Effects of aquatic vegetation on sediment transport

Resumen

In the margins of continents there are the coastal zones, regions of remarkable biological productivity. Coastal zones include, among others, wetlands, that are land areas inundated permanently o seasonally, characterized by the presence of aquatic vegetation adapted to the hydric soil. Therefore it is necessary to consider the effect of aquatic canopies in wetlands investigations. These areas are governed by physical forces originated from tidal currents, waves, winds, night convection and floods. The aim of this thesis is to study the sediment transport in wetlands in which fluid is dominated by a) progressive waves, b) nearly isotropic turbulence and c) extreme flooding events. The study of sediment transport under progressive waves corresponds to Chapter 3. The experiments were conducted on a laboratory channel, and three vegetation models (rigid, flexible and real plants of Ruppia maritima) were considered with different plant densities. Also different wave frequencies were used. The aim of the study was to understand the relation between the morphology of canopies (flexibility and density) in front of the physical forcing, and its effect on sediment resuspension from a bed of cohesive sediment. The experiments showed that the sediment resuspension is related to TKE. Some of the canopy densities (real and flexible plants, and rigid plants at high density and high wave frequencies) reduce TKE, and thus reduce sediment resuspension, in contrast to the sparse rigid canopies at low wave frequencies, that increase TKE and enhance sediment resuspension. Chapter 4 describes the effect of canopies on sediment resuspension by nearly isotropic turbulence. The experiments were conducted on an oscillating grid chamber, with either submerged rigid and flexible canopies with different plant densities. This chapter was aimed to investigate the protecting capacity of soil erosion by submerged canopies, in front of turbulence. The concentration of suspended sediment is a linear function of the TKE. All of the canopies used in the experiment reduce sediment resuspension, except sparse canopies of flexible plants that increase TKE by the freely movement of its blades. The third part of this thesis (Chapter 5) is the study of extreme flooding events of different intensities. The experiments were made with emergent rigid vegetation on a laboratory channel, and floods were simulated by discharging particle-laden water into water free of particles. The flood was composed by a mean flow and a gravity current that advanced through the channel across the vegetated area. The aim of this study was to understand the effect of canopies in flooding events and the depositional rates from this particle-laden gravity current. The canopies can reduce the maximum velocity of the flow. The progression of the gravity current that follows the mean flow is modified by the presence of plants from an inertial-dominated regime to a viscous-dominated regime. Emergent plants also enhance sediment deposition. Chapter 6 illustrates the general discussion, and Chapter 7 presents the overall conclusion of this thesis. Chapter 3 is an adaptation of the published article in Continental Shelf Research. A copy of this article can be found in the appendix.