عنوان مقاله [English]
The environmental issues and loss of natural resources are the important causes of management strategies creation of the environment crisis. The influx of quicksand to settlements, communication ways and economical installations in the form of barchan dunes, is one of the most important of environmental problems in the South of Haj Ali Gholi playa. Barchan dunes originate from the characteristics of prevailing wind and little ability to access the sand that is crescentic shape, and have been formed from mobile sand.
A side view of a barchan shows a rather flat aerodynamic structure. When viewed from above, a barchan presents a crescentic shape with two horns pointing downwind. A sharp edge divides the dune in two areas: the windward side and the slip face. Because of a boundary layer separation along this sharp edge, a large eddy develops downwind and wind speed decreases dramatically. Therefore, the incoming blown sand is dropped close to the brink line.
The aim of this study is the assessment of morphodynamic indices of barchan dunes in Chah Jam Erg using experimental, mathematical and statistical methods. By obtained results, we can comprehend a correct understanding from 3D morphology and morphodynamic characteristics of barchan dunes and their behaviors, to manage the environment of desert regions using systemic approaches.
The study field is located in south of Haj Ali Gholi playa, central IRAN. Haj Ali Gholi playa situated in southwestern of Shahroud city to south of Damghan city. Chah Jam Erg, with an area of about 25260 hectares, is one of the most important ergs of Haj Ali Gholi playa that is located asymmetrically along northeastern - southwestern edge of the playa with a length of 10 to 12 Km. The study area is located at latitudes between 35° 45΄ and 35° 50΄ North, and at longitudes between 54° 40΄ and 55° 10΄ East.
First, the study area was investigated by satellite images of Google Earth and 1:50000 maps. Then, for explanation of steady state index, the most important morphometric parameters of 15 barchans were measured by linear sampling in the studied field, and the relation study of between parameters and the evaluation of the steady state model were performed using analysis of them by statistical tests.
In context of defining the stability index of Barchan system, the rate of sand flux was investigated using empirical equations in the given extent with dimensions of Barchan height and width.
Results and Discussion
In the study area, we can see various barchans, most of them having same and similar shapes. Generally, the barchans of this region have a height of 2.1 to 17.9 meters, a length of 19.5 to 307.85 meters and a width of 6.3 to 165.6 meters.
When system maintains its life and function and has a maximum efficiency, there is the stability in the landscape. Until the needed energy of landscape maintain was supplied, and the flow of material balance occurs in the system, it remains stable. In this study, for exploration of stability index of the barchan, the method of sand flux calculation is used. So that, first, the input flux (qi) and output (qo) of barchan system was calculated using the relations (1) to (7). Then, using the proportions between input flux (qi) and output flux (qo), the occurrence probability of the three states was recognized in barchan system as defined in following:
1) , in other words, the input flux is the equal with output flux. Thus, the barchan system is the stable.
2) , in other words, the input flux is more than output flux. Thus, the barchan system is the unstable with developing trend.
3) , in other words, the input flux is less than output flux. Thus, the barchan system is the unstable with vanishing trend.
The results obtained relation study, show that relationships between movement rates and morphological parameters are following from simple linear, power and exponential equations. The relationships between barchan morphometric parameters represents a maximum significant exponential reverse relationships between movement rate and barchans height parameter with R square of 0.915 and Std error of estimate of 0.133.
To define the steady state index, the annual displacement rate of 15 barchans was divided into 5 groups according to comparison test. Also, a zero amount that is presenting the absolute steady state was added to the previous groups. A total, the steady state index includes the 6 groups in studied barchan dunes. Then, the steady state index of each barchan dune is determined by the groubs. The results show that group 6, with annual displacement rate more than 12 meters, have the maximum number and highest rates of displacement, and have the lowest steady state.
The base of applied geomorphology analysis is according to systemic approach. Systemic geomorphology is based on recognition of geomorphic forms and processes and relationships among them. Efficiency of this stand point, when there are linear and nonlinear reciprocal relationships between parts and elements of system, is very important.
The morphology of barchan system reflects the dynamic sets of processes that their goal is the formation of barchan landscape. Development and evolution of Barchan system is the function of various feedbacks between forms and processes. These feedbacks lead to structural changes in the flow of matter and energy in barchan system. In other words, the structural and functional interaction of form and process will be developed barchan system.
The results show the exploration of stability and steady state models for studied barchan dunes. The stability model was involving the input and output of energy and material in the barchan system that eventually is formed a flow structure. This structure is representing the stability and equilibrium of barchan system in time. Also, the obtained results from investigation of steady state index show the barchan dunes, with annual displacement rate more than 12 meters, have the maximum number and highest rates of displacement, and have the lowest steady state. Thus this group is demanding the more attention and planning requirements for stabilization of quicksand and environmental management of mobile barchan dunes.
Keywords: Barchan, Morphodynamic, Stability index, Steady state index, Chah Jam Erg.