عنوان مقاله [English]
The study of the past morphological changes in floodplain unconfined reaches and the relationship with the natural and human-induced controlling factors is widely recognized as a useful tool to define the evolutionary trend in order to plan correct river management or sustainable river restoration. Morphological evolution involves bed level fluctuations, as a consequence of incision and aggrading, as well as plan form changes, concerning channel width, position and pattern. Following appreciable variations of boundary conditions, due to either natural (e.g. climate change) or anthropogenic (e.g.water extraction, damming or sediment mining) causes, rivers may experience long-term morphological evolution, which will eventually affect large portions of their watersheds Karoun is the biggest river of Iran which is emanate from the Zagros mountain and empties into the Persian Gulf. During the past two decades, numerous sedimentary islands have been made in Karoun river bed within Ahvaz city that caused the river ecosystem faced with various issues and has created some changes for Ahvaz city. This study examines geomorphological changes of the river and also the reason of creation and development sedimentary islands within the city.
In the first phase of this study, Landsat7 satellite images of 2000 and IRS (internal revenue service) of 2011 and aerial photos of 1334 and also statistics hydro metric stations have been used. According to aerial photos, the base map and geomorphology were produced in ArcGIS environment. Then, by comparing satellite images, geomorphological changing procedure of river bed was extracted from 1954 to2011.
In this study, check path length of110km is divided into 4phases (periods) and then geomorphological map was produced for 1354.In next years, the other changes were measured against the base year. In the next level (phase), the statistics on the rate of hydro metric stations and rainfall data were analyzed by software excel. This analysis ,6th linear regression polynomial and drawing diagrams monthly and yearly average , average maximum instantaneous flow rate of each of the stations was drawn from 1961to2011. Preliminary observation was done during several stages for more and better recognition and understanding of the region in Farvardin94. And after that the sediment samples of islands were taken selectively. After transferring the deposits to the lab, a depth isolated from the sediments were taken into granulometry analysis and its sedimentology variables like cumulative frequency percent, tilt and sorting were revealed.
Results and discussion
The river's path has suffered changes and many displacements from 1334, the period which has been studied, to 1391. According to the territorial sections of the river that its value given in table1, it is seen that there has been lots of landscape changes in river channel.
The average changes show that in overall, during the period of study of this research, the least amount of landscape changes during the 57year old period were in the first interval with average 330m and the most amount of landscape changes were in the third interval of the river which means metropolitan area of Ahvaz with average 430m.
In term of river channel pattern and its relationship with changes rate has been seen in the second level. Evaluation of historical maps shows that arterial pattern (the second interval) in the first level and meander patterns (the third and fourth intervals) has the most amount of changes and displacements while the direct intervals (the first interval) is more stable than the other intervals.
Result show significant changes in the river's morphology during these 57years. Including width changes in river bed and river channel especially within Ahvaz city. mendar variations and formation of bar islands were formed. River islands in this area can be divided into the following 3categories:
1. Longitudinal islands which are long and droplet like are formed in the middle of the channel. Their overlapping structures (they consists of both large and small sediments) indicates various flood period.
2. Transverse islands which widen rather suddenly, occurs when sediments are deposited in the center of the river due to gradient losses.