Martin Detert

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.

4 Pressure- and velocity-measurements above and within a porous gravel bed at the threshold of stability

Abstract Experimental results of measurements characterising the pressure and velocity above and within a porous gravel layer are presented. The goal of this study is to give a better understanding of the flow in the hyporheic interstitial under the influence of turbulence in the main flow. Latest developments in measuring techniques were applied: miniaturised piezoelectric pressure sensors (MPPS) were used to measure turbulent pressure fluctuations inside the gravel layer. Velocity measurements were carried out by a 3D-particle tracking velocimetry system (3D-PTV) using miniaturised endoscopic stereo setups within artificial gravel pores. Additionally, in the main flow a 1D-acoustic doppler current profiler (1D-ADCP) was used. Within the main flow, alternating faster and slower fluid packets with a size scaling with the flow depth were observed. Pressure fluctuations rms( p ) as well as velocity fluctuations rms( u , v , w ) decrease exponentially with increasing gravel depth, mainly within the first two layers of gravel grains.

Proof‐of‐concept for low‐cost and non‐contact synoptic airborne river flow measurements

ABSTRACT This article presents an example for an image-based method to determine a small rivers’ surface velocity field, bathymetry, and its flow discharge derived thereof. While common river measurement techniques typically need to be in contact with the waterbody, at least to measure transects of water depths, the new approach is completely without direct contact to water. The scenery is recorded by an off-the-shelf action camera mounted to a low-cost quadcopter. During the analysis, image frames are orthorectified and georeferenced by an approach that combines structure from motion and multiview stereo algorithms. Particle image velocimetry based on tracer particles present in the river is utilized to compute flow velocities. Instantaneous surface flow velocity fields are used to compute the mean velocity field as well as to derive water depth estimates by turbulence metrics. The latter provide a basis to verify the bathymetry gained from the reconstructed subaqueous three-dimensional scenery. Finally, flow discharge is estimated based on the above findings. The new method will expand the scientific knowledge on river flows for diverse disciplines such as hydrology, biology, and river engineering. Due to the low cost of the measurement instrumentation being deployed, it has the potential to be applied by a broad user group.

Estimation of the Washout Depth of Fine Sediments from a Granular Bed

An exponential distribution of the bed-pressure fluctuations is used to estimate the depth within a porous gravel bed from which fine sediment of a given size can be removed. The coarsest grain size of the fine sediment that might be washed out is of O ( 10−1 ) in relation to both the gravel grain size and the equivalent grain roughness. A higher equivalent grain roughness results to a larger absolute cleaning depth, whereas the averaged gravel grain size is seen to be less important. The results are successfully tested for plausibility against the grain size distributions of an armored gravel bed and its underlaying bimodal layer as found in situ in the river Rhine. However, qualitative and quantitative experimental data for an in-depth validation remain to be performed.

Size Ratio of Fluvial Grains’ Intermediate Axes Assessed by Image Processing and Square-Hole Sieving

AbstractThe comparability of grain sizes emerging from different methods are discussed, including image-based grain-size analysis. Waterworked gravel-bed surfaces from laboratory and field experime...

Automatic high-throughput measurement of live aquatic snails from images

Andrea Irniger, Manfred J. Morari, Anja Bürkli and Martin Detert Hunziker, Zarn und Partner AG, 5000 Aarau, Switzerland; Hetzer, Jäckli und Partner AG, 8610 Uster, Switzerland; Department of Aquatic Ecology, EAWAG, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland; Institute of Integrative Biology, ETH Zurich, 8092 Zürich, Switzerland; and Laboratory of Hydraulics, Hydrology and Glaciology (VAW), ETH Zurich, 8093 Zürich, Switzerland