حساسیت شاخص های شکل و تحولی مئاندرها به پویایی رودخانه‏ های کوچک (مطالعۀ موردی: رودخانۀ قره سو در کرمانشاه)

نوع مقاله: مقاله علمی پژوهشی

نویسندگان

1 دانشیار گروه جغرافیا، دانشکدة ادبیات و علوم انسانی، دانشگاه رازی

2 دانش ‏آموختة ژئومورفولوژی، گروه جغرافیا، دانشکدة ادبیات و علوم انسانی، دانشگاه رازی

چکیده

برای بررسی وضعیت پویایی موج‏ها و مئاندرهای رودخانة قره‏سو مسیر رودخانه در چهار دورة زمانی بر اساس عکس‏های هوایی سال‏های 1334 و 1346 و تصاویر ماهواره‏ای IRS سال‏های 1383 و 1388 در محیط نرم‏افزار GIS رقومی‏ شد و فاکتورهای شکل مانند شعاع مئاندرها، طول موج، دامنه، و عرض ساحل طغیانی و شاخص‏های تحولی مانند ضریب فعالیت مجرا، میزان جابه‏جایی عرضی، و مساحت تصرف‏شده توسط فعالیت مجرا از آن‏ها استخراج شد. نتایج آزمون‏های آماری نشان داد که در چهار دورة زمانی هیچ اختلاف معنی‏داری بین فاکتورهای شکل رودخانه وجود ندارد؛ ولی شاخص‏های تحولی و شاخص‏های ترکیبی نشان دادند که رود در سه دورة زمانی اول به سمت تعادل ایستا و در دورة چهارم به پویایی تمایل دارد. این شاخص‏های تحولی می‏توانند با دقت بیشتر به شیوة دیگری نیز استخراج شوند. از دو دوره تصویر ماهواره‏ای 1386 و 1394 پهنای رود ترسیم شد و ترکیب آن‏ها با دستور یونیون بازه‏هایی از رودخانه را، که دچار تغییرات مساحت و جابه‏جایی ‏شده بودند، به‏دقت نشان داد. در رودخانة قره‏سو پویایی رودخانه به بازة مستقیم‏شده در میان‏دست رود و همچنین چند بازة پایین‏دست آن متمرکز شده است.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

Sensitivity of Form and Evolutionary Parameters of Meanders to Small Rivers Dynamics, (Case Study: Ghere – Sou River in Kermanshah)

نویسندگان [English]

  • Iraj Jabbari 1
  • Tahere Rahimi Javid 2
1 Associate Professor of Geography, Faculty of Literature, Razi University, Iran
2 MA in Geomorphology, Faculty of Literature, Razi University, Iran
چکیده [English]

Introduction
Shield (2000), using some geometric activity parameters, explains how reservoir can be effective on downstream river channel migration. After that, Magdaleno and Fernández-Yuste (2011) recognized that these parameters may complement the classical form parameters and represent the real functioning of the river corridor, in geomorphological analyses of meander dynamics. However, the effectiveness of these indices is not clear on other channels where meandering is not very developed and it is not clear if or not the geometrical parameters can indicate the type of functional and dynamics of this type of rivers.
Materials and methods
We have selected an area about 52 km wide along the total length, 219 km, of the Ghere- Sou River as the study area; this is because of location of Kermanshah Plain on a fault line that allows smaller changes. The study of the sinuosity of the river during the 54 years (1955 to 2009) shows that the number of meanders has been increased and decreased constantly.  The meanders are included just a quarter of windings, at this study.
To study the dynamics of sinuosity, we used form parameters such as the radius of curvature, wavelength, amplitude, meander length, and evolutionary parameter such as bankfull width, magnitude of channel lateral migration, area occupied by the active channel and channel activity coefficient to determine the evolution of the meander belt in the central sector of Ghere-Sou River.
At first, aerial photos from 1954 and 1967 (black and white; approximate scale 1:55,000 and 1:20,000) and satellite images of IRS 2004 and 2009 (colour; with 2 m. resolution) were digitized and the factors was measured in ArcGIS. Using the Kolmogorov-Smirnov normality test for three form factors (wavelength and the radius of curvature and amplitude), Friedman tests were examined for abnormal data and ANOVA test for normal data.  Finally, the spatial distribution for the morphology parameters was analyzed in order to determine if they showed a change downstream. Comparing the form and geomorphic activity parameters elucidated which groups of parameters are powerful to show the dynamics of river.  
Results and discussion
Ghere-Sou River has curvatures as sinuosity and meanders at 81 points, totally. The curvatures numbered 1 to 81 from up to downstream. Comparing these points at four years (1955, 1967, 2004 and 2009) showed that sinuosity has decreased at some points and it has increased at some other points. Thus, we can see that the number of arcs has been changed from 77 to 79 and then 77 and finally to 65. The sinuosity characteristics of arcs have been changed through these periods. However, three parameters including radius of curvature, wavelength, and amplitude have suffered from very little fluctuations. The results of Friedman and ANOVA show that there is not significant difference between them. However, other parameters including meander length, bankfull width, magnitude of channel lateral migration, the area occupied by the active channel and channel activity coefficient have indicated a progressive trend from 2005 to 2009.  
A glance at the average bankfull width of river in different years show that this factor was about 108 m in 1955, 95 m  in 1967,  and 77 m in 2004 and finally reached to 89 m in 2009. This means that river has been more dynamic during last 5 years and it has added 11 meters to its width on average. The same is also true about average of lateral migration of river, so that the first until 2.5, 0.92, and 3.5 m per year, respectively. The total area occupied by means of channel activity had increased in the period. Channel activity coefficient is reached from 2.51 at the first period to 0.82 at the second period and finally to 2.76 at the third period.  Therefore, the river is desired to have a state of static equilibrium at the first and second period and now try to have a state of dynamic equilibrium. The evolutionary changes are conducted in GIS and cause to promote the analysis capacity.
The river has been divided into 11 reaches in the two Google earth images in 2005 and 2015 and they were union together at the ArcMap. The results illustrate that the river has had up to 70% overlapping in the fifth reach. However, the changes of river have become more apparent at sixth reach while going to be channelized. This situation is to be the same downstream because of river balancing nature.
Conclusion
This study about the dynamics of Gher-Sou River elucidated that only the form parameters (wavelength, amplitude, curve radius) don’t explain evolutionary characteristics and we need some morphological parameters such as magnitude of channel lateral migration, the area occupied by the active channel and channel activity coefficient. Some compositional parameters such as radius of curvature to the average bankfull width can also show these changes. These results confirmed by Magdaleno and Fernández-Yuste (2011) highlighted dynamic equilibrium of Ghare-Sou River in Kermanshah plain since 2004. The dynamics are coincided with urbanization and the expansion of activities and projects implementation.
Further study on the parameters may be a cause to produce more accurate parameters in relation to evolutionary characteristics; for example, the remaining of active channels overlapping may provide better results in some cases. The parameter is obtained from overlaying of two periods and calculation of non-intersected area. Using GIS, it can be obtained by combination of different layers of the area.  

کلیدواژه‌ها [English]

  • river dynamics
  • Ghare-Sou River
  • geomorphology
  • channel change
  • meander
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