Volker Weitbrecht

Large scale PIV-measurements at the surface of shallow water flows

Abstract To measure the flow dynamics at the surface of shallow water flows over a large measuring field, a simple and reliable method has been developed using the advantages of Particle Image Velocimetry (PIV). Besides the determination of mean flow conditions and turbulent flow characteristics, this method makes it possible to track two-dimensional large coherent structures, which are the dominating flow phenomena in many shallow flow applications. As basic equipment, a commercial PIV software system has been used. The measurements are carried out at the water surface, which means that no laser light sheet is needed. Depending on the time scales of the flow and camera characteristics, it is even possible to work with a constant light source. A particle dispenser to provide a homogeneous distribution of particles on the water surface is also presented. Because floating particles have a strong tendency of sticking together, different types of particles and special coatings have been tested to reduce this problem. A laboratory application of this method is presented to analyze the effects of shallow dead-water zones on exchange processes in rivers where large coherent two-dimensional flow structures in the mixing layer dominate the flow characteristics.

Migration of silver from commercial plastic food containers and implications for consumer exposure assessment

Food storage containers with embedded silver as an antibacterial agent promise longer durability of food. For risk assessment the release of this silver into the stored food and resulting human exposure need to be known. For the purpose of exposure assessment, silver migration from commercial plastic containers with declared content of ‘nano-’ or ‘micro-silver’ into different food simulants (water, 10% ethanol, 3% acetic acid, olive oil) was quantitatively determined by ICP-MS and the form of the released silver was investigated. The highest migration of silver was observed for the acidic food simulant with 30 ng silver cm−2 contact surface within 10 days at 20°C. In a second and third use cycle, migration dropped by a factor of up to 10, so that the maximum cumulated release over three use cycles was 34 ng cm−2. The silver release over time was described using a power function and a numerical model that simulates Fickian diffusion through the plastic material. The released silver was found to be in ionic form, but also in the form of silver nanoparticles (around 12%). Consumer exposure to the total amount of silver released from the food containers is low in comparison with the background silver exposure of the general population, but since natural background concentrations are only known for ionic silver, the exposure to silver nanoparticles is not directly comparable with a safe background level.

Flow distribution in solar collectors with laminar flow conditions

Abstract This paper presents the results of an experimental study conducted in a water solar flat plate collector with laminar flow conditions to analyze the flow distribution through the collector. LDA-measurements were carried out to determine the discharge in each riser, as well as pressure measurements to investigate the relation between junction losses and the local Reynolds number. Analytical calculations based on the measured relations are used in a sensitivity analysis to explain the various possible flow distributions in solar collectors. In addition, a simple parametric approach to predict the flow distribution without performing a pipe network calculation has been realized.

Breaching of overtopped river embankments controlled by apparent cohesion

Laboratory experiments were conducted to obtain detailed experimental data of high temporal and spatial resolution on the breach of homogeneous non-cohesive embankments due to overtopping. These results supplement earlier data for this embankment type by providing information on the breach discharge, longitudinal and transversal breach profiles, erosion rates and pore-water pressures within the embankment. Based on detailed experimental results, two breach phases are classified. Shear and tension failure mechanisms were identified from the analysis of the obtained cross-sectional breach profiles. Furthermore, the temporal cross-sectional analysis of the used non-cohesive granular material shows a different behaviour regarding the lateral breach widening and the breach slope failure. The data analysis indicates that the apparent cohesion represented by the pore-water pressure influences the stability of the breach side slopes and hence the complete breach process. This finding is confirmed by a dimensional analysis of the experimental data and compared with information from earlier work.

Laboratory Measurements on Turbulent Pressure Fluctuations in and above Gravel Beds

The statistics of pressure fluctuations above and within three types of porous granular beds such as in gravel bed streams, rivers, and man-made canals are investigated by data gained via laboratory flume experiments. The flow conditions examined include a diversity of hydrodynamic loads that increase up to the point where single grains are moving from time to time, without causing severe modification to the bed texture and the related positions of the pressure sensors. Analysis is performed by means of histograms and spectral techniques and vertical intensity profiles. Two simplified equations are found that describe the vertical decrease for the standard deviation of the measured fluctuations indicating drag and lift, respectively, nondimensionalized by the mean bed shear stress. The former fluctuation is described by a crude linear fit, whereas the latter clearly shows that the lift intensity decreases exponentially in the porous bed with a decay distance of one to two times the equivalent grain roughness. Within the subsurface layer the standard deviation reaches a nonzero constant, mainly dominated by long-wave pressure fields that are convected in the outer flow. These findings can be used in future sediment transport models that use force balance approaches to determine incipient motion conditions.

A low-cost airborne velocimetry system: proof of concept

ABSTRACT This Technical Note proves the applicability of a low-cost airborne velocimetry system to measure large-scale surface velocity fields. The measurement equipment consists of an ultra-light action-cam and a ready-to-fly low-cost quadrocopter. Video recordings were performed from heights between 45–74 m covering a total reach length of 310 m, while spruce chips were added as tracer particles. Each lens-corrected frame was automatically ortho-rectified to riparian ground reference points. The positional error of each point was computed to be within 0.17–0.39 m, so that the magnitude of the related descaling error was below ±2%, and the error of apparent ground velocity is approximately 0.03 m s−1. These values describe the uncertainty added to the subsequently calculated particle image velocity field. The final raster resolution was 1.0×1.0 m2 with 50% overlap. A comparison with the velocity profiles measured by a 3D acoustic Doppler current profiler indicates that the proposed new type of velocimetry system is capable of measuring with relatively high accuracy.

