and Turbulent Dissipation The link between mangrove root density and turbulent dissipation was explored in a coastal mangrove forest that is exposed to an energetic wave environment. Measurements of turbulent kinetic energy (TKE) dissipation were collected over millimeter to centimeter scales within clusters of mangrove pneumatophore roots (‘canopies’) spanning regions from the unvegetated mudflat to the densely vegetated forest. High-resolution root geometries were reconstructed using a newly developed photogrammetric method. The frontal area density of the vegetation (a) was compared to the TKE dissipation rate estimates at the same height above bed. Mangrove tree (and hence, root) density was greatest along a narrow band between the mudflat and forest: the forest ‘fringe’. Temporally variable turbulence was maximum in the fringe and was often elevated in the forest relative to the unvegetated mudflat. The largest dissipation rates (4.5 x 10⁻³ W kg⁻¹) were measured as breaking waves propagated over root canopies in very shallow water. Dissipation rates were reduced, but often remained intense (e.g., between 10⁻⁵ – 10⁻⁴ W kg⁻¹) under non-breaking waves at the fringe, likely indicating turbulent generation in pneumatophore wakes. Turbulence was positively correlated with root density and wave height and was negatively correlated with water depth. Substrate grain size distributions in the fringe were larger (sandier) than those offshore and onshore, suggesting intense turbulence may winnow fine-grained sediments from the fringe. Observations of Turbulence in Mangrove Root 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 6.5 x 10⁻⁴ W kg⁻¹), 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 x 10⁻³ m² s⁻²), and eddy viscosities (up to 1.9 x 10⁻³ m² s⁻¹). 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⁻¹), and therefore may enhance the sediment entrainment occurring in mangrove forest fringes. Understanding the Hydrodynamic and Morphodynamic Feedbacks in Mangroves Field experiments were carried out within a wave-exposed coastal mangrove forest to quantify the change in bed level throughout a tidal cycle using high-resolution velocity and bed level measurements collected in situ. Experiments spanned the unvegetated mudflat, mangrove forest fringe and forest, where data were collected during single tidal cycles (flood-ebb) during a two-week period. Cross-wavelet transforms of the velocity and bed level measurements were often highly correlated (≥ 90% squared coherence) over a range of frequencies, spanning those corresponding to short-period wind and swell waves to long-period infragravity waves. Bed level change events associated with short-period waves were more frequent in the mudflat and fringe, while those associated with long-period waves were much more frequent in the forest. Net bed level changes over single events were nearly normally distributed, indicating similar numbers of events resulting in accretion and erosion, regardless of the across-shore position. Still, bed level change events exceeding the observed net tidal elevation change occurred infrequently. This pattern suggests that the net tidal elevation change within the mangrove forest must be related to the frequency of these infrequent events. Large bed level changes were often associated with high bed shear stresses (0.3 – 1.5 N m⁻²) and turbulence of O(10⁻³ W kg⁻¹), particularly on the mudflat and in the forest fringe. Across all experiments, the mudflat and forest site experienced net accretion (4.5 and 6.8 mm, respectively) while the fringe experienced net erosion (-9.5 mm). This work suggests a dynamic role for waves in mangroves: short-period waves stir sediment on the mudflat and forest fringe, while infragravity waves help advect entrained sediment inside the forest.
Type of thesis
Norris, B. (2019). Small-scale turbulence and its Influence on forest-scale morphodynamics within a coastal mangrove forest (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12408
The University of Waikato
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