Assessment of the sustainability and instability of bank and bed of the Masil-Muchan river (Astaneh city - Markazi province)

Extended abstract 1- Introduction Rivers are one of the dynamic landforms of natural landscapes that are subject to changes due to environmental variables and human interactions during different times and locations. Changes in the river, erosion of the bed and river bank are natural processes of alluvial rivers that destroy the surrounding agricultural land and damage human installations around the river. Human activities, such as the degradation of basin vegetation and the uncontrolled harvesting of river water, cause more pressure on the  river and the destruction of the plant covered area along the river and the change of the river ecosystem. Therefore, the erosion of bed and bank and channel changes, finally, the vulnerability of  the channel, as social and economic issue, often causes irreparable damage to residents and facilities in riverside. Curitit (2014) predicts the erosion and sustainability of the riverbank in Stony Clove Creek in Catskills, using the analysis of the Bank Assessment for Non-point. Source Consequences of Sediment (BANCS). This study showed that the results obtained through the BANCS method may lead to improved  management of the future of the Stony Clove Basin. Hosseinzadeh et al. (2005) studied the efficiency of the Rasgen classification system in the rivers of Babol and Talar in the Caspian Sea coastal plain. The results of this study showed that the prediction of the river type using this method can not be substituted as real research on the river and can lead to failure of management plans. 2- Methodology The study area is a reach of 5.65 km in the Masil-Muchan river. The river originates from the northern and eastern mountains of Amarat village and joins the Shara River, one of the main branches of the Gharache river. The average discharge of Masil-Muchan river is 0.23 cubic meters per second and its Sediment discharge rate is 0.76 cubic meters per day. This river is located in Astaneh city, Markazi province. The channel vulnerability model, by dividing the average shear boundary stress (erosion forces) and shear stresses for each cross section of river, calculates the ratio of erodability both quantitatively and qualitatively, and based on that, the amount of damage to the channel is determined. The calculated data for the first stage for this model included active channel width, valley width, calculated area of ​​protected areas and no sediment production in studied basin, channel slope, channel width in bankfull, channel depth in bankfull, boundary and critical shear stress, Manning coefficient and flood width are considered. The required data for the second stage of the channel vulnerability model is as follows: Determining the type of sediment substrate: sediment types are classified into three general categories: a) non cohesive; b) cohesive deposits; and c) concrete bedrock, which the type of sediment was determined on the basis of field observations. Determination of bank erosion class: In this model, bank erosion classes are divided into three erosion classes: low, moderate and high erosion, that their type at each cross section was determined on the basis of field observations and comparison with the photograph. 3) Determination of erosion class of the channel in Schumm class:  According to Schumm river model, the rivers are classified into six classes. The type of erosion class of river channel, was determined using field observations and cross-sectional profiles, and comparing it with Schumm river classes at each cross section of the river, then the river erosion class number entered the vulnerability model. Depending on type of Schumm erosion class and bank erosion, the degree of erosion is different in both classes. In the third stage, using the data provided, the ratios and classes of effective parameters, the rate of channel vulnerability is calculated in all cross sections of river. the ratios and classes of effective in rate of channel vulnerability are as follows: 1) risk of channel erodability rate, 2) risk of channel entrenchment, 3) At the end is calculated channel vulnerability. 3- Results In this study, the channel vulnerability model was used to investigate and determine the Instability and vulnerability of the channel in Masil Muchhan river. With this aim, 8 cross sections were selected in a reach of 5.55 km between villages of Ghaleh and Sarsakhti and calculated shear stress and channel erosive forces on the basis of field data and some of the effective parameters in the channel,  and at the end was determined vulnerability of the river by describing all parameters as a table in the main article. Based on the calculations, it was found that all cross sections of the Masil Muchhan river are subject to moderate to high vulnerability; cross sections 1, 2, 3, 4, and 5 have a moderate vulnerability and cross sections 6, 7 and 8 have a high vulnerability risk that Is described the causes of erodibility difference, vulnerability and cross-sectional profile of all cross sections is expressed in the original article. 4- Discussion & Conclusions The results of the channel vulnerability calculations showed that all cross sections of the Masil Muchan river are subject to erodibility and thus vulnerability. Of the 8 cross-sections that were evaluated, 5 cross-sections (sections 1, 2, 3, 4, and 5) have a medium vulnerability risk and three cross sections (6, 7, and 8) are at high vulnerability risk. The results of vulnerability were investigated in two levels: the results of vulnerability in two levels showed that at the level of morphometric indices of the channel, all of these three cross sections showed a high degree of slope and depth of bankfull. These parameters increase the flow velocity at bankfull discharge in these cross sections of river. In addition to the above, all three sections have the highest critical shear stress, which, in case of bankfull discharge, causes the highest amount of shear stress and thus erodibility. In these sections, despite the high Wet perimeter and low amount of hydraulic radius, which reduces the flow of power, but the high shear stress and flow velocity in bankfull discharge, increases the power of river and ultimately creates a high risk of vulnerability. Angle of repose on all banks was obtained by a similar number and therefore not considered as an effective parameter in the channel vulnerability. As a result, the largest amount of erosion occurs for banks which are more sloping. In all sections except sections 5 and 6, the left banks have more erodibility due to greater slope.