بررسي فعاليت‌هاي تكتونيكي بر ژئومتري رودخانه (مطالعه موردي: رودخانه خرتوت از حوزه آبخيز اترك)

Investigation of Tectonic Activity Effects on the River Channel Geometry and Hydraulic (A Case Study: KhartutRiver)

مطالعه سیستم‌های رودخانه‌ای در ارتباط با جنبه‌های مختلف محیط طبیعی شامل هیدرولوژی، هیدرولیك، فرآیندهای فرسایش و رسوب‌گذاری و همچنین شكل كانال‌های رودخانه‌ای است. برای درك صحیح از شرایط طبیعی رودخانه، استفاده از مدل‌های مفهومی و پردازش تئوری، یك ضرورت محسوب می‌شود. در این تحقیق مدل هیدرولیكی HEC-RAS از طریق الحاقیه HEC-Geo-Ras جهت بررسی خصوصیات هندسی و هیدرولیكی رودخانه خرتوت واقع در استان خراسان شمالی مورد استفاده قرار گرفت. پس از اجرای مدل، تجزیه و تحلیل نتایج خروجی از مدل برای دبی با دوره بازگشت 25 ساله، برای 25 بازه كه 10 بازه آن محل تلاقی گسل‌ها و 15 بازه محل اتصال شاخه‌های فرعی به رودخانه اصلی بوده، انجام شده است. بهترین معادله از روابط رگرسیونی كه دارای كمترین خطای استاندارد و ضریب تشخیص بالاتر بود، انتخاب گردید. از طرفی جهت بررسی تأثیر فعالیت‌های تكتونیكی بر مشخصات هندسی و هیدرولیكی رودخانه، از تحلیل مورفومتری و شاخص نسبت پهنای كف دره به ارتفاع آن (Vf) استفاده شده كه این شاخص فعالیت‌های تكتونیكی را در 4 طبقه، فعالیت كم، متوسط، زیاد و خیلی زیاد نشان می‌دهد. در برخی مقاطع وجود گسل‌ها سبب افزایش عمق، عرض، شیب و شعاع هیدرولیكی و در بعضی مقاطع سبب كاهش این پارامترها شده است. نتایج نشان می¬د هد كه فعالیت تكتونیكی در طول بازه سبب فروافتادگی یا برخاستگی كف بستر رودخانه می‌شود كه تأثیر بسزایی در تغییر وضعیت ژئومتری رودخانه دارد.

