Investigating the Banks Sensitive to Erosion with an Emphasis on Geological, Geometric, and Hydraulic Characteristics: A Case Study of Bashar River

Extended abstract Introduction The vital need for river management is understood and predicts bank erosion processes. River bank erosion is one of the sediment major sources for many rivers across the world. Bank erosion is a key process in fluvial dynamics, affecting a wide range of physical, ecological and socio-economic issues in the fluvial environment. These include the establishment and evolution of river and floodplain morphology and their associated habitats, turbidity problems, sediment, nutrient and contaminant dynamics, loss of riparian lands, and associated threats to flood defense and transportation infrastructure. Moreover, recent studies have shown that the contribution of bank-derived sediments to catchment sediment budgets may be higher than what previously thought. The study of the factors influencing Bank Geomorphology and Erosion requires basic information on geological, hydraulic and geometric characteristics. Therefore, the main aim of this paper is to study the stream banks susceptible to bank erosion in Bashar River.   Methodology   2-1- Geometry and Morphology of Beshar River  In order to understand the mechanism of the lateral erosion, the morphology of the Bashar River was identified through digitizing the Landsat satellite image in the ENVI software. After making necessary corrections, the geometric properties and the river morphological parameters were determined in the AutoCAD software. The most important indicators used in this study included the central angle of the grunge, the bending coefficient, relative radius and Meandander width and length. To calculate each of the indicators, refer to the guide for river morphological studies.   2-2-Geology of Beshar River: In order to study the geological condition of the  Beshar river, the geology map of Bashar watershed was prepared using a map of 1/400,000 Geological organization; then the map of Bashar's main river was overlapped with the geological formations map and the geological map of the river was extracted.   2-3- Shear Stress The reach of Beshar river with a length of 10-km was selected and the hydraulic, morphologic and geologic characteristics were studied. Then, the critical shear stress was computed using the empirical relations based on the soil parameters.   2-3-1- Near-Bank Shear Stress In order to determine the erosion reaches of Beshar river, shear stress coefficient was used which was calculated according to the ratio of the curvature radius to the river's width.   2-3-2-Total Shear Stress Shear Stress (t) is a measure of the force of friction from a fluid acting on a body in the path of that fluid. In the case of open channel flow, it is the force of moving water against the bed of the channel. Shear stress is calculated as: 2-3-2- Critical Shear Stress   2-3-3-Relative Bed Stability Index   3- Results The results showed that Bashar river has more than 100 large and small meander. Curvature coefficient in reach located in the kilometers of 7-14 and 75-85, is equal to 1.4. Critical shear stress index indicated that the reach of Beshar river located in 10km is instable (τc> τ and RBS< 1 and FS<1). The results showed that the shear stresses calculated for the east, west and bed cross-sections are instable (facror safety<1). Also, the rate of bed stability index is less than one in most sections. The results inicated that the shear stresses calculated for the cross sections are more than thershold amount. The average shear stress on the eastern and western banks are 8.85 and 14.87 kg / m2, respectively Whereas Permissible shear stresses average in the eastern and western banks are 61.93 and 61.76 kg / m2, respectively. Therefore, shear stress average of the river bank is less than the Permissible shear stress average. Also, the shear stress average of the river bed is equal to 17.44 kg / m2 and the bed Permissible shear stress mean of river bed is 78.17 kg / m2. Therefore, the average shear stress tolerance is lower than the shear stress average on the bed. 4- Discussion and Conclusions Results indicated that the reach of Beshar River, crossing the silt formations, has a high level of sensitivity to erosion bank. More than 50 km of the Beshar River crosses Asmari formation. The long length of Beshar River contains alluvial sediments, which is located on Asmari formation. The effect of shear stress on the cross-sections of Beshar River indicated that the shear stresses on the riverbed and bank are more than the Permissible shear stress. Also, the bed stability index is less than one, which indicates the instability of Beshar River in the selected reach, which is in line with the findings of Ghaffari et al (2006), who reached similar results in terms of Babol River. On the outer side, the amount of shear stress in the river bank is less than the main channel, so that in the river meanders, the amount of shear stress and the amount of bank erosion increased.