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Hydrodynamics of fluvial wakes past large in-stream natural objects and their implications for riverbed morphology

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 411877768
 
Final Report Year 2023

Final Report Abstract

Diversity of fluvial environments is tightly linked to spatial heterogeneity in abiotic factors mediated by turbulent flows. Central to this topic are flows evolving around natural in-stream objects - turbulent wakes. Our research aimed to expand the knowledge of wake’s dynamics from small-scale laboratory environments to large-scale natural systems subjected to hydrologic variations from low flow to flooding events. To achieve this objective, our methodology exploited the synergy between controlled field-based experiments, case field studies on natural rivers, numerical experiments with eddy-resolving techniques and hydrodynamic theory of shallow flows. The experimental program of the project yielded a comprehensive dataset consisting of 46 field experiments and 12 field case studies that assessed turbulent wakes during both low flow and flooding events in natural rivers. So far, the most important finding of the project is the development of a theory of intermodal behavior for mixing interfaces in fluvial networks. In this theory, the dynamics of shallow wakes are demonstrated to control the fundamental processes of momentum and mass exchange at natural fluvial confluences. We show that for correct predictions of these dynamics the contribution of lateral advective and turbulent fluxes of momentum should be added to the effect of bed friction, which before was considered as a single factor governing conventional shallow wakes. These theoretical findings agree well with the results of our field-based experiments. Our theory and experimental data expand the application of shallow wakes model in riverine environments, which was before traditionally focusing on the near field flow dynamics around in-stream objects (e.g. boulders and riparian/aquatic vegetation patches), to the scale of river reaches. However, the small-scale wake dynamics have also received a full account in our experimental and theoretical work. Particularly, we demonstrated that at the interfaces between ambient flows and bleed flows behind model patches of riparian vegetation, the flow dynamics are similar to shallow mixing layers with strong stabilization effect. Stabilization can be enhanced by lateral advection fluxes of momentum formed in vertically confined flow due to lateral displacement (e.g. flow jetting) induced by vegetation patches. During floods, patches collect drift wood, leaf litter and macro-plastic, which are consolidated by fine sediments and increase the solid volume fraction. An increase at the leading edge, typical for patches on the floodplains of rivers with natural hydrologic regime, can change the dynamics from porous- to solid-like object behavior, which needs to be accounted to avoid large underestimates in theoretical predictions based on simplified laboratory experiments. Thus, the outcomes of our project provide a solid theoretical ground for understanding wakes in river environments that has also an important applied value for river management and restoration.

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