[1] The continuous measurement of river discharge for long periods of time is crucial in water resource studies. However, the accurate estimation of river discharge is a difficult and labor-intensive procedure ; thus, a robust and efficient method of measurement is required. Continuous measurements of flowrate have been carried out in a wide, shallow gravel bed river (water depth % 0.6 m under low-flow conditions, width % 115 m) using Fluvial Acoustic Tomography System (FATS) that has 25 kHz broadband transducers with horizontally omnidirectional and vertically hemispherical beam patterns. Reciprocal sound transmissions were performed between the two acoustic stations located diagonally on both sides of the river. The horizontal distance between the transducers was 301.96 m. FATS enabled the measurement of the depth-and range-averaged sound speed and flow velocity along the ray path. In contrast to traditional point/transect measurements of discharge, in a fraction of a second, FATS covers the entire cross section of river in a single measurement. The flow rates measured by FATS were compared to those estimated by moving boat Acoustic Doppler Current Profiler (ADCP) and rating curve (RC) methods. FATS estimates were in good agreement with ADCP estimates over a range of 20 to 65 m 3 s À1 . The RMS of residual between the two measurements was 2.41 m 3 s À1 . On the other hand the flowrate by RC method fairly agreed with FATS estimates for greater discharges than around 40 m 3 s À1 . This inconsistency arises from biased RC estimates in low flows. Thus, the flow rates derived from FATS could be considered reliable.Citation: Kawanisi, K., M. Razaz, K. Ishikawa, J. Yano, and M. Soltaniasl (2012), Continuous measurements of flow rate in a shallow gravel-bed river by a new acoustic system, Water Resour. Res., 48, W05547,

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Abstract:The acquisition of reliable discharge estimates is crucial in hydrological studies. This study demonstrates a promising acoustic method for measuring streamflow at high sampling rate for a long period using the fluvial acoustic tomography system (FATS). The FATS recently emerged as an innovative technique for continuous measurements of streamflow. In contrast to the traditional point/transect measurements of discharge, the FATS enables the depth-averaged and range-averaged flow velocity along the ray path to be measured in a fraction of a second. The field test was conducted in a shallow gravel-bed river (0.9 m deep under lowflow conditions, 115 m wide) for 1 month. The parameters (stream direction and bottom elevation) required for calculating the streamflow were deduced by a nonlinear regression to the discharge data from the well-established rating curve. The crosssectional average velocities were automatically calculated from the acoustic data, which were collected on both riverbanks every 30 s. The FATS was connected to the internet so that the real-time flow data could be obtained. The FATS captured discharge variations at a cut-off frequency of approximately 70 day À1 . The stream exhibited temporal discharge changes at multiple time scales ranging from a few tens of minutes to days.

Observation of a shallow tidal channel was executed to examine turbulent evolution in ideal condition of weak, stratified flow for a complete cycle of semi-diurnal tide. Duration of this study allows a maximum velocity variation and therefore gives comprehensive interpretation of the relevant processes such as momentum and heat transfer, turbulent kinetic energy (TKE) production and dissipation rates. Under weak stratification condition dominant through the observation period testing the parameterization of dissipation rate in the Mellor-Yamada type models revealed that close to the bed perhaps due to boundary effects experimental dissipation closure constant values were remarkably larger than commonly used value of 16.6. Also, the effect of extending critical Richardson number Ri cr to infinity on parameterization of stability functions, eddy viscosity, and turbulent Prandtl number in Kantha-Carniel model was also examined. It was found that despite Kantha-Carniel model reconstructed bottom boundary layer (BBL) turbulence characteristics well during the flooding tide, it underestimates those parameters during the ebbing tide partially due to underestimation of turbulence macroscale.

