Zonation and analysis of stream bank erodiability in upstream of Neka River, Mazandaran

Streambank erosion is a complex process that has many contributing factors. Geomorphologically, erosion of the streambed and influx of sediments and large woody debris into the channel lead to changes in river flow, floodplain development and river channel morphology. Accordingly, stream bank erosion is an important management problem along the alluvial rivers. The main aim of this study is to analyze the erosion susceptibility of the riverbank and to map the erosion zoning at reach scale.

In this paper, the Bank Erosion Vulnerability Zonation (BEVZ) method is used to investigate the erosion of the river bank. This method was first proposed by Bandyopadhyay et al. (2014) for zoning the hazard of river bank erosion on the River Haora of India. In this method, six parameters of river bank slope, meander index, longitudinal river gradient, soil erodibility factor, vegetation cover and anthropogenic factor were used. In this study, the above parameters were used with slight modifications and addition of lithology factor as well as changes in scoring. The study area was a reach of the Neka River (Iran, Mazandaran) from the village of Sefidchah to the Gelevard Dam with a length of 17 km. Path of the river was traced using Google Earth images and in Arc GIS software from both sides of the river, a buffer width of 20 meters was determined to assess erosion. After preparing the above data layers and classifying them into ARC GIS software, the final map was obtained using the overlay sum and, based on minimum and maximum scores. The erodibility of the river bank was classified into five groups. To analyze the erodibility of the riverbank, the amount of channel displacement was measured over a 13-year period (2006-2019). The channel displacement was measured at a constant distance of 250 m and the mean displacement was calculated.

More than 66% of the river's slope lies between 0.001-0.01 m/m. Lithologically, 79.5% of the river flow is composed of alluvial sediments. In river banks, 63% of the vegetation is of very low density, including grasslands and scattered trees and approximately ten percent of the riverbanks consist of dense and very dense trees. The slope of the river bank was less than 10 degrees in 96.6% of the river's course. Bank height varied from 1 to 2 meters in 59% of the river channel. The meander index is 60.7% of the river's length between 1-1.5, and 26.4 percent of the river's route is in the convex side. The anthropogenic factor in the study area is gravel extraction that include approximately 34% of the river route. With the overlapping of information layers, the final map of river bank erodibility has been divided into different classes including very low, low, moderate, high and very high. Of the entire study reach, 65 and 20 percent of the riverbanks have moderate and high erodibility, respectively.By comparing the centerline of the channel in the 13-year period 2006-2019, the channel displacements in the reaches of 1 to 4 were 0.87, 0.85, 0.83 and 0.42 m/y, respectively. There is a correlation coefficient of 0.84 between the percentage of high erodibility of river bank and the annual channel displacement rate, which indicates the lateral displacement of the river was higher in the reaches with higher erodibility. Among the effective parameters in erosion, lithology is the first rank with 33%, which is due to river crossing of loose alluvial deposits. Another factor is vegetation that, due to low density, cannot withstand erosion, so it is ranked second in river bank erosion with 31% impact.

The results of river bank erodibility map show that 20% of river banks have high erodibility potential.There is a significant relationship between the displacement of the river channel and the erosion rate of the river bank, which confirms the high probability of precision and accuracy of this map. This map provides a spatial view and is suitable for locating and planning any river-related human activities such as road, bridge, dam, sand extraction, and so on. Therefore, by identifying and analyzing the factors affecting river bank erosion at each reach, it will be possible to prioritize human activities more accurately and avoid potential hazards.