A field study of turbulent flows in shallow gravel-bed rivers

The study of turbulent flows has always been a challenge for scientists. Turbulent flows are common in nature and have an important role in several geophysical processes related to a variety of phenomena such as river morphology, landscape modeling, atmospheric dynamics and ocean currents. At present, new measurement and observation techniques suitable for fieldwork can be combined with laboratory and theoretical work in order to advance in the understanding of river processes. In this Ph.D. dissertation, an Acoustic Doppler Velocity Profiler (ADVP) suspended from a deployable structure allowed the investigation of turbulent gravel-bed river flows. The ADVP, which was developed by the Laboratoire d'Hydraulique Environnementale (LHE), permits to obtain over the entire water depth, three-dimensional quasi-instantaneous information on the fluctuating velocity flow field in the production and inertial subranges of the spectral space. Improvements on the ADVP data quality were made with the implementation of a correction methodology for errors due to data aliasing. This reduced the range-velocity ambiguity in Doppler-based instruments. The results presented in this dissertation contribute to the understanding of transport and mixing processes in river flows. They are based on three sets of field measurements made in gravel-bed rivers with blockage ratios of h/D84≈3.0 and aspect ratios B/h between 23 and 32. The measurements were made during low-water periods. The fieldwork provided results on the mean and instantaneous velocity field. The flow was divided into three inviscid vertical layers with different mean field and Reynolds stress characteristics: the roughness layer, the blending or intermediate layer and the surface layer. In the lower layers of the flow three types of mean velocity profile were found: mono-logarithmic, s-shaped due to bed perturbations and double-logarithmic downstream bed perturbations. The determination of the shear stress distribution for each of these profile types is studied. In double-log profiles, the friction velocity and roughness length determined for the outer logarithmic layer are required for the velocity profile parameterization. The s-shaped profiles are described by a tangent-hyperbolic function in the lower layers compatible with an external log layer. Limitations of 2D open-channel theories to parameterize the velocity distribution and to characterize the bottom drag are discussed. Bottom drag occurs in the predominant momentum direction. The direction changes as a function of the local bed forms. To estimate bottom drag one has to consider the actual momentum transport direction which varies with the flow depth. The wall effect of the riverbank is visible until y/h≈5. Bottom topography produces important secondary mean motion in the flow. A permanent structure of the flow was described in the upper layers, near the surface (z/h<0.80): the Surface Layer Organized Movement – SLOM. It is composed by local jets (CH regions) and by lower velocity regions (CL regions) associated with a compensatory secondary motion with streamwise vorticity. Lateral momentum transfer exists between adjacent CL and CH regions. In one river, the bed form presented signatures of possible streamwise ridges. All the turbulent characteristics of the flow respond to the periodic riverbed morphology. The strips were produced during high water events affecting river processes on the long-term. The D-shaped profiles are investigated. They relate to the CL regions and are formed where the velocity is lower near the surface. The maximum velocity is situated at around zUmax/π~0.80. The occurrence of D-shaped profiles shows a dependence on the local Froude number; the SLOM might be an inviscid response to the bed forms. Two distinct approaches in the study of the turbulent velocity field were made: an analysis of the mean turbulence characteristics and the analysis of particular instantaneous features in the turbulent flow. The normal Reynolds stress distributions are anisotropic because streamwise turbulent intensity (TI) dominates the TKE (50 to 80% of the total energy with maximum at z/h≈0.70). Spanwise and vertical contributions vary along the flow depth. The surface and the bottom layers exert a strong influence on the vertical TI profile inducing a parabolic distribution. Self-similarity of the flow is only found inside the blending layer. In the roughness layer all Reynolds shear stresses become equally important. Empirical formula established by previous authors to describe the Reynolds stress tensor components are analyzed. The diffusive terms in the TKE budget equation are negligible and consequently the pseudo-dissipation may be considered equal to the actual dissipation ε~ ≈ ε. Production mostly happens inside the boundary layer and is mainly due to the gradients of the streamwise component. Dissipation is more widely spread in the flow with a maximum value near the bottom. Production and dissipation follow the exponential laws e5.1z/h and e3.9z/h, respectively. The repartition of the flow energy through the eddy scales is studied and a characteristic curve is found having as parameters the scale corresponding to the peak energy (Λmax), and a dimensionless parameter representing the energy dispersion through the scales (σ+). Empirical power laws are presented for the turbulence scales, Taylor (λ), Kolmogorov (η), integral (L), energetic (Λ) and mixing length (lm). The relationship between the different scales is also studied. With conditional sampling techniques, particular features of the turbulent velocity and their role in flow dynamics were described: detection and analysis of bursting packets inside the boundary layer and the study of the passage of large-sale uniform momentum regions (UMR), which here were called Streamwise Velocity Pulsation (SVP). Innovative tools are applied in the study of the instantaneous velocity measurements: wavelet decomposition and multiresolution analysis, empirical mode decomposition (EMD) and phase averaging based on the Hilbert transform. SVP corresponds to a non-periodic passage of large-scale UMR with streamwise velocities alternately higher or lower than the mean. This phenomenon was identified and characterized. The SVP Strouhal number is within the range 0.13<SSVP<0.32 (defined with the local depth averaged velocity). A vertical linear phase shift of ≈-3/4π of the front exists. The front has a first quarter moon shape with an apex at z/h≈0.35. It scales with h in the vertical and with 3.1–7.3h in the streamwise direction. In the lower layer the fronts have an average angle of 15°. In phase with the SVP, cycles of large-scale sweeps and ejections exist. Ejections dominate shear stress production with higher instantaneous amplitude and lower permanence. A periodic variation of the bottom drag is related nonlinearly with the SVP (direct effect). Due to the 3D bursting process triggered by the passage of an UMR, the bottom drag is enhanced (indirect effect). The SVP may be generated by the combined shedding effect produced by boulders which are randomly and widely spaced in the riverbed. Several identical bursting packets composed of sequential ejections and sweeps ({x-z} shear events) linked to spanwise vortical cells were visualized and characterized. The feedback of these was evaluated in the velocity profile, Reynolds stresses and TKE budget. These packets are highly energetic. The sampled bursting packets are independent. The time interval between packets is stable and the most probable value corresponds to a Strouhal number of Si=2.5 (defined with the local friction velocity). The bursting Strouhal number is Sb=12.5. Fronts from large-scale high- and low-speed wedges are associated with the bursting packets. Near the bottom the fronts have a concave shape and an angle of 18° with the horizontal

