پهنه بندی آسیب پذیری فرسایش کناره ای (مطالعۀ موردی: رودخانۀ هفت چشمۀ قزوین)

Introduction
Bank erosion is a severe problem to any fluvial system as it can generate up to 90% of the total sediment yield from a catchment It is also considered a hazard because it causes loss of lives and properties. The bank erosion and displacement of river planform change, each year, large areas of agricultural land and residential areas and coastal installation are exposed to destruction. The Haft Cheshmeh river located in the village of Razjard in Rudbar Alamut region ( Moaalem kelaye), northeast of Qazvin province‚ with longitude 50˚10ˈ14 and latitude 36˚ 20ˈ27, experiences severe bank erosion in several parts of its course.. This area located on the southern hillside of Alborz and south of Alamut in the mountainous area. The study area is on the southern hillside of Alborz in the Alamot section. The length of the studied route of the river is approximately 11 km and it flows near the villages of Razjerd‚ Rashteghon‚ Mianber and Shengher.
materials and methods
In this research were used from the topographic map (1:25000) of National Cartographic Center of IRAN (NCC) and Google images for the demarcation of the Haft Cheshmeh basin boundary and flow path of the river. Meander curve, vegetation cover, and other parameters of the bank have been measured from from Google images and field observations. In this study, the method Bank Erosion Vulnerability Zonation (BEVZ) has been used for investigate the factors affecting (rainfall erosivity, lithological factor, bank slope, meander index, river gradient, soil erosivity, vegetation cover, and anthropogenic impact ) the instability and bank erosion of Haft Cheshmeh river.
After collecting data and using from available parameters, the reaches and cross sections are selected. The base on six parameters data, general weightage values have been assigned to each of them.. this model Interprets data by weighting variables and It is actually a proposed model based on GIS. Finally, it prepares a map of the river zoning according to the parameters mentioned. Based on this zoning, the severity of the vulnerability is divided into 5 categories: very high, high, medium, low and very low. At the end when all cross sections are scored‚ map of cross sections is prepared and zoned based on weights assigned to areas susceptible to instability and bank vulnerability.

results and discussion
The bank slope map of the study area indicates that the most of the river banks of the Haft Cheshmeh river belongs to the moderate to gentle category of slope. The right side of reach 10 is in Very steep category in terms of side slope. According to the results, In the case of the meander index, the first reach is in the meander class and the rest of the reaches have straight pattern. In the soil erodibility factor, most bank river were in the class very gentle to gentle. Also in the left bank of reach 1, the right and left bank of reach 8 are in very high class. In the lithology factor, all reaches belong to the moderate category and only The lower part of the side of the reach 8 belongs to the very high category. There are also bed samples of all reaches of river in the low category.
The river gradient is considered as the triggering factor of river velocity that controls river erosion. Longitudinal slope at all reaches in Haft Cheshmeh river ranges from gentle to moderate category. Although in the height of 1710-1720 and 1700-1690 are in high category. In the vegetation parameter, most reaches are in scattered class and also reaches 4, 5 and 9 have high vegetation. In the human activities parameter, reaches 1-5 and 8.9 are in the medium category due to agricultural and garden land use. In reaches 6 and 9, human activity is in high category.
According to the results, all reaches except reaches of 5 and 9 have received average score. All of these reaches had moderate erosion vulnerability. According to the scores obtained and field observations, reaches 1 to 4 had a lot of vegetation. Also the soil erosion parameter in these reaches is in the gentle category and the meander index has the lowest score. The right bank of reach 5 has a slow erosion rate‚ but the left bank of this reach is high erosion. Because there is an bend at reach 5 that causes the left bank slope to increase and despite much vegetation, erosion has increased and the roots of the trees on this bank also show a lot of protrusions. But on the right bank, the gravely point bar has caused the wall slope to decrease and the amount of shear stress applied to it is low. As a result, it is less vulnerable to erosion. The right and left bank of reach 9 are very vulnerable, because in field observations, was observed in upstream of this reach a sedimentary barrier that caused the flow path to change and Channel walls on both sides become steep. At this reach, shear stress is high and also human activities have the most score.
Conclusion
The results showed that most of reaches in Haft Cheshmeh river are in moderate condition in terms of susceptibility to erosion. But in 8 and 9 reaches, due to high human activity in the river, they were in the middle to high class in terms of risk. Also, in all reachs except reach 10, are in the middle to high class due to the sensitivity of the side materials to erosion. A study of erosion sensitivity at different reach of Haft Cheshmeh river showed that more erosion occurs in areas where there is no vegetation or vegetation is scattered‚ also it is more susceptible to erosion. This study showed that vegetation and bank protection plays a key role in riverbank sustainability in this region . Evaluation of erosion using Bank Erosion Vulnerability Zonation method showed that vegetation is a protective factor along the river and human activities and meander index are two driving factors in the development of vulnerability and erosion.
Keywords: Zoning, Bank erosion, Channel vulnerability, Haft Cheshmeh river, Qazvin