Interactions between currents and the spatial structure of aquatic vegetation

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2009.Includes bibliographical references (p. 79-85).Vegetation is present in nearly all aquatic environments, ranging from meandering streams to constructed channels and rivers, as well as in lakes and coastal zones. This vegetation grows in a wide range of flow environments as well, from stagnant water to highly turbulent flows dominated by waves and currents. Feedbacks between the dominant currents and the vegetation not only significantly alter the velocity structure of the flow, but play a large role in determining the spatial structure of the vegetation as well. This thesis examines these interactions through field experiments, review of existing literature and theoretical and analytical models. The first study describes a set of experiments in which vegetation was added to the point bar of a stream meander during base flow. During the next flood event, this vegetation proved to be destabilizing as a portion of the vegetation scoured away and the cross section of the open channel showed clear patterns of erosion. The secondary circulation present in the meander was significantly altered as well. In the second study, the relationship between tidal currents and the spatial distribution of seagrass meadows is examined. Seagrass beds range in their coverage from continuous meadows, to spotty swaths dominated by discrete patches. The relationship between this area coverage and tidal currents, explained by the principles of percolation theory, helps describe why certain distributions of seagrass within a meadow are more stable than others.(cont.) Drawing on the principles and examples established in the first two sections, the final section describes an analytical model for predicting vegetation coverage in a rectangular open channel. The model can allow for fixed banks, such as those in a concrete-lined channel, or can allow erosion of the boundaries, as is possible in natural streams. These two versions of the model show notably different results. Ultimately, this thesis presents multiple cases of the interactions between currents and aquatic vegetation and showcases an important example of a multi-disciplinary research approach in fluid mechanics.by Jeffrey T. Rominger.S.M

Hydrodynamic and transport phenomena at the interface between flow and aquatic vegetation : from the forest to the blade scale

Thesis: Ph. D. in Environmental Fluid Mechanics, Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2014.116Cataloged from PDF version of thesis.Includes bibliographical references (pages 227-235).From the canopy scale to the blade scale, interactions between fluid motion and kelp produce a wide array of hydrodynamic and scalar transport phenomena. At the kilometer scale of the kelp forest, coastal currents transport nutrients, microorganisms and spores. But, kelp forests exert a drag force on currents, causing the flow to decelerate and divert as it encounters the canopy, affecting the fate of species transported by the current. We identify a dimensionless flow-blockage parameter, based on canopy width and density, that controls both the length of the flow deceleration region and the total flow in the canopy. We further find that shear layers at the canopy edges can interact across the canopy, providing additional exchange between the canopy and the surrounding water. At the sub-meter scale, kelp blades are the photosynthetic engines of kelp forests, but are also responsible for the majority of the fluid drag force on the plants and for acquiring nutrients directly from the surrounding water. These blades are highly flexible structures which move in response to the local fluid forcing. Recent studies documenting changes in blade flexural rigidity in response to changes in flow demonstrate a need for understanding the role blade flexural rigidity plays in setting both drag forces, and nutrient flux at the blade surface. We create a model physical system in which we investigate the role of blade rigidity in setting blade forces and rates of scalar exchange in a vortex street. Using a combination of experimental and theoretical investigations, we find that, broadly, forces are higher for more flexible blades, countering the adage that "going with the flow" is beneficial. Below a critical value of the dimensionless blade rigidity, inertial forces from the rapidly deforming blade become significant, increasing the likelihood of blade failure. Nutrient transport is also affected by blade rigidity. As blades deform, they alter the relative fluid motion at the blade surface, affecting nutrient fluxes. We develop a novel experimental method that simulates nutrient uptake to a blade using the transport of a tracer into polyethylene. Through these experiments and modeling, we demonstrate that increased blade flexibility leads to increased scalar transport. Ultimately, blade flexural rigidity affects both mass and momentum flux.by Jeffrey Tsaros Rominger.Ph. D. in Environmental Fluid Mechanic