Allen Bateman

An Open Channel Flow Experimental and Theoretical Study of Resistance and Turbulent Characterization over Flexible Vegetated Linings

Hydraulic engineers and scientists working on river restoration recognize the needfor a deeper understanding of natural streams as a complex and dynamicsystem, which involves not only abiotic elements (flow, sediments) but alsobiotic or biological components. From this point of view, the role played byriverine vegetation in river dynamics and flow conditions becomesessential. Hydro-mechanic interaction between flow and flexible plantscovering a river bed is studied in this paper and some previous works arediscussed. Experimental tests and measurements of turbulence on the flow in anopen channelwere performed using plastic plants seeded in a gravel bed. Characterization of flowresistance (friction factors) due to vegetation flexible roughness fordifferent plant densities was attained; furthermore, measuring detailedturbulent velocityprofiles within and above submerged and flexed stems allowed us to distinguishdifferent turbulent regimes. Some interesting relationships wereobtained between the velocity field and the deflected height of the plants, suchas a linear fit between the non-dimensional flexural parameter and the relativedeflection of the plants. Turbulent stresses weremeasured showing two different regions: above and inside the vegetationdomain. The spectral interaction between the plant oscillations, their wakes and theturbulence at different heights, forces strongly anisotropic Reynolds tensors andin order to clarify turbulent processes and their complexstructure, theoretical concepts (Taylor, Kolmogorovs K41) and several dataanalysis (autocorrelation functions, integral scales) were applied.

A new integrated, hydro-mechanical model applied to flexible vegetation in riverbeds

This paper suggests a simple newnumerical scheme for calculating the vertical velocity profile, turbulent shear stress distribution, and canopy deflection for flow in vegetated channels. The scheme is derived from the simplified, steady Reynolds equation (momentum balance) and its vertical integration. This study includes an experiment that compares and calibrates the numerical model to flume data. Fourteen runs were performed in a laboratory flume, of which nine involved natural grass (cultivated barley) and five involved plastic plants (PVC). Turbulent diffusion coefficient, mixing length, and a resistance equation (drag coefficient Cd vs. Reynolds number) were the input parameters for the numerical model. Parameters with physical meaning were calibrated. This scheme offers a low computation time and good estimation of plant deflection and velocity and stress profiles

A 2D finite volume model for bebris flow and its application to events occurred in the Eastern Pyrenees

Abstract FLATModel is a 2D finite volume code that contains several original approaches to improve debris-flow simulation. Firstly, FLATModel incorporates a ‘stop-and-go’ technique in each cell to allow continuous collapses and remobilizations of the debris-flow mass. Secondly, flow velocity and consequently yield stress is directly associated with the type of rheology to improve boundary accuracy. Thirdly, a simple approach for entrainment is also included in the model to analyse the effect of basal erosion of debris flows. FLATMODEL was tested at several events that occurred in the Eastern Pyrenees and simulation results indicated that the model can represent rather well the different characteristics observed in the field.

Tsunamis generated by fast granular landslides: 3D experiments and empirical predictors

ABSTRACT Landslides falling into water bodies can generate impulsive waves, which are a type of tsunamis. The propagating wave may be highly destructive for hydraulic structures, civil infrastructure and people living along the shorelines. A facility to study this phenomenon was set up in the laboratory of the Technical University of Catalonia. The set-up consists of a new device releasing granular material at high velocity into a wave basin. A system employing laser sheets, high-speed and high-definition cameras was designed to accurately measure the high velocity and geometry of the sliding mass as well as the produced water displacement in time and space. The analysis of experimental data helped to develop empirical relationships linking the landslide parameters with the produced wave amplitude, propagation features and energy, which are useful tools for the hazard assessment. The empirical relationships were successfully tested in the case of the 2007 event that occurred in Chehalis Lake (Canada).

Vortex rings impinging on permeable boundaries

Experiments with vortex rings impinging permeable and solid boundaries are presented in order to investigate the influence of permeability. Utilizing Particle Image Velocimetry, we compared the behaviour of a vortex ring impinging four different reticulated foams (with permeability k ∼ 26 − 85 × 10−8 m2) and a solid boundary. Results show how permeability affects the stretching phenomena of the vortex ring and the formation and evolution of the secondary vortex ring with opposite sign. Moreover, permeability also affects the macroscopic no-slip boundary condition found on the solid boundary, turning it into an apparent slip boundary condition for the most permeable boundary. The apparent slip-boundary condition and the flux exchange between the ambient fluid and the foam are jointly responsible for both the modified formation of the secondary vortex and changes on the vortex ring diameter increase.

