Effects of emergent and submerged natural vegetation on longitudinal mixing in open channel flow

Abstract

[1] An awareness of mixing processes is imperative in understanding the transport of pollutants in open channel flows, important for environmental impact studies. To date, controlled laboratory studies of the effects of vegetation on mixing processes have used simulated plants. This may neglect some of the important variables introduced by the presence of natural vegetation. In this study natural vegetation was planted within a laboratory channel, and a series of experiments quantifying velocity, turbulence, and longitudinal mixing were conducted over a time period sufficient to allow growth of the vegetation to impact on the mixing processes. In emergent conditions the results generally confirmed previous artificial vegetation and modeling studies, showing that vegetation reduces the magnitude of longitudinal shear dispersion. Additionally, measureable change in longitudinal mixing was observed primarily as a function of flow depth but also of plant age. Normalization using previously suggested parameter combinations failed to yield predictive trends. Submerged tests uniquely covered natural vegetation with a significant wake zone, and from this it was observed that longitudinal mixing is primarily a function of the degree of submergence. Overall, this paper presents a new data set quantifying the effects of natural vegetation on longitudinal mixing processes and illustrating deficiencies in previous understanding and predictive expressions based on idealized artificial vegetation.

Hydraulic roughness due to submerged, emergent and flexible natural vegetation in a semiarid alluvial channel

“…This adjustment to the velocity defect law indicated velocity fluctuations due to the stress created by vegetation. In accordance with the theoretical mixing layer model (Raupach et al, 1996), adopted by Shucksmith et al (2010), the submerged and emergent vegetation from Jacu River (Fig. 3) had Elder (1959) and in shear zone (0.6 < a < 1) and canopy top zone (1<a < 2) of semiarid Jacu River from semiarid of Brazil under submerged, emergent and naturally bended vegetation Beach Morning Glory (Ipomoea pes-caprae).…”

“…In the emergence condition it was estimated at the canopy top by the equation used by Murphy et al (2007) and Shucksmith et al (2010):…”

“…One area of research that, until recently, has been largely overlooked is the influence of channel vegetation on the mixing characteristics of flow, which is also important to evaluate the spread of nutrients and pollutants through channel treatment facilities that feature vegetation (Shucksmith et al, 2010). In order to asses this eco-engineering ability and enhance the success of restoration and protection attempts, more insight is needed into the interaction between vegetation, currents, waves, sediment transport and water quality (Dijkstra and Uittenbogaard, 2010).…”

“…Velocity profile and turbulence in submerged flow were investigated by Shucksmith et al (2010) considering an analogy with the mixing layer model developed by Raupach et al (1996), in which there are three distinct zones. The first zone has a slow moving zone through vegetation, nominated wake zone.…”

Feasibility of the Porous Zone Approach to Modelling Vegetation in CFD

“…Experimental data collected by Shucksmith et al (2010) has been used for validation of the CFD model. They investigated longitudinal dispersion in a vegetated channel, emergent or submerged depending on flow rate.…”