Methodological study on landslide hazard assessment by interpretation of aerial photographs combined with AHP in the middle course area of Agano River, Central Japan

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  • 空中写真判読とＡＨＰ法を用いた地すべり地形再活動危険度評価手法の開発と阿賀野川中流域への適用
  • クウチュウ シャシン ハンドク ト AHPホウ オ モチイタ ジスベリ チケイ サイカツドウ キケンド ヒョウカ シュホウ ノ カイハツ ト アガノガワ チュウリュウイキ エノ テキヨウ

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Based on interpretation of aerial photos for landslide slopes and micro-morphological components that reflect activity of mass movement, a landslide hazard rating system has been developed by combining with Analytic Hierarchical Process (AHP) . The AHP method decomposes the process of subjective decision of person into a layer structure and expresses the process qualitatively. This study adopted AHP for landslide hazard assessment by hierarchical structuring of factors and weighting of the factors that contribute to reactivation of landslides. Factors and items were determined by brain-storming of experts in geomorphology and landslide investigation works. The first hierarchical level is landslide hazard evaluation. Five geomorphic factors as the second hierarchical level were selected as follows ; 1) Distinctiveness of main scarp, 2) Surface feature of landslide body, 3) Position of landslide body on the slope, 4) Position of cracks in the landslide body, 5) Stability conditions of landslide toe. Those correspond to three principle factors such as geomorphological evolution processes, landslide activity and destabilizing possibility. The third hierarchical level consists of the optional items under each geomorphic factor of the second level. Factor 5) consists of 5) - 1 erodibility of the landslide toe and 5) - 2 instability perceived from the shape of the landslide toe as level IV.<br>Final weight coefficient for each item is calculated by multiplying values of hierarchical level II, III and IV (Table 1) . Hazard levels are classified into hazardous, slightly hazardous, moderate and safe categories, according to the score that accommodates the total weight for each landslide. The highest 5 % of cumulative frequency distribution of the scores for all detected landside slopes are evaluated as hazardous. Scores between 5 - 30 %, 30 - 60 % and 60 - 100 % in descending order are classified into slightly hazardous, moderate and safe, respectively.<br>312 landslides have been evaluated by this system, all of which developed on Tertiary sedimentary and volcanic rocks in the middle course area of Agano River. Since the results of AHP hazard rating are concordant with those of subjective evaluation of aerial photos, this system can well support conventional hazard evaluation. Also, as active landslides were indicated with high hazard levels, this system can be used for primary landslide hazard assessment without detailed field survey.

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