The Investigation of the Impact of Hydro-geomorphology and Land Use Changes on Zanjanrood Stream Stability

Rivers' pattern is seldom stable and is always subject to change. Rivers' banks have long been the most important areas of population settlement, especially in the semi-arid regions of Iran. This has made the recognition and assessment of the geometric deformation of rivers an important applied topics. Alluvial beds of a river are divided into two groups of stable and unstable. While in a stable bed, the walls and the floor are stabilized, in the unstable bed, the walls and the floor are not constant and have a changing state. Zanjanrood is regarded as the natural bio-economical artery of Zanjan province. In the present study, by recognizing the temporal and the spatial hydro-geomorphological changes of Zanjanrood during the years 1955 and 2011, and in the western part of the river, which is more important for agriculture, the amount of morphological changes is calculated and presented in the form of a map. The method of the study was analytical by spatial-local comparison based on the direct and the indirect observations using satellite images, aerial photos, and maps.
In this method, the intervals were classified into distinct parts based on the morphology-hydrodynamic similarity. Then, by applying spatial analysis on the raster maps, the pattern and variation of the changes were estimated by plotting tangential transects on the extension and restriction areas in the right and left sides. By determining the extension and restriction areas in river sides, the type and amount of stream’s variation in various reaches was determined. Afterwards, the total areas of extension and restriction on both sides of the stream were computed. Transects are rectangles with constant angles whose area variations indicate the changes of the river banks in different time ranges. This technique was used to determine the area dominated every progression and regression area in order to analysis and assess of the main factor(s) affecting this change. The results of the study showed that the outcome of the action-reaction of the factors affecting Znjanrood changes in the studied area had been a stable trend over the past 56 years. The most important factors involved in this trend were the development of the agricultural activities, the increase of management actions to stabilizing the river for agriculture and infrastructure proposes, and dam construction. However, the effect of slope and geological conditions (neo-tectonic) should not be neglected as well. Discussion and conclusion: The results indicated that the overall trend was to reduce the width of the channel to its axis. However, the process has increased from the first period (1955-1985) to the second (1985-2011) and the third periods (1985-2011), so that at the end of the given time intervals, there was an increase in the relative stability of the stream. The change in flow to the outlet reduced the erosion power of the river flow and increased the stability and relative stabilization of the bed. The development of the fairly stable gardening activities dispersed on the flood bed played an important role in the stability of the stream in the studied area, so that the gardens and fields on the stable river main bed sides exhibited great changes in the images and photographs.
The role of the human factors in the Zanjanrood Basin due to its relative geographical position (east to northwest transit), has been the basis for the development and launch of several road construction projects (railways, roads and freeways) during the period of 1955 to 2011 in Zanjanrood and caused the change of Hydro-geomorphological-Hydrodynamic systems. A quantitative assessment of the Zanjanrood river bed changes shows that the rate of change, overcoming the general trend of stability and stabilization, has increased from the first period to the last period. Indeed, while the stabilization rate in the first period was 0.16 km2, in the second and final periods, it was respectively 1.91 and 2.01 km2. Progressions are concentrated in the beginning and end of the research (Transects 1, 2, 6, and 9 to 13), which affects the lands of the villages of Nikpey, Darelik, Dolanab, Naji, Idalou, and Baghloja and Saifabad. The recessions are also focused on transects 3 to 8, excluding transect number 6. Based on the results, Zanjanroud shows the overall stability trend. The adjacent area of the upper and lower parts of the studied research (the areas around the villages of Nikpey and Baghloja) are among the most sensitive areas of the research.