River Velocity Research Articles

River Surface Analysis and Characterization Using FMCW Radar

Today’s warning systems for floods or droughts require sensor applications that provide a vast array of information. Current systems provide insight into either the water level or the river velocity. In order to obtain additional parameters for the characterization of the flow behavior in a noncontact manner, a frequency-modulated continuous-wave radar with chirp sequence is used. Since these sensors provide range and velocity information but also enable gathering of additional parameters. The presented measurements in this contribution were performed at four different rivers with a commercially available radar sensor. The results of the classical postprocessed 2-D fast Fourier transform are used as basis for an imagewise processing approach to obtain additional features for classifying the behavior of the river surface. For this purpose, the framewise enveloping velocities are depicted in a time sequence. Due to processing the detected reflection patterns in relation to time, a characteristic pattern for river flow profiles can be extracted. By reducing the information using time averaging, characteristic features for different flows can be extracted from the spatial envelope velocity distribution. In particular, the resulting insights lead to characteristic features for single flow distributions that enable novel monitoring systems with new possibilities to classify rivers using radar sensors.

Advances in selection mechanism of aquatic insects on water flow velocity and substrate in small mountain rivers

As a major group of macroinvertebrate benthos, aquatic insects can reflect the status of river ecosystem to a great extent, and are one of the important biological monitoring indicators. Sediment and flow velocity are very important environmental factors which can affect community structure and biological characteristics of aquatic insects in rivers. However, there were few studies on the relationship between benthos and the two factors at home and abroad. In future, it is suggested to use physical model to simulate and construct the coupling relationship between diversity index of aquatic insects, flow velocity and sediment, and can reveal the selection mechanism of aquatic insects on velocity and sediment, so as to provide scientific basis for the protection and restoration of aquatic habitat of small rivers in mountainous areas.

Soil Loss Estimation within the Nun River Basin using Revised Universal Soil Loss Equation (RUSLE) integrated in Geographic Information Systems (GIS)

River bank erosion can cause undesired river bank expansion, distortion of the river course thereby slowing the river velocity and hindering the water carrying capacity of the river. Slow velocity of the river course can result in fast sediment accretion and accumulation at the bottom of the river. If these trends are not checked, the river may wind down and at best begin to become a delta at its middle course. The focus of this study is to apply GIS and analytical RUSLE equation to estimate soil loss within the Nun Basin.
 Core sediment samples were collected from relatively undisturbed areas (nine different stations) using Uwitec Triple sediment cutter. The sediment samples were processed in the lab for sediment particle size analysis (PSA) and Pb-210 atmospheric deposition with age using alpha spectrophotometer. Spatial rainfall data of the study location were employed to generate the rainfall erosivity map in order to determine the erosivity factor (R). Particle size distribution analysis was validated using Scanning Electron Micrograph (SEM) and integrated with Geographic Information Systems (GIS) was employed to create the soil erodibility map which was subsequently used to determine the erodibility factor (K). The cover management factor (P) was determined using the land use land cover map and the resulting data were then employed to model the river bank erosion around the study area.
 Result of the study revealed that the maximum annual average soil loss rate was estimated to be 0.66 tons/ha. /year around the Nun River based on GIS application using the Revised Universal Soil Loss Equation (RUSLE).

Robust Image-Based Streamflow Measurements for Real-Time Continuous Monitoring

The volumetric flow rate in rivers is essential to analyze hydrological processes and at the same time it is one of the most difficult variables to measure. Image based discharge measurements possess several advantages, one of them being that the sensor (camera) is not in contact with the water, it can be placed safe of floods, its mounting position is very flexible and there is no need of expensive structures/constructions. During the last years several image-based methods for measuring the surface velocity in rivers and canals have been proposed and successfully tested under different conditions. However, these methods have been used and configured to perform well under the particular conditions of a single recording or single site. The objective of this paper is to present a system which has reached a Technology Readiness Level (TRL) 9. The system is able to measure the volumetric flow under different conditions day and night and all year long, the system is able to perform in rivers or canals of different sizes and flow velocities and under different conditions of visibility. In addition, the system is capable of measuring the river stage optically without the need of a stage, but it can also integrate external level sensor. Important for a wide set of customers, the system must be able to interface with the various common signal input and output standards, such as 4–20 mAmp, modbus, SDI-12, ZRXP, and even with customer specific formats. Additionally, the developed technology can be implemented as an edge or as a cloud system. The cloud system only needs a camera with Internet connection to send videos to the cloud where they are processed, while the edge systems have a processing unit installed at the site where the processing is done. This paper presents the key aspects needed to move from prototype with TRL5-7 and lower toward the presented field proven system with a TRL 9.

