Horizontal Acoustic Doppler Current Profiler Research Articles

On the Performance of a Horizontally Mounted ADCP in an Energetic Tidal Environment for Floating Tidal Turbine Applications.

Incident flow measurement is key in the tidal industry for conducting power performance assessments. This paper explores the use of a horizontally mounted Nortek Signature 500 Acoustic Doppler Current Profiler (ADCP) as a means for incident flow measurement onboard a utility-scale tidal turbine. This study shows that the measurement range of an ADCP mounted horizontally in highly dynamic tidal flow (up to 4 m/s) is less than the maximum range stated by the manufacturer. The ability for the horizontal ADCP to accurately resolve velocities in a multi-beam configuration is also analysed. Effects from both vertical shear and beam selection result in incident flow velocities that differ from a single horizontal beam recording. The maximum measurement range of the instrument is found to depend on current speed and on the proportion of data loss that is acceptable to the user. The ability of the ADCP to record data from the free-stream velocity two equivalent diameters upstream of the O2, as set out by IEC TS 62600-200, is considered. It is found that at this distance, there is 90% data loss. Accepting only 10% data loss across all flow speeds resulted in a maximum range of 31 m for a Nortek Signature 500 in this study. While some limitations of an ADCP deployed horizontally in highly energetic tidal flow are identified, the benefits of mounting the sensor close to the rotor facing horizontally into the incoming flow mean that valuable data are still produced for tidal turbine operators.

Statistical modeling of undertow on a natural beach

ABSTRACT To better understand the physical mechanism of undertow, field observations were conducted during barred and planar beach states on the Hasaki coast in Japan. The two observation periods spanned 21 days from May 13 to June 2 in 2016, and 13 days from May 9 to May 22 in 2017, respectively. A horizontal acoustic Doppler current profiler was used to measure the undertow velocity and water level. The observed data were statistically analyzed to investigate the characteristics of the spatial and wave-energetic distributions of the undertow exceedance probability, PE . The results reveal that when the wave energy flux level was high, the undertow velocity increased, as did its PE . By contrast, the undertow PE during a low-wave-energy level decreased with greater water depths. Here, a Weibull distribution was applied to explain PE of the undertow. The Weibull parameters: scale and shape were estimated by the combination of normalized values of the wave energy flux, relative surf zone locations, and normalized water depths, and this generalized equation was considered a statistical model for estimating undertow PE . A comparison of the statistical model against the measurement demonstrates that the model accurately predicted the undertow PE with a small error.

Acoustic based assessment of cross-sectional concentration inhomogeneity at a suspended sediment monitoring station in a large river

Establishing and operating a harmonized sediment monitoring system along large rivers such as the Danube River is a challenging international task. As an element of such a system, a new monitoring site with state-of-the-art instrumentation is currently under development in the Upper-Hungarian section of the Danube River. The monitoring station will consist of a near-bank optical backscatter sensor and a horizontal acoustic Doppler current profiler (H-ADCP). As previous studies showed, the suspended sediment concentration (SSC) that is continuously measured with near-bank sensors can significantly enhance the temporal resolution of sediment transport monitoring. However, sediment plumes from tributary inflows upstream of the monitoring station can alter the detected near-bank concentrations, eventually biasing the sediment load estimation. Such an influence is likely in the cross-section of the planned monitoring station, therefore, a thorough preliminary analysis of the cross-sectional variation of the SSC was performed, based on expeditionary sediment measurement campaigns. Between 2018 and 2021 24 campaigns were carried out at different hydrological regimes, where physical sediment samplings together with fixed and moving boat ADCP measurements were performed. The cross-sectional variability of SSC and its influence on the sediment load estimations were assessed based on the moving boat ADCP measurements, after calibrating the backscatter signal with more than 500 physical samples. Based on the results, we identified different cross-sectional patterns of the SSC which is apparently governed by: (i) the actual hydrological situation considering both the main river and the tributary, and (ii) the local river morphology. Based on our findings, we suggested a correction method that accounts for the above effects, using which the near-bank SSC can be reliably converted into total suspended sediment load.

