Geomorphology
elnaz piroozi; Aghil Madadi
Abstract
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 ...
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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.
elnaz piroozi; Aghil Madadi; Sayyad Asghari Saraskanroud
Abstract
1-Introduction Rivers are dynamic forms of natural landscapes with different changes at different times and places. The effects of river adjustment caused by the natural factors require much longer span to reveal. However, sometimes the natural factors such as river floods, landslide, or earthquake can ...
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1-Introduction Rivers are dynamic forms of natural landscapes with different changes at different times and places. The effects of river adjustment caused by the natural factors require much longer span to reveal. However, sometimes the natural factors such as river floods, landslide, or earthquake can lead to canal adjustments in a very short time (Chaiwongsaen et al, 2019:153(. In contrast, human activities can have a significant and rapid impact on natural processes and trends, resulting in a short time scale for river adjustments (Rinaldi & Simon, 1998:57). River canal instability plays a major role in erosion, destruction of beaches and riverbanks. This role becomes more significant when the canal and bed of the river is alluvial (Rezaei Moghadam, 2012:33). One of the key issues in studying the erosion and stability of rivers is the initiation of sedimentary particle movement. The motion of sediments occurs if the bed shear stress (available shear stress) induced by the flow exceeds a certain critical value. An alluvial canal, either artificial or natural, persists to deform its boundary by itself while transporting water and sediments. Therefore, erosion and riverbank instability have created major concerns worldwide over the past few decades and significant amount of money have been spent to sustain the riverbanks. Givi-Chay River which is almost 54 kilometers long, is one of the permanent rivers of Ardabil province, Iran; problems of bed and bank erosion are evident in different areas of this river and they damage agricultural lands and adjacent river installations. In addition, a review of the research shows that sufficient studies have not been carried out so far to reveal the stability, erosion and sedimentation process in Givi Chay River.Therefore, this study aimed to analyze and evaluate the erosion stability of Givi Chay River channel. 2-Methodology 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), bedrock maps and the Givi-Chay River area at a scale of 1:2000 hydrological data from two Abegharm stations (upstream of the dam) and Firoozabad (downstream of the dam) and field data are used. In addition, to control the results obtained by quantitative methods, field studies are applied for confirmation and verification. ENVI 5.3, Arc GIS 10.5, Excel, and HECRAS 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, it is 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.The increased shear stress in the riverbed increases the load of the floor and the scour of the bed, which can affect the riverbanks as erosion, destruction, and rupture of the walls. Direct measurement of shear stress is a difficult task and therefore researchers have developed methods for indirect calculation of shear stress. Existing shear stress (boundary), lateral shear stress, and critical shear stress were calculated by means of equation 1, 2, and 3, respectively: (1) (2) (3) Relative Stability Index Calculation (RBS):Judet has introduced this index as the ratio of critical bed velocity to actual bed velocity. Olsen et al (1997) defined this index as the ratio between the critical shear stress and the shear stress of the sides. Relative stability index (RBS) was obtained using the following equations: (4) (5) (6) 3-Results and Discussion Investigation of the morphology of the intervals shows that in the first, second and fourth intervals the conduit is sinusoidal and in the fourth interval, the pattern is meandering. In addition, according to the results of the study, the first, second and third intervals are developed in a very meandering manner and the fourth interval is just a meandering one. shear stresses in sections 4, 3 (second interval) are more than other sections, and given the direct relationship between shear stress and depth and width of sections, even under current conditions there will be phenomena such as scouring and damaging river bank and rivers. In addition, in terms of critical shear stress, the highest shear stress is in sections 3 and 7. Due to the relative stability values, sections 5 and 7 are stable and other sections are unstable. In the first period, the river flows into a valley bed, and in parts formed by erodible formations and at sections close to the dam, the river width is approximately increased. Therefore, sections 1 and 2, which pass through alluvial terrace sediments, are in unstable condition. In the second interval and immediately after the Givi Dam, the river passes through the valleys overlooking the Givi town, where the width of the bed due to the types of the banks decreases and the riverbed contains coarse sediments covered by broken rocks. In other parts of the city of Givi, erosion conditions prevail and large volumes of flanking material (especially during floods) are eroded and loose flanks lead to the widening of canals and intra-canal ridges, and these sediments are clearly visible in bends, middle islands and marginal lands and steep banks. At sections where the river width is excessive and the slope decreases, the stability factor is almost high (sections 5 and 7). At the beginning of the third period, Firouzabad area is located on path of the flood of the previous interval and by joining Sanghor Chay, the river enters the mountainous part and the coastal areas have deep valleys with steep slopes. Along the river, due to collision with high mountains and rocky outcrops, the alternate route has a meander and river changes are subject to valley changes, and the meandering state is seen throughout the valley. In the fourth period, the river width is reduced and the riverbed is covered with coarse sediments, which extends to Ghezelozan. 4- Conclusion(S) According to the study results, in the plain interval, the main factor affecting the river meandering is the alluvial formation; here, 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. According to the values of shear stress, the lowest boundary and bank shear stress is in sections 5, 6 and 7 and the highest is in sections 4, 3, 11 and 12. The highest critical shear stress is in sections 3 and 7 and the lowest is in sections 4, 2 and 12. The study of the relative stability of the river shows that the river is more unstable in sections crossing the old and new alluvial terraces, and in sections where the river width is high and the bed slope and flow rate have a decreasing trend, the coefficient of stability is relatively high. The third and fourth intervals are mountainous and semi-mountainous, respectively. In these intervals the river width is small and there is no agricultural land use .Lithologically, most of the third and whole of the fourth period consist of Eocene igneous and pyroclastic formations and they are resistant to erosion and the existing alluviums are the result of transport of water from sediments of other intervals.Therefore, the morphology of the river is affected by lithology and according to field evidence, the interval is stable .But the results of using mathematical and experimental methods have introduced the third and fourth intervals as unstable . Therefore, it can be acknowledged that the methods used in this study apply to the study of stability in rivers and alluvial intervals
Aghil Madadi; Elnaz Piroozi; Leila Aghayary
Volume 5, Issue 17 , March 2019, , Pages 85-102
Abstract
Introduction
One of the most striking natural hazards in the world is flood which generates a lot of financial and human losses every year. It can be said that in comparison with other natural hazards, it occurs with high abundance and in vast expanses. Some of its causes can be severe or prolonged ...
