Field measurements of settling velocities of fine sediments in Three Gorges Reservoir using ADV

The Three Gorges Reservoir (TGR) is suffering from unexpected fine sediment deposition, to better understand the fine sediment transport processes, field measurements were conducted at the Zhongxian and Fengjie reaches. A method based on the sediment diffusion equation was proposed to measure the settling velocities using the Acoustic Doppler Velocimeter (ADV). The backscatter acoustic intensities (BSI) received from the ADV were calibrated against the sediment concentrations measured via water sampling, suggesting a linear relationship in double logarithmic coordinate system. The instantaneous sediment concentration was calculated using the derived relationship, and then the settling velocity was obtained through the proposed procedure. The settling velocities of the fine particles in the TGR were found to vary with the water depth. Most of the effective settling velocities were within the range of 0.1–10 mm/s, which were larger than those of the primary particles, indicating that the flocculation was likely to occur in the TGR. Additionally, it is suggested that the turbulent motion played an important role in the flocculation in the TGR.     

Using ADV backscatter strength for measuring suspended cohesive sediment concentration

Laboratory experiments were conducted at two institutes to reveal the relationship between acoustic backscatter strength and suspended sediment concentration (SSC). In total, three acoustic Doppler velocimeters (ADVs) with different frequencies (5, 10 and 16 MHz) were tested. Two different commercial clays and one natural sediment from Clay Bank site in the York River were checked for acoustic responses. The SSCs of selected sediments were artificially changed between a selected low and a high value in tap or de-ion water. Each ADV showed quite different backscatter responses depending on the sediment type and SSC. Not all devices had a good linear relationship between backscatter strength and SSC. Within a limited range of SSC, however, the backscatter strength can be well correlated with the SSC. Compared with optical backscattering sensor (OBS), the fluctuation of ADV backscatter signals was too noisy to be directly converted to the instantaneous changes of SSC due to high amplification ratio and small sampling volume. For the more accurate signal conversion for finding the fluctuation of SSC, the ensemble average should be applied to increase the signal-to-noise ratio. There are unexpected responses for the averaged backscatter wave strength: (1) high signals from small particles but low signals from large particles; and (2) two linear segments in calibration slope. These phenomena would be most likely caused by the different gain setting built in ADVs. The different acoustic responses to flocculation might also contribute somewhat if flocs are tightly packed. This study suggests that an ADV could be a useful instrument to estimate suspended cohesive sediment concentration and its fluctuation if the above concerns are clarified.     

Measurements of coupled fluid and sediment motion over mobile sand dunes in a laboratory flume

The relationship between turbulent fluid motions and sediment particle motions over mobile sand dunes was investigated by using a laser Doppler velocimeter and an acoustic backscatter system in laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory. Profiles of acoustic backscatter from particles and at-a-point turbulence data were collected while translating both measurement devices downstream at the speed of mobile dune bedforms. The resulting data set was used to examine the frequency （recurrence frequency） at which the fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation （σ） of the local velocity and the magnitude the acoustic signal resulting from backscatter from suspended particles. The slope of the downstream and vertical velocity recurrence frequencies generally indicated a gradually increasing recurrence time with increasing elevation. The recurrence frequency for acoustic backscatter data was not strongly variable with elevation. The closest correspondence between the recurrence frequencies of sediment backscatter and vertical velocities at the 1σ magnitude threshold was in a region defined by X/L〈0.4 and y〈6 cm. The downstream velocity was most closely related to backscatter in a small region at 0.4〈X/L〈0.8 and less than 3-4 cm from the bed.     

PREDICTION OF THE GRAIN SIZE OF SUSPENDED SEDIMENT： IMPLICATIONS FOR CALCULATING SUSPENDED SEDIMENT CONCENTRATIONS USING SINGLE FREQUENCY ACOUSTIC BACKSCATTER

Collection of samples of suspended sediment transported by streams and rivers is difficult and expensive. Emerging technologies, such as acoustic backscatter, have promise to decrease costs and allow more thorough sampling of transported sediment in streams and rivers. Acoustic backscatter information may be used to calculate the concentration of suspended sand-sized sediment given the vertical distribution of sediment size. Therefore, procedures to accurately compute suspended sediment size distributions from easily obtained river data are badly needed. In this study, techniques to predict the size of suspended sand are examined and their application to measuring concentrations using acoustic backscatter data are explored. Three methods to predict the size of sediment in suspension using bed sediment, flow criteria, and a modified form of the Rouse equation yielded mean suspended sediment sizes that differed from means of measured data by 7 to 50 percent. When one sample near the bed was used as a reference, mean error was reduced to about 5 percent. These errors in size determination translate into errors of 7 to 156 percent in the prediction of sediment concentration using backscatter data from 1 MHz single frequency acoustics.     