TXT-tool 3.034-2.1: A Debris Flow Regional Fast Hazard Assessment Toolbox

The rapid development of GIS techniques permits the regional scale assessment for natural hazards. In this work different declared as GNU/GPL open source codes for debris flow hazard assessment have been developed for risk management and educational porpoises. The toolbox here presented manages the large amount of regional spatially distributed geographic information and includes: shallow landslide susceptibility assessment tools; an in-channel debris flow triggering mechanism evaluation tool; and the stochastic debris flow propagation tool “DebrisDice”. The tools are presented as executables in order to enhance the usability. Input/output are in ASCII grid format.

Closure to “Tsunamis generated by fast granular landslides: 3D experiments and empirical predictors” by FRANCESCO BREGOLI, ALLEN BATEMAN and VICENTE MEDINA, J. Hydraulic Res. 1–16. doi:10.1080/00221686.2017. 1289259

The Authors thank the Discussers for their valuable comments and particularly for providing further information on the attenuation of the water surface displacement η for tsunami wave propagation angles θ = 0°. The Authors’ experimental set-up, being confined in a narrow water tank, did not allow for an extensive analysis of propagation angles apart from the main slide direction θ = 0° (the x direction following the Authors’ paper notation). Lacking their own experimental observations the Authors suggested to refer to other authors investigations (e.g. Mohammed & Fritz, 2012) who proposed a simplified angular attenuation of wave to be proportional to cos(θ ). More complex relations between wave first crest amplitude a and propagation angle exist (e.g. Panizzo, De Girolamo, & Petaccia, 2005; Heller & Spinneken, 2015), although they are related to wave generated by block failures. The Authors agree with the Discussers that the angular attenuation of tsunamis generated by granular landslides is of primary importance to evaluate the related hazard in confined water bodies, and welcome the Discussers’ new experimental

Coherent structures in oscillatory flows within the laminar-to-turbulent transition regime for smooth and rough walls

ABSTRACT We investigate coherent structures present in oscillatory boundary layers over smooth and rough beds for Reynolds numbers between 103 and 104, in the transition to turbulence regime. A two-camera 2D- particle image velocimetry (PIV) system was used to visualize coherent structures in an oscillatory-flow tunnel. The obtained results show that smooth-bed flow is populated by vortex-tube structures that are formed due to the Kelvin–Helmholtz instability. Three types of coherent structures are observed in the rough-wall experiments: vortices randomly distributed in space; turbulent bursts – spatial structures without a clear shape compared to vortices but with a longer life than the former; and shear layers of vortices originating due to flow separation from some of the grains on the bed. The reported study contributes to the description of coherent structures in oscillatory flows that are captured with the PIV technique, particularly a new structure in flows over rough beds in the transition to turbulence regime.

Morpho-fluvial analysis of headwater catchments: an example from the Central-Eastern Pyrenees

Abstract Geomorphometry of headwater catchments has been poorly reported in the Central-Eastern Pyrenees. This study presents a series of parameters obtained for Central-Eastern Pyrenean headwater catchments. The database consists of 3,005 first- and 655 second-order catchments. These catchments have been digitalised, identified, and attributed a value for each parameter. The parameters investigated are divided into three groups: relative to catchments, relative to streams and morpho-hydrological ratios. Histograms reveal similarities between orders for some parameters such as mean slope or orientation, while stream orders seem to condition metrical parameters (area, perimeter, stream length). Streams have been fragmented to assess different values for slope. Values for slope over a small portion of the stream near the outlet seem to show clearer differences between orders. With regard to morpho-hydrological ratios, catchments show better distinctions between orders for the Melton and Lemniscate ratios than for the form factor or the basin elongation. The power–law relationship between catchment area and stream length recognised for large fluvial systems is shown here to follow a linear trend at small values. An attempt to identify the morpho-structural regionalisation differentiating the Axial Pyrenees from the pre-Pyrenees is made based on the parameters. However, applying the methodology to other environments could improve the context of the current results. Similar studies could also benefit from the development of such databases.