Evaluation of Morphodynamic Effects of Floods on the Morphological Changes of the Coastline of the Sedij Delta (East of Jāsk)

The coasts of Jāsk and its eastern regions are of great importance in developing the country's southern areas due to their strategic location, such as the establishment of military centers and the pipeline project to transfer Goreh oil to Jāsk. Monitoring and evaluating the changes in coastlines, mainly due to natural events such as floods in these areas where coastal structures are located, can provide valuable information to the environmental planners for the optimal implementation of construction projects. This research aimed to evaluate the changes in the geomorphology of the coastline due to the occurrence of floods in 6 years and 4 major floods in the years 2014, 2017, 2019, and 2020 at the base of the Sedij Delta. Landsat satellite data and the NDWI index were used to extract geomorphological units and coastlines. Coastline changes were also calculated using the Digital Shoreline Analysis System (DSAS) extension in the two geomorphological units of the sandy beach and the estuary of the Sedij Delta. The results showed that during the occurrence of these floods, according to the NSM index, the changes in coastlines in the mouths of the estuaries have a trend with sedimentation. The maximum progression of the coastline after these floods was 503 meters, and sandy beaches have an erosion process. The maximum rate of retreat of the coastline is 256.16 meters. The study of changes in the area of coastal barriers during the flood period showed that these floods have led to severe erosion of the coastal barriers, and about 171.79 hectares have lost their area at the base of the Sedij Delta. This research showed that in addition to the river processes, the intensification of marine processes, such as the height of waves, especially on sandy beaches, can significantly affect the erosion of coastlines in the studied area.
 
Extended Abstract
Introduction
In coastal areas, the occurrence of floods, as well as the hydrodynamic processes of the sea, affect the coastal geomorphology. The climatic conditions governing the coastal areas also have a decisive role in the morphological characteristics of the coasts in connection with the morphogenesis processes. Sedij River is located in the east of Jāsk City. The statistics of the Sedij hydrometric station showed the occurrence of high-flow floods in this area, and the analysis of satellite images has also recorded the severe changes caused by these floods. Among these floods is the January flood of 2020; due to the severity of the rainfall in the region, it left a lot of damage to both the humans’ part and the destruction of the residential areas of the villages around the Sedij River and in the geomorphological units, which destroys the river and its banks, and extensive changes in the mouth of the estuaries and changes in morphology. Therefore, evaluating and monitoring changes in the coastlines and geomorphological evolutions of Sedij River deltas during significant floods can provide coastal planners with a new approach to investigating the role of floods on coastlines. In this research, the Sedij delta coast has been divided into three areas in terms of location. In terms of geomorphology, it has been divided into the coastal barrier, estuary, and sandy beach units, and the changes of coastlines in each flood and the changes during the entire study period in these units have been evaluated. This research was conducted to evaluate the stability of different parts of the coastlines of the Sedij Delta and the estuaries related to this delta, and it has taken a step towards identifying the more vulnerable parts during coastal floods and protecting the beaches in this area.
Methodology
In this study, the evaluation and monitoring of the changes in the coastlines of the Sedij Delta during the occurrence of 4 major floods from 2014 to 2021 have been discussed. The floods of January 20, 2014, January 26, 2017, February 4, 2019, and January 11, 2020, considered the most significant floods of the studied period, were used in this research. Landsat OLI satellite data have been used to monitor changes in coastlines. Radiometric and atmospheric corrections were performed to eliminate the negative effects of atmospheric factors on satellite images. NDWI spectral index was used to extract coastlines. The extracted coastlines were entered into the DSAS software, and the changes in the coastline were evaluated in the ArcGIS software. In this study, the indices of Linear Regression Rate (LRR), End Point Rate (EPR), Net Shoreline Movement (NSM), and Shoreline Change Envelope (SCE) were evaluated.
Results and discussion
During significant floods, the coastal lines of the estuaries and the sandy beach have changed the form of progress and regression. Average coastline changes were obtained based on LRR, EPR, NSM, and SCE indices in each geomorphological unit for each flood and simultaneously for 4 floods. The results showed that due to the floods of 2019 and 2020, when the amount of discharge and precipitation was higher, the changes in the coastline in the mouth of the estuaries, the sandy beach, and even the area of the coastal barriers were also higher. Also, evaluating the changes in coastlines between 2014 and 2021, as a result of major floods, showed the progression of coastlines in the estuary geomorphological unit and retreat in the sand beach unit. According to the LRR index, the sandy beach coastline of the Sedij Delta has receded by an average of 1.63 meters per year. The highest rate of regression was in the 3rd zone. At the mouth of the estuary, after every river flood, a volume of sediment accumulates at the outlet of the river, and this can provide the basis for the advancement of the coastline in this area after every flood. The highest rate of progress is 0.90 meters per year and is also observed in the 1^st zone, most of which is located in the mouth of the Sedij estuary. Coastal dams also lost about 171.8 hectares (38%) of their area after the flood of 2020 compared to before the flood of 2014, which indicates the simultaneous effect of river and sea processes on the erosion of the coastline. Furthermore, by moving away from the mouth of the Sedij River, the effect of waves on the coastline increases, which is the feeding or coastal sediment. The investigation of marine parameters such as wind speed and wave height during floods for two floods in 2014 and 2020 showed that the wave height at the time of the January 2020 flood reached 3 meters, which is about 1.2 meters higher than the wave height at the time of the 2014 flood, which indicates a significant change in the height of the sea waves and as a result the increase in coastal erosion in the Sedij River Delta.
Conclusion
Makrān coast is a very dynamic area in terms of coastal geomorphology that is simultaneously affected by the processes governing land and sea. These changes are severe in many cases and are mainly seen in sedimentation and erosion processes in the coastline. The data evaluations of this research showed that, in general, the effect of floods during the studied period was the regression of the coastline. In zone 1, which is the place where the Sedij River enters the sea, a large amount of sediment enters the coastline at the same time as the floods occur, and this prevents severe erosion in this area and provides a relative balance of the erosion and sedimentation processes there. However, the erosion process increases in the area far away from the mouth of the Sedij River on the sandy beach. The sandy beach area has played a dominant role in the evolution of the coastlines of this section due to the lack of proper sediment feeding and the intensification of marine processes, such as the increase in wave height and the activity of currents parallel to the coast during floods. This section of the coastline has provided the basis for more severe erosion than the mouth of the estuary. Despite the erosion process in the area of the sandy beach, this beach is more suitable for the development of the coastline than the mouth of the estuary because it is possible to migrate the mouth of the estuary during floods, which can hurt the coastal facilities and structures.
Funding
The present article is the result of a doctoral thesis research and supported by the Iran National Science Foundation (INSF), (Project code: 4003100).
Authors’ Contribution
Authors contributed equally to the conceptualization and writing of the article. All of the authors approved the content of the manuscript and agreed on all aspects of the work.
Conflict of Interest
Authors declared no conflict of interest.
Acknowledgments
We are grateful to all the persons for scientific consulting in this paper.
Key words: Coastline changes, DSAS, Coastal barriers, Sadij, Flood