Effect of Topographic Threshold Conditions on Gully Erosion)Case Study: Alla Marvdasht -Lamerd & Fadagh - Larestan)

Gully erosion and sedimentation have high priority in Fars Province due to the losses from destruction of lands, roads and civil structures. In the present study, in order to evaluate the threshold for the development and initiation of gully topography in Fars province, two regions were selected including Allamarvdasht, Lamerd and Fedagh, Larestan. 30 gullies were selected in each mentioned area. Morphometric parameters were measured and topography threshold was drawn. To evaluate the effect of measured parameters on topography threshold, each gully was clustered based on parameters and cluster analysis. To determine the effect of dominant hydrologycal processes on gully initiation and development, the multivariate analysis was employed using SPSS (version 14) software. Using the power relationship between watershed area and slope of each gully, the dominant hydrological processes in gully initiation and development were determined by means of Excel software and were compared with coefficients of foreign regional studies. The results showed that in the two studied regions, the most effective hydrologycal process was surface runoff. Results also demonstrated that in these regions changing the shape of upstream basin topography to circle and their expansion have increased the threshold of watershed topography of gully erosion. Increasing the sodium absorbtion ratio and removing the ground cover reduce the threshold of gully erosion topography. Increasing the depth of gullies and soil organic matters resulted in greater relative threshold of topography.