The use of ADCPs for suspended sediment assessment in river applications is attractive because acoustic beams are ranging from remote positions (i.e., from water surface and river side bank). This simplifies field procedures, prevents the measurement volume from disturbance, which is typical for standard sampling techniques, and enables the sediment transport monitoring during high level conditions when direct sampling is unsafe or not possible. Despite these clear advantages, the assumptions regarding the applied backscatter and sound attenuation models may restrict the applicability of acoustic methods. Indeed, the usually applied semi-empirical models (Equations (1)–(13)) were mostly derived for coastal applications, where suspended particles are well rounded and sorted within the sand spectrum. Differently, rivers present heterogeneous sediments, depending on the actual sedimentological, hydrological and hydraulic conditions. In addition to that, in riverine applications, the mean water depth usually fixes the acoustic beam range that depends again on the specific study site and may span from 0.1 to 10 m as an order of magnitude.
Since sound scatter and attenuation determine the measurement range of an ADCP, also the combining of river heterogeneities in terms of sediment features and water depth may give rise to not a trivial pursuit when trying to infer suspended sediment concentrations by using ADCP echo profiling. This encouraged a thriving of a variety of different methods, which are tailored on specific sedimentological, hydrological and hydraulic conditions with the shortcoming of being site specific. Although this may be still useful to solve specific problems, scientifically speaking, guidance should be given regarding the applicability of each method, which requires a clear explanation on features and limitations of the applied backscatter-attenuation method with respect to actual conditions and instrument frequency and deployment (i.e., down- or side-looking instruments).
4.1. Implications of the Acoustic Features Assessed from the Observed PSDs
The backscattering coefficient and sound attenuation normalized coefficients depend on the actual PSD that is particularly relevant for the bimodal distribution typical for river sediment, with clay-silt content usually forming the wash-load portion of suspended sediment and the remaining part being sand transported in full suspension. This was in our study the case for the Parana system, where sand concentration suspended from the riverbed clearly dominated the resulting backscattering strength (“Parana main channel” and “Colastiné: suspended-load” in Figure 10
), while the clay-silt content resulted in appreciable viscous attenuation coefficient (“Colastiné: wash-load” in Figure 11
c,d). In addition to that, the actual PSD differently affected the acoustic coefficients: the sand content may be investigated by assuming the backscattering coefficient, Ks2
, of mono-size distributions entailing small changes in the representative mean size. On the contrary, the deviation for mono-size values of the acoustic coefficients (Ks2
) appeared relevant for the case of clay-silt content.
Despite this, the high concentration of fine fractions resulted in negligible variations of backscattering strength and moderate values of scattering attenuation coefficient. On the contrary, viscous attenuation coefficients of observed clay-silt concentrations may result in comparable dB values to sand backscattering strength variation (with a magnitude changing in the range of 35–55 dB), although this would require long acoustic beams (i.e., 20–30 m) such as it would be possible with horizontally aligned beams projected by a side-looking ADCP. However, side-looking deployment in the Parana River appears particularly challenging, while the use of down-looking ADCPs to measure river discharge is a standard practice.
It is worth noticing that using a down-looking ADCP in the Parana River would limit the acoustic beams on average to 5 and 8 m for secondary and main channels, respectively, which would also result in a moderate value for the two-way round-trip attenuation (in dB) due to viscous attenuation. This occurrence was exploited in previous studies by using a single frequency ADCP working at 1200 and 600 kHz, which produced reliable correlations between sand concentrations and measured echo levels [15
], and between mean grain sizes of suspended sand and the ratios of echoes for those two frequencies [13
Furthermore, in the case of the large Parana system, the correlation assessed between viscous attenuation coefficient and clay-silt concentration (Table 3
) was limited by a small variation in fine sediment concentration, although field campaigns encompassed different hydraulic conditions. This occurrence may be related to the huge extension of the Parana system that somehow produces a homogenous base flow of very fine sediment (i.e.
