Flow resistance in environmental channels : focus on vegetation

This thesis aims to improve the reliability of the determination of flow resistance in environmentally acceptable channels and floodplains. Special emphasis was placed on addressing the hydraulic effects of vegetation. For this reason, laboratory flume studies with living vegetation were employed. The most notable finding was that, when compared to leafless conditions, the presence of leaves increased the friction factor up to seven-fold. This was strongly dependent on the flow velocity. In addition, the linkage between flow resistance, channel properties, and physical habitat was investigated. For this purpose, field studies were conducted in degraded, restored, and natural channel reaches. To determine friction factor f or Manning's n for non-submerged woody vegetation, a new procedure based on the measurable characteristics of vegetation and flow was developed. A major advantage of this procedure over the old methods was its ability to estimate the flow resistance of woody vegetation in both leafless and leafy conditions. In determining the velocity profile and flow resistance caused by submerged flexible vegetation, the approach developed by Stephan (2002) was found to be suitable. However, a new formulation was proposed for the shear velocity based on deflected plant height. This modification offered better practical applicability than the original formulation, which requires complicated turbulence measurements. In the field studies, the experimental results for friction factors were, excluding those for low flows, in agreement with the values presented in the literature. Overall, the gathered field data from degraded, restored, and natural channel reaches formed a reference data set, which could be useful in other similar restoration or engineering projects. The field studies showed that both flow resistance and cross-sectional geometry were vital factors in determining local hydraulic conditions. The parameters defining these two factors were found to be simple but nonetheless valuable in evaluating the success of a project which aims to restore local hydraulics. A new procedure for applying the success criteria in the post-project evaluation of local hydraulics was developed.reviewe

Flow Resistance in Open Channel Due to Vegetation at Reach Scale: A Review

Vegetation on the banks and flooding areas of watercourses significantly affects energy losses. To take the latter into account, computational models make use of resistance coefficients based on the evaluation of bed and walls roughness besides the resistance to flow offered by vegetation. This paper, after summarizing the classical approaches based on descriptions and pictures, considers the recent advancements related to the analytical methods relative both to rigid and flexible vegetation. In particular, emergent rigid vegetation is first analyzed by focusing on the methods for determining the drag coefficient, then submerged rigid vegetation is analyzed, highlighting briefly the principles on which the different models are based and recalling the comparisons made in the literature. Then, the models used in the case of both emergent and submerged rigid vegetation are highlighted. As to flexible vegetation, the paper reminds first the flow conditions that cause the vegetation to lay on the channel bed, and then the classical resistance laws that were developed for the design of irrigation canals. The most recent developments in the case of submerged and emergent flexible vegetation are then presented. Since turbulence studies should be considered as the basis of flow resistance, even though the path toward practical use is still long, the new developments in the field of 3D numerical methods are briefly reviewed, presently used to assess the characteristics of turbulence and the transport of sediments and pollutants. The use of remote sensing to map riparian vegetation and estimating biomechanical parameters is briefly analyzed. Finally, some applications are presented, aimed at highlighting, in real cases, the influence exerted by vegetation on water depth and maintenance interventions

Hydraulic resistance to overland flow on surfaces with partially submerged vegetation

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96239/1/wrcr13661.pd

Effect of near‐bed turbulence on chronic detachment of epilithic biofilm: Experimental and modeling approaches.

The biomass dynamics of epilithic biofilm, a collective term for a complex microorganism community that grows on gravel bed rivers, was investigated by coupling experimental and numerical approaches focusing on epilithic biofilm‐flow interactions. The experiment was conducted during 65 days in an artificial rough open‐channel flow, where filtered river water circulated at a constant discharge. To characterize the effect of near‐bed turbulence on the chronic detachment process in the dynamics of epilithic biofilm, local hydrodynamic conditions were measured by laser Doppler anemometry and turbulent boundary layer parameters inferred from double‐averaged quantities. Numerical simulations of the EB biomass dynamics were performed using three different models of chronic detachment based upon three different descriptors for the flow conditions: Discharge Q, friction velocity u*, and roughness Reynolds number k+. Comparisons of numerical simulation results with experimental data revealed chronic detachment to be better simulated by taking the roughness Reynolds number as the external physical variable forcing chronic detachment. Indeed, the loss of epilithic matter through the chronic detachment process is related not only to hydrodynamic conditions, but also to change in bottom roughness. This suggests that changes in the behavior and dimensions of river bed roughness must be considered when checking the dynamics of epilithic biofilm in running waters

Determining leaf area index and leafy tree roughness using terrestrial laser scanning

Vegetation roughness, and more specifically forest roughness, is a necessary component in better defining flood dynamics both in the sense of changes in river catchment characteristics and the dynamics of forest changes and management. Extracting roughness parameters from riparian forests can be a complicated process involving different components for different required scales and flow depths. For flow depths that enter a forest canopy, roughness at both the woody branch and foliage level is necessary. This study attempts to extract roughness for this leafy component using a relatively new remote sensing technique in the form of terrestrial laser scanning. Terrestrial laser scanning is used in this study due to its ability to obtain millions of points within relatively small forest stands. This form of lidar can be used to determine the gaps present in foliaged canopies in order to determine the leaf area index. The leaf area index can then be directly input into resistance equations to determine the flow resistance at different flow depths. Leaf area indices created using ground scanning are compared in this study to indices calculated using simple regression equations. The dominant riparian forests investigated in this study are planted and natural poplar forests over a lowland section of the Garonne River in Southern France. Final foliage roughness values were added to woody branch roughness from a previous study, resulting in total planted riparian forest roughness values of around Manning's n = 0.170–0.195 and around n = 0.245–330 for in-canopy flow of 6 and 8 m, respectively, and around n = 0.590 and around n = 0.750 for a natural forest stand at the same flow depths

