Analysis of the riverbed backscattered signal registered by ADCPs in different bedload transport conditions – field application

Acoustic Doppler current profilers (ADCP) were deployed to investigate the backscattering (BS) signal in three navigable rivers, in different bedload transport conditions. This study aims to demonstrate that the BS strength, as an additional variable to the apparent bedload velocity, improves the characterization of the bedload transport using ADCPs. The M9 -3 MHz and the vertical beam M9 - 0.5 MHz showed decline of the BS strength as the bedload intensity increased, whereas the RDI -1.2 MHz was relatively insensitive. The correlation between the median grain size and the BS strength for the 0.5 MHz was linear, for the 3 MHz the BS strength was attenuated in the active layer, and for 1.2 MHz, it revealed a parabolic distribution. Moreover, the analyses of the ADCP measured variables, using wavelet transformations and unsupervised machine learning, highlighted the importance of the spatial and temporal variance and transient nature of the bedload transport

Bedload transport analysis using image processing techniques

Bedload transport is an important factor to describe the hydromorphological processes of fluvial systems. However, conventional bedload sampling methods have large uncertainty, making it harder to understand this notoriously complex phenomenon. In this study, a novel, image-based approach, the Video-based Bedload Tracker (VBT), is implemented to quantify gravel bedload transport by combining two different techniques: Statistical Background Model and Large-Scale Particle Image Velocimetry. For testing purposes, we use underwater videos, captured in a laboratory flume, with future field adaptation as an overall goal. VBT offers a full statistics of the individual velocity and grainsize data for the moving particles. The paper introduces the testing of the method which requires minimal preprocessing (a simple and quick 2D Gaussian filter) to retrieve and calculate bedload transport rate. A detailed sensitivity analysis is also carried out to introduce the parameters of the method, during which it was found that by simply relying on literature and the visual evaluation of the resulting segmented videos, it is simple to set them to the correct values. Practical aspects of the applicability of VBT in the field are also discussed and a statistical filter, accounting for the suspended sediment and air bubbles, is provided

Bedload Monitoring by means of Hydro - Acoustic Techniques

The bedload distribution through the alluvial streams contributes in shaping of the river morphology. Therefore, bedload transport data are fundamental requirement for proper management of engineering practices in complex river systems. However, measurement series of bedload transport are seldom available and therefore statistically unreliable. The conventional methods can be notoriously hard and labor-intensive, entailing significant stochastic and systematic uncertainties mostly due to the spatio-temporal variability of the bedload as well as the instrument direct disturbance of the riverbed. Thus, the use of non-intrusive surrogate techniques could significantly reduce that uncertainty. Recently, many studies have demonstrated that the hydro-acoustics instruments are promising technique for bedload measurement. These sensors do not disturb the riverbed and are easy-to-deploy for long and frequent measurements. Some studies have reported strong correlations between the acoustic Doppler current profilers (ADCP) bedload measurements and the conventional bedload samplers. However, most of these methods are site-specific and require detailed calibration. This study investigates the capability of different ADCPs to measure the bedload velocity and bedload concentration. For those purposes, two laboratory and several field campaigns were conducted. The first series of laboratory experiments were performed in the hydraulic laboratories at UNIBO and UOttawa, focusing on evaluation of the apparent bedload velocity and the scattering processes occurring at the riverbed. The second campaign was conducted at NTNU laboratory aiming to validate the previous results and to further examine the acoustic parameters and signal processing configurations. At the same time the backscattering strength sensitivity towards the bedload concentration was fully examined. Two ADCPs were deployed at the same time on 0.7 m mutual distance (M9 Sontek, 1 MHz and 3 MHz and Stream Pro RDI, 2 MHz). Side and planar looking camera were deployed to measure the bedload velocity, active layer thickness and surface bedload concentration and a bedload trap was installed at the end of the flume to monitor the bedload transport rate. Different bedload transport conditions were reached by utilizing various sediment materials, (e.g., sand and gravel) and by adjusting the hydraulic conditions. De-spiking and filtering were applied to the raw data, and the temporal average of the apparent bedload velocity was spatially normalized. The percentage of filtered erroneous velocity data from the ADCP time series demonstrated a strong correlation with the surface concentration of mobile particles. In all experiments the normalized apparent velocities measured by the M9 corresponded well to the bedload velocity of the imagery data, better than those measured by the StreamPro, which appeared to underestimate the bedload velocity by a factor of 2-22. These deviations resulted from the different signal processing configurations, the acoustic geometry, and the immobile sediment bed. The backscattering (BS) strength was de-spiked and corrected by adapting the basic sonar equation for riverbed scattering. For the M9 the BS strength decreased as the bedload concentration increased, independently of the particle velocities and sizes. The BS strength registered by the StreamPro resulted in almost constant values for all transport conditions. Additional tests were performed using ultrasound velocity profilers developed by Ubertone. These results confirmed that the internal pressing and echo profiling resolution are crucial in the determination of the correct bedload velocity. The field experiments were conducted in two relatively large rivers in Germany (Oder and Elbe River) and in one small river in Albania (Tommorice River). Stationary measurements were performed using four different ADCPs (M9 Sontek, Rio Grande RDI 0.6 MHz and 1.2 MHz, RiverPro RDI 1.2 MHz) working at four different frequencies. The raw apparent bedload velocities were de-spiked and filtered in a stream-wise direction. Then, functional correlations were observed between the magnitudes of the apparent velocities and the bedload transport rates measured by pressure-difference bedload sampler. Each ADCP yielded different results because of the different frequency, backscatter sensitivity and acoustic penetration in the active bedload layer. In addition to the frequency, other acoustic parameters such as the percentage of the filtered data, transducers width, beamopening and grazing angle, the pulse length, contributed to the different acoustic bedload sampling. More precisely, the lower apparent bedload velocity was obtained when lower acoustic frequency, longer pulse lengths and larger beam focusing were used. The kinematic model was successfully applied for the middle frequencies (1.2 MHz and 1 MHz), which gave the best correspondence to the empirical estimation of the bedload active layer thickness and concentration. The field data helped to understand the influence of the riverbed deformation as well as the acoustic sampling problems. The future research should focus on a more extensive examination of the internal processing algorithms to eventually clarify the best processing configuration and use of the ADCPs. Therefore, closer cooperation between the researchers and the manufacturers of the ADCPs is fundamental for better understanding and possible adaptation of the ADCPs to measure even more accurately the bedload velocity and BS strength. A complete model of bedload transport rate based on apparent velocity and BS strength should be fully developed in addition to further testing of the BS strength