Evaluation of flow resistance equations for high gradient rivers using geometric standard deviation of bed material

A dataset of 2184 field measurements reported in the literature was used to evaluate the predictive capability of eight conventional flow resistance equations to predict the mean flow velocity in gravel-bed rivers. The results reveal considerable disagreement with the observed flow velocities for relative submergence less than 4 and for the non-uniformity of the bed material greater than 7.5 for all the equations. However, the predictions made using the Smart and Jäggi (1983), Ferguson (2007), and Rickenmann and Recking (2011) equations were closer to the observed values. Furthermore, bedload sediment transport also reduces the predictive capability of the equations considered in this study except for the Recking et al. (2008) equation, which was developed considering active bedload transport. The performance of flow resistance equations improves when corrected by considering the geometric standard deviation of the bed material. Here we present an empirical approach using the whole dataset and its subsets for accounting for the additional energy losses occurring due to the wake vortices, spill losses, and free surface instabilities occurring due to the protrusions from the bed. The results obtained using the validation dataset shows the importance and usefulness of this approach to account for the additional energy losses, especially for the Strickler (1923) and Keulegan (1938) equations.

Finding the optimum groin layout for the Konaweha river banks protection via 2D numerical modeling

River bank erosion has become a critical issue, especially for river bends with an alluvial soil type. Mostly, river bank erosions have caused structural failures because many national roads in Indonesia are placed near the river bend areas. Groins can reduce flow velocity, thereby increasing riverbank stability. However, most groins were installed in the river without impacting the river flow due to the improper design. This paper analyzes the use of groins placed at the river bend of the Konaweha River to protect the river bank with a length of 250 m from erosion. The evaluation employs 2D numerical modeling using MIKE21 FM to observe the influence of the groin on the river velocity, flow distribution pattern, and water level. Sixteen model scenarios with four groin configurations were tested for 25-year discharge, 2-year discharge, normal discharge 600 m3/s, and low discharge 197 m3/s to achieve the most effective plan. Based on the simulation results, Scenarios with five groins are recommended to reduce the flow velocity along the outer river bank from 2 to 0.3 m/s, thus minimizing the erosion. In addition, the spacing of groins being twice the groin length is recommended.

Depths Inferred from Velocities Estimated by Remote Sensing: A Flow Resistance Equation-Based Approach to Mapping Multiple River Attributes at the Reach Scale

Remote sensing of flow conditions in stream channels could facilitate hydrologic data collection, particularly in large, inaccessible rivers. Previous research has demonstrated the potential to estimate flow velocities in sediment-laden rivers via particle image velocimetry (PIV). In this study, we introduce a new framework for also obtaining bathymetric information: Depths Inferred from Velocities Estimated by Remote Sensing (DIVERS). This approach is based on a flow resistance equation and involves several assumptions: steady, uniform, one-dimensional flow and a direct proportionality between the velocity estimated at a given location and the local water depth, with no lateral transfer of mass or momentum. As an initial case study, we performed PIV and inferred depths from videos acquired from a helicopter hovering at multiple waypoints along a large river in central Alaska. The accuracy of PIV-derived velocities was assessed via comparison to field measurements and the performance of an optimization-based approach to DIVERS was quantified by comparing calculated depths to those observed in the field. We also examined the ability of two variants of DIVERS to reproduce the discharge recorded at a gaging station. This analysis indicated that the accuracy of PIV-based velocity estimates varied considerably from hover to hover along the reach, with observed vs. predicted R2 values ranging from 0.22 to 0.97 and a median of 0.57. Calculated depths were also reasonably accurate, with median normalized biases from −4% to 9.9% for the two versions of DIVERS, but tended to be under-predicted in meander bends. Discharges were reproduced to within 1% and 4% when applying the optimization-based technique to individual hovers or reach-aggregated data, respectively. The results of this investigation suggest that, in addition to the velocity field derived via PIV, DIVERS could provide a plausible, first-order approximation to the reach-scale bathymetry. This framework could be refined by incorporating hydraulic processes that were not represented in the initial iteration of the approach described herein.