Observations of sediment resuspension and settling off the mouth of Jiaozhou Bay,Yellow Sea

We deployed bottom-mounted quadrapod equipped with acoustic Doppler current profiler (ADCP), acoustic Doppler velocimeter (ADV), and optical backscatter sensor (OBS) over two semidiurnal tidal cycles along the western coast of the Yellow Sea, China. In combination with shipboard profiling of CTD and LISST-100, we resolved the temporal and spatial distributions of tidal currents, turbulent kinetic energy (TKE), suspended sediment concentration (SSC) and particle size distributions. During the observations, tidal-induced bottom shear stress was the main stirring factor. However, weak tidal flow during the ebb phase was accompanied by two large SSC and median size events. The interactions of seiche-induced oscillations with weak ebb flow induced multiple flow reversals and provided a source of turbulence production, which stripped up the benthic fluff layers (only several millimeters) around the Jiaozhou Bay mouth. Several different methods for inferring mean suspended sediment settling velocity agreed well under peak currents, including estimates using LISST-based Stokes’ settling law, and ADCP-based Rouse profiles, ADV-based inertial-dissipation balance and Reynolds flux. Suspended particles in the study site can be roughly classified into two types according to settling behavior: a smaller, denser class consistent with silt and clay and a larger, less dense class consistent with loosely aggregated flocs. In the present work, we prove that acoustic approaches are robust in simultaneously and non-intrusively estimating hydrodynamics, SSC and settling velocities, which is especially applicable for studying sediment dynamics in tidal environments with moderate concentration levels.     

ADP在太湖沉积物再悬浮分析中的应用

分析太湖的悬浮物浓度时,使用传统的过滤与称重的方法,难以在短时间内取得大量的数据,并且进行大范围调查时困难较多,特别是在计算悬浮物浓度随时间的变化率时,根据导数的定义其时间间隔应足够小,此时上述方法显然难以得出较为准确的结果．本文从声学后散射原理出发,通过对声学多普勒三维流速仪(ADP)所接收的回声强度在传播距离上的校正,得出了当悬浮物粒径组成较为稳定时,该强度能反映水体中悬浮物浓度(SSC)的结论,并基于2002年在太湖乌龟山的一次为期一周的湖流观测结果,分析了经校正后的回声强度与太湖中悬浮物浓度间的指数相关关系,通过实测资料对上述关系进行了验证,结果表明该经验公式适用于太湖,其回声强度的变化能反映水体中SSC的变化规律,为大范围调查水体中悬浮物浓度提供了更加快速而有效的方法．     

A new method for evaluating errors in high‐frequency ADV measurements

The acoustic Doppler velocimeter (ADV) measures three‐dimensional velocities in a small, remote sampling volume at high frequencies, however, these measurements incorporate errors that are intrinsic to the measurement technique. This paper demonstrates a new method for calculating the total measurement errors, including sampling errors, Doppler noise and errors due to velocity shear in the sampling volume associated with single‐point ADV measurements. This procedure incorporates both the effects of instrument configuration and the distribution of errors between velocity components for any probe orientation. It is shown that the ADV can characterize turbulent velocity fluctuations at frequencies up to the maximum sampling rate and that Reynolds shear stress errors are very small. Copyright © 2000 John Wiley & Sons, Ltd.     

Estimating sand concentrations using ADCP-based acoustic inversion in a large fluvial system characterized by bi-modal suspended-sediment distributions

Quantifying sediment flux within rivers is a challenge for many disciplines due, mainly, to difficulties inherent to traditional sediment sampling methods. These methods are operationally complex, high cost, and high risk. Additionally, the resulting data provide a low spatial and temporal resolution estimate of the total sediment flux, which has impeded advances in the understanding of the hydro-geomorphic characteristics of rivers. Acoustic technologies have been recognized as a leading tool for increasing the resolution of sediment data by relating their echo intensity level measurements to suspended sediment. Further effort is required to robustly test and develop these techniques across a wide range of conditions found in natural river systems. This article aims to evaluate the application of acoustic inversion techniques using commercially available, down-looking acoustic Doppler current profilers (ADCPs) in quantifying suspended sediment in a large sand bed river with varying bi-modal particle size distributions, wash load and suspended-sand ratios, and water stages. To achieve this objective, suspended sediment was physically sampled along the Paraná River, Argentina, under various hydro-sedimentological regimes. Two ADCPs emitting different sound frequencies were used to simultaneously profile echo intensity level within the water column. Using the sonar equation, calibrations were determined between suspended-sand concentrations and acoustic backscatter to solve the inverse problem. The study also analyzed the roles played by each term of the sonar equation, such as ADCP frequency, power supply, instrument constants, and particle size distributions typically found in sand bed rivers, on sediment attenuation and backscatter. Calibrations were successfully developed between corrected backscatter and suspended-sand concentrations for all sites and ADCP frequencies, resulting in mean suspended-sand concentration estimates within about 40% of the mean sampled concentrations. Noise values, calculated using the sonar equation and sediment sample characteristics, were fairly constant across evaluations, suggesting that they could be applied to other sand bed rivers. © 2018 John Wiley & Sons, Ltd.     

