MORPHOMETRIC CHARACTERISTICS OF THE HYDROGRAPHIC NETWORK FROM CULA RIVER BASIN, REPUBLIC OF MOLDOVA. Abstract

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1 Department of Geography. Valahia University of Targoviste Annals of Valahia University of Targoviste. Geographical Series Tome 14/2014 Issues 1: MORPHOMETRIC CHARACTERISTICS OF THE HYDROGRAPHIC NETWORK FROM CULA RIVER BASIN, REPUBLIC OF MOLDOVA Viorica ANGHELUȚA Institute of Ecology and Geography, Academy of Sciences of Moldova, 1 Academiei Street, MD-2028, Chisinau, Republic of Moldova, (373 22) ieg@asm.md, vioricaangheluta@gmail.com Abstract The paper presents the main morphometric characteristics of the Cula River basin according to the Horton Strahler classification. The drainage model is obtained as the result of a combined analysis of the laws of the number of stream segments of successively higher order, of their length summed and their average lengths. The analysis of obtained model and of basin morphometry elements have contributed to given basin evolution stage establishment. Keywords: morphometric pattern, hydrographic network, geometric progression, slope orientation 1. INTRODUCTION The progress of modern society in the context of sustainable development requires, along with many others, a detailed knowledge of the natural and physical environment, as support for social and human activities. In this context, must be mentioned the efforts of scientific researchers in finding and quantification of a number of detailed parameters, in the basis of which is performed comparing these parameters within the hydrographic basin, them being regarded as basic units in all management actions of water resources (Zăvoianu et al., 2012). For the verification of morphometric characterization methodology of hydrographic network Cula River basin was chosen. The final aim of this study is finding the relationship between the agents of action, in this case water, and the landforms generated by it along time evolution, so that it can "decode" the information stored in created landforms. In the Cula River basin, under the long action of flowing waters as specific forms of relief were created, such as: floodplains, terraces, leveling surfaces. Analysis of the hydrographic network and its characteristics is necessary, because it is an active and dynamic factor in the exchange of substance and energy. This flow of matter and energy, both in small basins and in the large contributes to the representation of the hydrographic network with its morphometric characteristics, which can be, in turn, determined, measured or calculated. Thus, knowledge of these characteristics represents an important step in the study of the physical components of the environment, as support for social and human activities (Zăvoianu et al., 2012). Cula hydrographical basin is at the heart of the Republic of Moldova, is a component part of Northern Codrii physical-geographical region from Codrii Plateau region. Cula River, right tributary of the Raut River, which then flows into the Nistru River, carries its waters on the territory of three districts: Ungheni, Calarasi and Orhei. 15

2 2. MATERIALS AND METHODS For the determination of morphometric characteristics of the Cula basin hydrographic network, information from 1:25000 scale topographic maps (1969) was used, which I deemed it satisfying for the degree of details that it provides. Morphometric characteristics for the entire basin have been obtained through succession of a series of steps, as follows: Digitization of hydrographic network (vector line type, which contains in the attribute table information about: hydronym in Horton - Strahler system of classification, the length of the river segments), then was checked that the vector does not contain errors of type: junctions, overlaps, etc. Classification of hydrographic network was always in the attention of researchers, which, depending the aim pursued, were taken as the basic criteria its configuration in the plane, or a number of dimensional elements (basin surface, courses length, depth, direction, river flow, etc.) (Gravelius, Horton, Panov, Strahler, Shreve, Scheidegger etc.) (Zăvoianu, 1978). The diversity of physical, geographical and geological conditions (rock, the tectonic, structure, etc.) are essential factors in defining of the hydrographic network plan configuration, but they are not sufficient to give a genetic basis for the classification of the rivers network. Because Horton - Strahler classification system is the best tested system, it was adopted as a basic system in what follows, it constitute a very good appreciation means of the order of any course, gives the possibility of performing comparative studies and statistical processing of data on classes of values (Zăvoianu et al., 2012). This system explains that the order of size of a river increases when it intersects another river of the same order. Thus, all water courses without tributaries receive value 1, and when the two 1st order rivers join together, resulting hydrographic network will receive order 2. Order of size of hydrographic network will increase only when two rivers of the same order join together, otherwise, when joining an inferior order river with one higher, the order will be picked up from higher order river (Zăvoianu, 1978). Then, there were analyzed the different morphometric elements of the basin and hydrographic network, because they have an important role in the formation and distribution of river flow elements, also, are used in the development and practical application of calculation methods of the hydrological parameters. 3. RESULTS AND DISCUSSION 3.1. Basin morphometry Basin and river network morphometry plays an important role in forming and distribution of river flow elements, being used in the development and practical application of the calculation methods of hydrological parameters. For this reason, was considered necessary to determine (based on topographic maps, 1:25000 scale in digital format) of the main morphometric elements of the basin (area, altitude, length, shape, mean and maximum width, average slope) and of the river system (hydrographic network density, length of river, riverbed slope, coefficient of sinuosity, coefficient of unplaite) (Neculau, 2010). Among the morphometric elements of hydrographic basin, the area is the most important morphometric index being commonly used in the hydrological practice due to role in forming of the water flow. The size of the basin surface has an important role in the manifestation and evolution of hydrological phenomena. In a small basin, such as Cula, evolution of the quantities of water into river corresponds to the precipitation regime. A rain is felt, immediately, in the growth of the river water, and a relatively short period of drought shall result in their decrease (Zăvoianu, 1978). 16

