تاثیر نوسانات تراز آب دریای خزر بر تغییرات مکانی خطوط ساحلی منطقه شهری نور و رویان

پایش روند تغییرات خط ساحلی و تحلیل میزان تغییرات مکانی آن در منطقه شهری نور و رویان، براساس داده‌های آماری بلند مدت نوسانات تراز دریای خزر در ایستگاه‌های ترازسنجی بندر انزلی و نوشهر و تصاویر ماهواره‌ای متناظر انجام شده است. بررسی داده‌های ترازسنجی نشان داد که پایین‌ترین و بالاترین تراز به‌ترتیب در سال‌های 1356 (28.5 متر) و 1372 (25.24 متر) ثبت شده است. با توجه به آخرین داده‌های تراز آب، مقدار آن در آبان ماه سال 1398 به پایین‌ترین سطح خود طی سه ده اخیر رسیده است (27.31 متر). به‌منظور استخراج خطوط ساحلی متناظر با سه تراز فوق‌الذکر، تصاویر ماهواره‌ای و روش‌های فیلترگذاری و آستانه هیستوگرام به ‌کار برده شد. تنها منبع تصویر ماهواره‌ای برای استخراج خط ساحل سال 1356 به‌عنوان پایین‌ترین تراز، تصویر ماهواره landsat سنجنده mss است. تصویر سنجنده tm ماهواره landsat برای استخراج خط ساحل تیر ماه 1374 و تصویر ماهواره sentinel 2 هم برای استخراج خط ساحل آبان 1398 استفاده شد. الگوی جابجایی خطوط ساحلی متناظر با ترازهای تاریخی ثبت شده از نوسانات آب دریای خزر نشان داد که اثر نوسانات تراز طی سال‌های 1356 تا 1398 به‌خوبی در تغییر موقعیت مکانی خط ساحل منطقه مطالعاتی انعکاس یافته است. 214 هکتار از اراضی ساحلی شهرهای نور و رویان بر اثر بالا آمدن تراز آب دریا از سال 1356 تا 1374 در طول حدود 14 کیلومتر نوار ساحلی زیر آب رفته است. پس از بالاآمدگی آب دریا و فرسایش اراضی ساحلی، از سال 1374 تاکنون، 65.7 هکتار بر محدوده اراضی ساحلی (اراضی مستحدث) اضافه شده است.

the effect of caspian sea water level fluctuation on the shoreline spatial changes in nour and royan urban areas

introductioncoastal areas are constantly changing physically and ecologically, depending on natural and human factors. the natural causes of coastline changes are assessed in three ways: short term changes including the effects of up and down currents, long term changes including climate change, periodic storms and waves, and accidental changes including sudden natural events. these changes affect the coastline and coastal areas and consequently have a negative impact on human life, human activities and maritime communications. thus, monitoring the coastal area is important for sustainable development and environmental protection. in this regard, to monitor the coastal area, extracting the coastline at different times is essential. the shoreline is one of the most important linear features on the earth’s surface that shows the dynamic nature. the progressive of caspian sea water during the years 1978 to 1996, which led to a rise in sea level of more than 2 meters, caused great and serious damage to various land uses (including industrial, official, commercial, residential, agricultural land uses and natural resources) and had a special effect on the coats biological resources and ecosystems. on the other hand, in recent year, the water level of caspian sea had decreased and it is regressing. the gradual decrease of caspian sea level can change the coastline in some parts of the coastal areas, especially in golestan province and endanger various species of aquatic organisms and plants, and maritime transport industry as well as water quality. due to the sensitivity and fragility of the coasts against erosion and pollution, the coast management needs an efficient and integrated management system to enable sustainable development in such areas.materials and methodsbecause the dynamic nature, noor township coastal area is exposed to erosion and permanent variability due to river, wind, tectonic, wave and tidal processes. the sea water progressive and regressive in study area causes the destruction of coastal facilities and tourism places. monitoring the trend of shoreline changes and analyzing the extent of its spatial changes in the urban area of nour and royan was done based on long term statistical data of sea level fluctuations (1840 to 2019) in the tide gauge stations of bandar anzali and nowshahr and corresponding satellite images. according to the shoreline extraction selected times in this study, the satellite images of landsat mss 1977, landsat tm 1995 and sentinel 2 2019 were used to detection the lowest and highest water level shorelines.results and discussionthe effect of caspian sea water level fluctuation during 1977 to 2019 is well reflected in the changing location of nour and royan shorelines. the shoreline of 1977, corresponding to the lowest recorded water level, is in a regressive position compared to the later periods shorelines and current conditions. after mentioned year and with the rising sea level, the position of the shoreline has progressed to land, which in 1995 reached its maximum progress. investigation of the amount of submerged coastal lands in 14 km of the nour and royan coastal strip due to the rising caspian sea water level illustrates that 214 hectares of these lands have been submerged. after this water rising and coastal lands erosion, from 1955 until now (except for the period of 2001 2006) caspian sea water level has been rising. the water level falling has affected the 2019 shoreline location (as an indicator for the last three decades’ lowest sea level) and caused its regressive from the land. following the shoreline progressive in the same 14 km coastline strip of nour and royan, 65.7 hectares have been added to the coastal land area (land reclamation).conclusionaccording to the latest water level recorded from caspian sea level, the sea level has reached its lowest amount in november 2019 during 1976 to 2019 which is equal to 27.31 m. investigation of long term caspian sea water level fluctuation during 1992 2019 signed that the sea level has dropped at a rate of 12.5 cm per year, which reached from 25.78 m in october 1972 to 27.31 m in november 2019; this shows a drop of 1.44 meters during these years. generally, changes in caspian sea water level occur rapidly and dramatically which is also confirmed by morphological and sedimentological evidence has taken place in this sea.