Suspended sediment concentration and the ripple–dune transition

Flume experiments were conducted in order to monitor changes in flow turbulence intensity and suspended sediment concentration at seven stages across the ripple–dune transition and at three different positions above the bed surface. Three‐dimensional velocity measurements were obtained using an acoustic Doppler velocimeter (ADV). Suspended sediment concentration (SSC) was monitored indirectly using ADV signal amplitude. Although limited to time‐averaged parameters, the analysis reveals that SSC varies significantly with stage across the transition and with sampling height. The statistical analysis also reveals an apparent uniformity of suspended sediment concentration with height above the bed in the lower half of the flow depth at the critical stage in the transition from ripples to dunes. This is also the stage at which turbulence intensity is maximized. Statistically significant correlations were also observed between suspended sediment concentrations and root‐mean‐square values of vertical velocity fluctuations. These correlations reflect the various levels of shear‐layer activity and the distinct turbulent flow regions across the transition. Conversely, time‐averaged values of Reynolds shear stress exhibit a very weak relationship with suspended sediment concentrations. Copyright © 2004 John Wiley & Sons, Ltd.     

Acoustic parameters inversion and sediment properties in the Yellow River reservoir

The physical properties of silt in river reservoirs are important to river dynamics. Unfortunately, traditional techniques yield insufficient data. Based on porous media acoustic theory, we invert the acoustic parameters for the top river-bottom sediments. An explicit form of the acoustic reflection coefficient at the water–sediment interface is derived based on Biot’s theory. The choice of parameters in the Biot model is discussed and the relation between acoustic and geological parameters is studied, including that between the reflection coefficient and porosity and the attenuation coefficient and permeability. The attenuation coefficient of the sound wave in the sediments is obtained by analyzing the shift of the signal frequency. The acoustic reflection coefficient at the water–sediment interface is extracted from the sonar signal. Thus, an inversion method of the physical parameters of the riverbottom surface sediments is proposed. The results of an experiment at the Sanmenxia reservoir suggest that the estimated grain size is close to the actual data. This demonstrates the ability of the proposed method to determine the physical parameters of sediments and estimate the grain size.     

Calibrating multi-frequency acoustic backscatter systems for studying near-bed suspended sediment transport processes

The utilisation of sound backscattered from sediments in suspension, to measure profiles of near-bed particle size and concentration, has been gaining increasing acceptance and usage by sedimentologists and coastal engineers over the past two decades. To obtain the sediment parameters from the backscattered signal requires an inversion to be conducted on the signal and this necessitates a system calibration. The calibration can be carried out by detailed acoustic and electronic measurements, or alternatively by measuring the backscattering from suspensions with known scattering characteristics. Here, we explore the latter approach and describe in some detail the calibration of a triple frequency acoustic backscatter system. The aim is to provide coastal scientists involved in using acoustics as a tool for sediment transport research, with a clear exposition of the calibration process. Suspensions of glass spheres of varying particle size were used as the calibration scatterers. To interpret the signal backscatter from the suspension of glass spheres a simple model for sphere scattering is presented. The results show that consistent calibration results can be obtained in a relatively simple and robust manner.     

Particle size distribution of bed materials in the sandy river bed of alluvial rivers

The particle size distribution of bed materials in the sandy river bed of alluvial rivers is important in the study of topics such as friction, river bed evolution, erosion, and siltation. It also can reflect the dependency relation between river bed sediment and flow intensity. In this paper, the critical pattern of sediment movement in the near-wall region of a sandy river bed was analyzed. According to the principle of momentum balance, the critical settling-rising condition of bed material in a sandy river bed was found to be instantaneous turbulent velocity equal to 2.7 times the sediment settling velocity in quiescent water. Based on a vertical instantaneous turbulent velocity with a Gaussian distribution, a theoretical relation for calculating the particle size distribution of bed materials in a sandy river bed without pre-known characteristic grain sizes was developed by solving a stochastic equation. The for-mula is verified using measured data, and the results show that the proposed formula was in accordance with the measured data. This study has theoretical significance and practical value for determining the bed material particle size distribution of the sandy bed of alluvial rivers.     

