Assessment of Geomorphologic Techniques for Identification of the Old and New Alluvial Cone for the Purpose of Specifying Susceptible Areas to Flood in Four Alluvial Cones in Folded Zagros

In this study, in order to separate active and inactive parts of alluvial cones, four alluvial cones were studied in folded Zagros. Separation of active and inactive parts of alluvial cones from the view point of flooding has been conducted by simple indices of geomorphologic such as drainage pattern, morphology of the cone surface and serration index, color of cones in satellite images, maximum depth of cone shear and weathering indices such as the rate of alluvials weathering, pitting of time cobblestones. In order to specify the roughness and maximum shearing depth of cones׳ surface, by using surveying camera, two sections of each alluvial cones surface were prepared. Separation of new and old parts of alluvial cones were conducted based on the colour of images and also specifying the drainage pattern of channels in new and old cones was performed based on the quick board satellite images. Also the study of weathering indices of the cones surfaces was carried out based on the field studies. The results obtained from this study show that the old or inactive cones often have convergence drainage pattern or tree network, while the new cones have mostly divergence, ramified and crossover drainage pattern. Inactive parts of cones have a darker color tone in comparing with the new cones in satellite images. Morphology of the old cones surface (due to the dominance of aback erosion and spread of Bad Lands toward the upstream) is more roughness and has more serration in comparing with the ones. Generally, maximum rate of shear in old cones is more than the new cones. Also weathering indices including pittings in time particles are completely different in old and new cones, so that the inactive parts have more weathering and soluble features and therefore have a thicker soil in relation with the new cones. The recent study shows that geomorphologic indices are a proper tool for separation of active and inactive areas of alluvial cones from flooding point of view. Nonetheless due to the effect of factors such as tectonic, climatic changes, lithology of the cones׳ upstream catchment areas and area of the cones, it is not possible to use all the indices in each cone.