Reconciling multiple spatial and temporal scales : hydrodynamics and sediment transport within mangrove forests
Mangrove forests are highly productive ecosystems, which provide many physical, societal and ecological services in tropical and subtropical regions. Accurate prediction of the morphological evolution of these areas, in the face of global sea level rise and changes in sediment supply, requires understanding of interactions between vegetation elements, water flows, and sediment transport. This talk will provide an overview of hydrodynamics and sediment transport within mangrove forests. These salt-tolerant trees are characterised by complex aerial root systems (pneumatophores), which protrude many centimetres above the seabed. As the tide propagates into the forest, tide and wave energy is converted into dissipative wake-scale turbulence, through the processes of vortex shedding and eddy generation. We present unique small-scale field measurements, which reveal ‘hotspots’ of intense turbulence, with values sometimes reaching those observed in surf zones. Turbulence was particularly elevated in the fringing regions between mudflat and forest, although there existed substantial spatial variability. Both scour and deposition were observed within the same regions. On the forest scale, the enhanced drag exerted on the water column by pneumatophores and tree trunks slows the flow and causes the water surface to tilt. We then observe a rotation of the obliquely incident flows toward an orientation nearly perpendicular to the vegetated/unvegetated boundary. The momentum balances governing the large-scale flow are assessed and indicate the relative importance of friction, winds and depth-averaged pressure forces. Drag coefficients were found to be 10–30 times greater than values usually observed for bottom friction, and the drag induced from pneumatophores was dominant relative to drag from the larger, but sparser, tree trunks.
The ability to predict turbulent stresses acting on the seabed and to formulate accurate drag parameterisations are essential to our ability to reliably model the interplay between vegetation and deposition and/or erosion. Knowledge of these processes is crucial for our understanding of the overall resilience of delta systems, which face an uncertain future worldwide.
Contact Remi Chassagne for more information or to schedule a discussion with the seminar speaker.