نوع مقاله : مقاله پژوهشی
1 دانشیار گروه جغرافیای طبیعی، دانشکده جغرافیا، دانشگاه تهران
2 دانشجوی دکتری ژئومورفولوژی، دانشکده جغرافیا، دانشگاه تهران
3 کارشناس ارشد هیدروژئومورفولوژی دانشکده جغرافیا دانشگاه تهران
عنوان مقاله [English]
River morphology is one of the main factors to control the geometric shape of river sediments which is determined by two parameters of braiding and sinuosity (Rust, 1977: 187). Sinuosity or sinuosity ratio represents how much curved a channel is (Charlton, 2007: 138) and braiding parameter represents the number of braids or bars in any wave length of meander. According to these two parameters, rivers are divided into four groups: straight, braided, meandering and anastomosing (Mousavi Herami, 1991: 261-263). There are differences in texture and size of sediments deposited in the channel, braided bars and point bars (Mousavi Herami, 1991: 264). The purpose of this research is to compare sediments from different forms of straight, braided and meandering channel in order to determine the changes of sediment size and texture in: different parts of an individual bar, among bars of three channel forms and between selected locations in upstream and downstream of river.
The study area is in Abdanan county which is located in Ilam province in the west of Iran on its most important stream which is Abdanan river. At the beginning, two appropriate locations of Abdanan river at the upstream (Posht-qaleh) and downstream (Hezarani) parts of this river were chosen after considering items such as topography, geology map and google earth images and also GIS software. Afterwards on each bar (braided or point) 4 points were chosen for sampling. Then, half a kilogram sediment sample was obtained from each point and after encoding, these samples transferred to the laboratory and granulometry analysis was carried out on the samples. Grains categories and their weight percent obtained using the granulometric analysis and necessary graphs were plotted by Excel and Gradistate. In the next step Statistical parameters such as mean (MZ), sorting (Iσ), skewness (SKI), Kurtosis (K) were calculated and the results were represented in phi unit.
Result and discussion
The upstream sediments of river are mostly classified in gravel, sandy gravel and muddy sandy gravel on triangular diagram of Folk (1954), but the downstream samples include sand and muddy sand sediments. Histogram of samples represents some modes on medium and very coarse sand size but the mode on gravel is predominant in all of samples. The latter point shows that samples are poorly sorted and have polygenetic origin. Breaks on cumulative plots represent the different groups of grain sizes which were moving in different mechanisms from suspension to traction. The percentage of coarse grains is high in most samples; and plots are parabolic. This represents the changes of energy in environment. Calculating the statistical parameters such as mean, sorting, kurtosis, skewness using graphical method of Folk and Ward (1957) showed that upstream and riverside parts of the braided channels underwent one-sided washing, so that their sorting is higher and percentage of coarser grains is more than downstream part of these channels which experienced washing from both sides. In the meander bars, sedimentological characteristics of sediments are close to a normal pattern of a meander. The difference, however, is that a newly formed river crossed the flood plain and reduced the sorting of sediments, so that the sedimentological characteristics of these sediments in downstream meander is similar to braided pattern. contrary to what expected, sorting decreases in the straight channels, due to gravels shed from sub-branches and slopes
the differences seen sedimentological characteristics of the samples are due to the role of branches of main stream, periodic floods of Abdanan river and sediment-shedding from channel slopes that has changed the texture and size of sediments. These changes in the samples which were obtained from Hezarani location in the meandering channel are due to development of river sub-branches and in the straight channel are because of sediments resulting from channel slopes. generally speaking, sediment texture and size at upstream (Posht-qale) vary from well to moderate sorted gravel in the north to poorly sorted sand in the south but this pattern is not seen in sediment texture and size at downstream (Hezarani) location where effects of river branches and floods are illustrated by increasing gravel and irregularity of sediment pattern.