3 Table 1. Hydrographic basin area and perimeter Basin Area (km²) Perimeter (km) Cula 537,2 146,1 Cula River basin perimeter is km, including km forms the right interfluve and km the left interfluve. The average elevation of river basin has a great influence on the hydrological processes (in particular by the role that has in the distribution of climatic parameters, which are then reflected on the characteristics of hydrological elements) (Chirilă, 2010). In the case of basins with a small area, average altitude has important hydrological significance determines, in general, both the amount of matter and energy received per unit area and intensity transfer in the basin. Cula River Basin presents an average altitude of m., maximum altitude of 380 m. and situated in the south-west of the basin. Figure 1. Numerical model of land, Cula River basin Table 2. Average and maximum altitude of basin Basin Average altitude (m) Maximum altitude (m) Minimum altitude (m) Cula 146, The maximum height of the Cula river basin not always recorded near maximum distance of confluence because, in time, rivers fragmented the lines of maximum height through regressive erosion, so that at present, the maximum heights are associated with hills, disposed in a direction perpendicular to the outflow direction of actual rivers. The length of the hydrographic basin is used in the dimensional characterization of basin and in the definition of its shape. For the determination of this parameter in the literature are indicated different methods with a higher or lesser degree of objectivity. In hydrological practice is frequently used the maximum length of the basin (L. in km). The width of the hydrographic basin, as length, represents a morphometric parameter used in defining the shape of the basin, can be considered a maximum and average width. The maximum width (l max in km) is determined on map as the straight line connecting two distant points in the 17

4 basin and it is perpendicular on length. The average width (lav in km) is obtained by reporting the basin surface (F in km 2 ) to its length (L in km): lav = F/L (4). Table 3. Maximum length, maximum and average width of basin Basin Maximum length (km) Maximum width (km) Average width (km) Cula 55,6 14,2 9,6 Stretching in length and width provides to hydrographic basins different forms: elongated, flattened, circular (fan-shaped) etc. Form of catchment basins presents importance due to the influence which it exerts on the timings of concentration of the waters at Collector River. In the basins developed especially in width, the high floods are formed and transmitted more quickly, having a erosive and transportation force greater than those produced in the elongated basins (others natural conditions being similar). Determining the shape of a basin can be achieved by qualitative appreciating (observation of cartographic representation of the basin) or using quantitative indices. Determination of quantitative indicators, which expresses hydrographic basins form was in the attention of several specialists, so that, at present, exist the possibility of calculating of such indices through multiple relations, that uses three morphometric parameters of the basin: area, length and perimeter. From these relations (referred of Zăvoianu, 1978), include: the form factor (proposed by R.E. Horton): Ff = F/L2, where: F = hydrographic basin area (in sq.km); L = hydrographic basin length (in km). ratio of circularity (introduced by V.C. Miller): Rc = 4F/P2, where: F = hydrographic basin area (in sq.km); P = hydrographic basin perimeter (in km). The form factor and ratio of circularity are below unit indicators with values becoming smaller degree of elongation of the basin increases (Pişota, 2003). ratio of form proposed by Horton, which has the square as the reference figure and it calculated using the formula: Rf = F / (P / 4) ², where: F = hydrographic basin area (in sq.km); P = hydrographic basin perimeter (in km). According to the values of ratio of form, Rf, hydrographic basins can be: elongate, with Rf <1; squares, if Rf =1; round, with Rf values between 1 and 1,274 (Zăvoianu, 1978). Table 4. The form factor, ratio of circularity and ratio of form Basin Ff Rc Rf Cula 0,17 0,1 0,4 The degree of asymmetry offers a synthetic image on the way in which the basin surface is distributed to the drainage major axis. It can be appreciated qualitatively, by observation of cartographic representation of the basin and quantitatively, by determining the asymmetry coefficient (Kas) (Pişota, 2003). 18

