報告書番号

V11010

タイトル（和文）

水・土砂流出に影響する流域環境の評価－地理情報にもとづく裸地発生可能性評価手法の開発－

タイトル（英文）

Assessment of watershed environment affecting water runoff and slope erosion -Development of probability evaluation method for bare slope occurrence by using digital geographic information-

概要 （図表や脚注は「報告書全文」に掲載しております）

地理情報を用いた裸地の発生可能性評価手法の開発を目的として、過去に記録的豪雨のあった奈良県熊野川上流域において、裸地斜面の分布と経年変化に関する調査を行った。数値標高データを用いて区分された193の集水域を区分し、裸地面積率と平均標高や傾斜などの環境要因を算出した。解析の結果、裸地面積率は平均標高および過去の伐採面積率との間に高い相関があることが分かった。また、空中写真を用いた経年的な調査結果から、裸地は1970年前後の大規模伐採および豪雨が発生した時期に拡大し、現在も谷筋に存続していることが明らかになった。さらに統計解析の結果、裸地の分布は標高、周囲500m内の伐採面積率、谷からの距離に規定されることが示されたため、これら3つの要因を用いた裸地化可能性指数を提案した。裸地化可能性指数を用いると、地理情報から土砂の発生源となる新たな裸地の発生可能性が高い斜面を簡易に抽出することが可能になる。

概要 （英文）

Distribution and long-term change of bare slopes were studied in the upper stream of Kumano river for evaluating occurrence probability of bare slope by using geographic information system. I divided the watershed into 193 small catchments and calculated average altitude, average slope and the area percentage of vegetation and geology classes. The area percentage of bare slope closely correlated with average altitude and the area percentages of clear-cut of forests in the past. Long-term series of aerial photography in a certain catchment revealed that most bare slopes created in around 1970s when forests were logged in large area and the highest annual maximum daily rainfall were recorded. Thereafter, nearly half of the bare slopes have persisted in the valley bottom until now. The result of decision tree analysis suggested that altitude, cutting area around 500m circle and distance from valley were three important factors which regulated the distributions of bare slopes in 10m resolution. For evaluating occurrence probability of bare slope, I proposed a simple indicator called as degraded probability index (DPI) which was calculated from the geometric mean of the scores of the three important factors. DPI easily enables us to construct a potential map of bare slope occurrence from available geographic information which suggests future risks of sediment discharge during heavy rain.

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