## 3 Materials and Methods
### Monitoring of the Transport of Suspended Sediment in the River Drava in Slovenia
#### 3.1.2 Acoustic Method  
Acoustic methods in hydrology were developed primarily for measuring the velocity and discharge of watercourses [33; 34]. However, we can use the backscatter and attenuation of acoustic signals in water to monitor suspended sediment in water bodies. A calibration within situ measurements is necessary to determine the concentration of suspended sediment from the backscatter. Acoustic backscatter depends on the concentration and size of the scatterers and is also impacted by attenuation due to suspended matter along the water column [35; 36; 31]. The most commonly used device for measuring water velocity and indirectly further parameters is the Acoustic Doppler Current Profiler (Figure 2). All results presented in this article were collected with a 1200 kHz Teledyne RDI WorkHorse Rio Grande. In this ADCP model, five different frequencies (75-1200 kHz) are possible, which differ in sound absorption (sound absorption increases with frequency). The measuring device working with this frequency can be used in rivers with a minimum depth of 0.4 m to 15 m and velocities up to 10 m/s, which are also the conditions at the selected measuring points on the River Drava. By crossing the stream, the water depth and velocity can be measured. With a computer connected to the ADCP via a radio link, all data were recorded and collected in real time. The parameters such as discharge, cross-sectional area, and average velocity are known immediately after the crossing. Based on the backscattered signal, the information about the concentration of suspended sediment can be calculated. The conversion of the backscattered signal into a concentration of suspended sediments is possible with another piece of conversion software, e.g., Aqua Vision, VISEA-PDT 4.0 (PDT--Plume Detection Toolbox), which allows a direct conversion of the measured data into a concentration of suspended sediments. In addition, the software allows a calibration of the measured backscatter with actual values of concentration of suspended sediments obtained with the direct methods. The theoretical background of the backscatter measured with ADCP is represented by the acoustic equation which links the discrepancy between the received and transmitted sound energy and the lost energy during the propagation of the sound wave [37]:  
\[\mathrm{S_{v}=C+log10\left[T_{T}R^{2}/L\;P_{T}\right]+2\;\alpha R+K_{C}\;(E-E_{r})} \tag{1}\]  
The equation for acoustic backscatter in dB (S\({}_{v}\)) is determined by several factors, including a constant value (C) in dB, the temperature (T\({}_{T}\)) of the Acoustic Doppler Current Profiler (ADCP) transducer in \({}^{\circ}\)C, the slant range (R) along the beam to the scatterers in m, the transmit pulse length (L) in m, the transmit power (P\({}_{T}\)) in W, the attenuation coefficient (\(\alpha\)) in dB/m, a scale factor (K\({}_{C}\)) in dB/count, the relative backscatter (E) equal to echo intensity in the count, and the received noise (E\({}_{r}\)) in the count. The values of C, TT, R, and PT are either recorded with the ADCP or provided by the factory. The value of E is derived from the Received Signal Strength Indicator of the receivers, and Er is the noise value when no signal is present. The typical value of Er is 40 counts, and the values of factor KC range from 0.35 to 0.55 dB/count. The attenuation coefficient \(\alpha\) is the sum of the water absorption \(\alpha_{w}\) and particle attenuation \(\alpha_{s}\), which are obtained based on empirical formulas that can be found in the literature [37]. The procedure of conversion of backscatter into a sediment concentration can be summarized in three steps: the conversion of backscatter into dB, the standardization of the instrument, the standardization of the area. The most important part of the measurement is the calibration with real values of sediment concentration.