عنوان مقاله [English]
Rivers through production, movement and storage of sediments are one of the most important factors that modifying the earth’s surface. Lateral migration of rivers has been always associated with the bank erosion of the streambed or channel wall due to turbulent flow condition of water. River Migration channel occurred in a corridor or region, so it in some cases creates the problems for whom living in this region. Many people lose their houses, farmland, infrastructure and even their livelihoods because of migration and erosion of the river channel. In this study, Aras River channel migration during the past 29 years (1987 to 2016) was evaluated using transects methods. This river have a great importance in relation to water supply in the northwestern parts of the country. Moreover, in the large distances, forms Iran boundary line with the countries of Armenia and Azerbaijan. Therefore, research on the lateral changes of river becomes necessary.
Topographic maps with scale of 1: 50,000, digital elevation model (DEM) with 27 m resolution, and satellite imagery (Landsat 5 TM satellite sensor& Landsat 8 OLI satellite sensor) are most important materials in this research. Studied channel reach of Aras River for two time periods, 1987 and 2016 were extracted by processing satellite images. Then, channel based on morphology and changes trend was divided into 13 transects, and quantitative indicators were calculated for each transect. Moreover, the extent of the lost or added land area on both banks of the river were calculated with respect to changes in transects area.
3. Results and Discussion
Study of lateral migration of Aras River showing high change in late 29 years. The average of channel migration rate in the study reach of Aras River is about 4.73 meters per year, which is a significant value. According to the studies, meanders of the river channel are active, and the formation of new meanders, meanders migration as a result of erosion and create cutoff frequently occurs with a relatively high rate. By comparing the mean values of the central angle and rate of channel migration can be said, in transects that planform is the developed meandering river, the rate of channel migration is higher. But in a few transects where the river tends to be a straight pattern. In the study area, according to the past changes trend, channel changes have occurred due to three major reasons: (1) channel migration in the floodplain due to the erosion of concave banks of meander loops, (2) occur cutoffs through development and nearing the base of meanders, which its effects can be seen as an abandoned channel, and (3) occur avulsion in the parts of the river channel. In fact, large quantities and unusual migration rate in some transects related to the avulsion. Most likely, the avulsion caused by the river flooding, especially in the spring and disturbances due to the confluences.
In some cases, combined these factors associated with interventions variables such as effects of confluences has caused the channel movement is very significant, and the unusual. The results indicated that the average of channel-migration rates for the Aras River (Between two dams) was about 4.73 meters per year during the past 29 years. During the period from 1987 to 2016 as a result of due to channel-migration of river approximately 171.48 hectares of land were added to the Iran and in contrast at about 376.26 hectares has been inaccessible. It is suggested that more attention be accurately assess changes in trans-border rivers.