This paper summarizes current practices for the estimation of flow resistance caused by floodplain vegetation in emergent flow conditions. The current state-of-the-art for the parameterization of vegetative form drag and associated flow resistance was explored with a view on practical applicability. Specifically, the dissimilar resistance behaviour of simply shaped rigid elements and foliated natural vegetation was emphasized by compiling and reanalysing data published by the authors’ research teams as well as others. It was shown that describing the key hydraulic properties of plants, geometry, and flexibility, with species-specific parameters is superior to the rigid cylinder analogy commonly used in hydraulic engineering practice. The discussion on the limitations of many existing approaches for the determination of vegetative flow resistance is intended to advance the use of modern practices such as the parameterization of vegetation density with the leaf area index, a parameter that can be derived us...

Flow resistance of emergent rigid and flexible floodplain vegetation

The presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed-shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed-shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges. Copyright © 2014 John Wiley & Sons, Ltd.

Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models

A two-layer approach for depth-limited open-channel flow with submerged vegetation is described. A momentum balance is applied to each layer and expressions for the mean velocities are proposed. The velocity is assumed to be uniform in the vegetation layer and logarithmic in the upper layer. The proposed relationship successfully predicts the mean velocity distribution when compared with the measured data. Using the velocity formula, the layer-averaged mean velocities in the upper layer and over the entire layer are derived. An expression for the roughness coefficient increased by vegetation is also presented, performing better for the roughness coefficient than other formulas. Another relationship is proposed for predicting the distribution of suspended sediment in depth-limited flow with submerged vegetation by using an eddy-viscosity profile. The predicted profiles moderately agree with the measured data. Comparisons with simulated data from the Reynolds-averaged Navier–Stokes equations with the k–ϵ mo...

A two-layer approach for depth-limited open-channel flows with submerged vegetation

The hydrodynamics of turbulent flow through submerged flexible vegetation is investigated in a flume using acoustic Doppler velocimetery (ADV) measurements. The flow characteristics such as the energetics and momentum transfer derived from conventional spectral and quadrant analyses are considered as the flow encounters a finite vegetation patch. Consistent with numerous canopy flow experiments, a shear layer and coherent vortex structures near the canopy top emerge caused by Kelvin–Helmholtz instabilities after the flow equilibrates with the vegetated layer. These instabilities are commonly attributed to velocity differences between non-vegetated and vegetated canopy layers in agreement with numerous experiments and simulations conducted on dense rigid canopies. The power-spectral density function for vertical velocity turbulent fluctuations at different downstream positions starting from the edge of the vegetation layer are also computed. For a preset water depth, the dominant dimensionless frequency is found to be surprisingly invariant around 0.027 despite large differences in vegetation densities. The ejection and sweep events significantly contribute to the Reynolds stresses near the top of the vegetation. The momentum flux carried by ejections is larger than its counterpart carried by the sweeps above the canopy top. However, the momentum flux carried by sweeps is larger below the top of the canopy.

The structure of turbulent flow through submerged flexible vegetation

The significance of riparian vegetation on river flow and material transport is not in dispute. Conveyance laws, sediment erosion and deposition, and element cycling must all be adjusted from their canonical rough-wall boundary layer to accommodate the presence of aquatic plants. In turn, the growth and colonization of riparian vegetation are affected by fluvial processes and river morphology on longer time scales. These interactions and feedbacks at multiple time scales are now drawing significant attention within the research community given their relevance to river restoration. For this reason, a review summarizing methods, general laws, qualitative cognition, and quantitative models regarding the interplay between aquatic plants, flow dynamics, and sediment transport in vegetated rivers is in order. Shortcomings, pitfalls, knowledge gaps, and daunting challenges to the current state of knowledge are also covered. As a multidisciplinary research topic, a future research agenda and opportunities pertinent to river management and enhancement of ecosystem services are also highlighted.

Flow dynamics and sediment transport in vegetated rivers: A review

Three-layer model for vertical velocity distribution in open channel flow with submerged rigid vegetation

Analytical model of the mean velocity distribution in an open channel with double-layered rigid vegetation

The theory of an eddy viscosity model is applied to the study of the flow in a compound channel which is partially vegetated. The governing equation is constituted by analyzing the longitudinal forces acting on the unit volume where the effect of the vegetation on the flow is considered as a drag force item. The compound channel is divided into 3 sub-regions in the transverse direction, and the coefficients in every region’s differential equations were solved simultaneously. Thus, the analytical solution of the transverse distribution of the depth-averaged velocity for uniform flow in a partially vegetated compound channel was obtained. The results can be used to predict the transverse distribution of bed shear stress, which has an important effect on the transportation of sediment. By comparing the analytical results with the measured data, the analytical solution in this paper is shown to be sufficiently accurate to predict most hydraulic features for engineering design purposes.

Zhonghua Yang

Papers

Three-layer model for vertical velocity distribution in open channel flow with submerged rigid vegetation

Flow dynamics and sediment transport in vegetated rivers: A review

Analytical model of the mean velocity distribution in an open channel with double-layered rigid vegetation

Two dimensional analytical solution for a partially vegetated compound channel flow

A genetic programming-based model for drag coefficient of emergent vegetation in open channel flows