A Study of Physiographic and Geomorphic Relationships to Identify Erosion and Sedimentation Areas in the Ladiz Watershed, Zahedan, Eastern Iran

Abstract

The Ladiz watershed is a mountainous area which is divided into 3 medium sub-watersheds. Taftan volcano is a brilliant morphological characteristic and one of the most special features of the watershed. Physiographical and geomorphologic data in GIS can be useful in recognition and analysis of erosion areas and the sedimentation in the catchment. Comparison between physiographical ratios such as Compactness Coefficient, Form Ratio, Circularity Ratio, Elongation Ratio, and the length and the width of the Equivalent Rectangle represent the stretching mode in the Siahjangle and Saadabad sub-watersheds, and which results in reduction of the effective time of runoff concentration in the Mianrud sub-watershed and can result in torrential flows or floods. The study of the Gradient Index (IP), Average Weighted Slope, Average Height and analysis of the Dimensionless Observed Curves added with the geomorphologic information and the maps indicate that the mountainous areas are spread with high slopes in the west and southwest parts of the watershed. The study of the Ladiz Slope Map has revealed that about 70 % of this area is mountainous and has greater than a 10% slope gradient. A Low Density Index in Mianrud is the reason for the spread of alluvial plains in the central parts of the watershed. The drainage pattern in the Ladiz watershed in treelike, but it has parallel and radial drainage patterns around the dominating peaks of Taftan. Geomorphology of the Ladiz area is the result of tectonic, volcanic, and erosive activities and the sedimentation process. The dominant landscape figures in the watershed are mountainous. morphological components in different parts of this area having various resistances against erosion and diverse lithological types. Shale Cretaceous-Eocene lithological units have created mounds and the soil profile shows a greater development in them, whereas sandstone units, basalt-andesitic and dacite flows have mainly formed most of the rocky highlands. Faults and folds are the most important factors which have provided elevated rocks on the surface of the Ladiz watershed. Expansion of a corrosion fault system, especially in sandstones and shale, has increased the amount of erosion in this watershed, and we can observe Pencil erosion in most of the flyshe slopes. Prismatic structure, which is the result of Taftan basalt flows, has created severe rocky faces in the western basin. Andesitic lava flows in the topography have conic shapes. Those forms which are a result of sheet and rill erosion are quite visible in different parts of the basin especially in the Cretaceous-Eocene flyshe units, and they are considered to be some of the most destructive corrosive elements during flood times in the Mianrud sub-watershed. Two of the most important shapes of erosion processes in the Ladiz watershed are the river sediments and the flood plain sediments. The peaks scattering pattern, which in fact indicates the distribution of rough areas all over the watershed, has its largest scattering in the special distribution and altitude range in the Mianrud sub-watershed. The main river channel pattern in the Ladiz catchment has special features, such as high slopes of the river channels, high stream power carrying particles as large as gravel, low sinuosity, and the presence of numerous dams in the main flow direction which confirms the presence of braided rivers with a gravel bed, much more than other rivers in the area.