Studying the effect of different in-stream fluvial turbines siting on river morphodynamics allowed us to witness the onset of a time-averaged, large-scale, alternate distortion of bed elevations, ...which could not be exclusively related to the turbine rotor blockage. The longitudinal profiles of this two-dimensional bathymetric perturbation resemble those of steady fluvial bars. In this contribution we generalize the problem addressing a spatially impulsive, asymmetric distribution of drag force in the channel cross-section. This is experimentally investigated through the deployment of differently sized grids perpendicular to the flow, and analytically explored as a finite perturbation of an open channel flow over an erodible sediment layer, as described by a coupled flow–sediment shallow water equation. The steady solutions of this fluvial morphodynamic problem, physically represented by alternate bars scaling with the channel width, highlight the importance of the resonant conditions in defining the spatial extent of the bed deformation. The equations further suggest that in very shallow flows any asymmetric obstruction may lead to an upstream propagation of the steady bars, consistent with previous studies on the effects of channel curvature. In broad terms, this study provides the preliminary framework to control the onset of river meandering through imposed finite perturbations of the cross-section. In a more applied sense, it provides a tool to predict non-local scour–deposition patterns associated with the deployment of energy converters or other flow obstructions.
Harmful algal blooms (HAB) are ubiquitous ecological and public health hazards because they are composed of potentially toxic freshwater microorganisms called cyanobacteria. The abiotic drivers for ...toxic HAB are investigated using a research station deployed in a eutrophic lake in Minnesota in 2016. This research station provides full‐depth water quality (hourly) and meteorological conditions monitoring (5 min) at the sampling site. Water quality monitoring provides chemical, physical, and biological measurements, that is, phycocyanin concentration, a photosynthetic pigment distinct to cyanobacteria. The high cyanobacteria biovolume (BV) in the epilimnion observed in mid‐July persisted until late September when it was distributed uniformly throughout the water column. A scaling relationship was developed among BV heterogeneity, thermal stratification stability, and surface water temperature. This relationship was verified in a dimictic lake the following year. The proposed scaling relationship is relevant to sampling protocols of HAB as it informs if the sample depth is representative of the entire water column. During the strongly stratified period, BV accumulated above the thermocline and in the photic zone, with distinct peaks forming occasionally both near the water surface and at locations with photosynthetically active radiation approximately equal to 10 μE/m2s. Our observations suggest that the temporal/vertical variability of cyanobacteria BV is strongly influenced by lake dynamics, thermal structure, seasonal temperature variation, and light availability. These observations demonstrate that cyanobacteria tend to move and accumulate in specific warm water layers, confined by the thermocline and determined by well‐defined light conditions.
Key Points
A cyanobacteria bloom was observed to be driven by prolonged warm surface water temperatures and strong thermal stratification stability
A strong correlation was quantified between cyanobacteria vertical heterogeneity, thermal stability, and surface water temperature
The depth of maximum cyanobacteria biomass follows the thermocline depth and well‐defined light conditions
With the expansion of hydropower, in‐stream converters, flood‐protection infrastructures, and growing concerns on deltas fragile ecosystems, there is a pressing need to evaluate and monitor bedform ...sediment mass flux. It is critical to estimate real‐time bedform size and migration velocity and provide a theoretical framework to convert easily accessible time histories of bed elevations into spatially evolving patterns. We collected spatiotemporally resolved bathymetries from laboratory flumes and the Colorado River in statistically steady, homogeneous, subcritical flow conditions. Wave number and frequency spectra of bed elevations show compelling evidence of scale‐dependent velocity for the hierarchy of migrating bedforms observed in the laboratory and field. New scaling laws were applied to describe the full range of migration velocities as function of two dimensionless groups based on the bed shear velocity, sediment diameter, and water depth. Further simplification resulted in a mixed length scale model estimating scale‐dependent migration velocities, without requiring bedform classification or identification.