, clay), relaxing the need of including clay-silt acoustic effects on the calibration of profiled echoes to investigate suspended sand, although this calls for further studies on the Parana system.
In the Danube case study, sand was not observed in full suspension by using the LISST-SL, and the backscattering strength appeared dominated by suspended clay-silt. A further research effort using a 1200 kHz ADCP to track suspended sediment concentration would require the assessment of the actual PSD backscattering coefficient, Ks2
. In fact, in this case, the backscattering coefficient of a mono-size distribution characterized with the particles mean size of the measured PSD was three to four orders of magnitude lower. This occurrence would produce a not reliable correlation between concentrations and backscattering strengths. In this case, therefore, there is a need of a priori
information about the actual PSD. In addition, the viscous attenuation coefficient appeared inverse correlated to std/a
ratio characterizing the measured PSD (Table 3
). In spite of that, the same arguments regarding beam range as the Parana case may be exhibited for viscous attenuation relevance with respect to corresponding backscattering strength variation in the Danube case. In this case, the beams projected by a down-looking ADCP would be limited even more, resulting into lower water depths (6 m on average) that would keep the two-way round-trip attenuation moderate.
Aiming to assist ADCP users on the basis of our findings, basic recommendations are discussed regarding instrument deployment (i.e., down- or side-looking ADCP) and frequency, the advantage of using multi-frequency, the need for a priori grain size information and the need for ADCP calibration. This discussion does not pretend to be exhaustive, indeed other sources of scatter and attenuation such as organic material, flocculated particles and sediment specific shapes (e.g., stick-shaped) are not encompassed in our study.
Furthermore, it is worth noting that the assessment of sediment features from ADCP recordings requires applying inverse method to solve Equation (1). These methods depend on the expected spatial gradients of sediment concentration as well as on the instrument frequency-deployment, although our work has not included the solution of the inverse problem. The inversion methods may be implicit or explicit [24
], the attenuation due to suspended sediment may be neglected [13
] or the concentration profile is assumed homogeneous [18
Our discussion is conducted regarding the performed analysis of acoustic features of samples which were observed in the field (i.e., the direct problem solution).
When using typical ADCP frequencies with a down-looking deployment, the vertical profiling results in shorter ranges than the distance for a complete signal attenuation (i.e., the distance for which the signal is significantly attenuated up to the noise level) because actual water depth limits the beam ranging. In addition to that, vertical gradients of sediment suspended from the riverbed likely result in relevant backscattering strength variations along acoustic beams with a negligible-moderate effect of sound attenuation due to sediment. In this case, the echo level recording from an ADCP may be successfully correlated to the concentration of suspended sand from the riverbed, neglecting or not, the viscous attenuation due to clay-silt fractions, which depends on the actual concentration of fine fractions. This is the case of tracking sand contents across a river section for a given hydro-sedimentological condition lasting the time of survey. The concentration of fine fractions is not acoustically investigated, although its effect on the used correlation should be carefully evaluated especially when considering sand concentration values from field campaigns performed with different hydro-sedimentological conditions that are likely characterized with different contents of clay-silt fractions.
Side-looking deployment usually extends the acoustic beams to the ranging distance for a complete signal attenuation, at the same time horizontal alignments at a given level in a river cross section are likely characterized with a homogenous concentration of suspended sediment. In this condition, the range length depends on sound dissipation and the variation of echo level profile may be correlated to clay-silt concentration change among time.
The instrument frequency may be fixed to boost the backscattering strength from sand or the viscous attenuation coefficient due to clay-silt particles. ADCP typical frequencies span the range from 500 to 3000 kHz that means a particle diameter changing from about 900 to 150 μm for the wave number-particle radius product, x
, equal to unity, which indicates the particle size of maximum scatterng effectiveness among an actual PSD at a given frequency. On the other hand, the viscous attenuation is not as much sensitive to a frequency change as the scatter processes (i.e.