Estimating sediment transport from acoustic measurements in the Venice Lagoon inlets

This paper presents the results of a 3-year-long (November 2004–November 2007) study based on the use of acoustic Doppler current profilers (ADCPs) to estimate the solid transport through the three inlets of Venice lagoon. In each of the three inlets instruments were mounted both on survey boats and deployed on the channel bed. The three bottom-mounted ADCPs were positioned in the central part of the inlets, continuously monitoring vertical profiles in the water column. Periodic transects along the investigated sections were collected by the boat-mounted ADCP. Both installations measured current speed and acoustic backscatter intensity. The latter expresses the attenuation of acoustic energy due to material in the water column.     

Sensitivity analysis of sediment flux derived by laser diffraction and acoustic backscatter within a reservoir

A combination of two indirect methods to measure sediment flux is presented in this study to evaluate suspended sediment transport in a hydropower reservoir. The acoustic backscatter signal (ABS) from an Acoustic Doppler Current Profiler (ADCP) is therefore applied in pre-defined transects within the reservoir in combination with a Laser In-Situ Scattering Transmissometry – stream lined device (LISST-SL). The stationary LISST-SL derived suspended sediment concentration (SSC) measurements are used to calibrate the ABS. From the LISST-SL measurements a time series of SSC is obtained. This enables, in addition, a comprehensive data analysis to evaluate the influence of natural fluctuations of the SSC on the calculated sediment flux, which should be taken into account when assessing sediment transport. Furthermore SSC measurements are done with the LISST-SL close to the reservoir bed. In areas close to the bed no information regarding the ABS is available from the ADCP measurements due to the side-lobe interference. In various studies the information from the last three valid cells is used for extrapolation. However, as result of a comparison of the LISST-SL measurements with extrapolated SSC values from the ADCP measurements it can be seen that, especially in deep reservoirs, this method has to be adapted to the in-situ conditions.     

Acoustic seabed segmentation from direct statistical clustering of entire multibeam sonar backscatter curves

A fast, simple method is presented to obtain acoustic seabed segmentation from multibeam sonar backscatter data, for situations where processed backscatter curves are already available. Unsupervised statistical clustering is used to classify multibeam sonar backscatter curves in their entirety, with the curves essentially treated as geometrical entities. High variability in the backscatter curves is removed by along-track averaging prior to clustering, and no further preprocessing is required. The statistical clustering method is demonstrated with RESON 8125 multibeam sonar data obtained in two bathymetrically complex environments. These are a sandwave field in Keppel Bay, Queensland, and an area of inter-island sand, reef, seagrass, and rhodolith beds in Esperance Bay, Western Australia. The resulting acoustic charts are visually compelling. They exhibit high spatial coherence, are largely artifact free, and provide spatial context to comparatively sparse grab samples with relatively little effort. Since the backscatter curve is an intrinsic property of the seafloor, the mappings form standalone charts of seafloor acoustic properties. In themselves they do not need ground truthing. Conceptually, use of the full angular backscatter curve should form the primary means of obtaining acoustic seabed segmentation. However, this is dependent on the scale and configuration of seabed backscatter features compared to the dimensions of the averaged swathe used to obtain reliable realisations of the backscatter curve.     

Unsupervised fuzzy classification and object-based image analysis of multibeam data to map deep water substrates, Cook Strait, New Zealand

A comprehensive 32 kHz multibeam bathymetry and backscatter survey of Cook Strait, New Zealand (∼8500 km²), is used to generate a regional substrate classification map over a wide range of water depths, seafloor substrates and geological landforms using an automated mapping method based on the textural image analysis of backscatter data. Full processing of the backscatter is required in order to obtain an image with a strongly attenuated specular reflection. Image segmentation of the merged backscatter and bathymetry layers is constrained using shape, compactness, and texture measures. The number of classes and their spatial distribution are statistically identified by employing an unsupervised fuzzy-c-means (FCM) clustering algorithm to sediment samples, independent of the backscatter data. Classification is achieved from the overlay of the FCM result onto a segmented image and attributing segments with the FCM class. Four classes are identified and uncertainty in class attribution is quantified by a confusion index layer. Validation of the classification map is done by comparing the results with the sediment and structural maps. Backscatter (BS) strength angular profiles are used to show acoustic class separation. The method takes us one step further in combining multibeam data with physical seabed data in a complementary analysis to seek correlations between datasets using object-based image analysis and unsupervised classification. Texture within these identified classes is then examined for correlation with typical backscatter angular responses for mud, sand and gravel. The results show a first order correlation between each of the classes and both the sedimentary properties and the geomorphological map.     

