## 4 Results and Discussion
### High-Water Event Dynamics of Suspended-Sediment Concentration  
The aim of this case study was to investigate the concentration of suspended sediment in the River Drava during a flood event. The detailed description of the measurement procedure and the results have already been published in [38], but in order to give a comprehensive picture of sediment transport in the Drava, some main conclusions are summarised below. During high-water events characterized by large flow rates, measuring sediment transport in rivers poses significant challenges due to hazardous conditions, large woody debris, and other floating objects on the river surface. It is a known fact that more than 50% of suspended sediment is transported during high-water events. Since such situations usually occur in very short periods of time, predicting them and collecting samples are very difficult if the system is not automated. However, when conditions are optimal, classical sampling methods are uneconomical, and errors in sampling and calculating the final concentration of sediment increase. Traditional measurement techniques such as Acoustic Doppler Current Profilers, hydrometric wings, and sediment samplers face considerable difficulties or are even impractical. Therefore, the introduction of invasive and automated methods for determining suspended-sediment concentration is crucial [2; 39; 40; 41; 42; 43]. As alternative measurement approaches or modifications to existing techniques, some novel technologies, such as remote sensing methods or automated sampling systems, may provide potential solutions by enabling measurements from a safe distance or offering enhanced instrument resilience to debris interference. The investigation of a high-water event occurred at the Duplek Bridge site, located between the cities of Maribor and Ptuj. At this site, the outflow channel is separated from the riverbed at the Melje Dam to supply the Zlatolice Hydropower Plant. The maximum capacity of the derivation channel is 500 m\({}^{3}\)/s, and when this capacity is exceeded, the extra water flows into the natural riverbed. This area's required ecological river discharge is 10 m\({}^{3}\)/s in winter and 20 m\({}^{3}\)/s in summer. In the autumn of 2018, a high-water event commenced on 29 October when the discharge of the River Drava surpassed 500 m\({}^{3}\)/s. The discharge within the natural riverbed began to escalate from 10 m\({}^{3}\)/s and reached a peak discharge of 1391 m\({}^{3}\)/s, lasting for a duration of three days. Discharge data were obtained from the Melje Dam's automated measuring station at the Dravske elektrarne site and were accessible online in real time. In order to determine the concentration of suspended sediments relative to discharge during the high-water event, water samples were collected manually using 1000 mL plastic bottles from the bridge. The sampling period continued until 4 November 2018, with only one sample per day collected during the last four days when the discharge was below 500 m\({}^{3}\)/s and decreased significantly. The collected samples were filtered through glass microfiber filters, and the concentration of suspended sediments was determined according to the standard SIST EN 872:2005 [44]. Filter trays were dried in an oven and weighed using an analytical balance (RADWAG AS 220.R2). The results of the suspended-sediment measurements during the high-water event in October 2018 are shown in Figure 9. The minimum discharge value was 92 m\({}^{3}\)/s on 5 November 2018, at 15:13, while the maximum was 1391 m\({}^{3}\)/s on 30 October 2018, at 18:33. The minimum concentration value was 105 mg/L on 5 November 2018, at 15:13, while the maximum was 2450 mg/L on 31 October 2018, at 00:28. The suspended-sediment concentrations were found to be different at rising water levels from those measured at the same water level at falling water levels.