ارزیابی تاثیرات سیلاب‌های شدید بر تحولات مورفولوژی خطوط ساحلی دلتای سدیج (شرق جاسک)

سواحل جاسک و مناطق شرقی آن به‌دلیل ویژگی‌های راهبردی ازجمله، استقرار مراکز نظامی، طرح لوله انتقال نفت گوره به جاسک و توسعه مناطق آزاد در توسعه مناطق جنوبی کشور از اهمیت زیادی برخوردار است. پایش و ارزیابی میزان تغییرات خطوط ساحلی به‌خصوص در اثر رویدادهای طبیعی همانند سیلاب در این مناطق که محل استقرار سازه‌های ساحلی است، می‌تواند اطلاعات ارزشمندی را در اختیار برنامه‌ریزان محیطی جهت اجرای بهینه طرح‌های عمرانی قرار ‌دهد. هدف این تحقیق ارزیابی تغییرات ژئومورفولوژی خط ساحلی در اثر رخداد سیلاب‌ها در یک دوره 6 ساله و درنتیجه 4 سیلاب شاخص در سال‌های 1392، 1395، 1397 و 1398 در قاعده دلتای سدیج است. به‌منظور استخراج واحدهای ژئومورفولوژی و همچنین استخراج خطوط ساحلی از داده‌های ماهواره‌ای لندست و شاخص ndwi استفاده شد. تغییرات خطوط ساحلی نیز با بهره‌گیری از افزونه سامانه تحلیل خطوط ساحلی (dsas) در دو واحد ژئومورفولوژی ساحل ماسه‌ای و دهانه خور دلتای سدیج و در سه بازه مطالعاتی محاسبه گردید. نتایج این تحقیق نشان می‌دهد در طی وقوع این سیلاب‌ها، با توجه به شاخص nsm تغییرات خطوط ساحلی در دهانه خورها دارای روندی همراه با رسوب‌گذاری بوده است. همچنین جابجایی طولی، در امتداد خط ساحلی صورت گرفته و بیشترین میزان پیشروی خط ساحل پس از وقوع این سیلاب‌ها، 503 متر است. برخلاف آن‌ها، واحد ژئومورفولوژی سواحل ماسه‌ای دارای روندی فرسایشی و بیشترین میزان پسروی خط ساحل 256 متر ثبت‌شده است. بررسی تغییرات مساحت سدهای ساحلی در طی دوره موردمطالعه نشان می‌دهد که این سیلاب‌ها منجر به فرسایش شدید آن‌ها شده است، به‌طوری‌که سدها در حدود 171 هکتار از وسعت خود را در قاعده دلتای سدیج از دست‌ داده‌اند. این تحقیق نشان داد که علاوه بر فرایندهای رودخانه‌ای، تشدید فرایندهای دریایی همانند افزایش ارتفاع امواج هم‌زمان با وقوع سیلاب به‌خصوص در سواحل ماسه‌ای می‌تواند تاثیر زیادی بر فرسایش خطوط ساحلی در منطقه موردمطالعه داشته باشد.

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.methodologyin 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 discussionduring 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.