An integrated suspended sediment transport monitoring and analysis concept

A new integrated suspended sediment monitoring strategy applying direct and indirect technologies is presented.Optical sensors continuously record the turbidity at one point in the channel cross section close to the river bank and are calibrated by water samples taken close to the sensor.Additionally measurements are performed to establish the distribution of suspended sediment in a cross section（bottle samples combined with acoustic devices）.Using correction factors（probe and cross-sectional factor） these monitoring methods are combined and it is,thus,possible to fully document the temporal and spatial variability of the suspended sediment transport and to estimate the suspended sediment load for certain time periods.This monitoring strategy was implemented at various measurement sites in Austria as well as at the Hainburg Road Bridge site on the Danube River.It has already been successfully applied for three years at this measurement site and suspended sediment loads during high discharges up to a 15 year flood event have been monitored.To evaluate the new monitoring methods the results were compared with load estimation methods found in the literature including averaging and ratio estimators as well as rating curves.The results prove that with the new methodology,the temporal variability of the suspended sediment transport can be detected more accurately compared with the other methods.They also demonstrate that the additional consideration of the spatial distribution of the suspended sediment concentration in the cross section is crucial as the mean concentration in the cross section can significantly exceed the concentration near the banks,especially at large rivers like the Danube River.     

Determination of the particle load based on detailed suspended sediment measurements at a hydropower plant

Suspended sediment particles contained in inflows of water systems of hydropower plants (HPPs) cause hydro-abrasive erosion of the hydraulic turbines and structures leading to significant maintenance costs, efficiency reductions, and downtime. Relevant parameters such as suspended sediment concentration (SSC), particle size distribution (PSD), shape, and mineralogical composition were measured with an online multi-frequency acoustic instrument and based on manually taken samples from the end of the sand trap of the Toss HPP in the Himalayan region, India. In the laboratory, the samples were analyzed using the gravimetric method, laser diffraction, turbidity, dynamic digital image processing, scanning electron microscope, petrography analysis, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The online instrument and the samples provided measurement results at a single point. To investigate vertical gradients in concentration and particle sizes, additional samples were collected 9 times at 7 relative water depths. The SSC, most particle sizes, and particle shape were found to be evenly distributed over depth except d90, i.e. the diameter which is not exceeded by 90% of the particle mass. d90 measured at 76% of the water depth was in the range of fine sand and was multiplied by 1.05 to obtain an average value representative for the entire depth. Improved methodologies to quantify both particle shape and size in an analytical model for hydro-abrasive erosion are proposed. Also, the PSD measuring performance of laser diffraction and dynamic imaging was studied and similar values of the median particle sizes were obtained from both instruments. Further, multi-frequency acoustic, turbidity and laser diffraction techniques were found suitable for SSC measurement at the test case HPP.     

一种新型海底沉积物声学原位测量系统的研制及应用

为了能够精确地测量海底表层沉积物的声学参数,自主研制了一种新型海底沉积物声学原位测量系统,与国内外传统的声学原位测量系统相比,该系统能够实时显示声波波形,调整测量参数,其工作方式除了站位式测量之外,还实现了拖行式连续测量,极大地提高了工作效率.根据前期海试情况,对海底仪器结构进行了重新设计,使之可以同时测量海底沉积物及海底海水的声学参数,同时建立了双向数字信道,解决了测量过程中系统信号的干扰问题.该系统的结构分为两部分:甲板控制单元和水下测量单元,整套系统通过主机控制程序进行控制,采用GPS定位系统测定仪器的大地坐标.为了检验系统的稳定性及准确性,分别进行了实验室水槽实验和海上试验.利用水声测量设备对测量系统进行实验室水槽标定分析,实验结果表明系统测量值相对误差仅为0.04%,测量结果具有较高的精度.海上试验在青岛胶州湾和东海海域进行,获得了试验区域海底沉积物声速和声衰减系数的测量数据,将测量数据与他人的研究结果进行对比分析,结果表明测量数据与前人研究结果一致,较为准确.该原位测量系统在站位式测量和拖行式测量中都能够快速准确地测量出沉积物声速和声衰减系数,可以作为海底底质声学测量的调查设备.