نوع مقاله : مقاله پژوهشی
عنوان مقاله [English]
Observation and interpretation of geomorphic and sedimentary features and processes conducted after and possibly during the flood event are fundamental to developing a better understanding of the mechanisms responsible for channel changes Extreme flood events increase stream power and the rates of erosion and accumulation in the river channel. The geomorphological effect of a flood depends on the size of the stream, magnitude, and frequency of the flood event and on the physical properties of the channel, banks, and floodplain. In October 2015, following the occurrence of severe and sudden rainfall, three large and devastating floods occurred with a maximum instantaneous discharge of 230 m3 / s from 6 to 8 October in Ilam province. The floods caused major changes in the morphology of Ilam's main streams and rivers and caused various sediments and deposits in the bed and rivers of Ilam. Since few studies have been done in this field, this research was conducted with the aim of investigating the geomorphic sediment response of Mountain Rivers on the creation of flood landforms upstream of Ilam dam to severe flood events.
100 sites were studied in waterways of the upper Ilam dam drainage basin after flood. The shape, location, dimensions and deposits of large depositional forms (bars) were documented. The bars are regarded as mega forms, strictly connected with the alluvial sedimentary style. Smaller depositional forms were not investigated because their formation was controlled by local hydrodynamic conditions, and moreover, they were quite rare in the coarse-grained channels. All waterways were divided into two groups: streams and rivers. This subdivision was necessary because both qualitative and quantitative character of depositional forms appeared to be different in these systems.
Results and Discussion
Most of the depositional effects in mountain streams are concentrated within the channel. Boulder mounds are the most typical bar type in the upper reaches of streams. Deposition of coarse-grained material takes place during the flood peak in areas of lower stream power between the main current tracts. Gravel–boulder longitudinal bars dominate in the lower reaches of streams; these are distinctly elongated forms located within the channel. Gravel–boulder side bars appear in slightly sinuous reaches of stream channels. The bars exist close to both banks. Overbank depositional forms of streams are relatively rare. Boulder berms are characteristic of large floods. A boulder berm is a coarse-grained levee formed immediately above the bank crest, in the zone of large velocity gradients during the flood peak. Longitudinal bars are formed in the central part of river channels. Their plan form is elongate, oval or rhomboid. Large compound bars commonly develop in wide zones of nearly straight channel courses. Unit longitudinal bars result from deposition in crossover zones in the slightly sinuous channel. Longitudinal bars play a significant role in the process of braiding. They diverge the current and enlarged, compound bars lead to new channel growth. A diagonal bar is the most abundant and characteristic macro form from the group of side bars. Diagonal bars are typical of river channel reaches characterized by slight sinuosity. They exist alternatively close to both channel banks, immediately downstream from gentle bends. A side compound bar is the second type of the bar distinguished within the group of side macro forms. This is a large-scale bar that exists in the zones of channel widening. These bar types were only noted in channels with tendency for braiding. A coarse-grained point bar is the next characteristic type of side macro form. It differs from other side-type bars by location in the channel, morphology, and mode of accumulation. Point bars exist in sharp bends. A gravel levee is formed on the lowermost terrace, close to the channel bank. Sometimes its formation was caused by the stems of trees growing along the river channel. The basic feature of stream alluvium is that lithofacies type is weakly dependent on parent depositional form. Generally, deposits are characterized by: very coarse grain size, lenticular shape of beds and imbricated structure. All bar derived deposits of the main rivers studied represent one lithofacies assemblage. Gravel and boulder clast-supported beds with imbricated structure or are the most abundant lithofacies derived from the bars.
There is a relatively regular spatial succession of depositional processes and forms along the mountain streams. In the uppermost reaches, only erosion takes place. Depositional processes ensue downstream. Formation of boulder mounds and boulder berms take place first. These forms are replaced by longitudinal bars and side bars in lower reaches. One phenomenon was characteristic both of streams and main rivers: the zone of increased deposition always follows the zone of erosion. Moreover, the ratio of fluvial deposition is proportional to intensity of upstream erosion. Both texture and structure of mountain stream alluvium indicate very weak relationship with parent depositional form type. Generally, deposits are characterized by: very coarse grain size, beds are of lenticular shape and their structure is typically imbricated. The most abundant bar types of main rivers are represented by one lithofacies spectrum. The most common lithofacies are clast-supported gravel and boulders with imbricated structure.