Sensitivity of Form and Evolutionary Parameters of Meanders to Small Rivers Dynamics, (Case Study: Ghere – Sou River in Kermanshah)

  Shield (2000), using some geometric activity parameters, explains how reservoir can be effective on downstream river channel migration. After that, Magdaleno and Fernández-Yuste (2011) recognized that these parameters may complement the classical form parameters and represent the real functioning of the river corridor, in geomorphological analyses of meander dynamics. However, the effectiveness of these indices is not clear on other channels where meandering is not very developed and it is not clear if or not the geometrical parameters can indicate the type of functional and dynamics of this type of rivers. We have selected an area about 52 km wide along the total length, 219 km, of the Ghere- Sou River as the study area; this is because of location of Kermanshah Plain on a fault line that allows smaller changes. The study of the sinuosity of the river during the 54 years (1955 to 2009) shows that the number of meanders has been increased and decreased constantly. The meanders are included just a quarter of windings, at this study.
To study the dynamics of sinuosity, we used form parameters such as the radius of curvature, wavelength, amplitude, meander length, and evolutionary parameter such as bankfull width, magnitude of channel lateral migration, area occupied by the active channel and channel activity coefficient to determine the evolution of the meander belt in the central sector of Ghere-Sou River.
At first, aerial photos from 1954 and 1967 (black and white; approximate scale 1:55,000 and 1:20,000) and satellite images of IRS 2004 and 2009 (colour; with 2 m. resolution) were digitized and the factors was measured in ArcGIS. Using the Kolmogorov-Smirnov normality test for three form factors (wavelength and the radius of curvature and amplitude), Friedman tests were examined for abnormal data and ANOVA test for normal data. Finally, the spatial distribution for the morphology parameters was analyzed in order to determine if they showed a change downstream. Comparing the form and geomorphic activity parameters elucidated which groups of parameters are powerful to show the dynamics of river. Ghere-Sou River has curvatures as sinuosity and meanders at 81 points, totally. The curvatures numbered 1 to 81 from up to downstream. Comparing these points at four years (1955, 1967, 2004 and 2009) showed that sinuosity has decreased at some points and it has increased at some other points. Thus, we can see that the number of arcs has been changed from 77 to 79 and then 77 and finally to 65. The sinuosity characteristics of arcs have been changed through these periods. However, three parameters including radius of curvature, wavelength, and amplitude have suffered from very little fluctuations. The results of Friedman and ANOVA show that there is not significant difference between them. However, other parameters including meander length, bankfull width, magnitude of channel lateral migration, the area occupied by the active channel and channel activity coefficient have indicated a progressive trend from 2005 to 2009.
A glance at the average bankfull width of river in different years show that this factor was about 108 m in 1955, 95 m in 1967, and 77 m in 2004 and finally reached to 89 m in 2009. This means that river has been more dynamic during last 5 years and it has added 11 meters to its width on average. The same is also true about average of lateral migration of river, so that the first until 2.5, 0.92, and 3.5 m per year, respectively. The total area occupied by means of channel activity had increased in the period. Channel activity coefficient is reached from 2.51 at the first period to 0.82 at the second period and finally to 2.76 at the third period. Therefore, the river is desired to have a state of static equilibrium at the first and second period and now try to have a state of dynamic equilibrium. The evolutionary changes are conducted in GIS and cause to promote the analysis capacity.
The river has been divided into 11 reaches in the two Google earth images in 2005 and 2015 and they were union together at the ArcMap. The results illustrate that the river has had up to 70% overlapping in the fifth reach. However, the changes of river have become more apparent at sixth reach while going to be channelized. This situation is to be the same downstream because of river balancing nature. This study about the dynamics of Gher-Sou River elucidated that only the form parameters (wavelength, amplitude, curve radius) don’t explain evolutionary characteristics and we need some morphological parameters such as magnitude of channel lateral migration, the area occupied by the active channel and channel activity coefficient. Some compositional parameters such as radius of curvature to the average bankfull width can also show these changes. These results confirmed by Magdaleno and Fernández-Yuste (2011) highlighted dynamic equilibrium of Ghare-Sou River in Kermanshah plain since 2004. The dynamics are coincided with urbanization and the expansion of activities and projects implementation.
Further study on the parameters may be a cause to produce more accurate parameters in relation to evolutionary characteristics; for example, the remaining of active channels overlapping may provide better results in some cases. The parameter is obtained from overlaying of two periods and calculation of non-intersected area. Using GIS, it can be obtained by combination of different layers of the area.