تحلیل تغییرات جانبی مجرای رودخانه زرینه‌رود با استفاده از روش‌های ژئومورفومتریکی

نویسندگان

چکیده

مجرای رودخانه­های آبرفتی سیستم­های دینامیکی هستند که در معرض تغییرات مختلفی می­باشند. در این رابطه، جابجایی مجرا و فرایندهای مرتبط باعث مخاطراتی از قبیل آب شستگی پل­ها، تخریب جاده­های ارتباطی و از بین رفتن اراضی می­شود. ازاین‌رو، پایش و نظارت بر تغییرات مجرای رودخانه­ها ازجمله اقدامات اساسی در زمینه مدیریت رودخانه­ها و دشت­های سیلابی می­باشد. در تحقیق حاضر، تغییرات جانبی مجرای رودخانه زرینه­رود (جیغاتی­چای) در طی 30 سال گذشته بررسی‌شده است. این پژوهش متکی بر کارهای میدانی و آزمایشگاهی، تصاویر ماهواره­ای، نقشه­های توپوگرافی و زمین­شناسی و داده­های ایستگاه­های هیدرومتری می­باشد. به­‌منظور تحلیل­های کمّی از روش­ها و شاخص­های ژئومورفومتریک به همراه مدل HEC-RAS استفاده شد. مجرای رودخانه در بالادست سد انحرافی نوروزلو به دو بازه (از ابتدای بازه مطالعاتی تا شهرک صنعتی شاهین­دژ به‌عنوان بازه اول و از این شهرک تا محل سد مذکور به‌عنوان بازه دوم) و از پایاب سد انحرافی نورزولو تا چاله ارومیه به سه بازه (از پایاب این سد تا شهر میاندوآب به‌عنوان بازه سوم، از میاندوآب تا روستای چلیک به‌عنوان بازه چهارم و از این روستا تا مصب به‌عنوان بازه پنجم) تقسیم­بندی شد. پلان­فرم مجرا در بازه­های بالادست سد انحرافی از نوع مئاندری با میان­برهای شوت (میان­پشته­ای) می­باشد. از پایاب سد نوروزلو تا شهر میاندوآب، درنتیجه دخالت­های انسانی تبدیل به شبه­مئاندری شده و سپس دوباره الگوی مئاندری ظاهر می­شود. با اینکه توان رودخانه در بازه اول نسبت به سایر بازه­ها زیاد است اما به علت مواجهه با یک بستر آرمورینگ و برخورد کناره­های رودخانه به واحد کوهستان، از انجام کار ژئومورفیک محدودی برخوردار می­باشد. درحالی‌که توان نسبتاً بالای رودخانه، افزایش پهنه­های سیل­گیر، پوشش گیاهی پراکنده، کنترل محدود متغیر زمین­شناسی و فرسایش­پذیری زیاد مواد کناره باعث شده است که بازه دوم دارای بیشترین میزان دینامیک عرضی باشد. از پایاب سد نوروزلو تا شهر میاندوآب به علت برداشت شن و ماسه بستر و کناره­ها، رودخانه در بستری عریض و گود افتاده جریان می­یابد و ارتباط آن با دشت سیلابی خود قطع‌شده است. این شرایط تا حدودی در بازه پایین­دست نیز دیده می­شود و بنابراین ازنظر دینامیک جانبی، غیرفعال محسوب می­شود. در بازه انتهایی نیز خاصیت چسبندگی رس با توان پایین رودخانه و شیب ملایم کناره­ها توأم شده و توسعه و مهاجرت مئاندرها از آهنگ پایینی برخوردار شده است. نتایج نشان­دهنده افت چشمگیر دینامیک جانبی مجرا در طی 15سال گذشته است. شاخص­های ژئومورفومتریکی برای بازه­های پایاب سد نوروزلو نشان­دهنده دینامیک بسیار پایین و تنگ­شدگی مجرا در طی این بازه زمانی می­باشد. دلیل اساسی این امر مربوط به کاهش شدید دبی رودخانه (تقریباً دوسوم) و فعالیت­های انسانی می­باشد. کاهش دینامیک جانبی در بازه­های بالادست سد نوروزلو به‌صورت افت نرخ مهاجرت جانبی مجرا، کاهش قابل‌توجه رخداد میان­برهای شوت و تمایل بیشتر رودخانه به الگوی مئاندری نمایان شده است.