Cambial growth periodicity studies of South American woody species: A review

This paper reviews histological studies of cambium activity in South American woody species and provides future research prospects. The majority of the studies almost exclusively describe radial increment and/or its periodicity. There are 15 papers concerning the cambial activity of 17 woody species from the South American flora and 3 exotic species in 4 countries that were published to date. Despite endogenous factors affecting the radial meristem, the seasonality of rains has been identified as the main factor influencing cambial activity in the tropics and subtropics. There is a lack of standardization and a need for improvement and discussion concerning the methods used. Moreover, radial growth studies conducted by monitoring cambium cell production are still scarce in South America, especially when considering the high diversity of the continents flora and ecosystems.Fil: Callado, Cátia Henriques. Universidade do Estado do Rio de Janeiro. Departamento de Biologia Vegetal. Laboratório de Anatomia Vegetal; BrasilFil: Roig Junent, Fidel Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Científico Tecnológico Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Tomazello Filho, Mário. Universidade de São Paulo. Escola Superior de Agricultura Luis de Queiroz. Departamento de Ciências Florestais; BrasilFil: Barros, Claudia Franca. Instituto de Pesquisas Jardim Botânico do Rio de Janeiro; Brasi

Organised turbulence over mobile and immobile hydraulically rough boundaries

33rd IAHR Congress: Water Engineering for a Sustainable EnvironmentThe present work is aimed at the study of near-bed organised turbulence over mobile and immobile, porous, hydraulically rough boundaries. The bed was permeable and composed of non-cohesive sediments. Two data sets were analysed, characterised by the same u*. The mobile bed data featured generalised sediment transport for all size fractions smaller than the d90. The fixed bed was obtained as result of an armouring process. Comparison of these data sets reveals differences on such parameters of the bursting cycle as the maximum shear stress and the transported momentum. These results point to a reorganization of turbulence, in the near-bed region, when the bed is mobile. The impacts of these reorganization are discussed, namely in what concerns the third order moments of the distributions of the velocity fluctuations

