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 m 2 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.
The subglacial hydrology of tidewater glaciers is a key but poorly understood component of the complex ice-ocean system, which aects sea level rise. As it is extremely dicult to access the interior of a glacier, our knowledge relies mostly on the observation of input variables such as air temperature, and output variables such as the ice ow velocities reecting the englacial water pressure, and the dynamics of plumes reecting the discharge of meltwater into the ocean. In this study we use a cost-eective Vertical Take-O and Landing (VTOL) Unmanned Aerial Vehicle (UAV) to monitor the daily movements of Bowdoin Glacier, northwest Greenland, and the dynamics of its main plume. Using Structure-from-Motion photogrammetry and feature-tracking techniques, we obtained 22 high-resolution ortho-images and 19 velocity elds at the calving front for 12 days in July 2016. Our results show a two-day-long speed-up event (up to 170%) caused by an increase in buoyant subglacial forces with a strong spatial variability revealing that enhanced acceleration is an indication of shallow bedrock. Further, we used the Particle Image Velocimetry (PIV) method to analyze water ow from successive UAV images taken while ying over the main plume of the glacier. We found that PIV successfully captures the area of radially diverging ow of the plume, and provides information on spatial and time variability as no other remote sensing technique can. Most interestingly, the active part of the plume features pulsating water jets at the time scale of seconds, and is 1 to 5 times smaller than its visual footprint dened by the iceberg-free area. Combined with an ice ow model or a non-steady plume model, our approach has the potential to generate a novel set of input data to gather information about the depth of the bedrock, the discharge of meltwater, or the subglacial melting rate of tidewater glaciers.
This paper presents a comprehensive study of the near-bed hydrodynamics at non-moving streambeds based on laboratory experiments in open-channel flows. Pressure and velocity measurements were made with an array of up to 15 miniaturized piezo-resistive pressure sensors within the bed and slightly above it, and a two-dimensional particle-image-velocimetry (PIV) system measuring in streamwise vertical or horizontal planes. Three different types of bed materials were studied covering typical natural streambed conditions. The range of the global Reynolds number covered in the experiments was from 20000 to 200000. This study provides new insights into the flow structure over gravel beds based on the PIV measurements in both streamwise vertical and horizontal planes. In a streamwise vertical plane, large-scale wedge-like flow structures were observed where a zone of faster fluid over-rolled a zone with slower fluid. The resulting shear layer was inclined along the flow at an angle of 10°–25° to the bed, and was populated with clockwise rotating eddies. This mechanism occurred with sufficient frequency and shape to leave an ‘imprint’ in the velocity statistics. Typically, the described flow pattern is formed near the bed and is approximately scaled with the height of the logarithmic layer, although the biggest structures extended over the whole flow depth. In a horizontal near-bed plane, turbulent structures formed a patched ‘chessboard’ pattern with regions of lower and higher velocities that were elongated in the streamwise direction. Their lateral extension was typically two to four times the equivalent sand roughness with lengths up to several water depths. The dimensions of the regions were increasing linearly with the distance from the bed. These findings are consistent with conceptual models originally developed for smooth-wall flows. They also support observations made in rough-bed flume experiments, numerical simulations and natural rivers. Spatial fields of bed-pressure fluctuations were reconstructed by applying Taylor's frozen turbulence hypothesis on time data obtained with an array of pressure sensors. Based on the conditional sampling of velocity patterns associated with pressure-drop events a distinct bed-destabilizing flow-pressure pattern was identified. If a high-speed fluid in the wake of a large-scale wedge-like flow structure reaches the vicinity of the bed, a phenomenon akin to a Bernoulli effect leads to a distinctive low-pressure pattern. The resulting force may exceed the particles' submerged weight and is assumed to be able to give an initial lift to the particle. As a result, the exposed area of a particle is amplified and its angle of repose is reduced, increasing the probability for entrainment.
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