بررسی اثرات جهت جغرافیایی دامنه‌ها بر حجم مخروط‌افکنه (مطالعه موردی: شیرکوه یزد)

Introduction

Alluvial fans are considered as one of the geomorphical features of dry and semi-dry areas which is the result of intense runoffs in the past and present. Thus, it can be considered as a Quaternary climate heritages. However, it is now visible in all climate conditions across the globe such as arid regions, Alpine environments, humid climates, and even tropical environments. Several studies have been done on their structure, morphology and hydrology. These studies have shown that cone-shaped characterization is associated with various and different parameters and factors which can be classified into six major groups including climatic factors and changes, geometric Properties of upper catchments, Lithological conditions, Tectonic variations, in particular in base level, land-use changes and geographic directions of slopes. The main goal of this study is to investigate the morphometric role of catchments in the volume of alluvial fans in the different slopes of Shirkouh Mountain in the south of Yazd with emphasis on geographical orientation.

Since Shirkooh is a granite mass relatively cone-shaped that has created a number of alluvial fans in all its aspects, and in terms of lithology, its various directions are similar, the orientation of slopes against the air masses can play a major role in the volume of alluvial fans.


Methodology

To do this project, we used satellite images, a digital elevation model with a resolution of 5 meters and topographic maps of 1: 50000 . Initially, using these data, the boundary of alluvial fans and their upstream watersheds were determined.So, 24 significant alluvial fans and their watersheds were identified that there were 9 of them in the eastern slopes with the southern direction, 6 of them in the same range with the northern direction, 5 in the western slopes and 4 in the northern slopes of the mountain. Using the quantitative model in the geographical information system, the volume of alluvial fans was calculated. Then, the boundary of the upstream watershed of each alluvial fan determined in ArcGIS 10.3 software and ArcHydro add-ons and their geometric characteristics include area, environment, average height, average slope, the length of the basin, and the Gravilius coefficient were measured individually. In fact, by converting the information layers into the Raster's format, quantitative geomorphic characteristics and morphometric parameters of the catchment were calculated in four directions. Then, the correlation between the volume of alluvial fans and watershed morphometric parameters were calculated in SPSS software for 4 geographic directions and is also shown in the graph.Finally, to illustrate the effectiveness watershed morphometric parameters on the volume of the alluvial fans,  We calculated multivariate regression between alluvial fans volume and morphometric basins in SPSS in four geographical orientations.


Results and Discussion
Eastern slopes with southern and northern directions extend to the vast plain of Ebrahimabad in the eastern slopes of Shirkouh and west of Mehriz city. Alluvial fans in this range are more numerous than in other slopes of Shirkouh and they have created bajada. However, in the slopes with northern direction, the watersheds are much wider compared to the opposite slopes. Therefore, the size and volume of the alluvial fans in the section are bigger and wider. While the correlation between the volume of alluvial fans and geometric characteristics of the upstream watershed is very high in the eastern slopes with the southern direction. So that the area, the environment and the length of the river show correlation above 95%. But the correlation for the slopes with the northern direction is very low in the range and the significant level between them is unacceptable.In the western slopes of Shirkouh, the geometric factors of the upstream watersheds are the main controllers of the volume of alluvial fans. The correlation between the volume of alluvial fans and geometric parameters of watersheds showed that among the eight parameters studied, only two of them includes Gravilius coefficient and drainage density played a small role in the formation and volume of alluvial fans, and other factors including form factor, area, environment, basin length, average elevation and slope Respectively played a major role in the volume of alluvial fans. But this case is different for the northern slopes of Shirkouh, So that there are the lowest correlations between geometric factors of catchment and volume of alluvial fans Relative to other geographic directions.


Conclusion

The results indicate that there is a high correlation between alluvial fans volumes and geometric factors especially area (0.99), environment (0.98), and the length of watersheds (0.95), in the eastern slopes with southern direction with a significant level of zero and close to zero. This applies to the western slopes and geometric factors includes area, environment and coefficient of form (with correlation 0.99), length of the catchments(0.94), average slope and elevation(0.85) have been the most important factors effect on the volume of the alluvial fans in the slopes. But the correlation between the volume of alluvial fans for the eastern slopes with northern direction includes only slope (0.64) and form factor (0.59) and for the northern slopes of includes Gravilius coefficient (0.43) and watersheds length (0.37) that have low significant levels. Therefore, in these slopes, geometric factors of watersheds played the least role in the volume of alluvial fans.