Driftwood: Risk Analysis and Engineering Measures

AbstractTransported driftwood and woody debris during floods may lead to accumulations and blockages at river bridges or weirs, and can result in excessive scour or an increase in backwater that may lead to flooding of the nearby areas. Although driftwood-related problems occur predominantly in forested mountainous regions, driftwood may eventually reach densely populated lowland areas. Prevalent retention structures, such as nets or racks, across the river often fail for larger alpine rivers because of the excessive structural loading or the resulting backwater rise that may overtop the flood embankments. Therefore, this paper presents an overview of driftwood risk analysis and retention measures in large alpine rivers. Several methods on how to determine the potential driftwood volume for a given catchment area are summarized. The most reliable data are thereby obtained with a detailed investigation of the catchment characteristics. Observations during past flood events may also add valuable information...

Laboratory flume experiments with the Swiss plate geophone bed load monitoring system: 1. Impulse counts and particle size identification

We performed systematic flume experiments using natural bed load particles to quantify the effect of different parameters on the signal registered by the Swiss plate geophone, a bed load surrogate monitoring system. It was observed that the number of impulses computed from the raw signal clearly depends on bed particle size, mean flow velocity, bed roughness, and to a minor extent on particle shape. The centroid frequency of the signal resulting from the collision of a bed load particle against the geophone plate was found to be inversely related to particle size but to be less sensitive to variations in mean flow velocity and bed roughness than the signal amplitude, which is also related to particle size. Combining frequency and amplitude information resulted in a more robust identification of the transported particles size over a wide range of sizes than using amplitude information alone.

Laboratory flume experiments with the Swiss plate geophone bed load monitoring system: 2. Application to field sites with direct bed load samples

The Swiss plate geophone is a bed load surrogate monitoring system that had been calibrated in several gravel bed streams through field calibration measurements. Field calibration measurements are generally expensive and time consuming, therefore we investigated the possibility to replace it by a flume-based calibration approach. We applied impulse-diameter relations for the Swiss plate geophone obtained from systematic flume experiments to field calibration measurements in four different gravel bed streams. The flume-based relations were successfully validated with direct bed load samples from field measurements, by estimating the number of impulses based on observed bed load masses per grain-size class. We estimated bed load transport mass by developing flume-based and stream-dependent calibration procedures for the Swiss plate geophone system using an additional empirical function. The estimated masses are on average in the range of ±90% of measured bed load masses in the field, but the accuracy is generally improved for larger transported bed load masses. We discuss the limitations of the presented flume-based calibration approach.

Erosion and sedimentation, second edition

The second edition of Prof. Julien’s well-known book has the identical contents as the first edition published in 1995, but an increase from 280 to 371 pages. Among the relatively few books available in this field of river engineering, this work is particularly user-friendly, given the clear and concise statements at the introduction of all chapters, the excellent artwork supporting understanding of the material and the exercises for furthering the learning process. Prof. Julien is to be congratulated for this excellent work, therefore. The book author is well known in the field of river engineering with experiences both in hydraulic modelling and in basic research. The book includes the following chapters: (1) Introduction, (2) Physical properties and dimensional analysis, (3) Mechanics of sediment-laden flows, (4) Particle motion in inviscid fluids, (5) Particle motion in Newtonian fluids, (6) Turbulent velocity profiles, (7) Incipient motion, (8) Bedforms, (9) Bedload, (10) Suspended load, (11) Total load, and (12) Reservoir sedimentation. The book ends with two Appendices, Bibliography and an Index. When comparing the two editions, it is obvious that all figures were redrawn, of which the quality meets top standards. Further, the second edition is divided into a theoretical part in usual white colour and an application part in grey, allowing us to distinguish visually better the portions, depending on the purpose of learning basics, or applying the knowledge to examples. The author has selected a modern way of offering his knowledge to readers by this approach. It is also observed that the second edition has a large number of additional exercises, whereas the theoretical part has remained more or less as in edition 1. The basic formulations of fluid mechanics are provided as equations of motion in Cartesian, cylindrical and spherical coordinates. However, these are not applied using numerical approaches but are simplified to basic problems of fluid mechanics, including, for example, flow around a sphere, its drag and fall velocity or laboratory measurement of the particle size. In Chapter 6, the standard knowledge on turbulent flow is presented for both smooth and rough plane boundaries, and then the deviations from the logarithmic velocity profile including the log-wake law, the modified log-wake law and the sidewall correction method are highlighted. Chapter 7 summarizes the facts known on incipient sediment motion, including the approach of Shields and its expansion to sediment mixtures. The interaction with turbulent coherent flow structures is not considered. Bedforms are discussed in Chapter 8, including a large accumulation of findings made at the author’s university from the 1960s. In Chapters 9–11, the most important equations describing bedload, suspended load and total load are introduced, yet one might have expected a description of their Journal of Hydraulic Research Vol. 48, No. 6 (2010), pp. 836–837 doi:10.1080/00221686.2010.534278