Acoustic doppler velocimeter backscatter for suspended sediment measurements: Effects of sediment size and attenuation

Abstract

Use of backscatter output from a 10-MHz acoustic Doppler velocimeter (ADV) for quantifying suspended sediment concentration was investigated in a laboratory setting. The ADV was immersed in a 125-liter solid polycarbonate circulation tank in which known concentrations of non-cohesive quartz sediments (63–500 µm size range) were mixed in freshwater to obtain homogeneous solutions. Six different size fractions under a wide range of concentration conditions up to 10 kgm${}^{-3}$ were used in the tests. Empirical relations between the acoustic backscatter strength and concentration for each sediment size fraction revealed that there are size-dependent upper and lower bounds on measurable concentrations. Above or below an optimum number of particles in suspension, the performance degrades due to insufficient scatterers which results in a lower bound, and high acoustic attenuation which results in an upper bound on concentration. Both upper and lower bounds increased with increasing sediment concentration, providing, in general, a wider applicability range for larger particles. The range of measurable conditions was broad enough to make the approach useful for sand sizes and concentrations commonly encountered in nature. Effect of sound attenuation was also examined in detail by correcting the acoustic backscatter for transmission losses due to spreading, absorption of energy by water and attenuation due to suspended sediments. The results showed that the effect of transmission losses varies significantly depending on sediment size and concentration, which especially becomes important at high concentrations. This is the first time this effect is quantified to show the direct impact of signal attenuation on suspended sediment measurements using an ADV.

Introduction

Quantification of the transport, deposition, and suspension of sediment in various water environments (i.e., rivers, lakes, estuaries, and bays) is of crucial importance for many aspects such as shoreline morphology, navigation, water quality, and fate of pollutants and bio-matter. Sediment kept in suspension by hydrodynamic processes accounts for the majority of the sediment transported past a location of interest [5], [37]. Both grains of sand and finer material may be transported in suspension, depending on the magnitude of the energy in the flow, among other factors. In many studies related to sediment transport, it is essential to use the best possible methods and equipment while measuring and recording the mass concentration of sediments in suspension due to the highly variable nature of suspended sediment. For example, in terms of environmental and ecosystem concern, sediments can be a source of nutrients and toxins that impact the health of organisms within the system. The high concentration of suspended materials can limit both light transmission and photosynthesis. From a navigational point of the view, the deposition of suspended sediments into shipping channels causes infill and may require occasional dredging to maintain a navigable waterway [14].

The quantification methods of suspended sediment concentration (SSC) can be classified into two main groups as direct and surrogate methods. Direct methods, which employ water sampling bottles or submersible pumps, are labor-intensive and might not be representative due to spatial and temporal variability of SSC in the water column. The main advantages of this method are its simplicity and providing information on sediment characteristics, such as shape, size, and mineralogy [3], [4], [13], [41]. The main shortcoming of the direct methods is the difficulty of taking adequate measurements, particularly when frequent sampling is required.

Surrogate methods, none of which involve the collection of water samples except for calibration of the devices, include acoustic and optical backscatter sensors, and laser diffraction methods. They provide high temporal resolution without distorting the flow, as the measurements are collected at distance. In some cases, the spatial variation in concentration may also be quantifiable. Methods employing acoustic signals tend to be less problematic logistically, as optical sensors are typically much more sensitive to biofouling in natural environments [8], [39]. Multi-frequency acoustic sensors also have the potential to provide information on the size distribution of the suspended sediment [8], [19], [21], [34], [40]. Laser diffraction instruments have been successfully used for several decades to measure volumetric particle concentration and particle size distribution, which are feasible over a limited range of sizes ranging from 2 to 500 microns [1]. However, measured light scattering from particles outside of these measurement size limits introduces errors, while particle shape and variability in sediment density influence the conversion from volumetric to mass concentration [2].

Acoustic backscatter from devices, which are originally designed to measure water velocities, have been applied successfully to various water environments over a few past decades to quantify SSC (e.g., Gartner [14], Sahin et al. [33], Sahin [34], Sahin et al. [35], Thorne et al. [39]). The basic principle for the measurement of SSC is that acoustic waves emitted by the device travels through a water-sediment mixture. While some of the acoustic waves are reflected back to the device, some of the waves will scatter and attenuate as a function of sediment, fluid, and instrument characteristics such as acoustic frequency, sediment concentration, and travel distance from the sound source. Each acoustic frequency has a minimum detectable particle diameter and a peak sensitivity for a certain particle (e.g., Thorne and Hanes [40]). Besides having successful applications of acoustics for estimation of SSC, it has also some limitations, and the factors such as variations in water density (salinity and temperature), sediment grain size and concentration, water depth (distance the signal travels from the transducers), water surface conditions may result in unreliable data.

