عنوان مقاله [English]
The assessment of river patterns is necessary in order to understand current conditions and the potential of their possible changes in the future. Geographers, Geomorphologists, and geologists have used channel shape as an effective parameter in classifying, analyzing, and predicting fluvial responses. In recent years, the changes in river systems have led to increased environmental damage caused by river processes in many countries, therefore managers have had to pay more attention and be more sensitive and change as an integral part of all river systems. River classification is one of the most effective tools for solving these problems. Brierley and Fryirs (2005) categorized the channel pattern based on three interrelated characteristics, including the number of channels, sinuosity, and lateral stability, into five types, straight, sinusoidal, Meandering, braided, and anastomosing. For this purpose, changes in the braided pattern in Jajrud river have been studied.
The studied area is located on the east bank of the Jajrood river, which is 19Km long (17 reaches) and located between the Latian and Mamlu dams. The Jajrood River originates from the heights of Alborz (Klon Bestak) and its most significant branches are the Fasham, Damavand, Migun, and Ahar rivers. Mamlu basin covers 1772.82 square kilometers, including Latian dam basin (701.19 square kilometers). The study area is located on the main channel of the Jajrood river, where the Latian and Mamlu dams were constructed.
Throughout this study, satellite images (years 2006 and 2019), aerial photographs (years 1957, 1973, and 1995), and fieldwork have been used as the main research methods. In order to conduct the fieldwork, the study area has been divided into 17 study subcategories, each subcategory being 200 meters long. Brace, Richards, and Warburden indexes were used to check the degree of braided pattern development. which are, respectively, the first index of the parameters of the length of barriers and islands (a) and the length of the study reaches (L), the second index of the parameter of the length of the sub-channels (l) and the length of the study reaches (L), the third index of the parameter of the active channel width ratio (b) the width of the channel (B) in the studied section. According to Richard’s and Brace’s Indices, the higher the output number, the more braided the river is. Brace and Richard indices are the reaches of investigation and the results of these two include the calculation reach. The Warburden index, however, is based on the ratio of the width of the branched channels to the width of the main channel which approaches Issue number one as branching increase.
Richards and Warburton indices were used to check of braided pattern. For estimating the effect of dam on the braided pattern of Jajrud River, the Wilcoxon test was used and the paired t-test was used to check the difference in the braided pattern in different reaches.
Results and discussion
The results based on the studies, conducted in 17 reaches of 19 km of Jajrud river for 5 time periods, show that these reaches did not have the same conditions in terms of the braided index.
These changes in the river pattern may indicate changes in environmental conditions along the river or the effect of the built dam on the river regime. The results of the study show that among the areas near the Latian dam, areas No. 1 and 2 have always had a single-channel state except in 1973 when it was low braided.
The most important reason for the single-channel pattern in the first and second reaches is the location of the channel, which is limited inside the valley. For the year 1957, when the river regime is completely normal, except in reaches 1 and 2, all reaches have an braided pattern. The trend has been almost constant. This trend in 1973, when the Latian dam is in use, in all indicators for reaches 3, 4, and 5, the braided coefficient is zero or low values. The explanation of the above conditions can be the water extraction and launching of the Latian Dam in the same period of time and the limited output of water, which has finally led to the single-channelization of the mentioned sections of the river. In 1995, the values of the braided coefficient with all three methods in the studied reaches show the lowest value of the braided coefficient compared to the previous and subsequent years. Based on the discharge data of the available hydrometric stations, the high discharge values at this point in time compared to other times were the most important reason for the reduction of the sediment load of the bed and the reduction of the braided pattern index. In 2006, the average braided index increased compared to 1995. This year, the highest braided index has been created in the last reaches of 13 to 16. The most important reason for this issue would be the construction of the Mamlu Dam at a short distance.
Based on the average values of the braiding index in the Jajrood river, from 1957 to 1995, the values of the braiding index decreased and then increased until 2019. Assessment of the impact of the Jajrood dam on the braided pattern indicates the significant difference between the Brice and Richards indices values before and after the dam construction. Based on the results of the paired t-test in the Brace and Richards indices, the values of the braiding index in the studied reaches from 1973 onwards have significant differences in several periods. According to the findings, the Brace and Richards indices have been more effective rather than the Warburton index to study the braided pattern in this study area.