Abstract

Background and purpose
Bank erosion is one of the geomorphic processes caused by natural and human factors. This phenomenon, as one of the types of water erosion, can create several geomorphic effects in the river. The increase in river erosion also causes the instability of the river and as a result changes the type of flow, changes the pattern of the river and changes the course of the river. So far, several methods of rapid geomorphic assessment have been used to investigate river bank erosion, and one of the most common and widely used in these assessments is the Channel Instability Index (CSI). The aim of our study was to know the applicability of the channel stability index to identify erosion-sensitive and unstable banks in the Qaleh Rudkhan Fooman river in Gilan.
Material and methodology
Qaleh Rudkhan river is located in the west of Gilan province in Fooman city. This river originates from the northeast heights of Sote village and flows into the Caspian Sea after traveling 40 kilometers from the Qaleh Rudkhan area. The area of the basin is 347.76 Km2. With the aim of studying the bank erosion and stability of the river bank of Qaleh Rudkhan, a part of the river path with a length of approximately 5 km was selected in near the city of Fooman. In this area, 8 reaches were determined based on river conditions (pattern and morphometric differences) and its sensitivity to bank erosion. Then, mapping of cross_sections, Channel morphometry, grain measurement of the bank sediments, state of compaction and stickiness of sediments, and examination of the vegetation characteristics of the bank and flood plain were carried out for each reaches. In eight reaches, using the channel stability index, the streambank erosion situation was investigated. In this model , the primary bed material ,bed/bank protection, degree of incision, degree of constriction, streambank erosion, streambank instability (percentage of mass movements), established riparian woody- vegetative cover, occurrence of bank accretion and stage of channel evolution model are scored.
Findings and discussion
Based on the findings of the channel stability index (CSI) on the left and right banks of Qaleh Rudkhan river, it showed that the index values ranged from 18.5 to 25.5. Based on these values, reaches one, two, three and four are in a very unstable state (high instability) in terms of channel stability, and reaches five, six, seven and eight are in a medium instability state. Based on the relative score method of bed sediments, bank protection and riparian vegetation are the most important factors affecting channel stability. Based on Pox-Whisker Plot diagram, the status of three variables of degree of incision, the available evidence of bank erosion, percentage of mass movements and occurrence of bank accretion are different in different reaches and as a result of these factors, the sensitivity of the river bank of Qaleh Rudkhan to instability is different.
Conclusion
The results of this research show that the three variables of the primary bed material, bank protection and established riparian woody- vegetative cover are the same in all reaches and are considered important factors affecting the stability and river channel stability. Based on the above findings streambank instability, occurrence of bank accretion, degree of incision and stage of channel evolution model are variable factors in each reaches, which cause differences and changes in the intensity of river bank erosion and river channel.