Turbulence in partly vegetated channels: Experiments with complex morphology vegetation and rigid cylinders

Vegetation is a fundamental feature of riverine ecosystems, playing a variety of valuable ecological and biological roles. Concurrently, the presence of vegetation and its interaction with the flow alter the mean and turbulent flow field, with implications on flow resistance, water conveyance and transport of mass and energy. The proper understanding of these vegetation-influenced processes is essential for solving the existing and future river management challenges, concerning both societal needs and ecosystem requirements. The objective of this thesis is to provide new insight on the flow-vegetation hydrodynamic interaction with a specific focus on partly vegetated channels, a configuration representative of natural settings. Indeed, in natural watercourses, vegetation is generally found along river margins, partly obstructing the river cross-section and laterally interacting with the flow. Riparian vegetation presents a complex morphology and, owing to its flexibility, exhibits a dynamic and reconfiguring behavior under the flow forcing. In the analysis of flow in partly vegetated channels, these flow-influencing characteristics have been generally neglected, simulating vegetation with rigid cylinders. In the current study, two main experimental campaigns were performed to investigate the turbulent structure of the flow in partly vegetated channels, simulating vegetation with natural-like plant stands (PN) and with rigid cylinders (PR). The PN tests aimed at investigating the effects of plant morphology, reconfiguration and dynamic motions on the turbulent flow field. Furthermore, the effects of seasonal variability of plants on flow structure were explored. Results showed that plant morphology and reconfiguration play a key role in the vegetated shear layer dynamics, significantly affecting the exchange processes across the vegetated interface. The PR test series was performed to investigate the effects of vegetation density on the turbulent flow structure. The results showed that, for rigid vegetation, the density directly affects the shear layer features, governing the onset of large-scale coherent structures. Finally, the impacts of embedding natural plant features in the simulation of partly vegetated flows were explored by comparing the shear layers induced by complex morphology vegetation (PN) and by rigid cylinders (PR). In addition, an existing model for velocity prediction was tested against the experimental results, showing the need to improve existing models for taking into account the peculiar hydrodynamic behavior of natural vegetation

Transport of fine sediment in vegetated flows

Complex interactions and turbulent flow structures take place across the interface, between vegetated regions and unobstructed main channel flow. For instance, in partly vegetated flows, different transport processes and sediment deposition affect the availability of nutrients and presence of pollutants. Within the vegetation, the turbidity is altered, influencing light accessibility and photosynthesis. The goal of this thesis is to improve knowledge on turbulent flow and fine sediment transport in partly vegetated flows. Experiments were carried out in the Aalto Environmental Hydraulics Lab:~60% of the flume width was unvegetated, while ~40% of the flume width was covered by a vegetated patch comprised of understory grass mat and artificial emergent flexible natural-like plants. The experiments used a combination of vegetation density and plant properties, well representative of conditions found in natural riverine flows. Instantaneous 3D velocities, suspended sediment concentration (SSC) and net deposition were measured in the fully developed flow region of the vegetated patch. Two transverse transects and several vertical profiles were measured. Two vegetation conditions, representing the seasonal changes due to lifecycle of riverine plants, were investigated: leafless and foliated. In addition to descriptive data analyses, equations from literature were applied and tested against the flume measurements to check if it was possible to use them for a reliable prediction under the examined vegetative conditions. The experimental data showed that effects of the presence of vegetation on flow field and fine sediment transport vary when the plant density increases (i.e. changing from leafless to foliated condition). The difference in streamwise velocity between the open channel and the vegetated region increased. SSC decreased, within the foliated vegetation compared to the main channel, in agreement with the decrease in velocity and increase in net deposition. Under foliated condition, the mechanical dispersion appeared to lead sediment transport, because turbulence declined rapidly. In the leafless case, the turbulence at the stem scale was the main player, determining high local fluctuations in transversal and vertical profiles for both SSC and streamwise velocity and a reduction in net deposition. Overall, the investigations on flow-vegetation-sediment processes performed in two different conditions, representing seasonal vegetation changes, showed that theoretical and empirical relationships used to predict patterns of velocity are less suitable for predictions within leafy vegetation, but they still are in good agreement within flows in leafless condition. For SSC patterns, the predictions through equations and assumptions used in unvegetated channel are hard to obtain and unreliable in both vegetative conditions

Flow hydrodynamics across open channel flows with riparian zones: implications for riverbank stability

Riverbank vegetation is of high importance both for preserving the form (morphology) and function (ecology) of natural river systems. Revegetation of riverbanks is commonly used as a means of stream rehabilitation and management of bank instability and erosion. In this experimental study, the effect of different riverbank vegetation densities on flow hydrodynamics across the channel, including the riparian zone, are reported and discussed. The configuration of vegetation elements follows either linear or staggered arrangements as vegetation density is progressively increased, within a representative range of vegetation densities found in nature. Hydrodynamic measurements including mean streamwise velocity and turbulent intensity flow profiles are recorded via acoustic Doppler velocimetry (ADV)—both at the main channel and within the riverbank. These results show that for the main channel and the toe of riverbank, turbulence intensity for the low densities (λ ≈ 0 to 0.12 m−1) can increase up to 40% compared the case of high densities (λ = 0.94 to 1.9 m−1). Further analysis of these data allowed the estimation of bed-shear stresses, demonstrating 86% and 71% increase at the main channel and near the toe region, for increasing densities (λ = 0 to 1.9 m−1). Quantifying these hydrodynamic effects is important for assessing the contribution of physically representative ranges of riparian vegetation densities on hydrogeomorphologic feedback