ANNALS OF DUNAREA DE JOS UNIVERSITY OF GALATI [311710]

ANNALS OF “DUNAREA DE JOS” [anonimizat], [anonimizat] X (XLI) 2018, No. 2

A review of bathymetric measurements from August 2018 campaign on the lower course of Danube River

Arseni Maxim1,*, Rosu Adrian1, Iticescu Catalina1, Georgescu Puiu Lucian1, Timofti Mihaela1, Violeta Pintilie1, Calmuc Madalina1, Roman Octavian2

1[anonimizat], “Dunarea de Jos” University of Galati, 111, [anonimizat]-800201, Galati, Romania

2[anonimizat], and Engineering (Cahul, the Republic of Moldova), “Dunarea de Jos” University of Galati, 111, [anonimizat]-800201, Galati, Romania,

* Corresponding author: [anonimizat]

Abstract

The presented paper shows a combination of different methods and techniques for high precision depth measurements. The results show the procedures for collecting field data from the bathymetric measurement campaign of August 2018, developed along the lower course of the Danube. Measurements were made over a total distance of 20 km. The main purpose of the measurements is to create bathymetric maps and to generate depth maps. [anonimizat], [anonimizat] 70 coordinate system. Repeated measurements in different quarters of the year will determine the morphometric changes in time of the riverbed bed on this study area. Furthermore, [anonimizat], it can be created a [anonimizat], like 1D/2D or 3D hydrodynamic modeling and flood inundation mapping.

Keywords: [anonimizat], riverbed 3D model, flood mapping

Acknowledgement: This work was supported by the project "[anonimizat]" acronym "DANS" financed by the Romanian Ministry of Research and Innovation.

1. Introduction

Nowadays the details of the bottom of a river or its detailed cartography play an important role in carrying out scientific research in the field of hydrology and hydrodynamics. In order to carry out detailed mapping of the bottom of a river, it is necessary to make bathymetric measurements. Bathymetry is a [anonimizat], lakes or canals. Bathymetric maps are made using depth measurements and it shows the exact shape and elevation characteristic of the ground below the water level. Different methods and equipment are used to measure depths below water. Initially, the bathymetric measurements were made by a [anonimizat] a [anonimizat] a counterweight was attached [1]. [anonimizat], it was determined the real depth at that point. [anonimizat], but with very low accuracy, a high cost of work and a very long time of the procedure.

During the time the progress of technologies was conducted to another type of bathymetric measurements and it is made with modern instruments that have a very high measurement accuracy. Modern bathymetric measurements are performed with the help of a single beam (SBES) or multibeam (MBES) echosounder. The basic principle consists in emitting a sound pulse from the transmitter up to the riverbed (in other cases to the ocean or sea bottom), and by determining the time of its emission – reception path it can be calculated the depth below the water level [2]. The depth value depending directly on the emission-reception time of the pulse.

Using this type of water depth measurements (SBES or MBES), what is depending on the purpose of the project and the necessary accuracy, many interesting discoveries have been made over time. At ocean and sea levels were discovered submarines or sunken ships [3]. At the level of the rivers, we can determine different types of habitats with MBES type of measurements [4, 5]. Other uses in river case are to determine the movement of the riverbed and the morphological differences during the time [6]. Another application of water depth measurements is the creation of 3D models of underwater terrain. The 3D model of the minor riverbed is essential for achieving hydrodynamic modeling and determination the risk areas that can be flooded [7].

The main purpose of this paper is a review of the bathymetric measurements on the lower Danube section. During the August 2018 measurements campaign, different data related to the depth of water, its velocity rate in various cross-sections of the river, the water temperature at the time of the measurements (to apply the measurement corrections) and the flow rate in these sections, was recorded Based on all these values and on the upcoming campaigns, will be analysed the spatiotemporal changes of the riverbed morphology, as well as the observation of the change of the flow rate and velocity of water that depending on the water level. Also, it can be mentioned that these determinations are very important by transposed them into a validated bathymetric model, and together with land surveying and photogrammetric measurements, this can be the basis of different hydrodynamic simulations in order to obtain flood risk and hazard map in the study area.

