بررسی هیدرولیکی- مورفولوژیکی اثرات برداشت مصالح رودخانه ای بر کف کنی و فرسایش کناری رودخانه(مطالعه موردی رودخانه خرم آباد)

Sand and gravel mining from the riverbed causes many environmental and social problems in the rivers because of disturbing the natural conditions of the bed and also, its changes. Some of these problems are subsidence of riverbeds and erosion of hydraulic structures its path, bank erosion and damage to land placed in riverbank, the destruction of dewatering openings, drainage of agricultural land placed in riverbank and also, the change of plant and animal environment. In studying the morphological changes of the rivers’ cross-sections, it is often assumed that the riverbanks are non-erodible or the erosion of river banks is less than the erosion of bed, so, river width is considered constant. A closer study of the changes in river alluvial, particularly, with the erodible banks shows that the river width will be faced with significant changes because of material mining.In this study, it has been tried to study the changes in the bed, bank erosion of the river and its causes and also to identify the vulnerable intervals by simulating hydraulic conditions of sediment flow in Khorramabad River with the use of model Fluvial-۱۲. Khorramabad River is located in ۳۶.۴۸-۴۵.۴۷ east latitude and ۱۰.۳۳ to ۵۰.۳۳ North longitude and of the important branches of Kashkan River. The case study includes an interval of Khorramabad River with the length of about ۷ kilometers before the inset of Kakasharaf River. IN this interval, Khorramabad River is a mature river in the plains with the average slope about ۱.۶%, and a flow with u-shaped cross-section. In mentioned interval, there a map with the scale of ۱:۱۰۰۰ that includes about ۹ kilometers of the river and the hydraulic studies were performed on this interval. In this study, ۱۳۷۲ cross-sections surveyed in ۲۰۰۳ were used. According to the changes in the sections along the way due to the construction of the bridge, the protection wall of the bank, river materials mining and encroaching the bed of Moradabad River, firstly the required sections were extracted from the topographic map in the GIS environment using HECGeo-RAS interface and then, these sections were modified in model HEC-RAS and inappropriate sections were removed. With this approach, totally, ۵۱ cross-sections were entered in model HEC-RAS to introduce a geometry of the river in the studied interval. For hydraulic modelling of the sediments of Khorramabad River in the studied interval, the model FLUVIAL-۱۲ was used. The model FLUVIAL-۱۲ was provided in ۱۹۷۲ by Cheng.it a mathematical one-dimension model is used to rout the flow and sediment in the natural and built waterways (Journal of ۳۸۳, Ian Management and Planning Organization, ۲۰۰۷). This model cannot model the unsteady flow. In model FLUVIAL-۱۲, digital techniques and physical relations are used to analyze the momentum, flow resistance and sediment transport. The main parameters required for calibration of the model, are roughness coefficient, Sediment transport equation, coastal erosion factor, bed erosion factor and the factors such as these. Model FLUVIAL-۱۲ has been tested with data from many different rivers in the world that most of the data was used to test and calibrate other models. Roughness coefficient of Khorramabad River in the suited area was estimated by field survey and engineering judgment and completing the checklist of SCS method. To calculate the Manning roughness coefficient which is used in hydraulic model HEC-RAS, the values of vegetation, artificial irregularity and additional roughness of barriers created in the river by the people were considered in the calculation of river roughness coefficient. To calculate and estimate the Manning roughness coefficient in floodplains and main section, US Soil Conservation Service (SCS) and also, field survey were used. The sediment transport was simulated by using the functions of the model, the Graph’s (۱۹۷۰) sediment transport equation, Yang’s (۱۹۷۲-۱۹۸۶) flow power unit, the equation of Engelund & Hansen (۱۹۷۶), the equation of Parker et al.(۱۹۸۲) for sand, the sediment equation of Akers and White, the equation of Meyer-peter & Muller for bed load (Shafaei Bejestan, ۲۰۰۵). Result revealed in the v-shaped sections and the channels with small sections, the bank erosion is developed after armoring the bed materials. Also, In the U-shaped sections and the channels with larger sections, the bed erosion is reduced and the development of bank erosion is inconsiderable after armoring the bed materials. Furthermore, In the sections that the section shape and slope make the stable section, no significant difference will be observed in terms of erosion and sedimentation after armoring the bed materials. As it is mentioned before, importance of studying and discussing the river mining becomes clear. Each river has the sediment transport capacity depending on its discharge. There is a reverse relationship between the transport capacity and the size of the constituent grains, so that the coarse grains have the less transport capacity than the smaller grains. So, determining the river transport capacity, the proper points to mine the materials according to the deposition intervals and the appropriate volumes of material mining according to the bed load requires the comprehensive studies on the potential of material mining in different parts of the country. Also due to illegal river mining activities in recent years, the need for the studies on improving the rivers based on the modelling the current situation and providing the protection plans are essential.