Modeling for estimation of Barchan plane geometry (Case study: Barchans in south of Chah Jam playa)

Different structures of sand dunes can be observed among sedimentary systems both rich and poor in terms of the amount of sediment load. Barchans are one of the most well-known types of sand dunes which form under unidirectional wind conditions and in the absence of much sand on the ground. The features include individual mobile dunes of crescentic shape، the two horns of which face the direction of dune movement. They are generally regarded as occurring in areas of limited sand supply، on planar surfaces، with low precipitation، sparse vegetation cover، and where winds are narrowly bimodal in direction. Theoretical framework: The earliest studies of sand dunes by geomorphologists were focused on the classification of different types of dunes، and in particular the factors responsible for the wide variability of dune shapes. Three factors controlling the shape of dunes were identified: vegetation، variability of wind direction، and the amount of sand available. The morphological characteristics of Barchan are affected by different temporal and spatial factors. Relationship among Barchan morphometric parameters are closely tied to its three-dimensional morphology. The identification of the relations and representation of their models will be affected in recognition of the behavior and function of this landscape in nature. The purpose of this study is to provide a modeling for the estimation and calculation of Barchan plane geometry parameters، such as perimeter and area، using its morphometric parameters، and by statistical models in south of Chah Jam playa.

The study area is located in the south of Haj Ali Gholi playa، Semnan province، central Iran. Haj Ali Gholi playa، situated in the southwest side of Shahroud city and towards the south of Damghan city، is the most important playa of Semnan province. The playa is influenced by different geomorphic and climatic processes. Chah Jam Erg، with an area of about 25260 hectares، is one of the most important erg of Haj Ali Gholi playa that is located asymmetrically along the 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. In order to achieve the goals of this study، first، the most important morphometric parameters of 52 Barchans were measured through linear sampling in the studied field. The most important Barchan morphometric parameters include the following: width of right side، width of left side، length of right horn، length of left horn، length of windward side، length of slip face side، area، and perimeter. These parameters، except for area and perimeter، were measured using the field sampling method in the study area، according to the pattern shown in Figure 2. The estimation method of Barchan area and perimeter is as follows: first، a coordinate system was considered on the earth surface، for each Barchan. Then، the coordinates of 24 points were determined on its perimeter using a mesh network. Sampled points in the field were drawn on graph paper with a scale of 1:200. Finally، the area and perimeter of each Barchan was measured by planimeter and curvimeter، respectively. In the next step، after the formation of a data matrix، the data were modeled using regression analysis in SPSS. For this purpose، first، simple and complex regression were examined. Then، the accuracy assessment of the models is determined through comparing their validity; and finally، the most suitable was selected. The preference indexes of models are the maximum R. Square، R.، Adjusted R. Square، Sig. and the minimum Std. Error of Estimate. Findings and

According to the measurements، Barchans of the study area have a length of between 19. 5 and 307. 85 meters، and a width of between 6. 3 and 165. 6 meters، generally. In order to model the relationships among the plan geometry and morphometric parameters of Barchan، simple and complex regression methods were examined، and the most suitable relationships were reported. The results of the regression analysis between area and perimeter with width، and length of Barchan have been presented in the table below.