The study area is located on the Lower Danube River between Km 159 and Mm 73 (Fig. 1). This is an important area for ​​monitoring the river bed morphology because this section of river has deep erosion processes at a large scale. The given sector represent a local interest place, because it is the single river crossing point, from one bank to the other, and connects the Galati County with the Vrancea County.

Fig. 1. Study area map and path of bathymetric measurements

2. MATERIALS AND METHODS

The depth measurement was carried out with a 9-beam acoustic profiler Sontek Hydrosurveyor M9. This type of system is designed to collect bathymetric, water column velocity profile, and acoustic bottom tracking data as part of a hydrographic survey. The total discharge through a measurement section is computed based on the mean water velocity in the water column and the cross-sectional area. For the purposes of a measurement, the section is broken into three key components: the Start Edge, the Transect and the End Edge. These components are summed together o calculate the total discharge as shown in figure 2-a.

3. Results and discusSion

To generate a continuous area of the bottom of the measured section it approximates the cell values in areas where data doesn’t exist. The analysis of quality of record conducted to the generation of a bathymetric depth maps using approx. 150000 records (Fig. 3). The maximum depth records were 34 m, and the minimum 0.3 m (Fig. 4).

Fig. 3. Depth records quality histogram

Fig. 4. Bathymetric depth map

Figure 4 represents a bathymetric map. It is a representation of the ground below the water level or the equivalent of the topography of the riverbed bed terrain. The digital terrain model (DTM) is a widely used product and provides a three-dimensional representation (X, Y, Z) of the studied terrain areas. In this research paper, the term DTM can be defined as "a regular matrix representation of continuous variations of space relief units" [8].

Following the collection of topo-bathymetric data and validation from the precision and quality point of view, a digital model was generated using the Topo to Raster interpolation method, with the help of the 3D Analyst Tools extension of ArcGIS geographic information program. The Topo to Raster method is a very accurate interpolation method, specifically designed to generate digital terrain models for hydrological analysis of the studied field [9]. This method is based on the ANUDEM program developed by Hutchinson (1988, 1989, 1996, 2000, 2011).

Compared to other interpolation methods like IDW, RBF or Kriging like is described by Arseni et al. (2017), the Topo to Raster interpolation method has a much smoother and flatter graphic representation, with lower sinuosity elements.

The measured cross-section has a total distance of 561 m (Fig. 5). Figure 6 represents the differences between the Sontek M9 measured cross-section profile and the interpolated cross-section profile obtained by bathymetric measurements with HydroSurveyor program. If we analyze statistically the value obtained by bathymetric determination and the values extracted from raster obtained by TopoToRaster interpolation it can be observed that the coefficient of determination tends to close up to 1 (Fig. 6).

To determine the discharge on this section of Danube river, it was measured a cross-section in two ways: from left to right bank (Fig. 8), and backward (Fig. 9). The total discharge value is important for another type of simulation, like validation of hydrodynamic models, in order to obtain the extent of the flooded area.

Fig. 5. Measured vs Interpolated cross-section profile

Fig. 6. Linear regression representation and coefficient of determination between measured and raster value depth

4. conclusions

The depth measurement and total discharge depend on used instruments and its precision. The site selection is a critical part of a discharge measurement and is fundamental to its success. It needs to avoid possible obstructions and sites immediately downstream of bridges, gates, and weirs. Flow should be uniform with minimal turbulence. During the edge measurements, is needing to keep the vessel as stationary as possible. During the transect, is needing to maintain a constant vessel speed and direction, to obtain the same discharge flow from the left or right side. Ideally, any vessel movement should be slow relative to the water velocity, and changes in heading should be gradual and only when necessary.

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