5 Figure 2. The hydrographic network of the Cula basin, classified by Strahler The asymmetry coefficient is calculated based on the relation Kas=2(Flf Frg)/F, where Flf and Frg are adjacent surfaces on the left and right side, respectively, of the main course, and F - hydrographic basin area. Table 5. Asymmetry coefficient of the hydrographic basin Basin Frg (sq.km) Flf (sq.km) Kas Cula According to this coefficient (Table 5), Cula river basin is asymmetrically developed on the left side, however, when the surfaces of the two sides are approximately equal, Kas tends to zero and the basin is considered symmetrical. Average slope of the hydrographic basin determines time which water, derived from precipitation, traverse until it reaches at Collector River. It is part of the relief with the greatest influence on the maximum flow, because represents the surface of the slope flood genesis. The high values represent favorable conditions for a rapid water drainage, for short water time of concentration, for the pronounced erosion and transport of solid material appropriately. The intensity of these phenomena is, certainly, conditioned by vegetation, soils and other. Table 6. Average and maximum slope of the hydrographic basin Basin Average slope (º) Maximum slope (º) Cula 6,0 18,78 Slope orientation is an indicator of the appreciation of the agents meteorological action, but he is also correlated with the land use, especially, in the hills, where anthropic pressure is increased. It is observed that the largest share of the slopes orientation is determined by the general sense of relief inclination: from northwest to southeast. The slopes oriented to the north, east and southwest, have same percentage. The slopes oriented to southeast, south and northwest occupy, together, about 40% of the total area (Cristuţiu, 2012). 19

6 Figure 3. Share of types of slopes according to exhibition in the Cula river basin. The tortuosity of rivers or their meanders was evidenced by tortuosity coefficient values (Kt). Noting with Ld length in a straight line of a watercourse and with Ls its sinuous length, tortuosity coefficient Kt can be calculated using the formula: Ks= Ls/ Ld (Cristuţiu, 2012). In general, courses are straight, slightly meandering. The highest value Cula river holds, and the lowest value is recorded on left tributary of the Cula river Bagu river. Table 7. Tortuosity coefficient of the main river courses Basin Culișoara Hirișauca Bagu Tortuosity Cula (left tributary) (left tributary) (left tributary) coefficient Ks 1,14 1,07 1,07 1,06 Hydrographic network density is an important quantitative index that characterizes, at the same time, the basin and the hydrographic network. Is denoted, symbolically, with D and expressed in km/sq.km, it is the ratio of the total length of permanent watercourses (ΣL) on a territory (often a hydrographical basin) and its surface (F): D = ΣL / F. Hydrographic network density determination method on basins was initiated by the German geographer Neumann, in 1900 and may be applicable for different orders basins. Value of this index reflects, of the one part, fragmentation degree of the relief, and on the other part, offers the possibility of assessing of water resources in a certain area, and allows the identification of flow concentration area (Pişota, 2003). Hydrographic network density influences the torrential regime of the hydrographic basin. A dense, permanent and periodical river network make that high waters and flood regime to tend for one torrential, while a hydrographic basin with a low density of the hydrographic network tends to an uniform flow regime. Starting from the density of the hydrographic network flow slope length was determined Lsp using expression proposed by Horton: Lsp = 1/2*D. Table 8. Hydrographic network density Basin D (km/sq.km) Lsp (km) Cula ,73 20

7 3.2. The hydrographic network morphometry For Cula River basin was achieved hierarchy of network valleys in Horton - Strahler system from 1:25000 scale topographic maps. Order of size of the hydrographic basin for Cula River basin is the superior order of size, that has river before shedding, meaning 6 and is achieved from the confluence with Culisoara river, this is a specific value of the hydrographic basins in that area (ex: Ciuluc, Copăceanca, Cubolta). Analyzing the hierarchy river network map from Cula River basin is observed that it presents tributaries on the left longer than on the right side; Cula River basin is more developed on the left side. Order of 5 size is represented by two segments with a length of 35, 38 km; order of 4 size is represented by 8 segments with a length of 39, 59 km; order of 3 size is represented by 41 segments, order of 2 size is represented by 178 segments and order of the first size is represented by 538 segments. The lengths of the segments of the first order 1 (L1) sums up only 48.9% (Fig. 5), although the number of segments in this order (N1) is 70%. Figure 4. Share of river segments and summed length in Cula river basin The drainage model analysis was achieved for the river segments of order 1-6, to determine the stage of development of the basin. For this was calculated the number of river segments, length of river segments and their average length (table 9). Table 9. The data for morphometric model of drainage Parameter Order The Sum of progression terms of ratio progression Number of m RC=3.422 ΣN=760 segments c Length of m RL=1.941 ΣL= segments c Average m rl= Σl= length of c ΣL/ΣN= segments

