جستجوی مقالات مرتبط با کلیدواژه « گیوی چای » در نشریات گروه « جغرافیا »

AbstractSoil erosion is one of the most important problems in the watersheds of Iran, which causes the loss of thousands of tons of arable soil every year. The aim of the present study is to zoning the risk of soil erosion in Givi Chay watershed (northwestern Iran). In this study, first, the effective factors for erosion in the region were identified and then the information layers of each criterion were prepared in Geographic Information System (GIS). Valuation and standardization of layers was done using fuzzy membership function and criteria weighting, using critic method. Final analysis and modeling was performed using the Multi-Attributive Border Approximation Area Comparison (MABAC) method as one of the Multi-Criteria Decision Making (MCDM) methods. According to the results of the study, slope, land use, soil and lithology had the highest weight coefficient, respectively. Also, the results of the study showed; 283.89 and 414.93 km-square of the area, respectively, has a very high and high risk potential, and very high-risk and high-risk areas in unstable and erodible formations, agricultural uses and gardens and slopes of 25-40 % are located. It can be said that the results of this study indicate the high potential of the study basin in terms of erosion occurrence and it is necessary to control erosion and conservation measures on the agenda of experts and land managers. In addition, the results of validation of the results showed that the use of MABAC method has a high relative accuracy for studying the risk of erosion.

River and river processes are considered as the most significant geomorphic systems which are active on the earth’s surface (Bag, 2019). Over time, many changes in the morphology and dynamics of the river system can occur. The effects of river adjustment caused by the natural factors require much longer time span to be revealed. However, there are few exceptions that the natural factors such as river floods, landslide or earthquake can induce channel adjustments in a very short time (Chaiwongsaen et al., 2019). On the contrary, human activities can have a significant and rapid impact on natural processes and trends, resulting in a compressed time scale for river adjustments (Rinaldi & Simon, 1998). Morphological responses may include subtle shifts in cross-sectional stream channel geometry or widespread landscape transitions, involving progressive or abrupt change over daily to millennial timescales. In order to sustainably manage river systems, it is necessary to further investigate the characteristics of variation in river morphology at various temporal and spatial scales (Minh Hai, 2019). Givi Chay River is one of the permanent rivers of Ardebil province in northwest of Iran and there is still no comprehensive study on this river. This study attempts to investigate the changes of morphological of the Givi Chay River over the time period 2000-2019.2. Study AreaGivi Chay River with almost 54 kilometers is one of the permanent rivers of Ardabil province. Two rivers of Hiro (which is emanated from Khalkhal altitudes) and Arpa chay(which is emanated from north to south), are linked to each other in downstream and the stream around Inalava village is departed toward westward and between altitudes of Khalkhal and Givi reaches to Givi city through a narrow valley. In this area, that river is called Givi Chay. The river flows into Ghezelozan after crossing the city of Givi and joining the Firoozabad River.

In this research, the topography map with a scale of 1:50000, geology map with a scale of 1:100000, and google earth and Landsat Eight images, including OLI sensor (2019), Landsat seven including ETM + sensor (2000), bedrock maps and the Givi Chay River privacy at a scale of 1:2000 hydrological data and field data were used. Moreover, to control the results obtained by quantitative methods it is used from field studies for confirmation and verification. ENVI 5.3, Arc GIS 10.5 and Excel software were also used for image processing and data analysis. The geomorphological parameters of the river and their variations including bending coefficient and central angle were measured. The curvature coefficient is one of the few criteria used in river shape segmentation using s=1/ (y.2), i.e., by dividing the valley length by wavelength for each arc (Pitts coefficient) which was calculated. The central angle of the arcs on each of the intervals was calculated using the relation A=180L/Rπ, where A is the central angle, R, of the fitted circle radius (Kornias coefficient). The lateral changes of the canal were investigated using transect method and calculation of river migration rate. According to the Transect method, lines with distinct distances from both sides of the duct are depicted as baselines. These lines are constant for the time periods studied and hence can be calculated quantitatively for duct movements relative to these lines. When the conduit is moved in the right direction, the area of the right-hand transect of the conduit decreases and is added to the area of the left-hand transect of the conduit, and vice versa. In this study, the Givi Chay River channel was divided into 23 transects based on morphology and channel change trends and quantitative indices were calculated for each transect. The Rm = (A/L)/Y relation was also used to calculate the channel displacement rate. In this respect, RM stands for transverse displacement intensity, A for area between two centerline lines, L is centerline length at time t1 and Y is number of years.

