بررسی فرسایش‌پذیری کرانه‌های رودخانه‌ هفتچشمه‌ قزوین با استفاده از تنش نزدیک کناره (nbs)

فرسایش کناره‌ای رودخانه یک فرایند پیچیده‌ طبیعی است که در مقیاس کانال عمل می‌کند. هدف پژوهش حاضر، بررسی وضعیت فرسایش کناره‌ای مناطق آسیب‌پذیر و عوامل ایجاد و تشدید فرسایش کناره‌ای رودخانه است؛ بر این اساس 11 کیلومتر از رودخانه‌ هفت‌چشمه در محدوده‌ روستای رزجرد تا روستای شینقر با الگوی سینوسی مطالعه شد. در این پژوهش با توجه به مورفولوژی رودخانه، محدوده‌ پژوهش به 4 بازه و 10 مقطع تقسیم شد؛ از بازه‌های مطالعه‌شده در چند نوبت بازدید میدانی شد تا داده‌های مدنظر جمع‌آوری و نیمرخ عرضی کانال تهیه شد. برای بررسی فرسایش کناره‌ این رودخانه از مدل تنش نزدیک کناره استفاده شد. نتایج به‌دست‌آمده در این پژوهش حاکی است در نسبت شعاع انحنا به عرض دبی لبالبی تمامی مقاطع به‌استثنای مقطع 9 فرسایش شدید دارند؛ در نسبت حداکثر عمق نزدیک کناره به عمق دبی لبالبی بیشتر مقاطع فرسایش کم تا متوسط داشته‌اند؛ به‌جز مقطع 9 و 10 در کرانه‌ چپ که فرسایش زیاد و شدید داشته‌اند. در نسبت تنش برشی نزدیک کناره به تنش برشی دبی لبالبی تمامی مقاطع فرسایش خیلی کم و کم داشته‌اند. درنهایت با توجه به بازدیدهای میدانی مشخص شد سطح اول مدل یعنی نسبت شعاع انحنا به عرض دبی لبالبی با ویژگی‌های منطقه تناسب بیشتری دارد و با واقعیت منطقه منطبق است.

Investigating the Erosion of the Haft Cheshmeh River Banks in Qazvin Using the Near Bank Stress Model (NBS)

Introduction: The flow of water and rivers is the most important phenomenon in the crustal processes that not only plays a role in the overall appearance of the earth but also determines the form of human life on the planet. As a dynamic system, a river always changes its morphological location and characteristics according to time, geomorphic, geologicalhydrological factors, and sometimes due to the human intervention. Bank erosion of rivers could cause a lot of damage to agricultural lands, buildings, riverside structures, roads, bridges, etc. every year. Inaddition, it could cause significant amounts of sediment to be transferred to dam reservoirs. The Haft Cheshmeh River basin is located in the Rozjerd region, in the northeast of Qazvin province and on the southern slope of Alborz. In the present study, the Haft Cheshmeh River in the area from Rozjerd to Shinqar villages has been studied for approximately 11 km. The pattern of the river in this sinusoidal range with an average curvature coefficient is 1.17.   Methodology: In order to obtain the required data, the Haft Cheshmeh River was divided into 4 sections and 10 crosssections, and from these crosssections, a crosssectional profile of the channel was prepared during 7 field works. To investigate the erosion of the Haft Cheshmeh River using the near bank stress model (NBS), the estimation of the stress applied to the shore is related to the slope flow, in which seven methods can be used in accordance with the conditions of the region. According to the characteristics of the Haft Cheshmeh river, in this study, three methods of the ratio of the radius of curvature to the width of the bankfull (Rc/Wbkf), the ratio of the maximum depth of the near bank to the depth bankfull (dnb/dbkf), and the ratio of the shear stress of the near bank to the shear stress of the bankfull ) were used. After obtaining the data by field sampling and calculations, according to the measured parameters, the degree of lateral erosion in different classes was classified from very low to severe.   Results: To investigate the erosion of the Haft Cheshmeh River, the Near Bank Stress Model (NBS) was used. At the second level, which measures the ratio of the radius of curvature to the width of the bank, all sections, with the exception of crosssection 9, have severe erosion. Only in crosssection 9, very little erosion has been observed. At the fifth level, which is the ratio of the maximum depth of the near bank to the depth of the bankfull, low and very low erosion from the first to the eighth sections on both sides of the shore were observed. Only the ninth and tenth sections have high and severe erosion; however, even on the right bank of the tenth section, erosion is low. At the sixth level, which is the ratio of the shear stress of the near bank to the shear stress of the bankfull, the erosion has low and very low values in all crosssections.   Conclusion: The use of the ratio of the radius of curvature to the width of the bankfull is appropriate for a time when a narrow radius has the maximum effect on the bend of the river. Therefore, the results obtained from the crosssections that were harvested at the site of the bend of the stream were consistent with the reality of the region and showed the instability of the sides. The results of the shear stress of the near bank to the shear stress of the bankfull did not correspond to reality. According to field observations and data obtained using the abovementioned three methods, the ratio of the radius of curvature to the width of the bankfull is close to reality. Although the results of the near bank stress model show the erosive crosssections with less intensity, if it is necessary to provide a quick and lowcost estimate of river intervals, it is reasonable to use the near bank stress model.   Keywords: Bank Erosion, the NBS Model, Haft Cheshmeh River.   References: Akhtar, M. P., Sharma. N., Ojha, C. S. P. (2011). Braiding Process and Bank Erosion in the Brahmaputra River. International Journal of Sediment Research, 26(4), 431444. Bandyopadhyay, S., Ghosh, K., De, S. K. (2014). A Proposed Method of Bank Erosion Vulnerability Zonation and Its Application on the River Haora, Tripura, India. Geomorphology, 16, 111121. Ghosh, K. G., Pal, S., Mukhopadhyay, S. (2016). Validation of BANCS Model for Assessing Stream Bank Erosion Hazard Potential (SBEHP) in Bakreshwar River of Rarh Region, Eastern India. Journal of Modeling Earth Systems and Environment, 95(2), 115. Islam, M. (2000). River Bank Erosion and Sustainable Protection Strategies. 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