Bedload Monitoring by means of Hydro - Acoustic Techniques

This study provides an evaluation of a hydro-acoustic technique for efficient quantification of the bedload transport in riverine environments. Stationary bedload measurements/experiments were conducted simultaneously at different field study sites and laboratories, using different acoustic Doppler current profilers (ADCP) working at four different frequencies. A post-processing procedure for the ADCP outputs was developed and the data was compared with other bedload transport measuring techniques (e.g., imagery data, physical samples). The 3 MHz delivered spatially averaged apparent velocity, as well as the 1 MHz for the sand experiments only. The apparent velocity for the 1MHz gravel corresponded to the true mean particle velocities. The 2 MHz severely underestimated the true particle velocities. The backscattering strength measured by the M9 decreased as the bedload concentration increased, independently of the particle's velocities and size; the backscattering strength registered by the StreamPro resulted in almost constant values. Each ADCP yielded different results because of the different frequency backscatter sensitivity and acoustic penetration in the active bedload layer. In addition to the frequency, other acoustic parameters such as the percentage of the filtered data, transducers width, beam opening angle, beam-grazing angle and the pulse length, contributed to the acoustic bedload sampling. The results permitted quantification of the bedload transport characteristic and development of a new updated methodology for continuous non-intrusive bedload transport measurements. An extensive examination of the internal processing algorithms is necessary to clarify the best processing configuration, in addition to collecting more data and performing a detailed uncertainty analysis

Bedload Velocity and Backscattering Strength from Mobile Sediment Bed: A Laboratory Investigation Comparing Bistatic Versus Monostatic Acoustic Configuration

