Heide Friedrich

Live-Bed Scour at Submerged Weirs

AbstractWeirs or bed sills are low-head hydraulic structures used for bed stabilization, raising upstream water level, and reducing flow velocity. During high-flow events, the weir is fully submerged in the river and scouring occurs both upstream and downstream of the weir. For a fully submerged weir, the scour mechanism around the weir is dependent on approach flow intensity (clear-water scour conditions or live-bed scour conditions) and flow regimes (surface-flow regime or impinging-jet regime) over the weir. The fast evolution of underwater mobile topographies and propagating bedforms increase the complexities of the scour process and the difficulties for scour measurement at the submerged weir under live-bed scour conditions. This paper develops a measurement and data-processing technique for the study of scour at submerged weirs under extreme measurement environments and investigates the scour process both upstream and downstream of submerged weirs under live-bed scour conditions. The experiments are...

Laboratory study of gravel-bed cluster formation and disintegration

Increased knowledge of clusters is essential for the understanding of sediment transport behavior and the monitoring and protection of aquatic life. A physical study using graded river gravels is conducted in a laboratory environment. Using photogrammetry and painted gravels, a cluster identification tool (CIT) is developed based on image subtraction between subsequent frames, allowing identification of any stable areas and groups of particles on the bed. This is combined with digital particle tracking (DPT) to present a novel approach for monitoring the formation and disintegration of clusters. Clusters from graded gravels are formed successfully during the experimental stage, allowing investigation into the complex dynamic behavior of cluster formation and disintegration in a simulated natural environment. Various anchor stone arrangements are used in the experiments. However, only about one fifth of the potential anchor stones on the bed surface enable cluster formation. In general, clusters classified as “typical” and “heap” are most common. Inspection of temporal cluster coverage of the test-bed surface shows that the proportion of clusters present on the surface tends to grow with time. Maximum cluster surface coverage of between 5% and 34% is observed. In addition, particles entering and departing from clusters are monitored. Most commonly, particles enter from directly upstream of the cluster, however >20% of particles approach from a direction >20 deg from the streamwise direction. Approximately 35% of all particles directly upstream of a cluster bypass the cluster.

Turbulent entrainment in sediment-laden flows interacting with an obstacle

Temporal entrainment characteristics and mixing processes of sediment-laden turbidity currents interacting with a rectangular obstacle are investigated through lock-exchange experiments. Building on the Morton-Taylor-Turner hypothesis, dependency of temporal entrainment on non-dimensional parameters is examined. Currents of varying density are analyzed during the slumping phase over smooth and rough substrates. Quantitative and qualitative observations of the currents are captured through high resolution, high framerate binary thresholding techniques. Additionally, siphoning techniques are used to compare the density structure of the currents before and after the obstacle. Upon interaction with the obstacle, currents are found to experience four stages of entrainment: (i) lateral entrainment stage; (ii) jet stage; (iii) collapsing stage; (iv) re-establishment stage. The entrainment parameter was within the range of other studies for both obstacle and no-obstacle cases. Reynolds, Froude, and Richardson num...

A practical comparison between Zhang's and Tsai's calibration approaches

With the rise of affordable processing power and off-the-shelf apparatus supporting 3D imaging, there is a growing need for reliable and fast calibration tools, enabling timely accurate data gathering. When confronted with a choice of camera calibration tools, Zhangs and Tsais are not only the most cited, but also the most widely available solutions. Zhangs calibration is often chosen by default, based on the assumption that it is more accurate. However, it typically involves extensive manual data gathering when compared to the Tsai approach. Here, we demonstrate that there is no significant accuracy gain between Tsais or Zhangs approach in terms of stereo matching, given the variety of readily available 3D devices tested. Further to this, the trade-off between measurement accuracy compared to setup and data acquisition time is decisively in favour of Tsai. This paper also covers a new algorithm for the extraction of points from images of checkboards attached to calibration objects.

Local scour at submerged weirs in sand-bed channels

ABSTRACT Submerged weirs are river training structures that are used for raising upstream water level, bed stabilization and reducing flow velocity. This paper presents an experimental study of local scour at submerged weirs in sand-bed channels. Two types of tests (coarse sand tests and fine sand tests) were conducted to investigate the effects of sediment size and tailwater depth on scouring at submerged weirs. The flow regimes over the weir are found to be independent of the sediment size, and the transition flow regime boundary can be expressed as a function of upstream Froude number and the ratio of weir height to tailwater depth. New equations, including the effects of sediment size, tailwater depth, flow intensity, and weir height, are proposed for prediction of equilibrium scour depths both upstream and downstream of the submerged weir. A new design method is given for estimating the maximum scour depths at the weir.

