شناسايي شواهد ژئومورفيك تشكيل درياچه سد لغزشي شور در استان خوزستان

Identifying the geomorphic evidences of the Shour landslide dam lake formation in Khuzestan Province

در صورتي كه محل وقوع زمين لغزش ها در مسير شبكه هاي زهكشي و جريان رودخانه ها باشد، باعث مسدود شدن مسير جريان آب و تشكيل درياچه هاي سدي مي شوند. درياچه سدي شور در اثر رخداد زمين لغزش كوه سه تنان و مسدود كردن جريان رودخانه شور در دره دوزويل شهرستان لالي در استان خوزستان شكل گرفته است. هدف از اين تحقيق، بررسي ارتباط رخداد زمين لغزش و شكل گيري درياچه و بازسازي وسعت و ويژگي هاي مورفومتري درياچه مي باشد. با استفاده از مطالعات ميداني گسترده، نقشه هاي توپوگرافي و زمين شناسي و نرم افزار Arc GIS، شواهد ناشي از رخداد زمين لغزش و تشكيل درياچه سدي شور بازسازي گرديد. سپس با استفاده از ارتفاع پادگانه ها و موقعيت آنها نسبت به بستر فعلي رودخانه، محدوده هر پادگانه مشخص و روي نقشه انتقال داده شد. در ادامه ويژگي هاي مورفومتري درياچه براساس مطالعات ميداني، مقاطع طولي و عرضي، ويژگي هاي رسوب شناسي و شواهد باستان شناسي تجزيه و تحليل شد. نتايج نشان داد به دليل عرض كم دره و بزرگي قطعات لغزشي به صورت بلوك هاي سنگي، دره دوزويل مسدود و درياچه شور در پشت آن شكل گرفته است. بر اساس بقاياي رسوبي پادگانه هاي درياچه اي و ويژگي هاي رسوب شناسي، رخداد زمين لغزش در يك مرحله بوده و درياچه شور در نتيجه تخليه تدريجي در 3 مرحله شكل گرفته است. با توجه به بقاياي پل تاريخي مربوط به دوره ساساني بر روي دره دوزويل و محل پارگي درياچه به نام پل دوزويل (توقسه)، احتمالا زمان تخليه درياچه مقارن با دوره ساساني(250 تا 650 ميلادي) بوده است.

In almost all physiographic environments from narrow and deep valleys to vast alluvial valleys، landslides in the water flow of rivers can create natural dams. Landslide dams، as a kind of natural dams، are results of blocking the normal flow of riverswhich are created by the mass movements of hillside. Studies on different types of landslides that have formed natural dams، show that generally 50 percent of dams are caused by falling rocks and soil and landslides. In Iran، the formation of landslide dams in relation to various factors such as topography، tectonic، geological processes and etc can be seen especially in the Zagros Mountains; Shour Dam Lake in the Lali، Khuzestan is a good example of this type of geomorphic phenomena. Above the Shour bridge and before enterance of the river to Gotvand dam، there are lake sediments in the path of Sarvand and Shour River which represents blocking narrow and deep Dozvill valley which include the river. Observing slipped stone blocks around the Dozvill valley shows occurrence of landslides، blocking the valley and formation of a dam lake in this area. Thus، a dam lake is formed and is called the Shour Lake from now; itis a dam lake caused by a landslide. This study is the first study to introduce Shour River landslide and the resulted dam lake; therefore، the aim of this study is introducing and understanding of the dynamics and the reconstruction of geomorphic evidence related to Shour dam lake landslide in Khuzestan province. Materials and methods This study was conducted based on field studies in April 2015. Different tools such as topographic maps of 1: 25،000، geological map of 1: 100،000، satellite images IRS and TM، Google Earth images، GPS and camera have been used in different stages of the current research. First، the causes of landslides and the formation of the lake were investigated and discussed.Then، given that 3 terraces were identified within the lake، using GPS the height of the terraces to the sea level was determined and using digital elevation model in Arc GIS software environment of the lake was drawn in 3 stages. In addition، using archaeological evidence this research tries to determine the approximate age of the Shour dam lake. Results and discussion Landslide Sa tanan is an example of the landslide occurred in the folded Zagros unit that is located on the western slopes of the mountain with the same name above the Dozvill valley with a depth of 120 m and a length of about 40 meters. Satanan Landslide which is occurred as sliding rock blocks of various sizes in the Bakhtiari conglomerate. Based on field studies the major cause of Sa tanan landslide is trenching for establishing the communication path andica- Lali، dissolution based erosion of Bakhtiari conglomerate، digging the base of Bakhtiari conglomerate as a deep valley created by shour River and crossing Lali- Andica through the study area. After the occurrence of the landslide، stone blocks of the western edge of the valley which some of them are height up to 2 to 3 meters، fell into the valley and because of the low width of the valley they blocked the path of the river. After the closure of the valley، Shour river water and Sarvand seasonal river accumulated behind the sliding dam and created the Shour dam Lake. Since surrounding the river is filled with tall mountains;so، after filling the lake andin the lowest elevation of lake(293 meters above sea level) and on the valley filled by thelandslide that used to be the path of the river before the occurrence of the landslides، the lake overflowed and discharged. Reconstruction of various stages of formation of the lake on the remains of the lake sediments shows that the highest level of sediments of the first terraces can be seen at an altitude of 280 meters، second terraces at an altitude of 270 meters and the third terraces at an altitude of 260 meters above sea level. As a result، Shour Lake has been formed in three stages in whichthe first terraces is related to the largest lake with an area of approximately 4 square kilometers and thethird terrace is related to the smallest lake. It should be noted that due to the low height of Sarvand River (260 meters above sea level) in the eastern parts of the valley and before joining the Shour River and also in the flow direction of the river، a small dam lake has been created. Locating the remains of the Togheseh historic bridge or Dozvill belonged to the Sassani era (250 to 650 AD) and on the valley and lake sediments remains، the possibility that the bridge after an overflow and drain the lake has been destroyed. Therefore، the probability of the occurrence of the landslides and the formation of the lake has been in the same timeframe(250 to 650 AD). Conclusion Shour dam Lake in the Zagros Mountain is an example of dam lakes that the reason for its formation، occurrence ofLandslides has been in the path river. The results show that because of the low width of the Dozvill valley and the largeness of the sliding parts as stone blocks، Dozvill Valley is blocked and the Shour Lake has been formed behind it. Because of the remains of the lake sedimentary terraces، Shour Lake has been formed in 3 steps and the Sarvand Lake in the Sarvand River has been formed in one step and at the same time simultaneously with the first stage of the Sour Lake. According to the remains of a historic bridge (which has been built in Sasanian dynasty) on the Dozvill Valley and also the tear of the lake named Dozvill bridge (Togheseh)،probably the creation and discharging of the lake is back to the Sasanian (250 to 650 AD) era.