واكنش سيستم‌هاي رودخانه‌اي به وقايع سيلابي بزرگ نمونه موردي: رودخانه مادرسو

The Response of Fluvial Systems to Large Flood Events

سيلاب سال 1380 حوضه رودخانه مادرسو در جنوب شرق درياي خزر يكي از شديدترين و مهم‌ترين سيلابهاي موثر بر چشم‌انداز طبيعي داخل كشور در دهه اخير بوده است. در مقاله حاضر از روش هاي تحقيق توصيفي- تحليلي، تاريخي و علّي استفاده شده، ابزارهاي مورد استفاده در اين تحقيق، عكس هاي هوايي قبل از سيلاب ، تصاوير ماهواره اي IRS وQuick Bird بعد از سيلاب ، نقشه هاي توپوگرافي و زمين شناسي محدوده، اندازه گيري هاي به دست آمده از ميدان بوسيلهGPS و تصويرهاي برداشته شده، و معادلات هيدروليكي بوده است. نتايج بدست آمده نشان مي‌دهد كه در اثناي‌اين سيلاب، ميزان توان و سرعت جريان در تمام آبراهه‌هاي فرعي و سراسر كانال اصلي بيش از ميزان توان بحراني و سرعت لازم براي غلبه بر تنش برشي بستر بوده و به همين دليل قادر بوده حجم زيادي از رسوبات درشت دانه را با خود حمل و موجب شكل‌زايي وسيعي از جمله تعريض و تعميق قابل توجه كانال اصلي، تغيير مقاطع عرضي كانال ‌هاي فرعي، تغيير در الگوي كانال‌ها و خلق چشم‌اندازهاي تراكمي جديد مانند مخروط‌هاي افكنه‌ گردد. تغييرات ناشي از سيلاب مرداد ماه سال 1380 نيز باعث عملكرد شديدتر سيلابهاي بعدي يعني سيلابهاي مرداد ماه سالهاي 1381 و 1384 حوضه (عليرغم دبي كمترآنها) و برجا گذاشتن آثار ژيومورفيك شديدتر شده است.

In the Summer of 2001, an intense thunderstorm in the southeastern Caspian Sea triggered a catastrophic flood in the Madarsoo Basin. Methods that included high-resolution aerial photographs, interpretation of satellite images, multi-date mapping, hydraulic calculations and field observations enabled documentation of the geomorphic impact of this flood on the Madarsoo River and its tributaries. Geomorphologic effects of this catastrophic flooding included the uprooting of trees up to 2 m in diameter from the main river channels; locally as much as 80% of the tree crown cover was removed from the riparian zones. Uprooting of the trees exposed the underlying flood plain to macro turbulent scour, leading to extensive removal of flood plain sediments that had accumulated over centuries. Other hydro-geomorphic impacts of this flood have been drainage network modifications, the creation of new alluvial fans, and a new meandering pattern and gully development, all of which have been documented. Subsequent to the mentioned flood, two other extreme floods have also affected this basin in August of 2002 and 2005. In addition to causing further landform modifications, these floods wiped out the infrastructure which had been reconstructed after 2001. All three floods were of the same conditions based on their time, climatic and triggering characteristics. The peak flood was estimated to be 700m3/s in August 2002 and 1060 m3/s in August 2005. The occurrences of the two above mentioned catastrophic floods and related processes were the result of triggering variables and the formation new dynamic environments by the main event (August 2001 flood), which prolonged the required time for recovery of the stream channels. Geomorphic instabilities lead to consecutive crises in this new environment. This condition accelerated geomorphic hazards in combination with the effects of the recent climate.