مخاطرات محیط طبیعی (Dec 2017)
Comparison of Stream Bank Erosion Hazard Methods Using NBS Model (Case Study: Galali River, Ghorveh)
Abstract
There are seven methods for estimating NBS, which include: channel pattern and bar assessment, ratio of radius of curvature to bank-full width (Rc/Wbkf), ratio of pool slope to average water surface slope (Sp / S), ratio of pool slope to rifle slope (Sp/Srif), ratio of near-bank maximum depth to bank-full mean depth (dnb/dbkf), ratio of near-bank shear stress to bank-full shear stress (τnb/τbkf) and use of velocity profile/isovel/velocity gradients. In this article, three methods were selected for estimating Near Bank Stress (NBS); Rc/Wbkf, dnb/dbkf, τnb / τbkf and then according to these methods, the erosion of the Galali river banks were determined at different levels from very low to extreme. Results showed that in the method radius of curvature to bank-full width (Rc/Wbkf), the bank erosion was moderate to very low and in the method near-bank maximum depth to bank-full mean depth (dnb/dbkf) in most cross-sections, the bank erosion was low and in method near-bank shear stress to bank-full shear stress (tnb/tbkf) the bank erosion was very high. Also, curvature coefficient shows that the river is sinuosity in cross-sections 1, 2 and 3 and is a meandering river in cross-sections 4 and 5. Comparing the results and field observation shows that the method near-bank maximum depth to bank-full mean depth is acceptable. Finally, in the Galali river, results of near-bank maximum depth to bank-full mean depth (dnb/dbkf) for assessment of the bank erosion (NBS) is better than method radius of curvature to the bank-full width (Rc/Wbkf) and near-bank shear stress to bank-full shear stress (tnb/tbkf). The Galali river has low to moderate bank erosion potential. Also, the river is sinuosity in upstream and is meandering in downstream and according to field observation, bank erosion potential in downstream of the Galali river is more that upstream.
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