نوسانات درياي خزر و تا ثير آن بر سواحل جنوب شرقي درياي خزر

Caspian Sea-level Changes and its Impact on the Southeastern of the Caspian Sea

سطح درياي خزر قبل از 1929 و از سال 1850تقريبا" در ارتفاع 25- و 26- و از سال 1929 تا سال 1977 سطح درياي خزر از حدود 5/25- به 29- متر كاهش يافته و زمينهاي بسياري از سيطره آب خارج شدند و همه فعاليتهاي كناره ساحلي مرتبط با آب دريا بويژه بنادر دچار اخلال گرديد. اما به طور ناگهاني از سال 1977 تا سال 1995 دوباره سطح آن سير صعودي داشته و تقريباٌ به 5/26 – رسيد در نتيجه قسمتهاي اعظمي كه از آب خارج شده بود مجددا" دچار آب گرفتگي شد . افزايش تراز آب نفوذ و سيطره آبهاي تقريباً شور در مناطق كشاورزي، پيشروي به سمت تاسيسات صنعتي، شهري و روستايي آسيب هاي بي شماري را به ويژه در بخشهاي كم شيب تحميل نموده است. بدليل مرفولوژي ناهمگون مناطق ساحلي تاثير نوسانات درياي خزر در خطوط ساحلي ايران يكسان نيست. در حاليكه سواحل بخش مياني با شيب تقريبا" زياد به خط ساحلي منتهي مي شوند سواحل بخش شمال شرقي ايران با شيب بسيار ملايم به خط ساحلي مي رسند. سواحل جنوب شرقي درياي خزر به دليل شرايط ويژه ساحلي و مورفولژيكي يكي از مناطق مهم براي بازسازي تراز درياي خزر محسوب مي گردد. هدف از اين تحقيق بررسي تغييرات تراز آب با استفاده از تصاوير ماهواره، نقشه هاي تاريخي و مشاهده ميداني بر روي سواحل جنوب شرقي درياي خزر در آخرين چرخه درياي خزر بين سالهاي 1929 و 1995 مي باشد. نتايج بدست آمده نشان مي دهد مورفولوژي ساحلي در شمال شرقي ايران تحت تاثير تغييرات تراز آب به كلي تغيير مي كند.

Introduction The Caspian sea level was nearly stable between 1850 and 1929, situated around at -25, -26 below oceanic sea level, but dropped around 3 m between 1929 and 1977 situated at -29 below oceanic sea level and a large sea bottom was emerged. It suddenly rose back to the former sea level between 1977 and 1995 situated around at -26 and it had major impact on the Caspian Coasts. A large area was drowned as a rapid sea level rise. Rising sea level had some serious problems on agricultural, industrial and residential zone specially on low-land coast, the study area. Due to different coastal morphology along the Caspian sea the impact of sea level change is not equal all over the Caspian Sea. While the south-western and southern Caspian shelves in Iran have steep offshore slopes and corresponding steep coasts, the offshore gradient in the south-eastern Caspian is gentle, which correlates well with the gentle gradient onshore. The south-eastern of the Caspian sea is sensitive on the sea level change and can be considered as a good target to reconstruct sea level changes. Methodology Remote sensing data (LANDSAT), old maps (1890) and field observations were used to show delta progradation and recent coastal evolution, as well as the impact of the last sea-level cycle between 1929 and 1995.All Landsat data were georeferenced using the 1/25000 topographic map and resampled to the 60 m pixel size. Old topographic map was geocoded through the information within map. Results and Discussion The last cycle of the Caspian Sea between 1929 and 1995 totally changed the coastal morphology. The impact of the recent sea-level cycle will be discussed for the four coastal types in the area: Miankaleh spit; Gorgan Bay; Gorgan River Delta; and Hassan Gholi Bay. All these morphological features have been changed due to rapid sea-level changes as discuss below. Miankaleh Spit and Gorgan bay Spit-lagoon system reacted to sea level changes in the last cycle, whereas Minakaleh spit extended further seaward during rapid sea level fall, the bay behind it decreased strongly and tended to dry out totally. During sea level rise it acted conversely because the spit was decreased in size strongly and the bay behind it increased double in size. Gorgan River Delta During sea level fall, Gorgan river experienced rapid progradation at a rate of 85 m per year but its morphology changed during sea level rise and retrograded at a rate of 140m per year. Hassan Gholi Bay exited until 1890 based on the historical map when the Caspian sea level was around at -25 m below oceanic sea level but dried out completely when sea level dropped until 1977. This area is very sensitive to sea level changes and any change in sea level variation will impact strongly on coastal morphology. Conclusion The southeastern of the Caspian sea experienced rapid coastal evolution between 1977 and 1995 in comparison with the other Iranian coasts, spit-lagoon system responded to Sea level changes and showed that how morphological features can develop even a short period of sea level changes. This rapid coastal evolution happened in the study area indicates that such area offer a very good target to reconstruct former sea level change. This study also shows that the shoreline shift is not a simple linear equation and many other factors such as longshore current must be considered.