Plain Language Summary
Sand and gravel sediment in river beds often forms wave‐like patterns called ripples or dunes, which are collectively referred to as bedforms. The flow of the river causes bedforms to form and slowly travel downstream, thus contributing to sediment transport, erosion, and deposition along the river and to the evolution of deltas. Predicting how fast bedforms move can improve our ability to estimate sediment load carried by rivers. In this study, experimental measurements of bedforms in a laboratory facility and in the Colorado River are used to better understand bedform movement. This study confirms that smaller bedforms move faster than larger bedforms and provides a simple equation to predict the speed of moving bedforms of different sizes based on the bedform length, the size of the sediment grains, and the depth of the river.
Key Points
Scale‐dependent bedform migration velocities are confirmed using laboratory and field measurements
Bedform velocity depends on stream depth, sediment diameter and shear velocity, reorganized into frictional and bankfull scaling parameters
A mixed length scale is proposed in a simplified predictive model
Introduction
Alcohol and cannabis use disorders are the most frequent comorbid substance use disorders (SUDs) among patients with eating disorders (EDs). EDs and SUDs involving alcohol and cannabis ...are independently associated with excess mortality.
Objectives
To investigate the impact of comorbid alcohol use disorder (AUD) and cannabis use disorder (CUD) on mortality in anorexia nervosa (AN), bulimia nervosa (BN), and unspecified eating disorder (USED) compared with matched control subjects.
Methods
This retrospective cohort study was conducted using Danish nationwide registers. The risk of mortality among ED patients with/without AUD and/or CUD was compared to matched control subjects with/without AUD and/or CUD using hazard ratios (HRs).
Results
Of the 20,759 included ED patients, 4.7% and 4.3% had AUD and CUD, respectively. The corresponding figures for the 83,036 control subjects were 1.0% (AUD) and 1.3% (CUD). ED patients without SUDs exhibited an increased risk of mortality compared to control subjects without SUDs (adjusted HR 2.9,
P
<.001). Mortality risk was higher among ED patients with AUD (adjusted HR 11.8,
P
<.001) or CUD (adjusted HR 4.6,
P
<.001) compared to control subjects without AUD/CUD. In addition, patients with AN, BN, and USED, who had comorbid AUD and/or CUD, exhibited an elevated risk of mortality compared to control subjects without AUD/CUD (AN: adjusted HR 11.3,
P
<.001; BN: adjusted HR 5.9,
P
<.001; USED: adjusted HR 10.9,
P
<.001).
Conclusions
Comorbid AUD and/or CUD increase mortality risk in patients with EDs. In order to reduce mortality in ED patients, prevention and treatment of AUD and CUD is important.
Disclosure
No significant relationships.
•Offshore floating wind turbine models are tested at laboratory scale.•Scaling parameters are presented to describe wind and wave forcing.
Display omitted
Two sets of experiments in the St. Anthony ...Falls Laboratory (SAFL) wave tank facility and atmospheric wind tunnel are integrated to provide a scaled representation of a floating wind turbine under heave and pitch motions due to ocean waves. The quasi-coupling is established by controlling the turbine rotor speed to generate a thrust force mimicking steady or fluctuating wind gusts in the wave tank, and by using two actuators to oscillate a miniature turbine in the wind tunnel. Measured pitch and heave motions under varying waves are scaled down using rotor geometry and the wake meandering frequency to study the effect of the floating platform kinematics on the evolution and characteristics of the oscillating turbine wake. For a limited case of experimental conditions results provide a phenomenological and quantitative description of the floating-turbine system under variable waves and simulated wind gusts. Specifically, we demonstrate that wind gusts contribute to increase the platform pitch range, and that periodic large scale flow patches of high and low momentum flow are generated by the oscillating rotor in the turbulent boundary layer and are coherently convected through the wake. Both mechanisms could amplify the pitch response of downwind floating turbine units within the offshore power plant, in particular if the wave and/or wind forcing frequencies happen to approach the pitch natural frequency of the floating system.
