Collapse of slopes bordering forest roads and debris flow caused by typhoon No.4 in the Takakuma Experimental Forest，Kagoshima University，in July 2007

Investigations of collapse of slopes bordering forest roads and debris flow caused by typhoon No.4 in July 2007 in the Takakuma Experimental Forest at Kagoshima University were carried out. The results were characterized as follows: (1) Several types of slope failures were observed within the study area: collapse of shoulder part resulting from the infiltration of rain water into pyroclastic fall deposits in the cutting slope; shallow landslide resulting from the infiltration and concentration of rain water into surface soil in the natural slope; and deep-seated landslide resulting from groundwater combining with an influx of rain water in the cutting and natural slope. (2) The sediment yield due to slope failures and bank erosion，and sediment discharge due to debris flow in the watershed of occurrence of debris flow were calculated based on a field survey. The amount of sediment yield was calculated to be 4,325㎥ (5,545㎥/k㎡)，and the amount of sediment discharge was calculated to be 3,600㎥ (4,615㎥/k㎡). (3) Driftwoods resulted not only from slopes covered with conifers but also those covered with broadleafed trees. The driftwood budget shows that 72㎥ was caught in the channel from a total driftwood yield of 226㎥ . In addition，125㎥ was caught in the lowest part of the watershed and 29㎥ flowed out to the outside of the watershed.2007年7月の台風4号に伴う大雨によって鹿児島大学附属高隈演習林で発生した林道法面の崩壊ならびに土石流の実態について調査した。得られたおもな結果は以下の通りである。(1)調査地でみられた斜面崩壊の形態として，林道法面における大隅降下軽石層とその上位の火山灰・軽石層で発生した肩部の崩落，自然斜面における風化土層中への雨水の浸透に起因した表層崩壊，林道法面および自然斜面における雨水の浸透および地下水に起因した深層崩壊が挙げられる。(2) 土石流が発生した流域における生産・流出土砂量を現地調査に基づき求めた結果，生産土砂量は4,325㎥ (5,545㎥/k㎡)，流出土砂量は3,600㎥ (4,615㎥/k㎡)である。(3) 現地調査の結果，流木は針葉樹林で覆われた斜面からだけでなく，広葉樹林で覆われた斜面からも発生していた。斜面崩壊に伴って生産された226㎥の流木のうち， 72㎥が河道内で捕捉， 125㎥が流域最下流地点に流出， 29㎥が流域最下流地点より下流に流出している