The mean curvature coefficient for the first period in 2000 was 1.48 and decreased to 1.40 in 2019. But in other periods from 2019, the bending coefficient has increased compared to 2000, as the bending coefficient from 1.23 to 1.25 in the second period and from 1.85 to 1.86 in the third period and from 1/15 to 1/18 in the fourth quarter it increased. In general, the lowest bending coefficient for each period is in the fourth interval and in a finite amount. In the first, second, and fourth intervals, most of the intervals in both study periods have a curvature coefficient of 1.5-1.5 and therefore the conduit plan is sinusoidal, but in the third interval more than 60% of the range has a curvature of 1.5 to 2 and therefore the interval pattern is in the form of a Meander. In the second and fourth intervals, the standard deviation of the bending coefficient is low and in the second interval is 0.19 in 2000 and 0.18 in 2019 and in the fourth interval is 0.14 in 2000 and 0.12 in 2019. In general, they indicate the existence of similar arcs. In the first and third intervals, the standard deviation is relatively high for both periods, indicating non-similar arcs.In both periods, the first and the third intervals were highly developed in the form of Meander and the fourth period were of the developed Meander type. However, during the second period during the period, the type of the rift from the developed meander changed to the highly developed meander and the central angle reached from 143.82 in 2000 to 163/50 in 2019. Due to the low valley bed and its alluvial slope, which is associated with complex mazes, and with increasing spiral arc and river energy concentration, the intensity of erosion reaches its maximum and where it is a maze arch, it is concentrated on both sides. A large amount of excipients flows into the bed. As the spiral energy intensifies, the width of the floodplain increases due to erosion. In the first interval, the central angle of the riverbed has decreased in 2019 compared to 2000, and with the decrease of the central angle of the river, the mean radius of tangent to the riverbed has also decreased and in other intervals has witnessed an increasing trend of the central angle during the study period. Increasing the central angle indicates that the river meanders are active and the morphology of the river has changed to a highly developed rudimentary twist, as well as a change in the central angle in bends that have not been removed and only changes have been made in it. In the third interval, the mean central angle in both periods is higher than other intervals. In fact, the river flows in a winding direction due to the geological resistance of the river and the low width resulting from this factor.The maximum amount of lateral changes in transect 12 was 1.51 m and in this transect during the study period 5.47 hectares decreased from left bank and added to right bank. The lowest location in Transect 20 is 0.54 meters, thus reducing the left bank to 1.13 hectares and adding to the right bank of the river. Transverse displacements have often occurred in parts of the river course where the riverbed has floodplains, and the riverbed in these areas is significantly wider and the slope greatly reduced and widened significantly. Agricultural lands and gardens are visible in these areas.

According to the results of the calculation of morphological indices, the average bending coefficient in the first period decreased in 2019 compared to 2000, but the coefficient of bending increased in the 2nd, 3rd and 4th intervals. The first, second, and fourth intervals have sinusoidal plans in both study periods, but in the third intervals in both periods, the interval pattern is a meander. In terms of the central angle index, in the second interval during the study period, the type of interval changed from extended to highly develop. However, the first and third intervals, in both study periods, are highly developed in the form of a meander and the fourth period are in the form of a developed meander. In the plain, the main factor affecting the river meandering is the alluvial formation, the slope is low and the meanders are inscribed and plain, whereas in the mountainous part the river changes are subject to valley changes and the meandering state is seen throughout the valley.The results of lateral conduit changes also showed that the average migration rate of the Givi Chay duct during the 19-year interval was about 0.87 m/year. It should also be noted that, during the period 2000 to 2019, approximately 39.52 hectares were generally added to the right bank of the river and 11.62 hectares decreased to the right bank.In general, changes in the Givi Chay River Plans have been attributed to the expansion of existing meanders, displacement of the river path, and increased curvature and formation of small meanders. Hydrological processes are caused by the process of supply of sediment and sediment discharge, dam construction and lithological resistance of the riverbed as well as human interference such as, encroachment of the riverbed, construction of bridges, sand harvesting.

Rivers have different patterns along their path, and understanding these patterns could help researchers understand and predict the responses of human activities and natural factors of the river. In addition, in the morphological analysis, the flow power is one of the most important parameters. Therefore, the purpose of this study was to analyze the channel form and to evaluate the river power in different return periods as well as to estimate the shear stress in the Givi Chay River using Landsat 8 images, topographic maps, geology, hydrological, and field data. Morphological and hydrological indices, as well as methods for analyzing the total river power, river specific power, and shear stress, were used to achieve the purpose of the study. The calculation of the bending coefficient showed that the river pattern was sinusoidal in the first and third intervals and was meandering in the second interval. According to the central angle characteristics, the first and the second intervals were in the highly-developed meandering form and the third interval was in the developed meandering form. Also, according to the results of the study, the highest amount of the total river power was in the first interval at the 4th, 5th, and 6th sections. The specific strength of the river was high in the third intervals and the 2nd, 1st, 5th, and 6th sections. At present, due to the low strength of the lithological formations in these sections, the erosion of the riverbed is observed in these sections. The lowest specific power was due to the high river width and low bed slope at sections 12 and 13. The highest amount of shear stress was in the first interval in sections 5, 6, 2, and the lowest shear stress was in sections 12 and 13. In general, channel formation and channel dynamics in the Givi Chay River were influenced by discharge, lithological resistance of the riverbed, and human interference (encroachment on the riverbed, construction of bridges, and dumping of construction waste and debris).