# Estimation of Changes in Sediment Transport along the Free-Flowing Middle Danube River Reach
## 3 Materials and Methods
### Bathymetric Data Collection  
Bathymetric data that were used as a basis for the estimation of the sediment transport were obtained from the databases of the Directorate for Inland Waterways--Plovput in Serbia, the former Environmental and Water Management Research Institute (VITUKI), and the Lower-Danube-Valley Water Directorate (LDVWD) in Hungary. These included 23 surveillance cross-sections in Hungary at an average distance of approximately 2000 m and 46 surveillance cross-sections in Serbia at an average distance of approximately 1000 m. All cross-sections were distributed along the sand bed reach, spanning from Baja (rkm 1479 + 000) to the proximity of the confluence of the Drava and Danube Rivers (rkm 1387 + 200) (Figure 2).  
Plovput is the state institution responsible for hydrographic surveys on international inland navigable waterways in Serbia. Plovput's typical cross-section single-beam surveying equipment includes the following:  
* 0.1% depth;
* DGPTS-RTK positioning with a measurement precision of +/ -20 cm;
* Data-processing and post-processing applications.  
VITUKI was a coordinator for hydrographic surveys on the Danube River until 2012, and the LDVWD is in charge of river management and navigation routes on the Danube reach downstream of rkm 1560 + 000. The hydrographic survey equipment of VITUKI included a single-beam ultrasonic echosounder with an accuracy of 2 cm and a total station on the shore. The accuracy of the total station was less than 2 mm at 200 m in the robotic survey mode. The equipment of the LDVWD consisted of a single-beam ultrasonic echosounder and an RTK GNSS receiver. The accuracy of both instruments was 2 cm.  
The echosounder used to survey cross-sections in Serbia was mounted on a 5 m long boat, specifically equipped for this activity. All collected data (which contained the x, y, and z coordinates of the measured points) are in the State Geographical Coordinate System. Before the commencement of a survey, the coordinates of boundary points were entered into the software. Survey lines ran along predefined cross-sections. Data on location and depth (x, y, z) were transferred to the software for hydrographic surveys. The software synchronized the data constantly. Thus, the boat position was supplied in real time. The speed of sound in water was entered according to the information provided by an SVP (sound velocity profiler) device. A differential GPS station was mounted at the reference point with known geographic coordinates. A station was connected to the boat with the radio signal, sending information on differential correction and providing the required accuracy for the performed survey. Depth information was obtained by using the time necessary for ultrasound waves to travel from the echosounder to the river bottom and back. Data on depth and location were synchronized in real time, and information was stored in ASCII format in the form of x, y (position), and z (depth) coordinates. After completing the survey, quality control was performed, spikes were removed, and data were stored in the database with cleaned x, y, and z coordinates for each cross-section.  
VITUKI surveyed cross-sections with a special motor boat. The total station on the occupied stance on the shore followed the active signal, coming from the echosounder beacon-receiver placed on the same rod with fixed height and without horizontal offset. The processing result was a 3D point (x, y, and z coordinate) list.  
The LDVWD surveyed the bed with satellite positioning (RTK GNSS) and single-beam ultrasonic depth measurement equipment based on a specific cross-section plan prepared in advance. To convert the satellite positioning coordinates into a Cartesian spatial coordinate system, original positioning in the global system and the officially accepted Field Transformation Procedure were used.