ارزیابی دینامیک مورفولوژیکی رودخانه‌ بابل‌رود، استان مازندران

Introduction
The morphology of rivers is constantly changing due to their dynamic characteristics. Water flow and sediment transport play a major role in the dynamic activity of the river and in interaction with environmental factors, they cause different behaviors in the river. River adjustment occurs in three groups: riverbed adjustment, lateral adjustment and whole river changes. In flood hazard mapping are usually evaluated probabilistically the hydrological characteristics of the flow, include the frequency and magnitude of the flow and do not consider the dynamics of the river morphology. Therefore, analyzing channel morphological changes with geomorphological approaches and methods to study river dynamics and instability reduces this problem. The aim of this research is to classify the morphological dynamics of Babol River for flood and river channel management. Another purpose of this study was to investigate the relationship between environmental variables and river morphology dynamics.
Methodology
In this study, the dynamics of Babol River were investigated based on the characteristics of river form and process, artificial features and river adjustment by morphological dynamic index (MDI) method, which has been proposed in the framework of IDRAIM (Rinaldi et al, 2015). River morphological dynamics index are (MDI) divided into three categories: morphology and process indices (M), artificial indices (A) and adjustment indices (CA). First, eight reach of Babol River (Mazandaran province) were selected in foothill and plain area. Geomorphological map of river landforms including river channel, lateral bar and point bars, floodplain and alluvial terraces has been prepared and drawn using Google Earth images of 2020, 2007 and fieldworks. To study the long-term changes of the river, aerial photographs from 1335 and studies conducted by other researchers were used. Some of the changes were assessed based on terrestrial evidence. Each indicator was scored and then the final value of MDI was calculated. River reaches were classified into several categories (very low, low, medium, high and very high) based on the morphological dynamics of the river.
Results and discussion
Eight river reaches were studied in this research. The reaches are in the range of 200 meters to -25 meters above sea level. Their slope varied from 0.001 to 0.018 m/m. The mean width of the channel ranged from 12 to 77 meters and the mean width of reaches, which includes channel and floodplain, ranged from 41 to 83 meters. The river confinement index was between 1.1 and 4.4 and confinement degree of the reaches varied between 18 and 100%. In general, this reaches of Babol River are mainly partly confined. The channel sinuosity coefficient varies from 1 to 2.2 and the river pattern in these eight reaches are straight, sinusoidal and meandering. Based on the total scores of MDI, reaches 1 to 4 were in the high class, reaches 5 and 6 were in the middle class, and reaches 7 and 8 were in the low and very low classes of river morphological dynamics, respectively. By calculating the ratio of each group of indicators to the maximum value of the same group, the relative value of each group was obtained. The value of MDI decreased from upstream to downstream, indicating a decrease in the morphological dynamics of the river in the downstream areas. The trend of MDI changes and its constituent indices with the width of the river channel are most consistent with exponential regression and indicate a decreasing exponential trend. These trend of changes are faster for the morphology and process index and the index of river adjustment, and then have a milder trend. Thus, upstream reaches are more active in terms of morphological dynamics than downstream reaches and show a significant relationship with the width of the river channel.
Active river width and confinement degree had a correlation coefficient of -0.92 and -0.78 with MDI index, respectively, which shows that with increasing each of these two variables, the morphological dynamics of the river has decreased. The variables of height, slope and confinement index had correlation coefficients of 0.68, 0.62 and 0.78 with MDI values, respectively, which indicated an increase in the morphological dynamics of the channel with increasing these variables. High correlation (0.94) of group M indicators with CA group indicates the high effect of morphology and natural processes of the river on the rate of adjustment and artificial factors could not have much effect on the natural characteristics of the river.
Conclusion
The analysis of the results shows that in the reaches with medium and high morphological dynamics index (MDI) where the river plan and channel geometric dimensions can be further changed, by identifying the previous river path and channel dimensions, different scenarios can be presented for zoning and flood hazard mapping. The study of the relationship between different variables and morphological dynamics indices (MDI) shows significant changes from upstream to downstream and the decrease in MDI downstream and high correlation coefficients between variables is consistent with the pattern presented in the model. Therefore, this model can be used in geographical types similar to the study area and has been required for other cases and has been validated.