ارزیابی کمی اثر سیلاب بر تغییرات مجرای رودخانه (مطالعه موردی: رودخانه سدیج استان هرمزگان)

Abstract

In this research, to evaluate the effects of these floods on the morphology of the Sadij River, the parameters of the channel activity (Ca) and the changes in the river banks were evaluated in 5 significant floods between 2009 and 2020. Landsat TM and OLI satellite images were used to extract river banks. According to the pattern, the river was divided into 4 reaches and the rate of change was analyzed using DSAS plugin in ArcGIS 10.5 software. In addition, by determining the center line of the river, the activity index of the channel (Ca) was calculated to determine the changes in the form and the river bed. Also to evaluate the role of the texture of sediments in the changes of the banks, after field sampling and laboratory work, the texture and granularity of sediments were determined. The results showed that the banks' response to floods with different discharges was different. Dominant trend of the entire river during the studied period was erosion, and most of the changes have occurred on the right bank of the river. In reach B, due to the cutting of the river meander by the flood flow of 2020, the maximum displacement of the channel and the highest amount of activity have been observed. Finally, the volume and discharge of different floods in the region have more influence than the type of sediments and the low slope in changes of the river at different reaches.



Extended Abstract



Introduction

Rivers and waterways have a dynamic system. These changes cause erosion and deposition on the banks and their movement and displacement. Sadij River is located in the east of Jask city. Field studies, satellite images, and hydrometric statistics of Sadij basin stations show that many floods have occured in this area every year. In addition to human damages, these floods caused the river to change its course, and erode the banks and the river bed. The study area of this research is located in the coastal plain of Oman Sea. So, satellite images, the system Digital DSAS, as well as active channel changes were used for evaluating the river channel changes due to five significant flood (January 24, 2009, January 20, 2014, January 25, 2017, February 4, 2019 and January 11, 2020) in four selected reaches. Also, to evaluate the role of the texture of sediments in the changes of the banks, the texture and granulation were determined using the hydrometric method and sieve shaker. The process of changes in the morphology of the banks and the amount of activity of the river channel from before the flood of 2009 to after the flood of 2020 was also done for the entire path and reaches, and finally, it aims to provide solutions to reduce the damage and erosion of a river during a severe flood.





Methodology

In the present study, Landsat TM and OLI satellite images related to before and after five significant floods were used. In order to reduce the negative effects of atmospheric factors, radiometric and atmospheric corrections were performed on the images in ENVI 5.3 software. Then, in order to more accurately evaluate the effect of floods on the morphological changes of Sadij River, it was divided into four reaches (A, B, C, and D) based on the planform and the changes in each reach. In the present study, the Digital Coastline Analysis System (DSAS) system was used to evaluate the quantitative changes of the river bank lines and the activity level of the river channel, to evaluate the activity of the river channel due to significant floods. Also, after a field working and sediment sampling, hydrometric and shaker methods were used to determine the soil texture.



Results and Discussion

In the investigation of the changes of the river bank lines, the results showed that the number of changes was related to the volume of flood discharge and the right bank had the highest numbers of changes. Also, among the indicator floods, the flood of 2020 had the highest numbers of changes due to the sudden interruption of the meander of the river in the B reach. In the investigation of the changes in the river banks before the flood of 2009 and after the flood of 2020, it was observed that according to the NSM index, the right bank has an advancing trend with an average annual change of about 8.53 meters, and the highest amount of advancement was in the B reach.

The highest amount of channel activity among the indicator floods was also related to the flood of 21 January 2020, which coincided with the flood and the sudden interruption of meander in the reach B and the change of the direction river, and the amount of activity of the river channel has reached about 57.13 meters. In examining the activity level of the river channel during 11 years, the amount of activity of the river against the flow of significant floods was low, and this shows that apart from the significant floods, the river was also affected by other floods with different discharges. Also, most bank line changes, based on the NSM index and the amount of channel activity (Ca) are related to the reach B, which has the largest number of meanders. The results of the soil texture also showed that the majority of sediments with more than 70% in the sampled reaches were uniform and did not have a significant effect on the change of the river.



Conclusion

According to the results, the DSAS plugin is a powerful tool for detecting and analyzing river bank changes. Investigations showed that with every flood, river bank erosion was observed on the left and right banks of the river. The amount of erosion and displacement of the channel depends on the volume of the flood in Sadij River, so that the flood of 2020 was the largest flood in the studied area due to the large volume of the discharge, which was separated the river meander. The effect of the flood on the activity of the river bed also shows that every flood event leads to the change of the channel bed and the amount of Ca increases with the increase in the volume of the flood, but despite the numerous floods between 2009 and 2020, the rate of Ca is not high during the studied years, and this shows, that the river rebuilds itself over time and moves towards relative balance. Also, the results of the sediment texture of the banks and river bed show that due to the relative uniformity of the sediment texture (sand) along the river channel, there is no direct relationship between the change of the river course and the sediment texture, and other important factors such as the flood volume can play a decisive role. Finally, the results showed that considering the morphology of the river, it is necessary to pay special attention to the morphology of the river when carrying out construction projects such as the construction of bridges and riverside structures.





Keywords: Flood, River morphology, DSAS, Sadij, Jask