کلیدواژه‌ها


عنوان مقاله [English]

Analyzing Lateral Changes of the Zarrineh-Roud River Channel Using Geomorphometric Techniques

چکیده [English]

Introduction
Rivers are fed by water and sediment, and therefore respond to the inherent fluctuation of these inputs, often resulting in a continuous dynamic state. These fluctuations can occur naturally (droughts, floods), or be human induced (dams, hydraulic control structures) (Sixta, 2004: 13). The fluvial system changes progressively through geologic time, as a result of normal erosional and depositional processes, and it responds to changes of climate, base level, tectonics, and human impacts (Kondolf and Piegay, 2003: 105). On the other hand, River channel self-adjustment and variability may create hazards for humans in the form of floods, bank erosion, or bed scour around bridge piers (Wohl, 2004: 29). In this study, lateral changes of the Zarine Roud river channel (Jigati-Chay) have been investigated in during the last 30 years. Zarrineh river channel in the upstream Norouzlou diversion dam was divided into two reaches) from the beginning of the study reach to Shahindejh industrial town as the first reach and from the this town to the Norouzlou dam as the second reach) and in the downstream dam to Urmia Lake divided into three reach (from downstream of this dam to Miandoab city as the third reach and From Miandoab city to Chelik village as the fourth reach and From the village to the Urmia lake as the fifth reach)
Methodogy
The most important data of the present study include topographic map scale of 1: 2000 (West Azerbaijan Regional Water Authority), geotechnical data (West Azerbaijan Regional Water Authority), Topographic map scale of 1: 50,000 and 1: 25,000, geological maps scale of 1: 100,000, Satellite images of Landsat (1985, 1990, 2000, 2007, 2015), ASTER (2000 and 2005), SPOT (2005), IRS (2005) and Google Earth. Also, data from Sari-Qamish and Nezam-Abad hydrometric stations located in the main stream and Qureh-Chay and Janaqa stations on tributaries were used for calculation of return periods and discharge–stage relation. Laboratory equipment including global positioning system (GPS) Van Veen Grab-Bottom Sampler, shakers, digital scales, and caliper were used for Sediment particle size analysis (granulometry). In this study, GIS & RS software includes Google Earth, ENVI and Arc GIS software with HEC-GeoRAS and Planform Statistics extensions; hydrodynamic HEC-RAS software and MATLAB software was used. In this research, field studies are basis for geomorphological analysis. The methodology and models used in this study can be summarized in processed satellite images to study changes river channel, the extraction of vegetation and land use; HEC-RAS hydrodynamic model to simulate the flow in the river channel and floodplain; Geomorphometric indices for quantitative analysis of planform and lateral changes in the river channel.
Results and discussion
According to Geomorphometric indices, field studies and the results of HEC-RAS model, morphological changes in the river channel Zarineh can be summarized:

Planform of Zarineh river channel tend to meandering pattern, but according to local control variables, threshold behaviors (meander- braided) observed.
During the period of the study (2015-1985), despite the cutoffs Occurrences and lateral migration, the mean of sinuosity and Central angle of channel meanders had not changed dramatically. This indicates that the river is in dynamic equilibrium (Table 1).
In the past 15 years, a decreasing trend in the migration rate of river channel can be seen. Reduce lateral dynamics channel especially in downstream reaches of Norouzlou diversion dam been remarkable. Reduced lateral mobility of channel primarily related to discharge decrease, especially bank full discharge as channel forming discharge, and thus reduce the stream power.
Narrowing of channel, which is primarily due to reduced lateral dynamics channel. This leads to the deposition processes of erosion processes have been overcome. Occupation and establish natural vegetation on the active point bar and encroaching agricultural land to near river land and river flood plains from other important factors are narrowing of Zarineh roud river.
Tend to single-channel meandering pattern in the reaches which previously had threshold behaviors. This process, especially resulted in overcome the depositional processes in multi– channel and reduce the occurrence of chute cutoff. In the second period (2000 to 2015), the chute cutoff occurrence remarkable has been decline which can be attributed to reduce the peak and bankfull discharges. A slight increase of sinuosity and the central angle, is due to limited cutoffs.