Online AUV Path Replanning Using Quantum-Behaved Particle Swarm Optimization with Selective Differential Evolution

This paper presents an online AUV (autonomous underwater vehicle) path planner that employs path replanning approach and the SDEQPSO (selective differential evolution-hybridized quantum-behaved particle swarm optimization) algorithm to optimize an AUV mission conducted in an unknown, dynamic and cluttered ocean environment. The proposed path replanner considered the effect of ocean currents in path optimization to generate a Pareto-optimal path that guides the AUV to its target within minimum time. The optimization was based on the onboard sensor data measured from the environment, which consists of a priori unknown dynamic obstacles and spatiotemporal currents. Different sensor arrangements for the forward-looking sonar and horizontal acoustic Doppler current profiler (H-ADCP) were considered in 2D and 3D simulations. Based on the simulation results, the SDEQPSO path replanner was found to be capable of generating a time-optimal path that offered up to 13% reduction in travel time compared to the situation where the vehicle simply followed a path with the shortest distance. The proposed replanning technique also showed consistently better performance over a reactive path planner in terms of solution quality, stability, and computational efficiency. Robustness of the replanner was verified under stochastic process using the Monte Carlo method. The generated path fulfilled the vehicle’s safety and physical constraints, while intelligently exploiting ocean currents to improve the vehicle’s efficiency.

Prerequisites for Accurate Monitoring of River Discharge Based on Fixed‐Location Velocity Measurements

AbstractRiver discharge has to be monitored reliably for effective water management. As river discharge cannot be measured directly, it is usually inferred from the water level. This practice is unreliable at places where the relation between water level and flow velocity is ambiguous. In such a case, the continuous measurement of the flow velocity can improve the discharge prediction. The emergence of horizontal acoustic Doppler current profilers (HADCPs) has made it possible to continuously measure the flow velocity. However, the profiling range of HADCPs is limited, so that a single instrument can only partially cover a wide cross section. The total discharge still has to be determined with a model. While the limitations of rating curves are well understood, there is not yet a comprehensive theory to assess the accuracy of discharge predicted from velocity measurements. Such a theory is necessary to discriminate which factors influence the measurements, and to improve instrument deployment as well as discharge prediction. This paper presents a generic method to assess the uncertainty of discharge predicted from range‐limited velocity profiles. The theory shows that a major source of error is the variation of the ratio between the local and cross‐section‐averaged velocity. This variation is large near the banks, where HADCPs are usually deployed and can limit the advantage gained from the velocity measurement. We apply our theory at two gauging stations situated in the Kapuas River, Indonesia. We find that at one of the two stations the index velocity does not outperform a simple rating curve.

Changes in the physical characteristics of the water column at the mouth of a torrent during an extreme rainfall event

Summary The city of Genoa (Italy) was hit by a severe flash flood on the 4th November, 2011. The effects of this event on the water column at the mouth of the Polcevera Torrent, the main water course flowing into the Port of Genoa, are presented in this paper. The hydrological characteristics were measured with two conductivity–temperature–depth probes equipped with a turbidimeter, one fixed on the port breakwater and one used at mobile stations around the mouth of the torrent. The dynamics were measured with a horizontal acoustic Doppler current profiler (H-ADCP) fixed on the breakwater. Data collected before, during and after the flash flood were analysed to quantify the changes due to the event. The weather conditions during the event showed extremely heavy rain associated with strong weather instability, the convergence of a low-level southerly flow and the persistence of a squall line over a restricted area. The temperature, salinity, turbidity and dissolved oxygen measurements taken during the event showed the strong influence of the weather conditions and the fresh water input of the torrent itself on the water column at its mouth, an influence that dissipated during the following days. Instead, the dynamics measured at the mouth of the torrent were affected more by the strong south-easterly wind and the sea than the flow of fresh water.

Calibration of Horizontal Acoustic Doppler Current profilers by three dimensional CFD simulations

This study presents a calibration method of horizontal acoustic Doppler current profilers (H-ADCP), based on 3D Computational Fluid Dynamics (CFD) simulations. Rigidly mounted H-ACCPs are currently used to continuously monitor water discharges. In this study three instruments were employed at separate sites in an unlined rock-blasted hydro power tunnel. The total discharge in the tunnel was predicted by fitting numerical and measured velocity profiles. The accuracy of the method was evaluated by comparing discharge predictions to results obtained by thermodynamic efficiency measurements. This study shows that 3D CFD simulations not only can be used to accurately calibrate H-ADCP, but also to assess the flow conditions at the locations of installation. An average error of 2% was obtained for the tunnel stretch that possessed the most uniform flow. However, the precision of the discharge estimates were confined by the CFD code's capability to correctly resolve the flow pattern.

