## 4 Results and Discussion
### Prediction of Suspended-Sediment Transport
#### 4.2.3 Studenska Brv, Maribor Measuring Site  
A case study of the suspended-sediment concentration in a selected River Drava profile, using indirect acoustic measurements (with an ADCP) and point sampling (direct method), is presented. Combining these methods is necessary to obtain a representative picture (temporal and spatial distribution) of sediment transport through the river profile. Figure 6: Q/c and Q/H (water level) relationships for Sturmov potok and Ptuj measuring sites. Figure 7: Flow rate, suspended-sediment concentration, and suspended-sediment annual balance B(t) at the Sturm monitoring station in 2005 and 2010. The measurement was performed on the profile of the River Drava in the area of Studenska brv in Maribor. The ADCP was operated from a bridge (Figure 2) with data collected and monitored in real time on a connected laptop. A wireless connection was established between the instrument and the laptop. Simultaneously, water samples were collected using a spot sampler (Figure 2) and later analysed in a laboratory. The measured velocity profile and the profiles where the water samples were collected are shown in Figure 8. The actual flow rate at the time of measurement was 287 m\({}^{3}\)/s. The streambed was characterized by uneven depths, with the greatest depth of 7.5 m reached near the left bank. There were two prominent points in the profile where the depth was shallowest (5.5 m), namely in the area of the bridge piers. These piers visibly influenced the dynamics of the water flow and caused significant sediment deposition at this location. The concentrations of suspended sediments in the collected samples were measured and are presented in the Table 2. The concentration was determined using a method in accordance with the Slovenian standard SIST EN 872:2005, which is further described in [11]. The backscatter signal and the concentration of suspended particles both depend on the attenuation of the particles, and conversely, the attenuation of the particles depends on the concentration of the suspended particles. Therefore, the concentration values must be optimized in an iterative process, which was performed automatically by the program VISEA-PDT [37]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **Depth [m]** & **Concentracion [mg/L]** \\ \hline V1-1 & 0.3 & 0.0090 \\ V1-2 & 0.9 & 0.0066 \\ V1-3 & 1.2 & 0.0116 \\ V1-4 & 1.4 & 0.0098 \\ V2-1 & 0.4 & 0.0052 \\ V2-2 & 1.2 & 0.0038 \\ V2-3 & 1.6 & 0.0038 \\ V2-4 & 1.9 & 0.0030 \\ V3-1 & 0.7 & 0.0022 \\ V3-2 & 2.1 & 0.0014 \\ V3-3 & 2.8 & 0.0020 \\ V3-4 & 3.3 & 0.0048 \\ V4-1 & 0.7 & 0.0020 \\ V4-2 & 2.1 & 0.0028 \\ V4-3 & 2.8 & 0.0034 \\ V4-4 & 3.3 & 0.0026 \\ V5-1 & 0.4 & 0.0104 \\ V5-2 & 1.2 & 0.0060 \\ V5-3 & 1.6 & 0.0066 \\ V5-4 & 1.9 & 0.0026 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Suspended-sediment concentration \(\epsilon\) in collected samples according to SIST EN 872:2005. Figure 8: Sediment concentration in mg/L showing sample collection sites and iteration with calculation of coefficients A and B in Equation (1) and velocity profile at Studenska brv measuring site. The relationship between the absolute value of the backscatter signal and the concentration of suspended sediments is given by the expression:  
\[10\ \mathrm{log}\ \mathrm{(C)}=\mathrm{A}\ \mathrm{I}+\mathrm{B}, \tag{2}\]  
C is the concentration of suspended sediments in mg/L, I represents the measured absolute backscatter in dB, and A and B are constants resulting from the measured values of the sediment concentration of the samples. The actual value of the concentration of suspended sediments was determined in an iterative substep, since both the strength of the sound signal reflection and the concentration of suspended sediments depend on the attenuating particles. The loss of sound energy due to absorption by particles and scattering in the water was taken into account.