Document Type : Research Paper

Authors

1 university of isfahan

2 university of I sfahan

3 university of Isfahan

Abstract

A major cause of landslide and relevant losses and fatalities is inappropriate land management, especially in mountainous areas and valleys. In this study, risk-vulnerability superimposed model was used to investigate the probability of occurrence of fatal landslides in Tarom Watershed. The risk-vulnerability superimposed model has been formulated to evaluate the landslide risk and vulnerability. These factors include topography, geology, number of streams, rainfall, frequency of faults, land use, density of roads, population density, and construction density, which were considered and analyzed in the present model. According to the model output (i.e., risk-vulnerability map), it was figured out that the eastern and northeastern parts of Tarom Watershed are exposed to highest levels of risk and vulnerability. That is, occurrence of a fatal landslide with financial losses is more probable in these areas, as compared to other parts of the watershed. Results of the present research showed that 17%, 35%, 23%, 16%, and 9% of the entire watershed area were classified as being exposed to very low, low, moderate, high, and very high risk-vulnerability levels, respectively. Advantage and superiority of this model over other models and methodologies for landslide study is simultaneous consideration of landslide occurrence risk and vulnerability of the study area to the landslide, making it capable of determining the areas of higher probability of fatal landslides with financial losses.

Highlights

 

Methodology

Model introduction

The risk-vulnerability superimposed model has been used to evaluate the landslide risk and vulnerability. Designed by the authors of the present research, this model was first used in this research to analyze landslides in Tarom Watershed, Iran.

Different stages of landslide risk and vulnerability assessment by the proposed model are as follows:

 

  • Preparing the required maps and information layers
  • Determining weighting factors for different classes of each factor and index (internal weight)
  • Calculating mutual weight (external weight) for factors and indices in MS Excel using embedded formulae like pairwise comparison
  • Calculating final weight of the factors by multiplying the wrights by the weight of the corresponding class (internal weight × external weight)
  • Introducing the final weights of each layer on the maps to come up with a new map with quantitative values of all factors and indices
  • Superimposing the layers of all factors contributing to landslide occurrence to prepare a landslide risk map
  • Superimposing the layers of all indices contributing to landslide vulnerability to prepare a landslide vulnerability map
  • Preparing a layer out of landslide-affected zones (regional landslide map)
  • Compositing and superimposing the risk map, the vulnerability map, and the regional landslide map to produce the final map (e., risk – vulnerability map)
  • Given the qualitative nature of the risk-vulnerability superimposed map, inputs to the model include the maps that have been extracted, compiled, and analyzed out of Tarom Watershed to finally demonstrate sensitivity of different points across the watershed to landslide. Results of different stages of the risk-vulnerability superimposed model are as follows.

The following factors were considered as effective for landslide occurrence in this research:

  • Topography (slope and height)
  • Geology
  • Precipitation
  • Density of streams
  • Land use
  • Density of faults

Indices affecting the vulnerability:

  • Population density
  • Construction density
  • Road density

Discussion

Analysis of the factors affecting the occurrence of landslide in the watershed and preparing the landslide risk map showed that, in general, the eastern part of Tarom Watershed exhibits a higher risk of landslide. Investigation of the factors contributing to the occurrence of landslide (e.g., high slope, height, heavy precipitation, dense fault system, and petrological characteristics) showed that the landslide risk is maximum in the northeast of the study area. Focusing on the vulnerability, the northern and eastern parts of the watershed were found to be more vulnerable due to the higher density of constructions, roads, and population, where more fatalities and higher financial losses were expected should a landslide occurs.

Conclusion

According to the model output (i.e., risk-vulnerability map), it was figured out that the eastern and northeastern parts of Tarom Watershed are exposed to higher levels of risk and vulnerability. That is, occurrence of a fatal landslide with financial losses is more probable in these areas, as compared to other parts of the watershed. Higher sensitivity of these areas to landslide was found to be linked to the higher slope, petrological characteristics, inappropriate land use, and hydrographic network. Results of the present research showed that 17%, 35%, 23%, 16%, and 9% of the entire watershed area was exposed to very low, low, moderate, high, and very high levels of landslide risk and vulnerability, respectively.

Keywords

Main Subjects

از دلایل عمدة وقوع زمین لغزش و تلفات و خسارات ناشی از آن، مدیریت نادرست زمین به ویژه در مناطق کوهستانی و دره‌ها می‌باشد. بنابراین لازم است تا با مدیریت صحیح این نواحی، از میزان خسارات ناشی از این پدیده کاست. هدف این مطالعه، استفاده از مدل همپوشانی ریسک- آسیب پذیری جهت بررسی احتمال وقوع زمین لغزش­های دارای خسارت زیاد در حوضه آبریز طارم است. عواملی که در این پژوهش استفاده شده اند شامل لایه­های توپوگرافی، زمین شناسی، تراکم آبراهه، بارش، تراکم گسل، کاربری اراضی، تراکم جاده، تراکم جمعیت و تراکم ساختمان می­باشد. در این مدل ابتدا تمامی لایه­های عوامل مذکور برای تهیه نقشه ریسک زمین لغزش، همپوشانی شده و سپس لایه ­های مربوط به شاخص­های موثر در آسیب پذیری حوضه تلفیق شده و نقشه آسیب پذیری زمین لغزش تهیه گردیده است. در نهایت با ترکیب و همپوشانی نقشه ریسک، نقشه آسیب پذیری و نقشه زمین لغزش­های منطقه، نقشه نهایی (نقشه ریسک- آسیب پذیری) به دست آمده است. نتایجی که بر اساس خروجی مدل یا همان نقشه ریسک-آسیب پذیری، حاصل شد گویای این مطلب است که قسمت­های شرقی و شمال شرقی حوضه آبریز طارم دارای بیشترین میزان ریسک-آسیب پذیری هستند. همچنین از کل سطح حوضه، 17% کلاس ریسک-آسیب پذیری خیلی کم، 35% کلاس ریسک-آسیب پذیری کم، 23% کلاس ریسک-آسیب پذیری متوسط، 16% کلاس ریسک-آسیب پذیری زیاد و  9% کلاس ریسک-آسیب پذیری بسیار زیاد را به خود اختصاص داده اند. چیزی که در این پژوهش داری اهمیت است این مطلب است که این مدل نسبت به سایر مدل ها و روش­های مطالعه زمین لغزش، بررسی ترکیبی احتمال وقوع زمین لغزش و آسیب پدیری منطقه در برابر زمین لغزش بوده و تعیین می­کند که احتمال وقوع زمین لغزش­های دارای خسارات جانی و مالی در چه نقاطی از منطقه بیشتر است.

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