Spatial and temporal variability of <i>p</i>CO<sub>2</sub> and CO<sub>2</sub> emissions from the Dong River in south China

Abstract. CO2 efflux at the water–air interface is an essential component of the riverine carbon cycle. However, the lack of spatially resolved CO2 emission measurements prohibits reliable estimation of the global riverine CO2 emissions. By deploying floating chambers, seasonal changes in river water CO2 partial pressure (pCO2) and CO2 emissions from the Dong River in south China were investigated. Spatial and temporal patterns of pCO2 were mainly affected by terrestrial carbon inputs (i.e., organic and inorganic carbon) and in-stream metabolism, both of which varied due to different land cover, catchment topography, and seasonality of precipitation and temperature. Temperature-normalized gas transfer velocity (k600) in small rivers was 8.29 ± 11.29 and 4.90 ± 3.82 m d−1 for the wet season and dry season, respectively, which was nearly 70 % higher than that of large rivers (3.90 ± 5.55 m d−1 during the wet season and 2.25 ± 1.61 m d−1 during the dry season). A significant correlation was observed between k600 and flow velocity but not wind speed regardless of river size. Most of the surveyed rivers were a net CO2 source while exhibiting substantial seasonal variations. The mean CO2 flux was 300.1 and 264.2 mmol m−2 d−1 during the wet season for large and small rivers, respectively, 2-fold larger than that during the dry season. However, no significant difference in CO2 flux was observed between small and large rivers. The absence of commonly observed higher CO2 fluxes in small rivers could be associated with the depletion effect caused by abundant and consistent precipitation in this subtropical monsoon catchment.

Applications of Second Log-Wake Law for Turbulent Velocity Distributions in Laboratory Flumes and Natural Rivers

AbstractNatural river velocity distributions are often studied by laboratory flume flows because of similarities of the bed shear stress effects between the two flows. Nevertheless, the velocity di...

Structure from Motion (SfM) method to characterize fluvial sedimentology in Way Semaka river in Lampung province, Indonesia

Structure from motion (SfM) has been applied recently in fluvial sedimentology. This application is due to the availability of many low-cost unmanned aerial vehicles/drones, which can help overcome challenging terrain, provide efficient and reproducible and high-accuracy topographic images and data. The current study describes the application of SfM to build a geomorphological model and estimate the surface water velocity of the Way Semaka River in the Bandar Negeri Suoh (BNS) region in the West Lampung area. Way Semaka is a river type with an extensive meandering system and stable extension tectonic regime relatively. River morphology approximately 96.62 m width and 4 m depth in straight (relatively) area, whereas in channel area approximately 171.22 m width and 5 m depth. The velocity range between 0.39 – 1.56 m/s based on image analysis proven by current meter measurements onsite with an RMS error of 0.25. Manual geomorphic unit level 2 analysis revealed that 9% of the coverage consisted of basins for the channel’s zone, 35% of the coverage consisted of convexity, 9% consisted of planar features, and 37% of the coverage consisted of the transition zone.

A model for predicting the lateral distribution of sediment transport in river bends

ABSTRACT Distribution of lateral velocity and suspended sediment concentration in river bends is highly non-uniform. Predicting such velocity and sediment distributions is prerequisite for many applications, such as river bank protection, sediment transport, deposition pattern, pollutant transport and flood control. Due to secondary flow development, flow structure in river bends has three-dimensional feature. In this case, one-dimensional mathematical models are generally not suitable, so two- or three-dimensional mathematical models have to be used. In this paper, field measurement of lateral distributions of flow velocity and suspended sediment concentration was conducted in three bends at the Karun River in Iran (namely, Jangieh, Khabineh and Malheh), and the measured velocity data were used for calibrating the Spooner and Shiono model. Based on the lateral velocity distributions by this mathematical model, sediment transport capacities were computed using five commonly used empirical formulas of sediment transport. The results showed that in the three river bends, the sediment transports formula by Yang agrees well with the measured lateral sediment concentration. The Van Rijn formula also predicts the results with suitable accuracy. The other three formulas by Ackers-White, Engelund-Hansen, and Karim-Kennedy significantly over-predict the sediment concentration compared with the measured data.