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Introduction
One of the most striking natural hazards in the world is flood which generates a lot of financial and human losses every year. It can be said that in comparison with other natural hazards, it occurs with high abundance and in vast expanses. Some of its causes can be severe or prolonged rainfalls, melting, breaking the dam and landslide, high waves, channel closure, rainfall intensity, type of rainfall, time and volume of rainfall, previous river conditions, drainage basin, inappropriate use, and falling of forest trees in the sources of the rivers. Knowing susceptible areas to floods is one of the basic measures in natural resource management and development planning. One of the most important flood management methods is flood zoning. The zoning of potential flooding is to identify and describe areas with potential for surface runoff. The Khiyav Chay Watershed Basin, with an area of 318 km2, is located in Meshgin shahr. Due to the specific circumstances of the region, such as topography, slope, and climatic conditions (sudden precipitation and spring precipitation, melting, flooding of rivers in the spring), there is a high potential for flood occurrence. Therefore, the purpose of this research was to study the area's potential for flood occurrence.
Methodology
In this study, ten factors of slope, height, rainfall, CN, runoff height, distance from the river, soil, lithology, vegetation, and user-use were identified as effective factors for flood formation in the region. Using Landsat 8 images including OLI and TIRS sensors and the Maximum Likelihood supervised classification method, in the ENVI 5.3 environment, the land use map was obtained. Then the user map was compared with the index table and integrated with the hydrologic group data, and the CN curve number was prepared. In the next stage, with mean precipitation and CN, and by using SCS method, ARC GIS software and Arc-Hydro and Arc CN-Runoff subtraction, the runoff height of the range was calculated. Also, the NDVI index, one of the most widely used indices for vegetation monitoring, was undertaken to prepare a vegetation map of the basin. Then, the other layers of information were provided in the GIS environment. The weights of the layers using the Critical method based on the correlation, interference, and standard deviation of the factors were determined. The final analysis and modeling was done using the WLC model as one of the methods of multi-criteria analysis techniques.
Discussion
By studying the zoning of the potential flood area of the study area and comparing it with each of the standard maps, it was concluded that the high risk areas were mainly in the hilly and mountainous areas of the area (slope over60%). Due to the slope and elevation of the area, the main role was with runoff, flood discharge, penetration, precipitation losses, and flow and water velocity. In these high risk areas, due to the fact that most of the formations belong to the formation of volcanic activity in the late third and early fourth centuries, the degree of permeability was very low but the runoff and CN amount were high. Secondly, areas with potential hazard were located within the urban boundaries of Meshkinshahr. In the city of Meshgin Shahr, on the east side, is the deep valley of khiyave chay, where the khiyave chay River flows. Two other radial valleys in the natural pathway formed the surface water stream, along which residential neighborhoods were developed that were subject to flood and extreme flow of surface water. Due to the fact that most of the city is made up of asphalt and residential surfaces, the permeability was very low, in contrast to the amount of runoff (99%) and CN (curve above 8).
Conclusion
According to the results of weighing, height factors with weight coefficients (0.173), lithology with weight coefficients (0.163), slope with weight coefficient (0.139) and rainfall with weight (133/0) were the most important factors on flood formation in the region. The results of the study showed that 13.33% and 22.88% of the study area were in high risk and high class. According to the final map, high-risk areas, in the first priority, were mainly in the hilly and mountainous regions of the region, but in the second priority they were within the urban boundaries (especially in the central regions of the city due to lower construction and permeability). The results of the study also indicated that due to the high potential of the study area in terms of the risk of flood, water protection and protection measures at the basin level should be considered. In addition, the simultaneous use of remote-sensing and GIS and using the SCS-CN model could be useful in preparing a flood zoning map.
