Shear layer over floodplain vegetation with a view on bending and streamlining effects

Juha Järvelä,Gerardo Caroppi

doi:10.1007/s10652-022-09841-w

Abstract

Shrubby and woody vegetation growing on floodplains profoundly influences hydrodynamic and transport processes in riverine systems. Existing hydrodynamic research is mostly focused on conditions with aquatic plants and rigid model vegetation. To appreciate the different hydrodynamic impacts of submerged floodplain and riverbank vegetation, a novel flume investigation was carried out. We simulated conditions found in riparian environments in terms of vegetation density, plant structure and flexibility, and presence of a grassy understory. Four experimental cases were defined so that vegetation exhibited different degrees of bending and streamlining. Extensive set of velocity measurements allowed reliable description of the double averaged flow. Vegetation morphology, with the flexibility-induced streamlining and dynamic motion controlled the magnitude and distribution of the vegetative drag, shaping the shear penetration within the canopy. The flows were highly heterogeneous, thus calling for spatially averaged approaches for the flow field investigation. The relative importance of dispersive momentum fluxes was high in the canopy bottom region where both Reynolds and dispersive stresses were small. The contribution of dispersive fluxes to momentum transport decreased with increasing reconfiguration. The results revealed the shear layers over floodplain vegetation to be dynamically similar to other environmental flows over porous obstructions. However, the velocity-dependent vegetative drag and deflected height introduced additional complexity in the flow simulation. Altogether our findings implied that accurate description of vegetated floodplain flows can be achieved only when plant morphology and flexibility are appropriately described in drag models.Article highlightsA novel experimental setup with flexible woody plants and grasses was used to model the hydrodynamics of vegetated floodplains.Plant morphology and flexibility controlled the vegetative drag, affecting key shear layer features, including the shear penetration.The spatially heterogeneous flows had higher dispersive stresses at the canopy bottom, where the total fluid stress was small.

Highlights

Riverbanks and floodplains are populated by shrubby and woody vegetation adapted to highly variable hydrological and hydrodynamic conditions [1, 2] (Fig. 1)
The investigation of flow spatial variability indicated that the double averaging methodology (DAM) should be considered as a key tool for the experimental and numerical investigation of flows with flexible woody vegetation, as is the case for flows with macro-roughness [55, 57, 63, 77]
We investigated the flow over submerged flexible woody plants, simulating conditions often found in floodplains of lowland rivers

Summary

Introduction

Riverbanks and floodplains are populated by shrubby and woody vegetation adapted to highly variable hydrological and hydrodynamic conditions [1, 2] (Fig. 1). Flow and vegetation periodically interact when overbank flows and floods occur. Such flows potentially cause economic and social losses [3, 4], and have the capacity to induce geomorphic restructuring of riverine environments [5] and to enhance pollutants remobilization and dispersion [6]. Riparian vegetation can provide flood protection by slowing down the flow and increasing storage capacity [11]. The importance of such ecosystem services has led to increased interest in restoring and promoting vegetation by recent water management policies, including the European Union Water Framework Directive [12]. Riparian vegetation has been in the focus of a wide range of ecosystem based management strategies and nature-based solutions for riverine environments [13,14,15,16]

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Conclusion