Bank stability and mophometry analysis of Galali river in order to its stabilization against erosion (Qorveh, Kurdistan)

Introduction The river bank protection against the erosion is considered as one of the main objectives of river improvement in sustainable development of water resources. The erosion of the river banks is causing damage to agricultural land, damage to structures such as bridges and roads, widening stream channel and environmental issues considerably. Bank erosion is a major cause of non-point pollution of water resources and increased sediment load in many rivers. Increasing banks erosion not only increases the sediment load but also causes river instability and changing flow and channel pattern. Thus, during the recent decades, sediment load and river banks have been created a large concern in the world and the large sums have been spent to stabilize banks. One of the important characteristics of the river banks erosion is permanent and active in comparison to other forms of the erosion, while other forms of the erosion are activated only during or shortly after the start precipitation. Interaction active forces of the water flow and the resistance forces to the flow due to bed and bank material were eroding the bank of the channel. Thus numerous engineering solutions have been created to strengthen the banks against the erosion. By conventional methods, the rock riprap or large boulders that are on the bank are used for bank protection beacuse they are easy to pick up. If rock riprap with appropriate dimensions and affordable is available, it can be widely used to protect banks of the river. Two methods are provided for determining the size of cobble for rock riprap. 1- Shear stress method, 2- flow velocity method. Shear stress method is used in this study. In shear stress method, stability of rock riprap on slope side depend on the amount and the velocity of flow or shear stress at the banks, slope angle and rock properties such as size, density and angle.
Methodology Gulalai catchment is located on 43 km from the Qorveh city, in the southeastern of Kurdistan province. Galali River is the main stream in this catchment. Length of the studied river reach in this paper is 3 km and from the bridge near the Shirvaneh village to the next bridge in the Gulalai village. In this reach, 5 cross sections were selected and the channel cross sections were measured using meter and survey indicator and parameters were measured such as bankfull width, average bankfull depth, banks and bed slope, bed and banks particle size, radius of curvature and flow velocity. Based on the measured data of the cross-sections, the required parameters in the measurement of shear stress were calculated. Shear stress is calculated based on Lane’s equation, upon effective size of rock for bank stability under shear force.
Results and discussion Results show that cross-sections 1 and 5 have the highest and sections 2, 3 and 4 have the least important factor of instability and the effective rock size for stabilization of cross-sections 1 to 5 were estimated 42, 26, 20, 22 and 60 cm respectively and given that βθ is less than 90 in all of cross-sections, so particles move down the slope toward the thalweg. The size of the rocks and angle of repose in different cross-sections are vary, section 1 and 5 are the large gravel and sections 2,3 and 4 are large cobble. The size of the rocks obtained for the rock riprap have been predicted as too large cobble for cross-sections 3 and 4, small boulders for cross-sections 1 and 2 and medium boulders for cross-section 5.
Conclusion Galali River has sinuses pattern in the most of its path and all its cross-sections are unstable so that providing management strategies is essential. Lanes shear stress method based on the effective size of rock riprap were used to estimate bank stable condition and given the constant stream flow, the most important influential factor of bank instability at different cross-sections is the particle size of the channel bank. Therefore after obtaining the required parameters, the safety factor of the bank, the size of rock riprap in moving threshold and the effective rock riprap were calculated for Galali river bank.