Morphological and morphodynamic analysis of Meanders in Shalmanrood River, Gilan Province

Meanders have active dynamic and are constantly changing. Morphodynamics of meanders are important in sediment patterns and processes and hydrological and ecological processes of flood plains. But human activities create changes in these processes. Hence, knowledge and understanding of the morphological changes of meanders and the natural dynamics of river systems are essential for the planning, conservation and reconstruction of river channels. In this research, the morphological changes of the Shalmanrood River (East Gilan) have been analyzed during a period of 51 years between 1967 and 2018. Changes in Shalmanrood meanders were evaluated for three periods of 1967-1994, 1994-2003 and 2003-2018. To do this, the aerial photos of the years 1967 and 1994 as well as the images of Google Earth in the years 2003 and 2018 were used. For each of these years, the river route was mapped and digitized. Changes in Shlmanrood meanders were analyzed with morphometric, morphodynamic and morphological indices. River geometric properties (morphometry) were measured with parameters including channel length, wavelength, amplitude channel, radius and central angle of curvature. Morphodynamic characteristics were examined with lateral channel migration, area occupied the active channel and channel activity rate and then, channel morphology and different channel migration models were evaluated. The mean values of the geometric parameters of Shalmanrood meanders were not significantly different at a significant level of 0.05 in the studied periods (51 years). Most of the Shalmanrood river arches have been developed meanders and it included 79, 81, 62 and 81 percent of the total arches in the years 1967, 1994, 2003 and 2019. Extremely developed arches have included 11 and 7 percent meanders in the years 2003 and 1397, which indicates changes in the forms of the arches in recent years.
The channel activity was measured at 1967-1994, 1994-2003, 2003-2018, respectively, 2.77, 1.22, and 0.48 meters per year, indicating a decreasing trend in this parameter. The amount of overlap of the river channel in the period from 1967 to 1994 was 13 percent but they have increased in period 1994-2003 and 2003-2018 to 78 and 86 percent respectively. This trend represents a decrease in the channel's morphodynamics over the past 25 years. The morphology of Meanders' changes in the first period (1967-1994) has changed dramatically and has changed slightly in the second and third periods, and has changed the general pattern of immigration. Accordingly, during the period of 1967-94, the complex changes and the growth of Meander with a value of 67% were the largest channel of immigration pattern. Approximately 81% of the channel was unchanged during the period 1994-2003, and the remaining 19% related to migration patterns, meandering growth and lobbing. In the period of 2003-2018, the patterns of change were almost the same as in the previous period. Reducing lateral channel migration and increasing the overlap rate of the channel, especially during the 2003-2018 periods, indicates a decrease in the geomorphic activity of river. The occupation of the surrounding areas of the river with agricultural and residential landuses, the construction of dikes and the consolidation of the banks in different parts of the river are among the factors influencing the reduction of geomorphic channel activities in the meander belt zone in Shalmanrood. In the current situation, the removal of sand and gravel from the channel bed and flood plain, the upstream parts of the study area, the bed lowering (incision), the presence of fine-grained and loose sediments in the size of the silt and sand in the bed and the bank and formation of developed meanders can increase the occurrence of channel interruption (cutoff) in the middle reaches and cause instability in the balance of the river. The results of this study show that geomorphologic study of meandering rivers using a total of morphometric, morphodynamic and morphological indices provides a more comprehensive and deeper understanding for the analysis of this type of rivers.