تحلیل کمّی و مورفولوژیکی نیمرخ طولی رودخانه‏ های البرز شمالی در استان مازندران

نیمرخ طولی رود یکی از مولفه‏های اساسی سیستم رودخانه‏ای است. در این تحقیق نیمرخ طولی 15 رودخانه در البرز شمالی، که بخشی از حوضه آبریز دریای خزر هستند، بررسی شده‏اند. نخست، با استفاده از مدل ارتفاعی رقومی، با قدرت تفکیک 12.5 متر شبکه‏های رودخانه‏ای شناسایی و نیمرخ طولی آن‏ها ترسیم شد. شکل نیمرخ طولی رود با شاخص تقعر (ci) و شاخص slk اندازه‏گیری شد. سپس، شاخص گرادیان طولی رود (sl) برای هر قطعه از نیمرخ طولی محاسبه شد و با استفاده از نقشه‏های سنگ‏شناسی و گسل‏ها تفسیر شد. نتایج نشان می‏دهد که شکل نیمرخ طولی رودخانه‏های البرز شمالی در دره‏های عرضی به‏صورت مقعر با چندین شکستگی است که بیانگر غلبه فرسایش در بلندمدت است. در دره‏های طولی البرز شمالی، نیمرخ طولی رودخانه‏ها به‏صورت محدب است که ناشی از ساختار چین‏خوردگی و بالاآمدگی سنگ‏هاست. بررسی شاخص sl نشان می‏دهد که عامل سنگ‏شناسی و گسل‏ها به‏ترتیب 53 و 45درصد در ایجاد شکستگی در نیمرخ طولی این رودها نقش داشته‏اند. از مجموع 98 خط گسلی، که به‏صورت عرضی رودخانه‏ها را قطع کرده‏اند، فقط بیست‏درصد آن‏ها موجب شکستگی در نیمرخ طولی رود شده‏اند. اثر گسل در نیمرخ طولی رودخانه‏های غرب و شرق استان مازندران به‏ترتیب 70 و 30درصد بوده است.

Quantitative and Morphological Analysis of Longitudinal Profile of Northern Alborz Rivers in Mazandaran Province

Introduction The longitudinal profile of river is one of the main components of the fluvial system, which is result from the interaction between lithology, tectonics, fluvial incision and base level change. The longitudinal profile of the rivers that are in a Equilibrium state have concave form, but several factors cause changes to the longitudinal profile of the river, including, lowering base level, rock resistance, structural control, sediment input, nonfluvial processes, bed load effects, human changes and insufficient time. Alborz Mountains are considered as one of the geomorphological zone of Iran, which geological, climatological and vegetation characteristics have created a special geographic landscape. In this study, longitudinal profiles of the northern Alborz rivers (Mazandaran province), which are part of the Caspian Sea basin, have been investigated. The aim of this research is to quantitatively analyze the longitudinal profile of the rivers in the region. Methods and Material The longitudinal profile of 15 rivers in the northern Alborz were investigated. The drainage network was extracted by the 12.5m DEM and Longitudinal profiles of the fifteen trunk channels were extracted using the ArcGIS 10.3. Geological maps were used to interpret lithology and fault distribution. The form of longitudinal profile was measured by the concavity index (CI) and SLK index. The longitudinal gradient index (SL) was calculated for each segment of longitudinal profile. concavity index was computed based on deviations from a straight line profile (Philips and Lutz, 2008). The SL Index analysis was performed with Hack’s (1973) method.The SLK index for the longitudinal profile of the river was calculated by normalizing the distances in the horizontal axis and the elevation in the vertical axis. To normalize values (SLK), in the horizontal axis of the graph, the normalized distances are represented as (d / D), where d is the distance between the specific points along the longitudinal profile and D is the total length of the profile. In the vertical axis of normalized height (e / E) it is shown that e is the height of specific points along the longitudinal profile and E is the height difference between the beginning and the end of the longitudinal profile (Vojtco et al., 2012). The maximum amount of concavity along the SLK profile was determined as Zmax. Results and Discussion The northern Alborz rivers are not smoothly concave profiles and there are numerous knickpoints in their longitudinal profiles. In all cases, the best fit (R2) of regression has been matched with polynomial equations of degrees 2, 3 and 4. the coefficient of determination of these regression equations was very high (>0.96). Some of rivers such as Kheyrood, Kojur and Noor have a convex profile with concavity index of 4, 4, and 11respectively. A number of other rivers such as Tajen, Babol, Sardabrood and Chalakrood with CI 8, 7.5, 9 and 8 respectively have more concavity than other rivers. Based on the relative concavity index (CIrel), 20% of the longitudinal profile of the North Alborz in the convex form (CIrel <0), 53% had a relatively straight or very low concave (0 Regarding the good correlation coefficient (0.73) between the SLK index and the CI index, the SLK index is used for clustering. The studied rivers were classified into three groups based on concavity of longitudinal profile and the position of maximum concavity. The first group includes rivers with a concavity of between 0.13 and 0.47 and the maximum concavity is in the second quartile of the profile. In this group, 80% of the North Alborz rivers are located and they are all in the transverse valleys. In the second group, the amount of concavity index of longitudinal profile was negative, that is, they have a convex shape and maximum convexity is in the third quartile. The rivers in the longitudinal valley are in this group. In the third group, the concavity is less than 0.1 and the maximum concavity is in the second quartile of the profile. The analysis of longitudinal gradient index (SL) in the northern Alborz rivers shows that 53% of knickpoints are due to lithologic changes, 45% due to the activity of faults and 2% due to dam construction. Of the 98 fault lines that crossing rivers, only 20% of them have broken the longitudinal profile. The effects of lithology on longitudinal profile were analyzed by statistical test.The results show that the sig value is less than 0.05 and the rejected zero hypothesis in the Safarood, Cheshmeh Kileh, Sardabrood, Chalous, Noor, Heraz, Talar and Nekah catchments, That is, the type of lithology has a significant difference in longitudinal profile formation. The effect of faults in the longitudinal profile of the rivers west and east of Mazandaran province was 70% and 30%, respectively. Conclusion In the longitudinal valleys, the rivers have a convex profile, where uplift (active tectonics) is dominant. In these valleys, the rate of uplift is greater than the amount of river incision and the river can not create a equilibrium profile.The longitudinal profile of the rivers in the transverse valleys is Concave–convex with erosion steps that indicate longterm predominance of erosional processes. Their profile is due to the high altitude difference, short distance to the base level (Caspian Sea) and lithological resistance. It seems that river icision has overcome the tectonics and the lithological factor has more effect on the longitudinal profile of the Northen Alborz rivers.