Prediction of Discharge in a Tidal River Using Artificial Neural Networks

Discharge predictions at tidally affected river reaches are currently still a great challenge in hydrological practices. In tidal rivers, water levels are not uniquely a function of streamflow. Here, the possibility to predict discharge from water level information from gauge stations at sea and in the river is explored. A hindcast model is established for a tide-dominated lowland site along the Mahakam River (East Kalimantan, Indonesia), using an artificial neural network (ANN) model. The input data for the ANN are gradually increased, by adding new input data in each step. The results show that the inclusion of data from tide predictions at sea leads to an improved model performance. The optimized ANN-based hindcast model produces a good discharge estimation, as shown by a consistent performance during both the training and validation periods. Using this model, discharge can be predicted from astronomical tidal predictions at sea plus water level measurements from a single station at an upstream location. Alternatively, the ANN model can be used as a tool for data gap filling in a disrupted discharge time-series based on a horizontal acoustic Doppler current profiler (H-ADCP). A forecast model is developed for the same river site that is located near the city of Samarinda. To this end, water level data, predicted tide levels, and at-site historical data are considered as input for the model. The discharge time-series derived from H-ADCP data are used for calibration and validation of the multistep ahead discharge forecast model. A good performance is obtained for predictions with a forecast lead time up to two days.

On the use of horizontal acoustic Doppler profilers for continuous bed shear stress monitoring

Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean. Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler (ADCP) to monitor bed shear stress, applying a prescribed boundary layer model, previously used for discharge estimation. The model parameters include the local roughness length and a dip correction factor to account for sidewall effects. Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data. We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP. To validate the results, co-located coupled ADCPs were used to infer bed shear stress, both from Reynolds stress profiles and from mean velocity profiles.From HADCP data collected over a period of 1.5 years, a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River, East Kalimantan, Indonesia. A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs. The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow, which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP. Bed shear stresses from the coupled ADCP are representative of a much more localized flow, while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales. Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods, and highlight the robustness of the method to uncertainty in the estimates of the roughness length.

Predictability of currents on a mesotidal estuary (Ria de Vigo, NW Iberia)

In this contribution, the implementation, validation and sensitivity analysis of an operational forecasting system (three-dimensional hydrostatic model) in the Ria de Vigo is presented. A set of sensitivity tests for different atmospheric and hydrodynamic typical periods was performed. The goal is to determine the relative importance of forcing mechanisms in order to evaluate the source of errors in predictions. Previously, validations for three periods of about 15 days were compared with measured data. Sea-level evolution reveals low errors and correlation values close to 1. Surface velocities were compared with high-frequency radar and horizontal Acoustic Doppler Current Profiler data, showing acceptable results on radar area, where tidal and wind circulation patterns are well reproduced by the model. Conductivity, temperature, and depth profiles were used to validate simulated temperature and salinity. While modelled temperature profiles show good agreement with measured profiles, eventual errors are detected on salinity. The sensitivity analysis took three variables into account: sea level, currents at open boundary conditions, and winds (two different configurations). The results show that the most important source of errors on simulated surface currents is wind. Errors on open boundary conditions seem to be limited on the outermost part of the Ria. Although the results presented mainly focus on the Ria de Vigo, the method and conclusions may be applied to other mesotidal estuaries. Moreover, this work will allow a more effective operational system focused on coastal management to be performed.

Suspended sediment load in the tidal zone of an Indonesian river

Abstract. Forest clearing for reasons of timber production, open pit mining and the establishment of oil palm plantations generally results in excessively high sediment loads in tropical rivers. The increasing sediment loads pose a threat to coastal marine ecosystems, such as coral reefs. This study presents observations of suspended sediment loads in the Berau River (Kalimantan, Indonesia), which debouches into a coastal ocean that is a preeminent center of coral diversity. The Berau River is relatively small and drains a mountainous, still relatively pristine basin that receives abundant rainfall. In the tidal zone of the Berau River, flow velocity was measured over a large part of the river width using a horizontal acoustic Doppler current profiler (HADCP). Surrogate measurements of suspended sediment concentration were taken with an optical backscatter sensor (OBS). Averaged over the 6.5 weeks covered by the benchmark survey period, the suspended sediment load was estimated at 2 Mt yr−1. Based on rainfall-runoff modeling though, the river discharge peak during the survey was supposed to be moderate and the yearly averaged suspended sediment load is most likely somewhat higher than 2 Mt yr−1. The consequences of ongoing clearing of rainforest were explored using a plot-scale erosion model. When rainforest, which still covered 50–60% of the basin in 2007, is converted to production land, soil loss is expected to increase with a factor between 10 and 100. If this soil loss is transported seaward as suspended sediment, the increase in suspended sediment load in the Berau River would impose a severe stress on this global hotspot of coral reef diversity.

