Environmental Erosion Research

fa بررسی فرسایش کناره رودخانه مِرِگ ماهیدشت Investigation of bank erosion the Mereg Mahidasht River مدلسازی و تحلیل زمانی و مکانی رخداد انواع مختلف فرسایش محیطی پژوهشي Research به طور طبیعی، فرسایش کرانه مشکلی جدی برای هر سیستم رودخانهای است. این پدیده از منابع اصلی تولید رسوب در جریان رودخانهها به شمار میرود که به تغییر کانال و رسوبگذاری در رودخانههای آبرفتی منجر میشود. هدف این پژوهش، بررسی فرسایش کناره رودخانه مِرِگ ماهیدشت در استان کرمانشاه است. برای پایداری بستر و کناره، از روش تنش برشی نزدیک کرانه (NBS) راسگن استفاده شد؛ بدین صورت که ابتدا برای اجرای محاسبات، اطلاعات رقومی برداشت شده به محیطGIS  منتقل شد و از طریق الحاقی  HEC–GEORAS ، پردازش دادههای ژئومتری انجام و اطلاعات به مدل عددیHEC_RAS  منتقل شد. سپس تمام محاسبات مقاطع برای اجرای مدل پایداری بستر و کناره (NBS) در محیط نرمافزار HEC-RAS انجام شد. با توجه به مورفولوژی، رودخانه مِرِگ به چهار بازه تقسیم و روش  (NBS) برای 44 مقطع انتخابی محاسبه شد. نتایج حاصل از مدل نشان داد که با توجه به شاخص نسبت حداکثر عمق نزدیک کناره به متوسط عمق دبی لبالبی، میزان فرسایشپذیری در بازه اول زیاد و شدید بود. در بازه دوم، غالب قوسها فرسایشپذیری زیادی داشت و در بازه سوم، اغلب مقاطع در گروه فرسایشپذیری متوسط قرار داشت. در بازه چهارم با توجه به الگوی سینوسی رودخانه و شیب کم، میزان فرسایش کم بود. در تمام بازههای رودخانه مرگ، بیشترین فراوانی فرسایش کم، متوسط و زیاد بود و غالب فرسایش در ساحل سمت چپ تمرکز داشت. افزایش شیب، کاهش پوشش گیاهی، افزایش الگوی پیچانرودی بودن و تغییر کاربری و تبدیل زمینهای اطراف رودخانه به کاربری کشاورزی، علت افزایش فرسایش در این بازهها بود.   1- Introduction The U. S. Environmental Protection Agency lists sediment as the most common pollutant in rivers, streams, lakes, and reservoirs defined bank erosion as a natural geomorphic process or disturbance that occurs during or soon after floods. By producing sediment load, it causes pollution and reduces the quality of drinking water. In addition to that channel enlargement, bank instability, degradation of physical habitat and numerous other geomorphic responses accelerate the process of bank erosion Bank degradation is the result of a process that combines the erosive power of water and the effect of gravity. Bank erosion is one component of the natural disturbance regime of river systems and is integral to long-term geomorphic evolution of fluvial systems and ecological sustainability. Bank erosion is, therefore, a desirable attribute of rivers. In this study, Near bank stress (NBS) Rosgen shore for a part of Mereg Mahidasht river has been investigated. Mereg River Due to its location at high altitudes, high length and variable width, its meandering pattern and flood nature have been prominent features of this river. To suffer from general and localized erosion during floods and to reduce possible damages in the future by examining the erodible points 2- Methodology In order to conduct erosion studies, first a digital model of the altitude of the region, a 1: 1000 map of the region that covers the river and part of the flood plain has been used. The first stage was preparation of input data in ArcGIS using the HECGeoRAS extension. HEC-GeoRAS helps in creation of the data needed for the HEC-RAS model and the transfer of data between ArcGIS and HEC-RAS. The next stage was done within HEC-RAS (5.0.3) using the river geometry prepared in the previous stage. The final stage consists of analyzing the results from the HEC-RAS model within ArcMap. Three input parameters must be specified: stream geometry, flow data, and the model plan to create the flood and inundation maps of the Mereg river in HEC-RAS. In order to create the river geometry for HEC-RAS, elevation data were needed. High resolution digital elevation model was obtained from 1:1000 topographic map that was prepared by Navandish Water Processors Consulting Engineers Company Company. The HEC-GeoRAS extension was used to set up the necessary features that would be needed for the HEC-RAS model (i.e., stream centerline, bank lines, cross sections, etc.). In addition to elevations, Manning’s roughness coefficient values were applied to each cross sections using Cowan method. 44 sections of different river sections were selected and measurements were performed in HEC-RAS environment. And then , for bank stability from the Near bank stress (NBS) Rosgen NBS method is used. Two Rosgen methods have been used: (ii) Ratio of radius of curvature to bank-full width & (iii) Ratio of near-bank maximum depth to bank-full mean depth 3- Results According to the morphology and meandering pattern of the Mereg River, the total reach is 28 km and It is divided into four reach. First reach: To study method of Near bank stress. In the first reach, 12 cross sections have been selected. In the first part, the river has a radius of curvature of 1.39. Bank Erosion of the Mereg River was low to severe in the first period. Second reach: the average slope of the Merege River channel was 0.34%. The value of the radius of curvature is 1.64, which is considered as a meandering river. In this reach, 12 cross section have been selected to measure the degree of erodibility. Most arches were highly erodible. Third reach: The slope of the canal in this part is 0.2%. The Mereg River has a radius of curvature of 1.7 and has a meandering pattern. And the rate of erosion of the shore in this section is estimated to medium or less. Fourth reach: The number of cross section studied in the fourth reach is 5 and this reach of the river has a radius of curvature of 1.2 and has a straight pattern. The rate of bank erosion has been low and medium.  4- Discussion & Conclusions Comparison of the two methods used to analyze the erosion of the Mereg River shows that the risk of erosion in all arches in the range of the river has been high erosion. And the reason is the intensification of hydraulic stress in these reaches. In the second and third reaches, the average bank erosion is calculated as medium to high. The reason for this is the reduction of the slope, which according to the longitudinal profile of the Mereg River, the average slope was 0.23%. In addition, there is a decrease in vegetation, an increase in the pattern of river winding and change of use and conversion of lands around the river to agricultural use in these periods. Comparison of the results obtained from different methods of estimating the risk of erosion with field observations showed that Ratio of near-bank maximum depth to bank-full mean depth in the river Mereg Mahidasht was closer to reality and It is suitable for checking the degree of erodibility of the bank.   پایداری بستر, تنش برشی نزدیک کرانه (NBS), رودخانه مِرِگ. bank stability, Near bank stress (NBS), Mereg River. 25 45 http://magazine.hormozgan.ac.ir/browse.php?a_code=A-10-863-1&slc_lang=fa&sid=1 roya panahi رویا پناهی r_panahi@sbu.ac.ir 10031947532846008041 10031947532846008041 Yes Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران Mohammad Mahdi Hosseinzadeh محمد مهدی حسین زاده m_hoseinzadeh@sbu.ac.ir 10031947532846008042 10031947532846008042 No School of Earth Sciences, Shahid Beheshti University, Tehran, Iran دانشکده علوم زمین، دانشگاه شهید بهشتی، تهران، ایران Seyed Meysam moshashaie سید میثم مشعشعی meysammoshashaee@gmail.com 10031947532846008043 10031947532846008043 No Navandish Water Processors Consulting Engineers Company, Kurdistan, Iran شرکت مهندسین مشاور آب‌پردازان نواندیش، کردستان، ایران