Large-Scale Particle Image Velocimetry to Measure Streamflow from Videos Recorded from Unmanned Aerial Vehicle and Fixed Imaging System

The accuracy of river velocity measurements plays an important role in the effective management of water resources. Various methods have been developed to measure river velocity. Currently, image-based techniques provide a promising approach to avoid physical contact with targeted water bodies by researchers. In this study, measured surface velocities collected under low flow and high flow conditions in the Houlong River, Taiwan, using large-scale particle image velocimetry (LSPIV) captured by an unmanned aerial vehicle (UAV) and a terrestrial fixed station were analyzed and compared. Under low flow conditions, the mean absolute errors of the measured surface velocities using LSPIV from a UAV with shooting heights of 9, 12, and 15 m fell within 0.055 ± 0.015 m/s, which was lower than that obtained using LSPIV on video recorded from a terrestrial fixed station (i.e., 0.34 m/s). The mean absolute errors obtained using LSPIV derived from UAV aerial photography at a flight height of 12 m without seeding particles and with different seeding particle densities were slightly different, and fell within the range of 0.095 ± 0.025 m/s. Under high flow conditions, the mean absolute errors associated with using LSPIV derived from terrestrial fixed photography and LSPIV derived from a UAV with flight heights of 32, 62, and 112 m were 0.46 m/s and 0.49 m/s, 0.27 m, and 0.97 m/s, respectively. A UAV flight height of 62 m yielded the best measured surface velocity result. Moreover, we also demonstrated that the optimal appropriate interrogation area and image acquisition time interval using LSPIV with a UAV were 16 × 16 pixels and 1/8 s, respectively. These two parameters should be carefully adopted to accurately measure the surface velocity of rivers.

Determination of Stage Discharge Rating Curve at Sungai Batang Melaka

The river is a natural source of water with distinctive hydraulic features. The hydraulic characteristic is a factor in morphology changes. The article describes a determination of a stage discharge rating curve at Sungai Batang Melaka that has been developed for the first time. The stage discharge rating curve is used to determine whether the rivers of this area are the cause of floods that have occurred in Malacca. Based on data collection for width and depth at four locations, enabling to evaluate and analysis the data of each cross-section for Sungai Batang Melaka. By observing the river velocity, the condition of the river flow in the area will be obtained. The two-point method and mid-section method is used to measure the flow rates in the rivers. Following the calculation using Chezy-Manning’s equation, the authors research shows that this river produces a good stage discharge rating curve. Overall, a stage discharge rating curve for each location shows it is associated with gradation and degradation rates. From the findings, the outflow, storage capacity, and discharge rating curve in Sungai Batang Melaka were successfully obtained. It will benefit future studies, especially in facilitating measures to be taken to avoid flash floods that keep occurring in Malacca.

Portable unmanned surface vehicle that automatically measures flow velocity and direction in rivers

We developed an automatic measurement system for flow velocity and direction in natural rivers using an autonomously controlled unmanned surface vehicle (USV). Oncoming mainstream velocity was measured by the propulsion force required for the USV in order to preserve the position at a measurement point. To conduct such a field mission, the system runs by changing four characteristic control stages: 1) calculation of the tentative propulsion force, 2) navigation to the target point, 3) velocity measurement by staying at the target, and 4) detection of flow direction by flowing downstream. More than 20 indoor tests were conducted under several hydraulic conditions by varying streamwise velocity, and the calibration formula was obtained by interrelating the oncoming velocity magnitude with the propulsion force required to remain at the target. The attitude control was provided with side thrusters to improve the yaw stability of the USV in the oncoming current. The adjunctive work of the side thrusters was very effective. Field tests were conducted to examine the reliability and accuracy of the present automatic flow measurements in a river. Both local velocity and direction in the river flow were measured well by the USV. Error analysis was conducted by comparing with the existing velocimetry results, and the USV was found to possess a sufficient ability to meet practical performance for the flow measurement in a calm river with about 1 m/s velocity.

