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By rotary empirical orthogonal function and coastal-trapped wave mode analyses, we analyzed current velocity data, collected from 2001 to 2016. The data were obtained by an acoustic Doppler current profiler, deployed upward at a location of 41°39.909′ N, 144°20.695′ E, on a 2630-m deep continental slope seabed off the southeastern coast of Hokkaido, Japan. The results indicate that the current intensifies toward the bottom and is directed nearly toward the shore, reaching an average speed of ~2.5 cm s−1 just above the bottom. The thickness of the along-slope northward component of the bottom-intensified current varied within the range of 50–350 m. We found that the current thickness change was caused by oceanic barotropic disturbances, produced by the intensification of the Aleutian Low, largely related to the El Niño–Southern Oscillation and modified through the excitation of bottom-trapped modes of coastal-trapped waves. This finding improves the prediction accuracy of the the bottom-intensified current change, being beneficial for suspended sediment studies, construction and maintenance of marine structures, planning of deep drilling, and so on.

The intensification of agricultural production brings problems related to water erosion, even to the upper parts of river basins. Soil particles that have eroded from unprotected agricultural land are often deposited in small water reservoirs, the efficiency or function of which might be compromised. This study presents an analysis of changes in the retention capacity of a small water reservoir over a period of 8 years. Within the study, a detailed bathymetry of the reservoir was conducted using an acoustic Doppler current profiler. The results, when compared to a 2008 geodetic survey, showed that the retention volume of the reservoir was reduced by only 2%, which was also confirmed by mathematical modeling. The possibility of strengthening the reservoir’s role in flood protection was also investigated. A flood wave with a return period of 100 years was estimated using a design storm approach. A simple numerical model was proposed to transform the flood wave through the reservoir by considering four different scenarios of the elevation of the initial water level. The model, which is based on a water balance equation, uses simple hydraulic relationships to control the discharge through the reservoir’s outflow objects. The results demonstrate that by reducing the initial water level, significant improvements in terms of the flood peak’s attenuation and a longer time to peak values could be achieved.

A simple but accurate interpolation procedure for obtaining the three‐dimensional distribution of three‐component velocity data, from moving acoustic doppler current profiler (ADCP) observation data, is proposed. For understanding actual flow structure within a river with complex bathymetry, the three‐dimensional mean velocity field provides a basic picture of the flow. For obtaining the three‐dimensional distribution of three‐component velocity data, in this work, anisotropic gridding was introduced in order to remove the random component of measured velocity data caused by the turbulence of the flow and measurement error. A continuity correction based on the pressure equation was used to reduce both random and systematic errors. The accuracy of the developed method was evaluated using three‐dimensional flow simulation data from a detached‐eddy simulation (DES). By using the procedure developed, the complex flow structure surrounding the spur dikes section in the Uji River was successfully visualized and explored. The proposed method shows superiorities in both accuracy and consistency for the interpolated velocity field, as compared to the kriging and inverse‐distance weighted (IDW) methods. Key Points Novel interpolation method to obtain velocity field is proposed Boat‐mounted ADCP survey lead to velocity with large fluctuation Proposed method provides a clear picture of the actual river flow structure

Morphological processes of the fluvial channel are controlled by liquid and solid fluxes through hydraulic forces exerted by flow and sediment transport, erosion, and deposition [1]. Large river deltas are complex alluvial systems of outstanding social, ecological, and economical importance, deeply influenced by human activity: dam, groins and dikes construction, meander cutoff. Since the 1880s intensive anthropogenic disturbances have affected the channel of the St. George branch, the southern distributary of the Danube River. The meander cutoff programme induced different hydrosedimentary impacts on the local distribution of river flow velocities, discharge, and sediment fluxes between the former meanders and the man-made canals [2]. The diversion of the flow induces strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander. We present new data on the hydrological and sedimentary processes along three meanders of St. George branch, as an example to analyse the human impact in the Danube Delta. Bathymetry, flow velocity, bed sediments, suspended-load concentration, as well as liquid and solid discharge data were acquired throughout several cross sections of both natural channels and artificial canals of the three cutoffs. We use for this investigation acoustic Doppler current profiler (ADCP) technology, in order to point out the distribution of the flow and sediment, along while the anthropic impact on the hydrosedimentary processes in each artificial canal and adjacent former meander. Additionally, 3D bathymetrical data (using multibeam technology) were acquired and very fine details of the bed were analysed. The data were collected on high waters, during a field campaign in June 2017.

