عنوان مقاله [English]
Coastal sand dunes are important morphological forms in Caspian coastal regions. These dunes are formed and developed under the influence of sediment influx, climate, impact of wind, waves, currents. These are parallel to the coastal characterisitics with enough space for expansion. The whole collection of these parameters are available in many coastal areas of the Caspian Sea , but most of the dunes in many parts have been destroyed due to human activities and only a few of these dunes have been remained in complete and intact form. The study area of this research is located in 10 Km east of Babolsar city (Mazandaran Province), south of Caspian Sea, that is coordinated in 36° 39' to 36° 46' N latitude and 52° 37' to 52° 58' E longitude.
Materials and methods
In this study, first of all the northern coast of the Caspian Sea was explored using satellite images and four stations with the highest density of not destroyed coastal sand dunes were identified and labeled with letters A to D eastward to investigate changes between groups and also inside each group at each station. Then sand dunes of each area were detected and coded using satellite images, based on geomorphologic features. Finally, field images were taken and morphometric parameters of 17 sand dunes along the shoreline including crest, lee, stoss and height were measured to determine the correlation among the properties of the components and also to analyze form of the graphs and statistical parameters.
Results and discussion
Parabolic dunes are considered as the most important types of coastal sand dunes in the east of Babolsar area. This Parabolic dune are divided morphologically into seven types: lunate, hairpian, hemicyclic, digitate, nested, long-walled transgressive ridge with secondary transverse and rake-like en-echelon dunes. Geomorphological situation of these sand dunes are mainly influenced by the prevailing winds in the area. It usually shows tip region that will contribute to prevailing wind direction. Pattern of wind speed and direction of rose diagram based on the data from climatology station of Babolsar can also reveal the major annual eastward winds in this region with 0.5 to more than 11 m/sec speed. Moreover, sometimes these parabolic-shaped sand dunes are different within a small geographic area in terms of geomorphology and direction. This indicates local annual changes in the wind conditions and directions in certain periods of time. Simple lunate dunes by 59.5 percent frequency are the most abundant types of the seven parabolic dunes group in the study area.Parabolic hemicyclic shape, hairpian, digitate, nested, long-walled transgressive ridge with secondary transverse and rake-like en-echelon dunes are the most abundant types of dunes in this group. Simple lunate form with 45 percent and hemicyclic form with 25 percent of frequency, totally more than 70 percent, are also the two most common types of sand dunes on coastal barrier system of Miankaleh. Therefore, parabolic sand dunes in east of Babolsar and Miankaleh show similar patterns of the seven parabolic dunes and are different from each other only in terms of scale and magnitude of morphometric parameters. They are related to the effect of key factors on distribution of these dunes. Also, for determination of type of relationship among the morphometric parameters measured in the parabolic dunes of this area, statistical calculations including a variety of linear and nonlinear simple regressions were carried out. Ultimately, the best model were applied in two steps firstly for all of dunes regardless their form and in second step based on the forms for the lunate and hemicyclic dunes. Statistical relationships of the measured components of these dunes regardless of their form indicate that the linear and cubic regressions of crest and stoss lengths are 0.771 and 0.778, respectively. This could properly be correlated with parameter of height. The following results were obtained from sand dunes form-based data: Type of dunes (hemicyclic) show proper correlation coefficient in variables of height and the best with lee-side squares regression. In type B dunes (lunate), powered crest length and squared stoss-side regressions reveal the best correlation coefficient with height of dunes. Therefore, the second best regression is the squared lee-side coefficient of hemicyclic dunes with the value of 1 correlation coefficient for the height component.
Due to the aboundance of simple lunate form with 59.5 percent frequency compared to the other seven forms of paraboplic dunes in this area, it can be concluded that the prevailing winds are unidirectional, blowing from west of Babolsar in eastward direction which is corresponding to the data obtained from climatology station of Babolsar. Statistic correlation between the measured parameters of these dunes indicate that the height, crest and stoss length parameters show the best correlation coefficient. In orther words, the dune height changes with the crest and stoss lengths whereas the lee length parameter has no effect on height. Because of the difficulties of field works in examination and measurement of height of sand dunes using their morphometric parameters such as length of peaks and sides, the application of satellite images is an easier method. It could be applied with field data for the purpose of morphometric parameters analysis on the areas with similar conditions.