Modelling the Energy Extraction from Low-Velocity Stream Water by Small Scale Archimedes Screw Turbine

In recent years, there has been a growth of interest in the development of micro-hydropower power generation, especially in the low-head turbine. Low-head turbines gained popularity due to their high efficiency, relatively low cost, ability to operate at a low flow rate, and low environmental impact. In this aspect, the Archimedes Screw Turbine (AST) could be the primary key to electrifying the rural area in Sarawak, Malaysia, which is surrounded by rivers. This study starts with a conceptual design based on literature review findings. Eventually, the small-scale prototype is then being built and tested in the laboratory. The experiment is set up to simulate the actual Sarawak river velocity to determine the relationship between key performance variables such as the inclination angle of AST and water flow velocity. The findings revealed that the 45° angle of inclination was the optimum angle of AST within the water velocity of 1.0 m/s until 1.5 m/s. At this angle, the highest revolution per minute (RPM) generated by the AST shaft was 179.8, and the highest torque recorded was 0.9Nm. The results were validated through statistical means. It was found that both angles of inclination and river water velocity are significant to RPM and torque generation (p < 0.05). Two statistical models were generated based on linear regression to explain the contribution of water velocity and angle of inclination as inputs to torque and RPM as outputs, with a Pearson R2 value of more than 60%. The maximum mechanical power generated is about 1.54 kW, with a maximum efficiency of 94.6%. The outcome of this study would be useful for designing a small-scale AST power generation system by utilizing a low-flow river (velocity < 1.5 m/s) as a power source. This study would contribute to the existing knowledge stock of small-scale AST, primarily to operate in low-flow velocity rivers. For the future study, it is recommended for conducting a pilot study to test the actual performance of AST in the Sarawak River or rivers with similar flow characteristics.

Machine Learning Analysis of Hydrologic Exchange Flows and Transit Time Distributions in a Large Regulated River.

Hydrologic exchange between river channels and adjacent subsurface environments is a key process that influences water quality and ecosystem function in river corridors. High-resolution numerical models were often used to resolve the spatial and temporal variations of exchange flows, which are computationally expensive. In this study, we adopt Random Forest (RF) and Extreme Gradient Boosting (XGB) approaches for deriving reduced order models of hydrologic exchange flows and associated transit time distributions, with integrated field observations (e.g., bathymetry) and hydrodynamic simulation data (e.g., river velocity, depth). The setup allows an improved understanding of the influences of various physical, spatial, and temporal factors on the hydrologic exchange flows and transit times. The predictors also contain those derived using hybrid clustering, leveraging our previous work on river corridor system hydromorphic classification. The machine learning-based predictive models are developed and validated along the Columbia River Corridor, and the results show that the top parameters are the thickness of the top geological formation layer, the flow regime, river velocity, and river depth; the RF and XGB models can achieve 70% to 80% accuracy and therefore are effective alternatives to the computational demanding numerical models of exchange flows and transit time distributions. Each machine learning model with its favorable configuration and setup have been evaluated. The transferability of the models to other river reaches and larger scales, which mostly depends on data availability, is also discussed.

Sustainable Improvement of Water Quality in Urban River Network by Rotating Overflow Weir—Case Study of Changzhou City in China

Water diversion is a practical and commonly used approach to improve the water quality of rivers in urban plains in China. This paper researches using rotating overflow weirs as the primary engineering solution to facilitate water diversion and improve the water quality of the river network in urban plain area of Changzhou, including hydrodynamic modeling, field tests, and application. The water diversion scheme, including clean water source, water transfer paths, quantity, and rate, was designed based on its current terrain and river systems. The Yangtze River was selected as a high-quality water source, and its two tributaries were utilized as clean water paths. Four rotating overflow weirs were proposed to create three stages of hydraulic gradient. Clean water can flow naturally and orderly in the researched urban river network without pumps. The required clean headwater level and clean water distributions were modeled hydrodynamically. Four temporary weirs were constructed at design locations to conduct field tests. And the water diversion scheme and its water quality improvement had been verified by field tests. Afterward, four permanent rotating overflow weirs were built. Currently, the monitoring results show the design water transfer scheme reaches expectations. The flow velocity in most rivers has been increased to 7cm/s from standstill in Changzhou's main urban plain area. The composite water quality of 93% river cross-sections monitored has been improved from Class V to Class Ⅲ and all have been improved to Class IV and above.