An automatic workflow to measure surface flow velocities in rivers is introduced, including a Python tool. The method is based on particle-tracking velocimetry (PTV) and comprises an automatic definition of the search area for particles to track. Tracking is performed in the original images. Only the final tracks are geo-referenced, intersecting the image observations with water surface in object space. Detected particles and corresponding feature tracks are filtered considering particle and flow characteristics to mitigate the impact of sun glare and outliers. The method can be applied to different perspectives, including terrestrial and aerial (i.e. unmanned-aerial-vehicle; UAV) imagery. To account for camera movements images can be co-registered in an automatic approach. In addition to velocity estimates, discharge is calculated using the surface velocities and wetted cross section derived from surface models computed with structure-from-motion (SfM) and multi-media photogrammetry. The workflow is tested at two river reaches (paved and natural) in Germany. Reference data are provided by acoustic Doppler current profiler (ADCP) measurements. At the paved river reach, the highest deviations of flow velocity and discharge reach 4 % and 5 %, respectively. At the natural river highest deviations are larger (up to 31 %) due to the irregular cross-section shapes hindering the accurate contrasting of ADCP- and image-based results. The provided tool enables the measurement of surface flow velocities independently of the perspective from which images are acquired. With the contactless measurement, spatially distributed velocity fields can be estimated and river discharge in previously ungauged and unmeasured regions can be calculated, solely requiring some scaling information.

Ocean currents are crucial in regulating Earth's climate, with a significant impact in the distribution of ocean properties. During the Calibration/Validation phase of the Surface Water and Ocean Topography (SWOT) satellite mission, we performed a high‐resolution, multi‐platform experiment to evaluate SWOT's ability to resolve small‐scale features, focusing on a ∼25 km‐radius anticyclonic eddy in the Western Mediterranean Sea. Acoustic Doppler Current Profiler (ADCP) recorded maximum velocities of 30 cm/s at 155 m depth and underwater glider data identified biconvex isopycnals, classifying the eddy as intrathermocline. SWOT successfully captured the sea level signal and surface geostrophic currents of the eddy, showing notable error reduction over conventional altimetry: 24% in sea level representation compared to glider observations, and 35% and 31% in horizontal velocity magnitude compared to Acoustic Doppler Current Profiler and drifter measurements, respectively. This study highlights SWOT's potential in resolving small‐scale ocean dynamics.

Field data on plastic pollution is extremely limited in Southeast Asian rivers. Here we present the first field measurements of plastic transport in the Mekong, based on a comprehensive monitoring campaign during the monsoon season in the confluence of the Mekong, Tonle Sap, and Bassac rivers around Cambodia’s capital (Phnom Penh). For improved accuracy in the estimation of plastic loads and distribution, we combined Neuston net multipoint cross-sectional water sampling with acoustic Doppler current profiler high resolution measurements. During the wet season, around 2.03 × 10 5 kg d −1 of plastic were released from Phnom Penh into the Mekong, equivalent to 89 g d −1 capita −1 , or 42% of all plastic waste generated in the city. Most plastic mass moved downstream at the surface. A smaller portion of plastics is mixed deep into the water column, potentially retained in the rivers, breaking down and resuspending over time. Overall, plastic waste from Phnom Penh and transported by the Mekong is a significant contribution to Southeast Asia’s plastic release into the ocean. This pollution represents a crucial risk to people in the region, as their livelihoods depend on fisheries from these water bodies.

