Vegetation impacts on near bank flow

Abstract

Vegetation is an important component of stream restoration designs used to control streambank retreat, but vegetation effects on near bank flows need to be quantified. The goal of this research was to evaluate how three-dimensional velocity structure and turbulence characteristics vary with three vegetation treatments: tree, shrub and grass. A second order prototype stream (Tom's Creek in Blacksburg, Virginia, USA) with individual reaches dominated by each vegetation treatment was modelled in a research flume using a fixed-bed Froude-scale modelling technique. One model streambank of the prototype stream was constructed for each vegetation type and compared to a bare control (only grain roughness). Velocity profiles perpendicular to the flume model boundary were measured using a three-dimensional acoustic Doppler velocimeter. Three-dimensional velocity records, turbulent kinetic energy characteristics, and Reynolds stresses were analysed. The addition of vegetation on a sloping streambank increased the free stream streamwise velocity as compared to a bare streambank. Velocity in the downstream direction decreased in the area close to the streambank boundary for all vegetation treatments. Tree turbulence intensity and Reynolds stress distributions were similar to the bare condition due to the sparse tree placement characteristic of mature forests. The turbulence caused by the upright shrub treatment increased turbulent kinetic energy and Reynolds stresses near the streambank, particularly at the toe. The flexible grass vegetation folded and protected the streambank, reducing shear stress near the boundary.

Flow Hydrodynamics across Open Channel Flows with Riparian Zones: Implications for Riverbank Stability

“…This can be observed for all cases shown in Figure 4, where the streamwise flow velocity at the riverbank does not exhibit a consistent change across the flow depth. This is in accordance to the observations of past studies, reporting non-logarithmic shapes for the streamwise velocity profiles past vegetation patches [11,33].…”

“…In this study it is found that both bed-shear stress estimates and turbulence intensity results offer consistent trends for the increasing potential of destabilization of the main channel and toe of the riverbank, for high and low vegetation densities respectively. However, as this study has demonstrated, bed-shear stresses cannot be estimated with certainty assuming logarithmic velocity profiles, nor other ways of estimation offer consistent results [11]. This makes it of even greater utility and urgency to identify relevant criteria, which can be used and associated to riverbank stability.…”

“…It can be seen from a few studies that riparian vegetation can drastically reduce the mean velocity at the riverbank, while increasing it at the main channel. Enhanced momentum and energy transfer take place at the bottom of the interface of the vegetated riverbank and the main channel [11]. Further, the slope of riverbanks can be another parameter affecting bank stability.…”

A combined field, laboratory and numerical study of the forces applied to, and the potential for removal of, bar top vegetation in a braided river

“…As noted by Paul et al (), vegetation also affects flow patterns by adding roughness and hence reducing the velocity in vegetated areas, introducing turbulence and inducing scour along the vegetation–channel interface, and forcing flow back towards the open channel (Bertram, ; McBride et al , ; White and Nepf, ). Vegetation can also have a protective role, reducing bank erosion in rivers, estuaries, lakes and coastal zones through deflection of flow away from banks (White and Nepf, ; Hopkinson and Wynn, ), dissipation of wave energy (Möller et al , ), and reinforcement of the soil matrix by plant roots (Pollen and Simon, ; Thomas and Pollen‐Bankhead, ).…”

Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management

“…Hession et al, 2003) and channel processes (e.g. Abernethy, and Rutherfurd, 1998;Darby, 1999;Hopkinson and Wynn, 2009). Therefore, any change or trend in vegetation over time can have an effect on response to flood events.…”