2.1. Study area description
The studied area, the Topla River basin, is located in eastern Slovakia in the district of Bardejov and it falls within the main catchment area of the Bodrog River. Its most important and therefore the main stream is the Topla River, which a right-bank tributary of the Ondava River. This important basin drains a large area with an area of approximately 1,595 km2. Due to its type, it is classified as a highland-lowland river with a length of 129.8 km. The upper part of the Topla River springs in the Cergov Mountains. Its river path (Fig. 1) points to the east and below the territory of Bardejov, it turns and heads to its lower part to the south. At this point it flows as a right-bank tributary to the Ondava River at a river km 34.2, below the village Sacurov in the district of Vranov. The flow of the upper part is navigable from Bardejov in a length of about 103 km and in this part it flows turbulently through a wide valley.
Under the village of Marhan in the Bardejov district, the Topla River calms down, the slope and velocity are lost and the flow is laminar. A calm stream, in places with silences and tones, flows smoothly to its mouth at the bottom of the Topla River (Lechman 2019).
2.2. Methodology to determine morphological parameters of the river basin
To determine the influence of hydro-morphometric characteristics of partial basin to the territory of the upper part of the Topla River, the followed hydro-morphometric characteristics were studied. The first group of factors included the catchment area, the flow length, the longitudinal slope and the forest cover. The second group contained parameters that combine several environmental factors, such as an indicator of the shape and density of the river network (Dobija et al. 1975; Allairea et al. 2015; Vogel 2011; Kimaszewskli 1978; Thorne 1998).
The essence is to determine the parameters of the river basin using numerical characteristics, which can then serve to study the relationship between river basin and type, intensity, fluvial processes or by comparison between different systems.
Catchment area – is an area of the plan view of the basin. In small river basins, the response to excessively heavy rainfall will depend to a large extent on climatic conditions, environmental factors as well as geological structure, land use, etc. Large river basins have a much greater potential to sustain extreme rainfall, but dynamics of water runoff from the area is much more complicated.
Flow length - is expressed as the length of the axis of the riverbed from its mouth to the source, therefore the beginning is the intersection of the centers of the main river and the tributary. The path of length is the line connecting points lying at the same distance from the shores. The length of the flow is most accurately determined by direct measurements and this is combined with height measurements, then we most often display them graphically, namely using longitudinal profiles or also using the river scheme system. The length of the stream is one of the most important characteristics of the river basin and according to the significance of the rivers it also reaches a sufficiently high value.
Longitudinal slope - the slope, height and exposure of the slopes of river systems significantly affect the regime of rivers and water content and climatic conditions, rainfall distribution and air temperature of the river basin and its surroundings. It expresses height the difference of the start and end points of the investigated river basin flow to length of the riverbed and is expressed by equation (1).
\(\text{i}= \frac{{H}_{max}-{H}_{min}}{L}\) * 100 = \(\frac{\varDelta H}{L}\)*100 (1)
where: i is the slope [%]; Hmax is the highest point [m]; Hmin is the lowest point [m]; L - line connecting Hmax and Hmin [m].
By its nature, the longitudinal slope influences the erosive activity of the river. The greater the slope the more intense the detachment and removal of the topsoil.
Density of the river network and its arrangement - The River structure is directly connected with the relief and geological structure of the basin. Individual parts of the River system differ from each other by the so-called density of the River networkthat is expressed as the length of the flows per square kilometer of concerned areas.
The mathematical formulation is given by equation (2):
$$\rho =(L+{L}_{1 }+{\dots L}_{x })/A$$
2
where: ρ - river network density [-]; L - flow length [m]; L1 - flow length [m]; Lx - flow length [m]; A - catchment area [m2].
It follows from equation (2) that the representativeness of the result is determined by the type of used maps and the evaluation method. An example is the division of the map of interest by a square grid and the density is determined in each of these squares. Areas of equal values are defined and marked graphically.
The impact of the arrangement of the river network on the river basin is manifested only significantly after the flood. It is unfavorable when the duration of the flood wave on the main stream and on tributaries is approximately the same. After at the confluence, both flood waves meet and a significantly higher resulting flood wave is created. The encounter of flood waves occurs first on fan-shaped river networks.
Basin shape coefficient - is based on the arrangement of river system and is of considerable importance in the creation of flows. Mathematical formulation of this coefficientis expressed by the equation (3):
where: α - coefficient of shape [-]; L - flow length [m]; A - catchment area [m2].
Forest cover - In addition to the hydro-morphometric parameters described above, other climatic characteristics of the natural environment, which greatly influence the type, course and extent of morphogenetic processes, are often analyzed in studies of river systems. Among the characteristics influencing the structure and dynamics of the river basin, it is necessary to mention in particular the geological structure and land use, which are most often expressed by the forestry factor. It characterizes the extent of river basin cover with forests, wooded areas, grasslands, arable land and non-cultivated land. The value of the afforestation index can be calculated by equation (4):
$$\lambda = \frac{{A}_{L}}{A}$$
4
where: λ - forest coefficient [%]; AL - total forested area [m2]; A - catchment area [m2].
The methodological procedures for obtaining hydro-morphometric data in this study were divided into three parts. In the first part of the survey, preparations and analyzes of the collected source data were performed in a GIS environment, which were key to the output. The second part was the identification and completion of all necessary hydro-morphometric characteristics of the Upper Topla River. Characteristics that were not found in the literature or in the GIS file were calculated by equations (1) - (4).