Parametrisation of aquatic vegetation in hydraulic and coastal research: The importance of plant biomechanics in the hydrodynamics of vegetated flows

Abstract

Understanding the complexity of the interactions between aquatic biological systems and their physical environment is a critical condition for the sustainable management of aquatic environments. In such ecosystems, aquatic vegetation holds a central place by influencing flow and turbulent processes, thus playing a major role both from an engineering and ecological point of view. However, the characterisation of the hydrodynamics of vegetated flows is facing several issues due to the complexity of the plant-flow interactions. In such a complex system, the plant's biomechanical properties are a key parameter governing the interplay between the living organism and its physical environment: a variation of the plant’s mechanics in time or space will generally imply changes in these regulation processes, which in turn can lead to major changes in the physical/ecological environment. The consideration of mechanical interactions, however, is fairly new to engineers, biologists and ecologists, as it defines an interface between engineering and ecology. As a consequence, this area of research has remained mostly unexplored and a lot of processes are to be discovered at the edge of the different disciplines. This thesis deepened the level of the understanding of plant-flow mechanical interactions and of the plant structural properties, in order to improve the parametrisation of aquatic vegetation in hydraulic and coastal research. Measurement techniques and experimental protocols were developed to collect plant biomechanical properties and a methodology for the collection of such data was provided based on the flexibility of the plant elements. New methods to design plant surrogates for hydraulic experimentation were further investigated based on mechanical similarity. In parallel, this thesis reviewed the measurement techniques commonly used to measure drag forces on submerged plants, and developed a theoretical framework to estimate random wave and random wave-plus-current induced drag forces on submerged plants. This framework is based on the definition of a drag coefficient for a given type of plant. Moreover, the common drag coefficient formulations were discussed, identifying possibilities for a standardisation of the formulations for oscillatory and steady flows. Finally, in order to understand the effects of the development of a biological community at an interface fluid/solid, the example of marine biofouling was investigated through flow visualisation. The methodologies developed in this thesis are to be used in ongoing projects and will trigger new research activities at NTNU and elsewhere.