Turbulent open channel flows over and within rough and permeable beds

Turbulence in open channel flows need to be clarified in several aspects. Perhaps one of the most difficult tasks is to understand the structure o f flows having small relative submergence for which standard boundary layer theory is not applicable. Furthermore, rivers and natural channels are often characterized by very porous and rough beds where water flows and interact with the free stream inducing transport mechanisms which play an important role in the flinctioning o f stream ecosystems. In order to design numerical models able to simulate such transport processes, there is a need to understand and predict the turbulence properties of both the free stream and the sub-surface flow. To achieve this objective, it is crucial to understand how the permeability o f the bed influences the velocity statistics in the free stream and in turn how the free stream induces turbulence within the permeable bed. The purpose of this study was to carry out a series of laboratory experiments to gain insight into: 1) the turbulence characteristics of open channel flows having small relative submergence; 2) the influence of permeability on turbulence in open charmel flows; 3) the turbulence properties o f the flow at the interface between the free stream and a permeable bed. The data obtained from these experiments have been analysed and interpreted on the basis of the double-averaged momentum and continuity equations. The results helped on clarifying the definition and meaning of key scaling parameters for flows with small relative submergence such as the friction velocity and the zero plane position. Furthermore, it is shown how permeable beds induce higher resistance to the flow than their impermeable counterparts having the same roughness texture. It is shown that this is mainly due to additional energy dissipation occurring within the porous bed. Measurements of velocities within the bed allowed identifying the dissipation mechanisms and the origin of turbulence in this flow region. Within the bed, two types of turbulence occur: one generated at the pore scale and another one, more energetic, imposed by the large eddies of the free stream.EThOS - Electronic Theses Online ServiceGBUnited Kingdo