, backscattering strength and scattering attenuation coefficient); the viscous attenuation coefficient presents a maximum in the clay-silt range with a moderate shift towards fine particles for the higher frequency (see functions for mono-size suspension in Figure 8
Therefore, the using of a specific frequency may be driven by the expected contributes in Equation (1), eventually decoupling sand backscatter from clay-silt viscous attenuation. For example, this is the case of dominant backscattering strength from fine sand when using a 1200 kHz ADCP (D = 380 μm, for x = 1) in a large river with a relatively low contribution of viscous attenuation due to clay-silt concentration. On the contrary, using a low frequency may noticeably reduce the backscatter from sand, which would rise the weight of clay-silt viscous attenuation in Equation (1).
The dependence of Equation (1) on frequency is further elaborated in a multi-frequency approach [21
], which is based on backscattering strength dependence on x
in a transitional region between the Rayleigh and geometric scatter regions. In this region, characterized with x
close to unit, and for moderate attenuation due to suspended sediment, Equation (1) may be inverted to assess the mean size of scattering particles. For ADCP frequencies, this region is represented by fine-medium sand; therefore, this multi-frequency approach was used to characterize the mean size of suspended sand from a riverbed [13
]. It is worth noting that for sand fractions, the acoustic coefficients in Figure 8
are well predicted by mono-size functions. In other words, in conditions of dominant backscattering strength of a given concentration of sand particles, a variation in Equation (1) should only be related to sand particles mean size variation.
A priori information about the actual PSD which has to be investigated may be extremely useful to choose the most appropriate acoustic system and method; however, too detailed information may appear wasteful. This simplifies as consequence the inversion of Equation (1) as well.
Indeed, one relevant objective of using an ADCP for sediment transport assessment is to simplify operations in the field and laboratory, which are usually required to measure detailed and reliable PSDs, and, at the same time, measuring streamflow discharge. In this perspective, while isokinetic-physical sampling and laboratory analysis remain the most appropriate techniques, the ADCP may be applied to extrapolate among time and space.
On the other hand, aiming to solve the inverse problem for a variety of hydro-sedimentological conditions without information about the actual PSD, an acoustic system may be implemented to span a large set of frequencies and beam ranges. This would eventually enable the application of different inversion methods for the investigation of a wide range of particles spanning from clay to sand, although this appears beyond the ADCP capabilities.
Indeed, a reasonable objective for an ADCP application in rivers may be the quantification of sand and clay-silt contents, at the same time, with a decoupled backscattering strength-viscous attenuation approach, by using multiple frequencies, although this will require further research efforts. However, this will not produce a detailed PSD such as from laboratory analysis and from the LISST-SL, but an estimation of two wide classes that may reflect wash-load and suspended-load of bed material.
The LISST-SL already provides a detailed PSD within a limited time in the field/laboratory, although the sampling effort remains. Indeed, the measurement range is 2.07–350 μm with limitations on the actual concentrations: inaccuracies may rise from multiple light scatter and too low scatter, which fix low and high threshold concentrations, respectively [44
]. These limitations rise doubts on the opportunity of having detailed PSDs carefully profiled with punctual sampling by a LISST-SL rather than an estimation of sand and clay-silt concentrations along beams by applying an acoustic method. Further speculating on that, the two technologies (i.e.
, laser and acoustics) may be usefully integrated.
A need of calibrating an ADCP is given, strictly speaking, regarding the assessment of the instrumental parameter Kt in Equation (1). Although this should be retrieved by means of laboratory tests, in the instrument setting and from the manufacturing, where it is a common practice to calibrate Kt by solving the direct problem by measuring the actual PSDs and echo intensity levels at the same time in the field. Besides that, another common practice is to include unpredicted and not directly investigated contributes to Equation (1) in that calibration. This may be the case of a moderate contribution of clay-silt concentration when tracking sand by investigating the corresponding backscattering strength.