Abstract:
از آغاز شکل‌گیری زمین، همواره250 مخاطره¬ی طبیعی فرسایش با آن همراه بوده‌است و در آینده نیز این پدیده‌ی مخرب، همچنان لندفرم‌های زمین را تغییر خواهد داد. هدف این مقاله، بررسی مورفومتری حوضه‌های آبریز در واحد مورفوتکتونیک زاگرس با استفاده از قاعده‌ی هورتن در زمینه‌ی وضعیت فرسایش است. برای این منظور، ابتدا کل واحد ژئومورفیک زاگرس به یازده حوضه‌ی آبی تقسیم و این حوضه‌ها در 271 زیرحوضه استخراج شد. در گام بعدی، رتبه‌بندی آبراهه‌ها به روش هورتن و استرالر صورت گرفت. سپس با استفاده از خصوصیات مورفومتری و روابط تجربی، وضعیت فرسایش‌پذیری زیرحوضه‌ها بررسی شد. ویژگی‌های مورفومتری نیز به سه دسته‌ی پارامتر شکلی، خطی و توپوگرافیک تقسیم شد. برای به دست آوردن طبقات فرسایش، پس از محاسبه‌ی پارامترها از همه‌ی عوامل برای محاسبه‌ی فرسایش میانگین گرفته و طبقه‌بندی آنها انجام شد. نتایج نشان داد هر چه مقدار میانگین کل پارامترهای مورفومتری کمتر باشد، شرایط برای فرسایش‌پذیری منطقه مساعدتر و وضعیت بحرانی‌تر است. طبقه‌بندی نشان می‌دهد که مساحت و طول آبراهه در فرسایش تأثیرگذار بوده‌است. در واحد مورفوتکتونیک زاگرس نیز طول جریان خشکی ارتباط مستقیمی با فرسایش دارد؛ افزایش تعداد آبراهه و طول آن در حوضه آبخیز بیانگر افزایش فرسایش است.
Extended abstract
1- Introduction
The purpose of this essay is to investigate the morphometry of catchment areas in the Zagros morphotectonic unit using the Horton principle of erosion status. Erosion is a natural phenomenon that has always been associated with the Earth since the formation of it, but over recent centuries it has taken an upward trend because of population growth, resources constraints, industry development and increasing human interference in natural ecosystems (Ghodsipour, 1395: 143). By eroding their bed for adjusting the bed profile, rivers increase their sedimentary load and its amount depends on the type of bed and the factors influencing the river speed (Majnoonian, 1378: 4). Production has always been expected to increase with the use of advanced agricultural methods, but unfortunately, this increase has been accompanied by a decrease in soil fertility due to erosion (Hudson, 1394: 469). The term “river morphometry” can be used to measure the geometric characteristics of a river. In fact, morphometry is a quantitative analysis of the geomorphic characteristics of the landforms of a region (Bayati Khatibi, 1388: 25). Morphometric analysis is one of the effective methods for prioritizing sub-basins, which can indicate the status of the basin drainage network (Mousavi et al., 1396: 250).
2- Methodology
For this purpose, the whole geomorphic unit of Zagros was first divided into eleven water basins. These water basins were then extracted in 271 sub-basins. The next step was to rank the channels using the Horton and Strahler methods. The Horton principle and the other researchers’ terms were used to investigate the erosion status of sub-basins. Morphometric characteristics divided into three categories: formal, linear, and topographic parameters. In order to obtain the erosion classes, after calculating the parameters, all the factors for calculating erosion were averaged and classified. According to calculations conducted, the coefficients were obtained that had to be classified and analyzed. The difference between the maximum quantity and the minimum quantity of the coefficients was obtained and the result was divided by five, and then it was summed with the minimum quantity of calculations and the obtained quantity, which was the smallest number, was coded as 1 and similarly, the next number was summed with the quantity obtained from (Max - Min / 5) and the code 2 was assigned to the answer obtained and all the numbers smaller than that. In the same way, up to 5 classes were classified.
3- Results
The result shows an inverse relationship between occurrence of erosion and average quantity of total morphometric parameters. In other words, a lower average quantity of the total morphometric parameters indicates a more prepared condition for erosion; therefore the situation will be more critical and threatening.
The classification shows that many factors have affected erosion, including; the area and length of the channel, permeability and inequality.
In the Zagros morphotectonic unit, the length of the dry stream is also directly related to erosion. Morphometric analysis shows the drainage network of the dendritic basin. A change in slope and topography might be a change in the flow-length ratio. Lithology, tectonics and the climate of that region have had a great impact on the erodibility of this unit. Also, the increase in the number of channels and their length in the drainage basin indicate an increase in erosion. Basins with very high ranking are likely to have high altitude, high slope and deep valley topography, which indicates a strong drainage structure and thus they are more exposed to intense motion of the brae.
4- Discussion & Conclusions
Separate geological facies and lack of cement between particles and soil granules are major factors leading to erosion, sedimentation and debris flows. Moreover, lack of cement between particles and granules in channels can cause lateral erosion, increased amount of sediment and reduced stability of soil aggregate and formations.
The erosion status of the Zagros morphotectonic unit was estimated based on the results and was classified into 5 groups. According to Figure (3), it can be stated that in this unit the erosion status is predominant, high and medium, which has been affected by lithological, tectonic and climatic factors. According to figure (4), very low erosion prevails over the coastal sub-basins of the Zagros unit, while low erosion is more centralized in some springhead and coastal sub-basins. Figure (7) indicates that many sub-basins with moderate erosion have a larger area than sub-basins with high erosion. As the sub-basins area shrinks, high erosion has prevailed. Very high erosion is scattered throughout the unit. In the sub-basins of this erosion status, the same logic prevails as in the sub-basins with high erosion and with the shrinking of the sub-basins, erosion has become too high.
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