کمی سازی تغییرات ژئومورفولوژیک رودخانه سفیدرود و بررسی ارتباط آن با ویژگی‌های هیدرولیکی سیلاب

رودخانه ها سیستم های دینامیک ژئومورفولوژیک هستند که دائماً در حال تغییر و تحول می باشند. این تغییر و تحولات بسته به شرایط توپوگرافیک، ژئومورفولوژیک، هیدرولوژیک و هیدرولیک متفاوت است. پژوهش حاضر با هدف کمی سازی تغییرات ژئومورفولوژیک رودخانه در سه بازه تقریباً 25 کیلومتری در طول رودخانه سفیدرود در استان های کردستان - بازه یساول؛ زنجان - بازه گیلوان و مازندران - بازه آستانه بررسی و ارتباط آن ها با شدت سیلاب انجام شده است. شدت سیلاب از ترکیب عمق و سرعت سیلاب با دوره بازگشت 25 ساله با مدلسازی هیدرولیکی به وسیله نرم افزار hec ras در بازه های مورد مطالعه بدست آمد. چهار شاخص شریانی کانال (b)، سینوسیته کانال (p)، نرخ جابجایی جانبی(rm) و پایداری کانال رودخانه(s) در بازه های مورد مطالعه بدست آمدند.  نتایج حاصل موید آن است که شاخص شریانی بازه یساول به دلیل دخالت های کم تر انسان و وجود پشته های رسوبی و جریانات چند شاخه بیشتر از دو بازه دیگر است. شاخص سینوسیته بازه آستانه به دلیل مورفولوژی جلگه ای کناره ها بالاتر بود و میزان جابجایی جانبی این بازه نیز تحت تاثیر همان عامل بیشتر بود. دوبازه گیلوان و یساول به دلیل محدود بودن کناره ها پایداری بیش تر و سینوسیته و جابجایی جانبی کم تری داشتند. نتایج بررسی ارتباط شاخص های ژئوموفولوژیک رودخانه با شدت سیلاب با دوره بازگشت های میان مدت نشان داد که برخلاف انتظار، در نواحی با شدت سیلاب بیشتر تغییرات کانال رودخانه کمتر است. که دلایل عمده آن را می توان با تاثیرگذاری هندسه مقطع کانال و شیب طولی کانال رودخانه مرتبط دانست. چرا که از نظر مورفولوژیک در نواحی عمیق کانال رودخانه، امکان جابجایی کم تر است لذا رودخانه پایدارتر می باشد. همچنین در شیب های تند جریان رودخانه اعم از پایه و سیلابی سرعت و در نتیجه شدت بیشتری دارد و کانال را بیشتر حفر می کند. کانال عمیق تر قاعدتاً توانایی عبور سیلاب های با دوره بازگشت کم تر را دارد و امکان سرریز شدن آب به اراضی مجاور و ایجاد مسیرهای جدید کاهش می یابد. 

quantification of geomorphological changes of sefidroud river and investigation of their relationship with hydraulic characteristics of flood events

introduction rivers are dynamic geomorphological systems that continuously change due to various influencing parameters. morphological changes in the river channel primarily involve adjustments in channel width, depth, slope, and river planform. the extent of lateral migration of the river channel depends on factors such as bank resistance to erosion, the duration and magnitude of flow, the curvature radius of the channel, and the flow capacity to transport sediments. quantifying geomorphological changes in rivers with an emphasis on flood hydraulics requires detailed analysis and complex modeling of the dynamic interactions between water flow, sediment deposition, and riverbed alterations. flood severity, characterized by high discharge, significant flow depth, and extreme flow velocities, can have substantial impacts on riverbed structures, including changes in depth, width, channel meanders, as well as sediment transport and deposition. the sefidroud river is one of the largest rivers in iran, directly and indirectly affecting the lives of a significant population. throughout history, there has been a strong inclination to settle near this river, with numerous urban and rural settlements located along its course. the occurrence of frequent floods in the past and the river’s tendency to alter its channel, especially in the lowland regions, make the delineation of floodplains and prediction of future river behavior essential aspects of this research. therefore, identifying and quantifying the geomorphological changes of the sefidroud river and investigating the relationship between these changes and flood severity during mid-term return periods are key objectives of this study.2- results  in the present study, four indices were calculated and used to quantify the river’s morphological changes: channel braiding index (b), channel sinuosity index (p), lateral migration rate, and channel stability. the study reaches showed minimal variation in terms of the braiding index, with the yasaval reach having the highest value, which is 0.07 higher than the astaneh reach, which had the lowest value. the lower braiding indices of the gilvan and astaneh reaches, compared to yasaval, can be attributed to the higher severity of human activities in the riverbed and the presence of the sefidroud dam upstream of the astaneh branch. regarding the channel sinuosity index (p), there is an inverse relationship between flow severity and shear stress with the sinuosity index. reaches with lower flow severity and reduced shear stress tend to have higher sinuosity. concerning the lateral migration rate, the results showed that this index is strongly influenced by the morphology and topography of the riverbanks. in the yasaval and gilvan reaches, which flow through mountainous regions, the lateral migration rate is lower (1.3 and 1.93, respectively) due to the rougher and more restrictive topography. the equations related to channel stability and migration rate indicate an inverse relationship: as the channel’s migration increases, its stability decreases, and vice versa. flood severity, which is the product of flow velocity and depth, can lead to significant changes in the river channel, meanders, and sediment distribution. the results indicate that, contrary to expectations, in areas with higher flood severity during a 25-year return period, river channel changes are less pronounced. this phenomenon is related to factors such as channel depth and longitudinal slope. deeper channels, where flood severity is greater, exhibit higher stability and therefore experience fewer changes. in contrast, wider and shallower sections of the river, with slower flow, tend to undergo more substantial alterations.3- discussion and conclusion  numerous studies have been conducted on river morphology changes across various regions worldwide, employing a wide range of methods. in this study, several of these methods were used to quantify and examine the geomorphological changes in the selected study reaches. however, a key aspect of this research is the relationship between flood severity and geomorphological changes in rivers. most studies in this field have focused on the impact of one or several extreme flood events on river channel changes, typically employing a before-and-after comparative approach. however, this research utilizes hydraulic modeling, based on validated 1:1000 maps and hydrometric data, to comprehensively examine the overall effect of flood severity on river morphology and channel changes. the results were then compared with geomorphological change maps, providing a broader perspective on the relationship between flood dynamics and river morphology.the results indicate that the braiding index of the yasaval stretch is higher than the other two due to lower human interference and the presence of sediment bars and multi-threaded flows. the sinuosity index in the astaneh stretch was higher due to the floodplain morphology of the riverbanks, which also contributed to a higher lateral migration rate. in contrast, the gilvan and yasaval stretches, with their more confined banks, showed greater stability and lower sinuosity and lateral migration rates.the study also examined the relationship between the geomorphological indices of the river and flood severity during mid-term return periods. contrary to expectations, in areas with higher flood severity, channel changes were less pronounced. this can be attributed to the influence of channel geometry and longitudinal slope. in deeper sections of the river, morphological changes are less likely, resulting in greater stability. additionally, in steeper slopes, both base flow and flood flows exhibit higher velocities, leading to greater channel incision. deeper channels are more capable of handling floods with shorter return periods, reducing the likelihood of overflow and the formation of new channels.