On the application of horizontal ADCPs to suspended sediment transport surveys in rivers

In the last five years, horizontal acoustic Doppler current profilers have come onto the market as an instrument designed to measure real-time horizontal current profiles from a permanent fixed mount. The focus of this paper is on the feasibility of using these commercial instruments to measure suspended sediment fluxes in rivers. Measurements are presented from a study site that was equipped with an optical turbidity meter and three horizontal ADCPs operating at 300kHz, 600kHz and 1200kHz between November 2009 and June 2010. Laser grain size measurements showed the primary particles of the suspended sediment to be predominantly silts with some clay and occasionally some fine sand. Measurements of sediment attenuation by the three horizontal ADCPs are presented for two high concentration events during which concentrations reached 2.5 and 8kg/m3, respectively. Very clear linear relationships are seen between sediment attenuation and particle concentration and the sediment attenuation is consistent with the theory for viscous absorption by fine sediments. Using the relationship between attenuation and concentration, we are able to reconstruct the concentration time series for events during which the turbidity data are unavailable. Our findings show that, when properly positioned, horizontal ADCPs can provide suitable measurements of sediment transport during extreme river flow conditions such as floods, at least for predominantly silt-sized particles. This ability is of primary interest for river survey applications since the long term sediment transfer is believed to be driven by these intense events.

River discharge monitoring using a vertically moving side-looking acoustic Doppler profiler

This paper presents an innovative river discharge monitoring system which uses a horizontal acoustic Doppler current profiler (H-ADCP) attached to a mechatronic system that provides vertical motion to different depths. Also, an extended index velocity method is developed which utilises velocity measurements from different depths to achieve more accurate discharge calculation under complex flow conditions. The prototype system was developed and installed near the estuaries of a river where backwater and tidal motion was known to be present due to small distance and altitude difference from the sea. Five experiments were performed, and in two experiments the proposed method performed roughly the same as the standard index velocity method. In the other three experiments the proposed method performed much better and the difference between the discharges was as large as 10%. The results indicate that a vertically moving H-ADCP can be used to calculate river discharge more accurately in complex flow environments.

Discharge estimation from H‐ADCP measurements in a tidal river subject to sidewall effects and a mobile bed

Horizontal acoustic Doppler current profilers (H‐ADCPs) can be employed to estimate river discharge based on water level measurements and flow velocity array data across a river transect. A new method is presented that accounts for the dip in velocity near the water surface, which is caused by sidewall effects that decrease with the width to depth ratio of a channel. A boundary layer model is introduced to convert single‐depth velocity data from the H‐ADCP to specific discharge. The parameters of the model include the local roughness length and a dip correction factor, which accounts for the sidewall effects. A regression model is employed to translate specific discharge to total discharge. The method was tested in the River Mahakam, representing a large river of complex bathymetry, where part of the flow is intrinsically three‐dimensional and discharge rates exceed 8000 m3 s−1. Results from five moving boat ADCP campaigns covering separate semidiurnal tidal cycles are presented, three of which are used for calibration purposes, whereas the remaining two served for validation of the method. The dip correction factor showed a significant correlation with distance to the wall and bears a strong relation to secondary currents. The sidewall effects appeared to remain relatively constant throughout the tidal cycles under study. Bed roughness length is estimated at periods of maximum velocity, showing more variation at subtidal than at intratidal time scales. Intratidal variations were particularly obvious during bidirectional flow conditions, which occurred only during conditions of low river discharge. The new method was shown to outperform the widely used index velocity method by systematically reducing the relative error in the discharge estimates.

Flow measurement with multi‐instrumentation in a tidal‐affected river

AbstractFlow measurements in a tidal‐affected river provide important information of cyclic flow pattern characteristics in temporal and spatial variations. Reliability and accuracy of measured flow data can provide for numerical model calibration and verification. In the present study, we measured tidally affected river flow in the Danshui River of northern Taiwan during the 2008 summer and fall. The measured flow data, produced by both state of the art and conventional instruments including particle tracking velocimetry (PTV), acoustic Doppler profiler (ADP), horizontal acoustic Doppler current profiler (H‐ADCP) and Price‐type current metre (PCM) were compared and correlated during several tidal cycles. The mean flows by ADP were similar to that by PCM and highly correlated with those by PTV and H‐ADCP. Index velocity rating between ADP and H‐ADCP separately computed during the flood and ebb tides shows good correlation for the ebb tide and poor correlation for the flood tide due to subsurface maximum velocity in the middle‐ to near‐bottom layers.

