بررسي تغييرات سريع خط ساحلي قاعده دلتاي سفيدرود به‎روش تحليل نيمرخ‎هاي متساوي‎البعد

مديريت پايدار سواحل نيازمند آگاهي از روند تغييرات خط ساحلي است و آشكارسازي تغييرات خط ساحلي، مي‎تواند سلامت ساحل را تضمين كند. پهنه مورد مطالعه اين پژوهش، بخش غربي خط ساحلي درياي خزر، در محدوده قاعده دلتاي سفيدرود، به‎طول تقريبي 15 كيلومتر است. هدف اين مطالعه، بررسي متغيرهاي موثر بر تغييرات سريع خط ساحلي طي شصت سال گذشته است. براي دست‎يابي به اين هدف از اطلاعات دِبي و رسوب سفيدرود، باد و نوسان‎هاي تراز آب به‎همراه عكس‎هاي هوايي، نقشه‎هاي توپوگرافي و تصاوير ماهواره‎اي چند زمانه به‎عنوان ابزارها و داده‎هاي اصلي پژوهش استفاده شده است. روش كار برپايه استفاده از نيمرخ‎هاي متساوي‎البعد (ترانسكت) در سه بازه مجزا، براي ثبت ميزان تغييرات خط ساحلي است كه با روي‎هم‎اندازي تصاوير موجود در نرم‎افزارهاي جغرافيايي و استخراج خطوط ساحلي تاريخي، دلايل فرسايش و رسوب‎گذاري در قاعده دلتاي سفيدرود، طي دوره زماني 1390-1334، بررسي و مشاركت عوامل موثر بر تغيير خط ساحلي در هر يك از بازه‎هاي سه‎گانه مورد سنجش قرار گرفت. نتايج نشان مي‎دهد كه تغيير در ميزان بده رسوب خروجي از سد سفيدرود با انجام عمليات شاس، بيشترين تاثير را در تغيير سريع خط ساحلي قاعده دلتا داشته است. طي عمليات شاس، ميانگين دلتاسازي با سرعت 26 متر در سال (1998-1981) به ثبت رسيده است كه نسبت به سرعت تغيير خط ساحلي در دوره قبل از عمليات با 19 متر در سال (1980- 1955) و 9 متر در سال پس از عمليات شاس (2011-1999)، تفاوت چشمگيري را نشان مي‎دهد. همچنين طوفان‎هاي دريايي نيز، به‎صورت مقطعي مي‎توانند نقش مهمي در تغيير خط ساحلي ايفا كنند.

Introduction Sustainable management requires knowledge and a good understanding about processes of shoreline changes. Shoreline change detection can ensure health of beach areas. The study area is located in the coastline of the Caspian Sea in an approximate length about 15 km in Sefidrud Delta. Delta Sefidrud have been formed in different periods that experienced symmetrical or semi-symmetric and asymmetric morphologies on the shoreline. General currents of the Caspian Sea have a west to east direction. Sea dynamics in the direction and sediment movement have the greatest impact on the shoreline. Hence, the shoreline should always be oriented towards the estuary of East River Delta, while aerial photography and satellite imagery shows something contrary. Thus, in addition of waves and the coastal currents dynamics no changes have occurred along this direction during the Holocene, so, some other factors may contribute to forming of the delta with periodical differences in geometry. Therefore, contribution of these factors should be characterized in the present curvature of the shoreline. The purpose of this study is to evaluate variables influencing rapid changes and dynamics of coastline over the last sixty years. Methodology To achieve the purpose of this research, some data including sediment discharge from rivers, wind and sea level change statistics, aerial photos, topographic maps and satellite images in several times have been used as the material of the research. Methodology of the work is based on transects in three separate portions. To do this, after atmospheric and geometric corrections of Landsat images and aerial photos in ENVI software, the images were processed for better visualization and interpretations. The images, then, have been imported into ArcGIS 9.3 software. The shoreline positions have been separately extracted from each of the images as distinct layer files. Given longitudinal and transverse movement of the shoreline, Kiashahr main road position, without any changes during this period, has been taken as constant milestone in a separate layer to measure rate of progress and retreat in the shoreline. The changes have been calculated relative to the main road. The entire shoreline is divided into three regions in the delta basal. With overlay of the images on the coastline, erosion analysis and sedimentation in the Sefidrud Delta base during the period 2011-1955, the contributing factors have been measured for each of the three regions of shoreline. Results and Discussion The results show that in the first period there has been a decrease in sea level changes. With the displacement of river along the course from the east to the west of Sefidrud Delta basal, drastic changes have occurred in the curve of the coastline. The changes have occurred in the two periods before and after the construction of SefidRood Dam. Shoreline changes in the second period with an increasing trend of sea level are dominated by the Shas operations. With creation of transects in this term, it is specified that coastline compared to the previous period has increased in the first and second zones and is reduced in the third. With reduction in sea level during the third period, Shas operations have been stopped and the top part of the delta has been eroded under the influence of waves. Conclusion The results indicate that changes in the rate of sediment output from Sefidrud Dam with Shas operations have the greatest impact on the delta basal to account for the rapid changes in shoreline. Aggradation during Shas operations has been recorded with average speed of 26 m per year (1998-1981). This shows a significant difference in acceleration of shoreline change in 19 m per year before the Shas operations and 9 m per year after the operations. Hurricanes can also play an important role in the changing shoreline occasionally. In the time periods, river mouth has more changes within the delta base. More changes have been related to the changes and irregularities in the shape of the coastline. With a fixed location in the mouth for a long time, sand bandwidth will be increased in the delta base. In aerial photos of 1955 and 1966 in addition of satellite images of the 1978, 1987 and 1998, when the changes have occurred in direction and movement from East to West of river estuary delta base, the changes is measured to be very high in the shoreline. It should be noted that in some parts of the shoreline there appears to be some critical points. The addition of new and old river estuary of Sefidrud, tributaries in the river estuary are also clearly visible. These areas as critical points can be viewed with a special look.