8 Analyzing the data in the table is observed that the number of river segments decreases once with increase the order of size and their average length increases with the order of size. This is achieved by three laws: the law of the number of river segments, the law of summed lengths and the law of average length. The law of the number of river segments refers to the fact that the number of segments of river of successive order of a river basin, tends to form a reverse geometric progression, where the first term (N1) is given by the number of courses of first order and the ratio is the confluence ratio (Rc) (Horton, 5). The confluence ratio (Rc) indicates how many inferior courses to form a course of higher order in the physical-geographical conditions, and was calculated as: Rc = Σ(Rci *Pi) / Σ Pi, where Rci = Ni/Ni+1, and Pi =Ni+Ni+1. Analysis of the partial confluence ratios values revealed the fact that Cula river basin behaves as an evolving basin, continuously subjected to fragmentation. The first term of the string, starting from the available general formula in a decreasing geometric progression, was calculated as: Ns = N1/Rc s-1. In the case of Cula basin s=6, so N6 = 538 / = For the calculation the other terms of progression was used the formula: Ni = Ns*Rc s-1. The calculated values are: 538; 158; 46; 13, 4; 1. ΣNc was calculated using the decreasing geometric progression property which imposes that the sum of terms is given by formula: ΣN = Ns*(1-Rc s ) / (1-Rc). The obtained value is: ΣN = 760 a value very similar to that obtained by direct counting. The real order size of the basin was calculated using the formula: s = 1 + (log N1 - log Ns) / log Rc. According to the obtained value, the real order size of the Cula basin is 6. The second law of the drainage pattern is the law of summed lengths which proves that the amounts length of successive orders river segments tend to form a decreasing geometric progression, whose first term is given by the total length of the courses in the order 1 (L1), and the ration is the ratios of the lengths (RL) (5). Total length ratio (RL) shows how many times smaller length of is a course a certain order towards the length of the courses which belong to the immediate superior order, and was calculated as a weighted average of the partial ratios: RL = Σ(RLi *Pi) / Σ Pi, where RLi = Ni/Ni+1; and Pi = Li+Li+1. The first term of the string was calculated as: Ls = L1/RL s-1. In the case of Cula basin s=6, respectively L6 = 384,81 / = 13,96. This value represents the length of the main course. The other terms of progression were calculated using the formula: Li = Ls*RL s-1. The total length of rivers network was calculated using the formula: ΣL = Ls*(1-RL s ) / (1-RL). ΣL = 13,96*(1-1,941 5 ) / (1-1,941) = 778,27 km. According to the law of the number of river segments and to the law of summed length it has been found that reporting these strings results in a new string li = Li/Ni (where i = 1,..., s which is the highest order of the basin), which is also a geometric progression, but this time is increasing. New data string obtained determines the third law, the law of average length, which formulated as: the average lengths of the successive order of rivers sectors in one basin, tend to form an increasing geometric progression, whose first term is the average length of the courses of the first order (l1) and ration is given by the ratio of the average length (rl) (Horton, 5). The ratio of progression rl was determined as the ratio: Rc/RL. rl = 3,422/1,941 = 1,763. The new terms of progression were determined according to the expression: 22

9 l1; l1*rl; l1*rl 2 ; l1*rl 3... l1*rl s-1. 0,715; 1,26; 2,21; 3,91; 6,90;12,17. Graphical representation of functions N (i), L (i) and l (s), where i = 1, 2,..., s was done in semi-logarithmic coordinates: on the x axis was carried on to orders of magnitude i, and on the y logarithmic axis has passed measured values of the number of segments of the river (N), length (L), average length of river segments (l). Points on the graph represents measured values and the lines that define each progression in part have been traced that pass through as many measured values. The drainage model (fig. 6) reveals the existence of qualitative and quantitative determinations in the hydrographic basin. First, the tip of the triangle, obtained at the intersection of graphs L (i) and l(i) represent the real order of size of the basin, namely, these two equations have common roots in the given point. Also, analysis of drainage model reflects the fact that rations are closely linked: 1<RL<Rc; 1<rl<Rc (Marinescu, 2006), so 1<1,941<3,422 and 1<1,763<3,422. CONCLUSIONS Cula River basin is of the order 6 (calculated 6.00), with elongated shape, asymmetrically developed on the left. The basin area is 537,2 sq.km, and the average density of the drainage network is 1.46 km / sq.km. From the analysis of morphometric model determined for Cula River basin can be concluded that the morphometric laws that define the drainage allow calculation of the indicators which can be used to characterize the situation in basin. From the analysis of these indicators observe that there is, in some cases, some less significant differences between the measured and calculated parameter values. The drainage morphometric model underlines the fact that, this basin has not reached the stage of maturity and it is in evolution, continuously subjected to fragmentation. Figure 6. The morphometric model of drainage for Cula river basin 23

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