تأثير خصوصيات بارش بر تغييرات مكاني و زماني حركت توده لغزشي منطقه توان در استان قزوين

The Effect of Precipitation Characteristics on Spatial and Temporal Variations of Tavan Landslide Movement

زمین لغزش را می¬توان حركت توده¬ای مواد دامنه¬های شیب¬دار تحت تأثیر نیروی ثقل توده و عوامل محركی مانند زمین¬لرزه، سیل و باران¬های سیل¬آسا تعریف نمود. این پدیده یكی از مخاطرات طبیعی است كه همه¬ساله خسارات جانی و مالی فراوانی را در مناطق كوهستانی، پرباران و لرزه¬خیز به همراه دارد. تشخیص زمان و مقدار تغییر شكل توده¬های لغزشی برای درك دلایل وقوع زمین¬لغزش و هشدار خطرات احتمالی ضروری است. در این تحقیق، مقدار جابجایی زمین¬لغزش منطقه توان واقع در شمال شرق استان قزوین با عامل ویژگی¬های بارش مورد ارزیابی قرار گرفت. بدین منظور ابتدا شبكه¬ای از نقاط ثابت در داخل و خارج توده لغزشی به تعداد 20 نقطه، برای پایش میزان جابجایی بر روی كاربری¬های مختلف توده لغزشی ایجاد و میزان جابجایی هر نقطه در 5 بازه زمانی با استفاده از سیستم موقعیت‌یاب جهانی (GPS) دوفركانسه اندازه¬گیری گردید. نتایج پایش در مدت 511 روز نشان داد مقدار كل جابجایی افقی نقاط دارای حركت در 5 بازه زمانی مورد پایش 1876 میلی‌متر بوده كه دارای نرخ حركت ماهانه 110 میلی‌متر می¬باشد. همچنین مقدار كل جابجایی عمودی نقاط دارای حركت در زمان مشابه 898 میلی‌متر بوده كه دارای نرخ حركت ماهانه 53 میلی‌متر است. سپس ویژگی¬های بارش منطقه نظیر مقدار بارش، نوع بارش، مدت بارش، حداكثر شدت بارش در بازه¬های زمانی 10، 20، 30 و 60 دقیقه و شدت متوسط بارش برای هر یك از 5 بازه زمانی محاسبه و استخراج گردید. رسم بردارهای جابجایی نقاط بر روی نقشه توپوگرافی منطقه مشخص نمود كه جهت حركت توده در جهت گرادیان ارتفاعی منطقه می¬باشد. نتایج نشان داد از میان ویژگی¬های مختلف بارش، تنها بین شدت بارش با میزان حركت توده لغزشی رابطه خوبی برقرار است و مقدار جابجایی، بیشترین همبستگی را به¬ترتیب با شدت بارش متوسط (854/0= R) و حداكثر بارش 30 دقیقه¬ای (675/0= R) دارد و بین سایر خصوصیات بارش (مقدار، مدت و نوع بارش) و حركت توده لغزشی رابطه معنی-داری حاصل نگردید.

1. Introduction Rainfall intensity and duration are one of the most important factors in the occurrence of landslides; therefore, nowadays rainfall is recognized as the most common and triggering factor in the occurrence of landslides. Since raw data of landslide displacement is very important to researchers in order to study the deformation process and laws of landslide displacement, the Global Positioning System (GPS) and computer sciences can be used as a tool to measure the landslide displacement. So monitoring and determining the relations between the amount of landslide movement and characteristics of precipitation (such as type, intensity, duration) and the direction of the sliding mass using GPS can be a turning point in the sustainable management of this natural disaster. 2. Study Area The study area refers to the Tavan landslide which occurred in 2010 covering an area of 40 ha in Qazvin Province in Iran. 3. Material and Methods After identifying the sliding mass, a network of fixed points on the inside (17 points) and outside of it (3 points) was created in order to monitor the movement, and the exact position of the points was determined using a dual-frequency GPS at 5timescales. The precipitation data and their characteristics such as the amount of precipitation, maximum intensity of 10, 20, 30, 60 minutes, average of precipitation, height of snow, average and total duration of precipitation were also calculated for each timescale. Finally, the relationship and correlation between the amount of displacement and the characteristics of precipitation was investigated by determining the vertical and horizontal displacement at the 5 timescales and the characteristics of precipitation for each period of time. 4. Results and Discussion In the total period of the study, 64 precipitation events happened in which the total of precipitation was more than 2 mm in 52 cases. Drawing displacement vectors of the points on the topographic map revealed that the direction of mass movement is consistent with the general slope of the region. The correlation between rainfall characteristics and the amount of horizontal and vertical displacement of the monitoring points at the five timescales are provided in Table 7. The results showed that among all the characteristics of precipitation, rainfall intensity features have more solidarity with the average of horizontal and vertical displacement of points at the five timescales. Moreover, no significant correlation was found between other characteristics of precipitation and the amount of displacement. Table 7. Correlation between rainfall characteristics and the amount of displacement Displacement Precipitation duration Average duration of precipitation Snow height Amount of precipitation Average intensity Precipitation intensity in time intervals (mm/hr) 10 20 30 60 Vertical 0.005 0.01 0.017 0.005 0.028 0.61 0.39 0.37 0.42 horizontal 0.091 0.117 0.102 0.059 0.854 0.364 0.598 0.657 0.52 5. Conclusion The analyses of displacement at the five timescales showed that the relative displacement has occurred at some points of the network. Although there have been slow vertical and horizontal displacement at all points in the network but the amount of movement is clearly visible at the points of 1, 2, 6 and 8. The total amount of horizontal displacement of moving points at the five timescales (511 days) was 1876mm with the monthly rate of 110 mm. The total amount of vertical displacement of moving points was also 898 mm with the monthly rate of 56 mm. The results also showed that among the different characteristics of precipitation, only the intensity of precipitation has a good correlation with the amount of landslide displacement. Previous studies have also achieved similar conclusions about the triggering role of precipitation and the relationship between the intensity of rainfall and the landslide occurrence. The highest coefficient of correlation was found between the average intensity of precipitation and the maximum rainfall of 30 minutes with the horizontal movement of the sliding mass. Generally, it can be concluded that several factors such as topography, soil, geology, land use and intensity of precipitation caused favorable conditions for the occurrence of Tavan sliding; however, in this landslide, rainfall intensity has played the role of a trigger.