Historical analysis of channel planform changes, displacement and stability in Neka river (Neka, Mazandaran)

Changes in river pattern is one of the most important issues of river engineering that affects the activities and construction structures along rivers.Changes in the parameters of the meandering can be due to changes by humans, changes in the hydrological regime of the river, ecological changes or due to geomorphology of the region.Knowledge and understanding of the morphological changes of meandering patterns and the natural dynamics of river systems are very important for the purposes of planning, urban planning, dam construction, erosion and sedimentation, road construction and protection and reconstruction of river channels.By studying the amount of changes in river parameters, especially the length and width of the channel in a region, the future of the meandering patterns and rivers can be predicted.Channel form is a good initial guide to determine the morphology and channel form changes in alluvial rivers.In fact, the most obvious feature of a river is its planform or geometric form.A river plan is called a planform, which shows the characteristics of a channel and a floodplain in an alluvial river.The term river pattern describes the planimetric (two-dimensional) shape of rivers.In the new classifications, channel patterns are classified into straight, meandering, braided, and anastomosing.The purpose of this study is to investigate the trend of morphological changes in the Nekaroud River from the city of Neka to where the river joins the Caspian Sea for a period of 35 years between 1364 to 1399.

Neka river originates from Alborz mountains in the southern part of Gorgan city and passes through the southern part of Behshahr and flows into the Caspian Sea.The area of Neka Basin is 1902 km.The Neka River is 130 km long in the mountains and 39 km long in the coastal plains.In this study, the Neka River with a length of 39 km in the coastal plain from Neka to the Caspian Sea has been studied.At first, Nekaroud was divided into three parts: upper, middle and lower part and the changes were studied separately and together.With this method, the amount of displacement, change of dimensions and pattern of bends were determined.Using the method of historical changes, the river route map related to different periods based on aerial photographs of 1985 (National Geographical Organization of Iran) and satellite images (from Google Earth) of 2006 and 1399 has been digitized in GIS software.In order to study the changes of the river in three time periods, from three indicators including morphometry which includes (channel length, arc radius, central angle, wave amplitude, wavelength), morphology which includes (different models of lateral migration including retraction, new meander,migration, confined migration, cutoff, growth, meandering change, double heading) and morphodynamics (transect method) have been used.Finally, the changes of the left and right sides of the channel in the studied statistical period were statistically tested in SPSS software.

The results of the study of satellite images and aerial photographs show that the values of the curvature coefficient, the central angle, the angles of the arcs along the river have decreased from the upstream to the downstream.Based on these morphometric characteristics, the river is classified into three parts including the developed meandering pattern, the undeveloped meandering pattern and the almost meandering pattern in the third reach.Although the morphometric values of the channel show that the Neka River has changed,however, the statistical test in the section with a developed meandering pattern showed that no significant changes were observed in the right bank during two different periods.There were no significant changes in the left bank of the river during 2006 and 2016.The study of Yemani and Hosseinzadeh (2013) on the Talar River and Yemani et al. (2014) on the Babol River had similar results. In these studies, similar to the Nekarud River, at the beginning of the river's entry into the coastal plain, i.e., the alluvial fan  section (the first part of the Nekarud), there is more instability and changes are still taking place during large floods; However, in the second and third periods, due to human intervention and protection, the channel deepened and finally, the main river was divided into several branches in order to transfer water to the rice agricultural lands, which caused the degree of instability and changes in the channel pattern in the four The last decade is at a minimum.

According to the studies, it was found that the Nekarood River has had limited changes in terms of morphology, morphometry and morphodynamics during 35 years.Changes in the bed of the Nekaroud River during 2006 to 1399 have been decreasing. The reasons for these conditions are the existence of large-scale cultivation around the river and the protection of the river bank, the creation of storage dams and water diversion in the middle part of the coastal plain and the construction of dams on this river (Golord Dam and other dams).With the construction of these dams, many changes were made in hydrological parameters such as flow velocity and discharge. Also, the amount of erosion and the amount and method of sedimentation have changed.