A high frequency, high resolution, seasonal research station was deployed to quantify a wide range of local meteorological conditions, water temperature, and water chemistry, including phycocyanin, ...in two different eutrophic stratified Minnesota lakes. The monitoring effort was coupled with discrete weekly sampling measuring nutrients, cyanobacteria composition, and microcystin concentrations. Our objective was to describe the vertical and seasonal distributions of cyanobacteria biovolume (BV) and microcystin concentrations (MC) using physical lake variables. Two types of BV distributions were observed above the thermocline upward in the water column. The first distribution depicted BV uniformly distributed over the diurnal surface layer (hSL), and the second BV distribution displayed local BV maxima. A quantitative relationship was developed to determine the anticipation of observing a uniform distribution as a function of the surface layer Reynolds number (ReSL), the dimensionless ratio of inertial to viscous forces. The uniform distribution was observed systematically for ReSL > 50,000. MC was observed to accumulate above the thermocline and have a vertical distribution similar to BV, thus depending on ReSL. This is important for directing sampling efforts, because it narrows the range of BV and MC heterogeneity above the thermocline, and suggests a vertical sampling protocol to detect potential maxima and compute representative depth-average concentrations. We explored the temporal variability of the MC to BV ratio, spatially averaged in the epilimnion (MCep/BVep). The maximum MCep/BVep occurred before the maximum BVep and specifically, during the onset of significant biomass growth in both lakes. This observation is notable because the maximum MCep occurs before the visual signs of enhanced cyanobacterial accrual are less recognizable to the public and to monitoring efforts. Our findings could have important implications for predicting MC distribution and guiding monitoring strategies for quantifying MC concentrations in small stratified lakes.
In the last decade several efforts were devoted to model sediment‐particle transport in rivers as a stochastic process. Experimental observations are therefore needed to validate these models and to ...provide the correct probability distribution of selected stochastic variables. The kinematics of sand particles is investigated here using nonintrusive imaging to provide a statistical description of bedload transport under incipient motion conditions. In particular, we focus on the alternation between motion (particle steps) and rest regimes to quantify the probabilistic distribution of the particles waiting time, which is suggested by many studies to be responsible for anomalous diffusion. The probability distributions of the particle step time and step length, streamwise and spanwise velocities, acceleration, and waiting time are quantified experimentally. Results suggest that variables describing the particle motion regime are thin‐tailed distributed, whereas the waiting times exhibit a power law distribution. A specific class of waiting times during which the grain is observed to oscillate without a net displacement is classified as active and is analyzed separately from the other, so‐called deep waiting times. The experimental results, obtained under five different transport conditions, describe grain‐scale kinematics and dynamics at different wall shear stress. They provide both a benchmark data set for validating particle‐transport numerical simulation and critical input parameters for the stochastic modeling of bedload transport.
Key Points
The alternation between motion and rest regimes of sediment particles is observed under low bedload transport
Particle waiting times are classified into “active” and “deep,” and their distributions are quantified experimentally
The particle waiting time decreases with increasing shear velocity, whereas the particle velocity, step length, and acceleration variance increase
We present an implementation of super-large-scale particle image velocimetry (SLPIV) to characterize spatially the turbulent atmospheric boundary layer using natural snowfall as flow tracers. The ...SLPIV technique achieves a measurement area of ~22 m × 52 m, up to 56 m above the ground, with a spatial resolution of ~0.34 m. The traceability of snow particles is estimated based on their settling velocity obtained from the wall-normal component of SLPIV velocity measurements. The results are validated using coincident measurements from sonic anemometers on a meteorological tower situated in close proximity to the SLPIV sampling area. A contrast of the mean velocity and the streamwise Reynolds stress component obtained from the two techniques shows less than 3 and 12 % difference, respectively. Additionally, the turbulent energy spectra measured by SLPIV show a similar inertial subrange and trends when compared to those measured by the sonic anemometers.
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