Conventional methods of measuring water discharge and suspended sediment concentration (e.g., water sampling and moving acoustic Doppler current profiler [ADCP]) present challenges in large tidal rivers due to temporal and spatial constraints. This study introduces a novel approach to monitor water discharge and suspended sediment discharge (SSD) in large tidal rivers. Total water discharge and SSD exhibit notable variability in tidal rivers due to the river–tidal interactions; understanding this variability and its causes is essential for effective tidal river management. From June to November 2023, a field study was conducted at Nanjing (NJ) to continuously monitor water discharge, suspended sediment concentration (SSC), and SSD in the tidal reaches of the Yangtze River using coastal acoustic tomography (CAT). Total water discharge ranged from 8,765 to 43,356 m3/s, with a mean of 27,825 m3/s, while tidal discharge varied between −11,998 and 9,983 m3/s, with a mean of 69 m3/s. SSC ranged from 0.02 to 0.09 kg/m3, and SSD ranged from 110 to 3,823 kg/s. Tidal variations in SSC and SSD were within ±0.04 kg/m3 and −1,252 to 1,410 kg/s, respectively. Over short timescales, tides caused instantaneous fluctuations in velocity, water discharge, and SSD, with tides contributing −40% to instantaneous water discharge and SSD at NJ. Over seasonal timescales, no significant wet/dry variations were observed in water discharge, SSC, or SSD during a few months of 2023. Long‐term CAT application (e.g., decades) is required to reveal trends in tidal river dynamics. Plain Language Summary Due to temporal and spatial limitations, traditional methods for measuring suspended sediment concentration (SSC) and discharge, such as moving acoustic Doppler current profilers (ADCP), fail to directly measure transect variations in water discharge, SSC, and SSD in tidal reaches of the Yangtze River. This study developed a new method using coastal acoustic tomography (CAT). Two CAT systems were utilized to continuously measure water discharge, SSC, and SSD at the Nanjing Tidal Station. The CAT results were highly consistent with traditional methods, showing a correlation coefficient greater than 0.9. This study demonstrates the potential of CAT for continuous, real‐time monitoring of water discharge, SSC, and SSD in large tidal rivers. The results showed that mean water discharge, SSC, and SSD are primarily driven by river flow at Nanjing, while tides induce instantaneous variations in water discharge and sediment transport. Key Points Coastal acoustic tomography enabled water discharge and suspended sediment discharge (SSD) monitoring in Nanjing tidal reach of Yangtze River Total water discharge and SSD at Nanjing varied from 8,765 to 43,356 m3/s and 110–3,823 kg/s from June to November 2023, respectively Tides can directly trigger instantaneous variations in sediment discharge, while average sediment discharge is river‐dominated at Nanjing

Field measurements of breaking waves and bubble depths were obtained using a stereo video system collocated with a submerged acoustic Doppler current profiler (ADCP) in the central North Sea. We discriminate between two bubble depths that define an active near‐surface layer and a deeper layer. The active layer intermittently sees short‐lived injected bubble depths from breakers whereas the deeper layer is dominated by persistent passive bubble plumes that remain visible for more than 50 mean wave periods. We augment traditional single‐beam bubble detection methods by utilizing all five beams of the ADCP to achieve broader spatial coverage of bubble plume measurements. The combined wave and bubble observations reveal that deep bubble plumes often occur offset spatially from surface whitecaps, suggesting that Langmuir‐type circulation plays a role in the formation and persistence of deep bubble plumes through vertical and horizontal advection.

Instruments drifting at the ocean surface are quasi-Lagrangian, that is, they do not follow exactly the near-surface ocean currents. The currents measured by three commonly-used drifters (CARTHE, CODE and SVP) are compared in a wide range of sea state conditions (winds up to 17 m/s and significant wave height up to 3 m). Nearly collocated and simultaneous drifter measurements in the southwestern Mediterranean reveal that the CARTHE and CODE drifters measure the currents in the first meter below the surface in approximately the same way. When compared to SVP drogued at 15 m nominal depth, the CODE and CARTHE currents are essentially downwind (and down-wave), with a typical speed of 0.5–1% of the wind speed. However, there is a large scatter in velocity differences between CODE/CARTHE and SVP for all wind and sea state conditions encountered, principally due to vertical and horizontal shears not related to the wind. For the CODE drifter with wind speed larger than 10 m/s and significant wave height larger than 1 m, about 30–40% of this difference can be explained by Stokes drift.