Modelling Spatio-temporal Flow Characteristics In Groyne Fields

摘要

The spatio-temporal distribution of residence times and water ages in groyne fields plays an important role for a number of biotic processes, particularly conditioning nutrient dynamics and phytoplankton growth. Experimental determination of these parameters involves time-consuming field work and is often limited to the water surface depending on the methodology applied. Computational fluid dynamics (CFD) can be used as efficient tool for direct modelling of residence times, limiting additional field work to collect data necessary for model calibration and validation. The objective of this study is to derive, test and apply a numerical particle-tracing method to model these flow characteristics based on the output of a three-dimensional hydrodynamic model for turbulent river flow. This allows for an evaluation of residence times and water ages at an arbitrary elevation within the water column. Virtual particles are released in a control volume, and their paths and travel times to and from the main stream are traced using a random-walk approach based on the spatial distribution of turbulent kinetic energy to account for turbulent fluctuations. The modelling approach was successfully validated for the water surface using field data measured in a groyne field of the Danube River in Austria. Particle-tracing experiments were carried out for several discharges to study the evolution of residence times and water ages with varying runoff. It was found that discharges at which overtopping of the groynes begins lead to significantly higher residence times and water ages than runoff higher or lower than this characteristic value. These findings are expected to be transferable for groyne fields of similar geometry and crest height.