Turbulence Driven Hyporheic Exchange Processes with Highly Permeable Channel Beds

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TURBULENCE DRIVEN HYPORHEIC EXCHANGE PROCESSES WITH HIGHLY PERMEABLE CHANNEL BEDSThe main objective of this project is to investigate the effect of spatial and temporal heterogeneity created by episodic turbulent events and coherent flows on hyporheic exchange with permeable channel beds. The fundamental challenge is the evaluation of the effect of coherent flow structures on hyporheic exchange processes and the ability to provide accurate predictions of fluid and solute influxes into and outfluxes from the hyporheic zone. This will be the first study that investigates the instantaneous flow field and will advance knowledge of the hydrodynamics as well as transport processes of hyporheic zones, which are critical to quantitatively analyze stream eco-systems. The numerical method proposed, i.e. Large-Eddy Simulation, provides impressive predictive accuracy in flows which feature pronounced periodic components and/or in which turbulence transport is dictated by the dynamics of large scale eddies as present in flows over permeable beds or around flow obstacles. This study will be the first to model explicitly the individual particles that constitute the hyporheic zone. Consequently, governing flow and transport processes are represented explicitly and can be studied in microscopic detail.Two distinct scenarios will be investigated, (a) flow over a flat gravel-type bed where hyporheic exchange is solely induced by large-scale turbulence created and induced by the texture of the bed and (b) flow over submerged and emergent boulder-type obstruction buried in a gravel-type bed, where hyporheic exchange is driven by both quasi-steady pressure gradients and large-scale coherent flow structures as a consequence of vortex shedding from the flow obstruction. Quantification of the exchange will be accomplished by adding a tracer study to the hydrodynamic investigations, with which mass and momentum exchange can be calculated directly. Further evidence of the effect of coherent structures on hyporheic exchange will be provided by a detailed statistical analysis of the mean and instantaneous flow field especially at the interface for instance with quadrant analysis, probability density functions, etc. The outcome of the proposed research is a comprehensive understanding of the physical mechanisms of turbulence driven hyporheic exchange and will provide very accurate quantification of exchange rates for different conditions.