When an acoustic instrument is calibrated, no additional sensor is needed to measure SSC. This approach then provides simultaneous measurements of velocity and concentration via one acoustic sensor that is typically much less susceptible to errors due to biofouling compared to optical sensors. However, the calibration can make the acoustic approach highly site-specific and in some cases strongly seasonally dependent. For example, the biological activity and flocculation can influence the acoustic response of the sensor [14], [17], [28], [32], [33], [34], [35].

The most commonly used acoustic devices are Acoustic Doppler current meters (including Acoustic Doppler Current Profilers (ADCPs), Acoustic Doppler Profilers (ADPs), and Acoustic Doppler Velocimeters (ADVs)). Acoustic profiling instruments (i.e., ADCPs and ADPs) employ range-gating of the reflected sound signal to resolve the spatial variations in their along-beam velocities, which can then be converted to velocity profiles in a Cartesian coordinate system assuming the particles travel with the same current speed. Profiling instruments have a distinct advantage over most other surrogate technologies in that they can provide data indicative of conditions spanning a substantial section of a river cross-section or water column [19], [24], [25], [36], [40].

ADVs are single-point measurement devices and very useful for the studies in a laboratory environment since they enable taking measurements in limited spaces due to the sampling volume very close to the instrument. They are readily portable and yield velocity estimates for a small (O(0.1 cm³)) sampling volume, typically 5–10 cm away from the instrument (Fig. 1). Hosseini et al. [20] showed that an ADV could be suitable and useful for determining the flow structure in turbidity currents, based on a study on low-density turbidity currents in a laboratory flume using a 10-MHz ADV. Elci et al. [11] estimated SSC in rivers using backscatter from a handheld ADV in quasi-steady flow. MacDonald et al. [28] studied the effect of cohesive sediment flocculation on ADV backscatter and concluded that the presence of flocculation modifies the backscatter properties of a suspension of sediment particles, compared to a suspension of the same particles in an unflocculated state. Xavier et al. [47] carried out a set of laboratory experiments to examine the relationship between glass microspheres of various sizes and their acoustic signals recorded by a 6-MHz ADV. They concluded that acoustic return signal amplitudes can be used to estimate concentrations of SSC, if appropriate corrections are applied to the backscattered signals, particularly due to the attenuation mechanisms. Öztürk [31] studied the sensitivity of the acoustic estimates of suspended sediment concentration to sediment size and size distribution using original sediment mixture and with well-sorted fractions obtained by sieving the mixture. A-10 MHz ADV was used in a controlled laboratory environment so that the suspended sediment concentration was readily controlled and known. The study suggests that suspended sediment concentration should be predictable based on the knowledge of backscatter intensity, dimensionless grain size, and grain size distribution. Most of the previous studies investigated the response of the ADV signal in relatively dilute SSC conditions (up to 1 kgm${}^{-3}$) for which attenuation of the acoustic signal due to water and particles in suspension may be neglected. A few number studies (i.e., Ha et al. [17], Li et al. [26]) observed a decreasing backscatter intensity trend with increasing SSC at larger concentrations and attributed this behavior to intense sound absorption under high SSC conditions without quantifying this effect.

This study investigates the response of the acoustic backscatter signal of a 10-MHz ADV to suspended particles of different sizes. The main aspects that differ the present study from the previous ones (e.g., Ha et al. [17], Li et al. [26], Öztürk [31], Xavier et al. [47]) are: (i) investigation of the ADV signal response under high concentration conditions (up to SSC of 10 kgm${}^{-3}$) in a laboratory environment for suspended sediment measurements, and (ii) providing a quantitative analysis on the effect of sound attenuation at high concentrations in detail for the first time. The method used to convert the acoustic backscatter information to SSC estimates and the experimental system that allows for measurements under controlled suspended sediment conditions are presented in Section 2. The results related to the effects of different particle sizes and signal attenuation on the acoustic measurements are given in detail in Section 3. The results are discussed in Section 4, and the conclusions are summarized in Section 5.