Table 1. Some geomorphometric indices of Zarrineh-Roud River



Index
Reach (1)
Reach (2)
Reach (3)
Reach (4)
Reach (5)


Ave. Sinuosity (2015)
1.16
1.30
1.08
1.21
1.44


Ave. Central Angle (2015)
85.1
113.5
65.9
98.2
138.8


Migration rate (1985-1990) (m/year)
0.801
2.923
1.716
0.980
1.265


Migration rate (1900-2000) (m/year)
0.820
3.132
1.667
0.804
2.210


Migration rate (2000-2007) (m/year)
0.215
1.412
0.355
0.175
0.387


Migration rate (2007-2015) (m/year)
0.113
0.664
0.030
0.031
0.087


D50. Bed material (mm)
90
15
3
0.2
0.06


Total bankline change, E (m/year)
0.3866
1.886
0.907
1.007
0.974


Normalized lateral movement, N (% width/year)
0.0034
0.0215
0.0076
0.0087
0.0085


Change in Active Channel Width (m/year)
-0.0043
-0.1803
-0.4008
-0.5492
-0.3261


Lateral Stability
0.9809
0.8211
0.9236
0.9030
0.9097


Actual Migration rate (m/year)
0.152
1.085
0.075
0.040
0.095



In the reach (1), meandering planform developed only in parts of the valley with the relatively developed floodplain. The morphology of this reach is largely controlled by geological variables. In most parts, the development meandering is limited due to connect to the mountain. Thus, in this reach, there are no neck cutoff. In the reach of (2) and reach (1), river channel planform is meandering with chute cutoff. In the fluvial geomorphology, chute cutoffs considered as a threshold behavior (transition from meandering to braided pattern). This type of threshold behavior due to the high variability of the bank materials erodibility, role of vegetation in bank stability and frequency of floods. In the reach (2), increased flood prone area, limited control of geology variable and high erodibility of bank materials cause increased lateral channel dynamics. This reach have the most lateral dynamics along studied reach of Zarrineh Roud River. In this reach, lateral mobility indices have the most spatial variability. Hydrometric data and satellite images shows that from 2000 to 2015 river erosion has been reduced due to decrease of discharge (especially bankfull and overbank) and stream power. The morphology of two reaches in downstream of Norouzlou diversion dam are controlled by anthropogenic variables. Due to sand and gravel mining, the planform and river bed morphology has changed. So that, can be said that in these reaches sediment extraction more than sediment bringing. This leads to entrenchment of channel bed and disconnected connection between floodplain and river channel. In the reach (3) (from Noruuzlou dam to Miyandoaab city), channel planform conversion into straight and undeveloped meandering channel. In fact, the channel tendency to direct pattern caused by human intervention include sand and gravel mining and canalization in Miyandoaab city around. As a result, this reach in terms of natural lateral dynamics is a passive reach. In reach (4) human intervention is relatively high, also. However, the central angle for the most part reflect the developed meandering planform. However, scroll bars sediments located in convex bank are extracted, that if this process continue leading to the artificial cutoff and conversion into straight pattern. In the reach (5), meanders are highly developed. In this reach, anthropogenic interference significantly decreased due to the decline of soil quality and overcome clay and silt in the bed and banks materials. But because of the gentle slope of bed and banks, stream power is very low.
Conclusion
In this study, lateral dynamics of Zarrineh roud river channel in the recent 30 years were investigated. For this purpose, a collection of Geomorphometric quantitative variables was combined with field resuls. Channel Planform in the upstream reaches of diversion dam is meandering whit chute cutoff. From downstream Norouzlou dam to Miandoab city, as a result of human activity has become quasi meandering and then again meandering pattern appears. Although stream power in the first reach is high compared to other reaches but because of the armoring bed and connection of banks to the mountain, the geomorphological work is the limited. While relatively high stream power, increasing flood prone area, sparse vegetation, low control of geology variable and a high erodibility of bank materials caused the second reach has the highest lateral dynamics. Downstream of Norouzlou dam to Miandoab city due to the sand and gravel mining, river flows in the wide and entrenched bed and its connection with floodplain is disconnected. This condition can also be seen in downstream reach. Therefore in terms of lateral dynamics are passive. In final reach the cohesion properties of clay combined with low stream power and gentle slopes reflected in the low migration rates. The results shows a decline in channel lateral dynamics over the past 15 years.

کلیدواژه‌ها [English]

  • Palnform
  • Lateral change
  • Geomorphometry
  • meander
  • Zarrineh Roud