Continuous measurements of discharge from a horizontal acoustic Doppler current profiler in a tidal river

Acoustic Doppler current profilers (ADCPs) can be mounted horizontally at a river bank, yielding single‐depth horizontal array observations of velocity across the river. This paper presents a semideterministic, semistochastic method to obtain continuous measurements of discharge from horizontal ADCP (HADCP) data in a tidal river. In the deterministic part, single‐depth velocity data are converted to specific discharge by applying the law of the wall, which requires knowledge of local values of the bed roughness length (z0). A new filtration technique was developed to infer cross‐river profiles of z0 from moving boat ADCP measurements. Width‐averaged values of z0 were shown to be predominantly constant in time but differed between ebb and flood. In the stochastic part of the method, specific discharge was converted to total discharge on the basis of a model that accounts for the time lag between flow variation in the central part of the river and flow variation near the banks. Model coefficients were derived using moving boat ADCP data. The consistency of mutually independent discharge estimates from HADCP measurements was investigated to validate the method, analyzing river discharge and tidal discharge separately. Inaccuracy of the method is attributed primarily to mechanisms controlling transverse exchange of momentum, which produce temporal variation in the discharge distribution over the cross section. Specifically, development of river dunes may influence the portion of the discharge concentrated within the range of the HADCP.

Subtidal water level variation controlled by river flow and tides

Subtidal water level dynamics in the Berau river, East Kalimantan, Indonesia, feature a pronounced fortnightly variation. The daily mean water levels at a station about 60 km from the sea are 0.2–0.6 m higher during spring tide than during neap tide. To explain the underlying mechanisms, a local subtidal momentum balance is set up from field data, using continuous discharge estimates inferred from measurements taken with a horizontal acoustic Doppler current profiler. It is demonstrated that terms accounting for friction and variation in the water surface gradient are dominant in the subtidal momentum balance. To further investigate the sources of subtidal water level variation, a generic method of analysis is proposed to decompose the subtidal friction term into contributions caused by river flow, by interaction between tidal motions and river flow, and by the tidal motions alone. At the station under study, mainly the river‐tide interaction term is responsible for generating fortnightly variation of the subtidal water level. The contribution from interaction between diurnal, semidiurnal, and quarterdiurnal tides to subtidal friction is significantly smaller. Provided that the reduction of tidal velocity amplitudes with increasing discharges can be predicted from a regression model, the results presented herein can be used to predict changes in subtidal water levels as a result of increased river discharges.

Horizontal current field, ADCP backscatter, and plankton distribution in Sagami Bay, Japan

AbstractThe dry mass of zooplankton and backscatter from an acoustic Doppler current profiler (ADCP) used in Sagami Bay, Japan, from 9 to 13 April 2005 were well correlated (correlation coefficient = 0.72, root mean square dry weight = 2.5 mg m−3). The horizontal current field and zooplankton distribution were estimated from shipboard ADCP data. Although sea surface temperature and chlorophyll a concentration observed by satellite were consistent with the current field, a counterclockwise circulation associated with the Kuroshio meander, zooplankton were mainly distributed downstream of the phytoplankton population by horizontal current advection.

A new monitoring system for river discharge with horizontal acoustic Doppler current profiler measurements and river flow simulation

A new monitoring system using horizontal acoustic Doppler current profiler (H‐ADCP) measurements and river flow simulation was developed to attain accurate and continuous monitoring for river discharge at a low cost. The H‐ADCP can measure the velocity profile along a horizontal line. In the numerical simulation the measured velocities were interpolated and extrapolated for a river cross section. As part of the simulation, we developed a river flow model with a new approach for data assimilation to reflect rationally measured velocities in numerical simulations. The new computational method is referred to as the dynamic interpolation and extrapolation (DIEX) method. To confirm the fundamental performance of the present system, H‐ADCP measurements were performed in the middle reach of the Edo River in Japan, and the river discharge was then evaluated using the DIEX method. The calculated velocity and discharge were compared with the results measured using an ADCP and a Price current meter. The vertical and lateral distributions of the calculated velocities are in good agreement with the measured results. The discharge calculated by the DIEX method is in good agreement with the observed data. The root mean squares of the relative error for the calculated discharge is 4.9%, showing that the accuracy of the DIEX method is the same as the conventional method using point sensors such as a Price current meter. The present monitoring system achieves accurate, automatic, and continuous monitoring of river discharge at a lower cost than direct measurement methods for discharge.