1. Introduction The analysis of drainage networks is a powerful tool to detect recent tectonic activity and uplift, as river channels are very sensitive to changes in the parameters that control their shapes and. Climate changes, tectonics and lithology affect river equilibrium conditions and hence, river geometry. A river is in dynamic equilibrium when erosion keeps pace with sedimentation and under these conditions a river can be considered to be graded. River channel profiles and various geomorphologic variables, including temporal variations of at-a-station hydraulic geometry (e.g., Park 1977) The results have implications for the late-Cenozoic topographic and geomorphic evolution of the southern Rocky Mountains as being related to erosional exhumation and epeirogenic uplift (Frankle, 2002). Geomorphological analyses allow the study of modifications that affect hydrographic basins, particularly modifications due to active tectonics, and the quantitative description of landforms. The evaluation of geomorphic indices may be used to appraise the influence of active faults on the hydrographic network. (Guarnieri, 2008). Ratios of Valley floor width to valley height (Vf) values being one of the best morphometric parameter for tectonic studies were calculated along of Tista River and its tributaries especially in the vicinity of Thrusts (Tajbakhsh, 2009). In the Sikkim area (northeast of India), longitudinal profiles of rivers are prepared and these profiles show six Knick points on Tista River and six Knick points on tributaries Rangpo and Rongli that indicates strong structural control in determining the course of the river located close to the major thrust zone (Tajbakhsh, 2009). The aim of this study is to investigate the interactions and possible feedbacks between morphometric and tectonic processes especially along Khartut River. Since any changes in the geometry of the river will affect to flood condition sand cause serious damages to the rivers. Analyze the temporal change of hydraulic geometry at individual cross-sections and its spatial changes along the river course as it flows from mountainous bedrock to alluvial floodplain valleys. 2. Study Area The study area is located between56° 54 '- 57° 7 ' E longitudes and 37° 55 '- 37° 59 ' N latitudes in the north Khorasan province covering an area of about 371.92 km2.The study area is characterized by deep gorges and rugged mountain. Topographical elevation varies between 973 m at outlet and 1747 m at Raz and Aqchil mountains and the average elevation is about 1349 m. Raz city and 7 villages are located around the main river channel. The upper portion of the area is rocky and remains under snow during the winter months. The middle portion is mostly covered with Sanganeh and Atamir formations and the lower portion is covered by Quaternary sediments occur along the main river. 3. Material and Methods The physiographic factors of the study area have been extracted in detail on the basis of DEM generated from 10 m counter from topographic maps at 1:25000 scale and elevation points from surveying in 1:500 scale. Natural conditions of cross sections in the river and each section of the coast were simulated with ArcGIS software and Hec-GeoRas extension. The calculation of the water surface profile was performed byHecRassoftware.25 cross-sections had chosen that included 10pointsare intersection of faults and15points are subsidiary water course confluence with the main river. Also the hydraulic parameters have been calculated in Hec- Ras software. Morphometric parameters have allowed us to comment about land form change in feature. For example Vf factor have direct relation to active tectonics and uplift. Morphometric calculations also conclude that the similar results according to tectonic activity, uplift, erosion and chances of flood. The ratio of valley floor width to valley height (Vf) may be expressed as (Vf) = 2Vfw / (EId-Esc) + (Erd-Esc)-3 Where Vf is the valley floor width to height ratio; Vfw is the width of valley floor; EId and Erdare elevations of the left and right drainage/ valley divides respectively and Esc is the elevations of the valley floor (Bull and McFadden, 1977). Then the river profile was preparedfor10 intersections of faults with a river to length of 400meters, with using ArcGIS software. 4. Results and Discussion Investigation of the relationships between geometry and hydraulic of Khartut River, has led in a series of mathematics and empirical relations between the characteristics of the river and the other parameters. These relations are obtained the regression from geometrical and hydraulic parameters .Regression relationship is established between the dependent variable means discharge and independent variables means width, depth, slope, hydraulic radius of the stream. The equations are given follows that: D = 0.241Q0.36 r= 56 -3 W= 0.003Q 2- 0.152Q + 15.25 r= 79 -4 S = 0.174Q-0.56 r = 58 -5 R= -0.0001Q2 + 0.0179Q + 0.2932 r = 55 -6 Where, D: Flow Depth (m), W: Flow Width (m), S: slop (m/m), R: Hydraulic radius (m), Q: flow rate (m/s3). Ratios of Valley floor width to valley height (Vf) values being one of the best morphometric parameter for tectonic studies were calculated along of Khartut river and its tributaries especially in the vicinity of faults. The values obtained for majority locations show relatively low values of Vf indicating the high tectonics activity is dominant. Thus it is inversely related to tectonic activity i.e. high values of Vf are associated with low values of uplift rates in the study area. In the present study, majority of Vf values are significantly lower than 0.5 in most of the main tributaries of each sub-basin and on all the investigated faults. Based on the classification, the river is active. Also Vf value for sections 4 and 7 are close to 0.1 that are very active. These sections are distinct for others hydraulic parameters. As the speed and shear stress increased in the two sections that could have an important role in the degradation of the bed river. Investigation of the river profiles for 10 sections, were examined knick points and the impact of geological factors. For example, river profiles of numbers 1 and 6 indicate that faults caused knick points as iffault1 has created 8meterheight change in less than200meterslength. Faults in sections2, 3, 5, and 10did not make any change in river profile but in sections 4, 7and 8,are causing changes in the river profile, that will impact on the water surface profile and then the rise up river flood zone. The uplift is significant in river profile of section 8(Fig 6). 5. Conclusion Knick point’s behavior is a key to understanding to the landscape response. A knick point may also be a disequilibrium steeping in response to a relative fall in base level. Such disequilibrium in Knick points in bed Rock Rivers are commonly triggered by surface uplift. The results show that tectonic activity during river is causing uplift and down lift of the bed river that has a significant impact on the changing status the river geometry and any change in the geometry of the river, increase flood zone, making river sand natural hazards. The water surface width responded significantly to changes in discharge in along Main River