Teildurchgängige Geschiebesammler für sicheren Rückhalt

Geschiebesammler halten oft bei kleineren Hochwassern unnötig Geschiebe zurück. Dadurch wird die öko-morphologische Gewässerkontinuität unterbrochen. Beobachtet werden zudem oft gefährliche, selbsttätige Entleerungen. Durch den Einsatz eines Leitgerinnes im Rückhalteraum von Geschiebesammlern und einem Abschlussbauwerk mit vorgelagertem Grobrechen können die Geschiebedurchgängigkeit bei geringem Abfluss erhöht und der sichere Geschieberückhalt bei Hochwasser verbessert werden

Continuously Fed Density Currents Over Porous Bed

Gravity currents impact significantly the environment and human's life. Understanding their internal structure and their dynamics is fundamental for modelling purposes. In this study we observe experimentally how bed porosity affects the dynamics of density currents. We will focus on density and velocity profiles of currents traveling over porous bed, sinking and entraining water from its cavities. This abstract presents the preliminary results of this study which are relative to density currents travelling over the reference substratum, a smooth impermeable bed. The tests presented will be used as a reference to observe how the water entrained from the bottom cavities can change the structure of the current and are herein used to validate the experimental techniques. Brine water is injected at constant discharge in a 7.5 m long, 0.3 m wide and 0.3 m deep flume while instantaneous velocity and density profiles are simultaneously acquired 3 m downstream of the inlet. Initial excess densities and inlet velocities allow to reproduce gravity flows in sub and super-critical regime

A Fachada da Natividade na Basílica da Sagrada Família em Barcelona: Da Encomenda a Antoni Gaudí às Estruturas Estereostáticas e aos Pretensos Simbolismos

info:eu-repo/semantics/publishedVersio

Experimental evaluation of the discharge capacity of flow constrictions by check dams in mountain rivers

Open check dams are built to retain bed load in case of major floods and may have filtering, sieving as well as dosing effects on sediment transport. The combination of an upstream reservoir with an open check dam is designated as bed load trap. These structures are crucial elements for flood protection in mountainous regions as the sediment transport capacity of mountain streams is ample, highly unpredictable and therefore an imminent danger for river dwellers. The contemporary design criteria for open check dams are based on theoretical approaches, laboratory experiments or engineering experience. A multitude of different opening types and shapes were studied and built in the past, mostly based on design parameters which are related to grading curves of the river bed. In a series of laboratory experiments, the opening geometry and position is currently optimized in view of sediment transition for frequent floods in order to ensure sediment dynamics downstream. Herein, the results of a first series of experiments are presented, which were dedicated to the evaluation of the discharge capacity of check dam openings, under the influence of bed load transport on a rough river – bed under supercritical flow conditions. Existing experimental and theoretical formulae for purely hydraulic discharges are reviewed and it is found, that the actually applied value for the orifice coefficient μ are too small. These results may also apply for bridges, which serve in some cases intentionally or unintentionally as open check dams

Effects of Lateral and Vertical Constrictions on Flow in Rough Steep Channels with Bedload

Two-phase flows occurring at flow constrictions such as bridges or open sediment check dams are complex, especially for steep rivers with bedload transport. Dangerous bedload deposition and backwater effects may occur in steep mountain rivers at bridges. In contrast, sediment deposition is desirable at open check dams combined with sediment traps. For design purposes, the discharge and bedload capacity across these flow constrictions must be known. The energy losses, discharge capacity, and bedload transport capacity of vertical and lateral flow constrictions are experimentally studied in a rough, 2% inclined, trapezoidal channel. Both free surface and pressurized flow conditions, as caused by lateral and vertical flow constrictions, respectively, were analyzed because both may occur at bridges and check dams. The experiments demonstrate that the vertical flow constrictions cause a faster increase in the backwater depth, with increasing discharge, than lateral constrictions. The resulting upstream flow conditions can be described by the upstream Froude number, defined as a function of the constriction dimensions (height and width). The bedload transport capacity through the flow constriction decreases with the upstream Froude number. The practical relevance of the findings is illustrated by a design example of flow constrictions at open sediment check dams