The diversity and abundance of phytoplankton and benthic diatoms in varying environmental conditions in Kok River, Chiang Rai, Thailand as bio-indicators of water quality

Abstract. Prasertsin T, Suk-ueng K, Phinyo K, Yana E. 2021. The diversity and abundance of phytoplankton and benthic diatoms in varying environmental conditions in Kok River, Chiang Rai, Thailand as bio-indicators of water quality. Biodiversitas 22: 1853-1862. The study of living organisms was important information for bio-indicators which was utilized to assess the quality of the environment. In the river ecosystem, the algae with which one organism was interesting accordingly considered the relationship of 2 groups algae, including phytoplankton and benthic diatoms and the physical-chemical parameters of Kok River Chiang Rai, Thailand. Phytoplankton and benthic diatoms were collected from five sampling sites along the Kok River during January (cool dry), March (summer), and June (rainy season) 2018, and assessed as bio-indicators to monitor environmental factors that represent water quality across varying conditions and periods. Fifty-seven species of phytoplankton and thirty-nine species of benthic diatoms were found. The phytoplankton mainly belonged to the phylum Bacillariophyta (diatom group). Physical and chemical factors affecting the dominant phytoplankton and benthic diatom species were subjected to canonical correspondence analysis (CCA). Results showed that planktonic and benthic Gomphonema lagenula positively correlated with ammonium nitrogen, recording highest abundance during the rainy season. Abundance of Achnanthidium straubianum, a planktonic and benthic diatom, and Navicula cincta negatively correlated with river velocity and water conductivity, with lowest numbers during the rainy season. Water trophic status evaluated from the main parameters of the AARL-PC score indicated that during the cool dry and summer seasons all sampling sites were classified as mesotrophic, while during the rainy season they were classified as meso-eutrophic. The combination of phytoplankton and benthic diatom can be used as bio-indicators of water quality in the Kok River and other freshwater ecosystems.

Application of Hydraulic Geometry to High Gradient Rivers in Southern Ecuador

AbstractUsing hydraulic geometry (HG) theory, we investigated three rivers typical of the Ecuadorian mountainous region with longitudinal slopes ranging from 0.8% to 10% and coarse bed material with d50 ranging from 3 to 54 mm and values of d90 up to 908 mm. Extensive field measurements were performed to characterize geometric and hydraulic properties at 33 sites. Using these field data, at‐a‐station HG relationships were obtained for top width, average flow depth, and average flow velocity. Dimensional and dimensionless downstream HG relations were obtained for these parameters as well as the channel bed slope using the bankfull discharge estimated with at‐a‐station HG. The correlation coefficients indicate that the dimensionless equations adequately represent the observed data for all quantities with the exception of slope. In addition to a low correlation coefficient, the exponent on non‐dimensional discharge was small in the downstream HG relation for slope. This small exponent indicates that slope is not dependent on discharge. Based on this finding, the non‐dimensional HG relations were reformulated using both dimensionless discharge and bed slope as independent variables. These new relations show improved correlation coefficients and demonstrate the role of slope in determining channel width, flow depth, and average velocity in high gradient rivers.

Physically Based Scaling Models to Predict Gas Transfer Velocity in Streams and Rivers

AbstractAir‐water gas transfer influences fundamental processes of aquatic systems such as photosynthesis, respiration, and greenhouse gas emission. Previous predictive models for gas transfer velocity (KL) often only apply to a small range of streams and rivers. Two “universal” scalings, KL ∼ 1/4 power of turbulent dissipation rate (ε1/4) and KL ∼ shear velocity (U*), deduced from classic thin‐film theory and surface renewal theory, have been reported for lake and marine systems, but have not been adequately tested for streams or rivers. To test the two scalings and understand how KL varies across systems, we compiled 588 field measurements of KL, and used inverse modeling based on long‐term high‐frequency dissolved oxygen time series to estimate KL of 35 streams and rivers capturing a wide range of discharge (0–221 m3/s). We found that the two scalings held for both the KL estimated by inverse modeling and the KL from measurements and outperformed 23 previous models in predicting KL. Furthermore, the main driver of KL variation shifts from bottom friction to other factors with increasing stream/river size. In most streams and a few rivers, correlation between U* and KL is high, and discharge‐KL relationships are strongly positive, indicating that turbulence generated by bottom friction dominates near‐surface turbulence, a fundamental control of KL, thus, the two scaling models are applicable to these systems. However, most rivers (mean discharge &gt;10 m3/s) show low correlations between U* and KL, suggesting biochemical factors and winds likely override bottom friction in driving near‐surface turbulence and KL.