Natural hydrocarbon seeps are ubiquitous along continental margins. Despite their significance, we lack a basic understanding of the long-term temporal variability of seep dynamics, including bubble size, rise velocity, composition, and upwelling and entrainment processes. The shortcoming makes it difficult to constrain the global estimates of oil and gas entering the marine environment. Here we report on a multi-method approach based on optical, acoustic, satellite remote sensing, and simulations, to connect the characteristics of a hydrocarbon seep in the Gulf of Mexico to its footprint on the sea surface. Using an in-situ camera, bubble dynamics at the source were measured every 6 h over 153 days and the integrated total hydrocarbon release volume was estimated as 53 m 3. the vertical velocity was acoustically measured at 20 m above bed (mab) and found to be approximately 40% less than the dispersed-phase at the source, indicating that the measured values are reflecting the plume continuous-phase flow. Numerical simulations predict that the oily bubbles with diameters larger than 8 mm reach the surface with a small footprint, i.e. forming an oil slick origin, deflection of which with wind and surface current leads to the formation of an oil slick on the surface. Nineteen SAR images are used to estimate the oil seepage rate from GC600 for 2017 giving an average discharge of 14.4 cm 3 /s. Natural hydrocarbon seeps have been reported from a broad range of oceanographic settings from the coast to the deep ocean over a wide variety of geological environments that affect the biosphere, the hydrosphere, and the atmosphere 1. Beside the anthropogenic sources, hydrocarbon seeps are the single most important natural source of oil and methane (CH 4) that enter the ocean 2. Best estimates indicate that 3 to 48 Tg/y of geo-CH 4 is released from marine seeps alone 3,4. Recent global estimates of crude oil seepage range from 0.2 to 2 Mt with a best estimate of 0.6 Mt that accounts for 47% of the global estimate. The large uncertainties in estimates of hydrocarbons released from offshore marine sources are due to a wide range of factors including size, rise velocity, contamination by surfactants such as gas hydrates, composition, upwelling and entrainment effects, and interaction of gaseous bubbles and oil droplets with the local ocean environment that vary constantly with time. To quantify the bubble/droplet size and rise velocity and the corresponding emission rate of hydrocarbons from the seafloor, investigators have primarily relied on snapshot acoustic 5-9 or optical 10-13 measurements. Based on these measurements, significant changes in hydrocarbon seepage and bubble venting have been inferred to occur over intervals of months to years 14. Sufficiently long time series of seepage characteristics rarely exist to conclusively address the day-today variability of bubble size and rise velocity or the spatial changes in vent location over an extended period. Time variation in seep emissions is of particular interest. It impl...

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[1] Long-term variations of streamflow in a tidal channel were measured using a Fluvial Acoustic Tomography (FAT) system through one transmission path. FAT is an innovative acoustic technology that utilizes the time-of-travel method to determine velocity between two points from multiple ray paths that traverse the entire cross-section of stream. Due to high spatial variability of flow distribution stationary ADCP measurements were not likely to yield true section-averaged flow velocity and moving-boat ADCP method was therefore used to provide reference data. As such, two short-term moving boat ADCP campaigns were carried out by the authors. In the first campaign, a couple of acoustic stations were added to the FAT system in order to resolve flow angularity in addition to the mean velocity. Comparing the FAT results with corresponding ADCP section-averaged flow direction and velocity indicated remarkable consistency. Second campaign was designed to capture the influence of salt wedge intrusion on the sound propagation pattern. It was found that FAT velocity measurements bias high if acoustic stations lay inside the cooler freshwater layer. Ray-tracing hindcasts suggest that installing acoustic stations inside the salt wedge may significantly improve function of output of the system. Comparing salinities evaluated from long-term FAT travel time records with nodal salinity measurements provided by conductivity-temperature sensors reveals the potential ability of FAT in measuring salt flux.

Continuous measurements of streamflow and water temperature were carried out in a shallow gravel-bed river during dam flushing operations using a new shallow acoustic system with two crossing transmission lines. The fluvial acoustic system (FAS) was equipped with four 25 kHz broadband omnidirectional transducers. The reciprocal sound transmissions were performed between the two pairs of acoustic stations, located on both sides of the river. The four-station FAS enabled the measurement of the stream direction in addition to the depth- and range-averaged sound speed, and therefore water velocity. Discharge values computed from FAS reports were compared to those estimated by the moving-boat acoustic Doppler current profiler (ADCP) and rating curve (RC) methods. FAS estimates were in good agreement with ADCP and RC estimates over a range of 50 to 180 m3 s−1. The temperature gradients induced strong sound scattering for the 25 kHz FAS. The fine suspended sediment particles also induced a gradual decrease in the signal-to-noise ratio of the FAS. Nevertheless, the FAS has been proved to be a reliable and accurate technique for the continuous measurement of cross-sectional average velocity/water temperature.