Despite the many advantages of using active ultrasound sonars, recent studies have shown that the specific acoustic geometry, signal-processing configuration, and complex surface-volume scattering process at the riverbed introduce several uncertainties in bedload estimation. This study presents a comparison of bedload velocity and bottom echo intensity measurements performed by monostatic and bistatic active ultrasound systems. The monostatic configuration is widely applied in the field to measure the apparent velocity at the riverbed with an acoustic current Doppler profiler (ADCP). Two laboratory investigations were conducted in two different hydraulic facilities deploying ADCP Stream Pro, monostatic and bistatic acoustic velocity profilers, manufactured by Ubertone. The bistatic instruments provided more accurate bedload velocity measurements and helped in understanding the acoustic sampling of the monastic systems. The bistatic profiles succeeded in measuring a profile over the active bedload layer, and the monostatic instruments resulted in different bedload velocity estimations depending on the acoustic resolution and sampling. The echo intensity increased in the cells measured within the active bedload layer with respect to the cell measuring the water column above. The cells that sampled the immobile bed surface beneath the bedload layer showed a reduction of the echo intensity compared with the cells above. The acoustic sampling, which combines the measurement volume geometry and internal processing, seems crucial for more accurate outputs. Future research about the use of monostatic instruments in the field should aim to define the best possible setting for the acoustic parameters at a given bedload condition that may be tuned by evaluating the backscattering at the river bottom together with the apparent bedload velocity

Bedload transport rate measurements using ADCPs with different frequencies

The complexity of fluvial hydro-morphological monitoring and the man ever-increasing data needs make an essential point of involving faster and modern measurement techniques as a common practice among scientist and engineers. The reliable bedload transport data is very important for optimized sediment management of big navigable rivers and reservoirs. DirectConventional sampling of the bed load transport rate is usually hindered by the turbulent near-bed conditions and the rough condition at loose bed. In relation, it is not continuous and it does not give information for the sediment dynamics.The acoustic Doppler current profilers (ADCPs) and its Bottom Tracking (BT) mode have been successfully used in evaluation of the bedload transport in the past. In the previous studies, the physical samples were necessary; hence, a calibration is required to determine the correct transport rate. However, the raw ADCP (apparent) velocity is not the average velocity of the bedload . The signal is noisy and contains erroneous data; therefore, it requires a proper filtering before calculating the transport rate. The recent laboratory studies showed that not only the working frequency and the Particle Size Distribution(PSD) are important, but also bottom roughness and the sampling frequency can significantly influence the estimation of the Doppler velocity. This study aims to develop a methodology of using the ADCP BT mode to measure the transport rate by only knowing the PSD and the frequency of the used instrument. The main idea is that the erroneous data that biases the results is being filtered. Measurements from few campaigns are presented. Four different ADCPs working on 0.6MHz, 1 MHz, 1.2MHz and 3MHz are employed to measure the bedload velocities and the water velocities. Simultaneously, the physical samples are taken from different positions in two rivers. The kinematic transport model is applied and the sensitivity of the acoustic properties towards different PSD are analyzed. Additionally, the bedload porosity of the active layer is estimated using empirical formulas that include the shear velocity. The log-law is applied on the water velocity profiles and the shear velocity is calculated. In general, the calculated transport rate and the physical samples presented a good matching and the eventual deviations are associated with the different positions of the ADCPs and the samples, the riverbed irregularities, etc. This study offers a non-intrusive technique that could significantly reduce the uncertainty in bedload measurements by introducing continuous and statistically more valid data. Nevertheless, the accurate calculation of the porosity and the thickness of the active layer still causes uncertainty in the results and remains to be investigated in the future studies

Evaluation of an acoustic Doppler technique for bed-load transport measurements in sand-bed Rivers

The bottom tracking (BT) feature of the acoustic Doppler current profilers (ADCP) have emerged as a promising technique in evaluating the bed load. Strong statistical correlations are reported between the ADCP BT velocity and the transport rate obtained by physical sampling or dune tracking; however, these relations are strictly site-specific and a local calibration is necessary. The direct physical sampling is very labor intensive and it is prone to high instrument uncertainty. The aim of this work is to develop a methodology for evaluating the bed load transport using commercial ADCPs without calibration with physical samples. Relatively long stationary measurements were performed in a sand-bed and sand gravel rivers, using three different ADCPs working at 3MHz, 1.2MHz and 0.6MHz. Simultaneously, bed load samples were collected with physical samplers, and the riverbed was closely observed with digital cameras mounted on the samplers. It is demonstrated that the kinematic transport model can yield a relatively good estimate of the transport rate by directly using filtered apparent velocity, the knowledge of the hydraulic conditions and instrument-related calibration coefficients. Additionally, the ADCP data can help in qualitative assessment of the physical sampling. Future investigation of the backscattering echo and further confirmation of the BT apparent velocity should be performed in laboratory-controlled conditions.publishedVersion© The Authors, published by EDP Sciences, 2018. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (http://creativecommons.org/licenses/by/4.0/)