Isolating roughness scales of gravel‐bed patches

There is a growing consensus that gravel-bed roughness should be parameterized based on bed-surface topography, not only sediment size. One benefit is the possible identification of various spatial scales of surface roughness and evaluation of their respective contributions to flow resistance (and also to bedload transport). The absence of relationships between roughness at the different scales is apparent in previous work, which currently limits roughness parameterization from topography and application in flow modeling. This study examines the use of moving-window detrending on gravel-bed digital elevation models (DEMs) for isolating roughness scales and their respective signatures. A large dataset of 35 water-worked gravel-bed patches from both the laboratory and the field was used for the analysis. The measured bed topography was separated into two distinct DEMs: one representing grains, the other representing small bedforms. For all DEMs, bed-elevation parameters measuring vertical roughness, imbrication, and spatial correlations were determined. Our results show distinct topographic signatures between grain and bedform DEMs. We show strong positive linear relationships between grain vertical roughness and the size of the bed-surface material. Surface sediment arrangement also determined bedform shape, with groupings of coarse sediment forming humps on the surface, and finer sediment sheltered in hollows. Patch-scale vertical roughness could not be estimated simply as the sum of grain and bedform vertical roughness. Instead, our results suggest weighted summation and the existence of universal weighting coefficients. Practical applications for studies on gravel-bed roughness and flow modeling using DEMs are discussed.

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...

A Merging Solution for Close-Range DEMs to Optimize Surface Coverage and Measurement Resolution

The process of efficient and effective DEM merging is increasingly becoming more important. To allow DEM analysis for features of different scales, an increase in surface coverage cannot result in reduced measurement resolution. It is thus inevitable that merging individual high-resolution DEMs will become common practice for applications such as hydraulic roughness studies for fluvial surfaces. This paper presents an efficient and effective merging solution, whereby accurate co-registration of individual DEMs collected from consistent viewpoints and standard averaging for overlapping elevations ensure seamless merging. The presented method is suitable for DEMs collected using any measurement technology, as long as individual DEMs overlap and are arranged on regular grids. The merging solution is applied to the study of a laboratory gravel bed measured with vertical stereo photogrammetry at the grain scale (>106 points/m2). We show that the approach can be integrated into the DEM collection workflow at the design stage, which optimizes the measurement performance. We present how resampling before merging can be beneficial to keep data handling requirements practical, whilst ensuring accurate surface representation. Finally, the effect of scale variation is studied, showing that seamless merging applies to DEMs with variable resolution.

Tsai camera calibration enhanced

The rise of Unmanned Aerial Vehicles for surveying and sensing tasks have created new challenges for quick calibration of sensing systems, which we feel is a critical issue. In this context, calibration is performed often and needs to be achieved as quickly as possible. An approach with minimal user-interaction, which preserves sensing accuracy would be ideal. We propose a version of Tsais camera calibration with an improved distortion model and two non-linear optimization phases to calibrate our UAV equipped with a stereo-camera system. We trialled our proposed calibration approach against three known reconstruction pipelines: an un-calibrated pipeline, a pipeline calibrated using Zhangs approach and a pipeline calibrated using Tsais original approach. Our findings indicate that our approach has competitive accuracy, while requiring far less user-interaction than Zhangs approach.

The development and internal assessment of a high-resolution, non-proprietary, stereo-photogrammetric setup for hydraulic experiments

Remote sensing of riverine gravel-beds has been shown to be fundamental to derive a theoretically driven definition of the hydraulic roughness and to understand the complex processes at the sediment-water interface. Commonly, 2D gravel-bed topography was recorded and analyzed, and only more recently technology allowed the measurement of high-resolution 3D Digital Elevation Models (DEMs). Equipment to do so is limited to Terrestrial Laser-Scanners (TLS) and proprietary stereo-photogrammetric systems and associated commercial software. Obtaining 3D DEMs of the gravel-bed allows the use of advanced statistical functions of riverbed elevations, like Probability Distribution Functions (PDFs) and structure functions, to characterize the spatial and temporal structural development of the riverbed surface. The promise of quick high-resolution data acquisitions, obtained with digital close-range stereo-photogrammetry, which can be employed at various locations, warrants detailed research into this area. In this paper we present the development of a high-resolution, non-proprietary stereo-photogrammetric setup, to be used for hydraulic experiments aimed at gravel-bed roughness characterization. Based on the quantitative assessment of the calibration process and stereo rectification of the images, means to evaluate the reliability of the system are described. It is shown that the extraction of the internal orientation of the two cameras and the external orientation of the stereo setup, as well as the rectification of the images to epipolar geometry, are crucial steps to successfully match corresponding pixels and obtain high-quality DEMs of a gravel-bed. Finally, surface plots of the measured gravel-bed topography are presented, showing how improvements are reflected in the quality of the DEMs.