長野県飯田市野底川流域の割沢支流域を対象地とし,山腹工事施工後7～17年を経過した8ヵ所において,土壌調査および植生調査を行い,山腹工事施工跡地における土壌及び植物の初期発達状況を調べた。その結果,土壌A,B層の厚さは,山腹工事の経過年数に従って増加していた。表層土層(貫入値>1.5cm/回)の深さは地点によってばらつきが大きかった。30～60cmの土壌全炭素,全窒素含有量とも,経過年数により増加し,0～30cmの土壌全炭素含有量も増加する傾向がみられたが,地点によってばらつきが大きかった。0～30cmの土壌全窒素含有量は山腹工事施工時の施肥の影響で,最初に高かったが,施工後14年まで減少する傾向がみられた。草本根系分布深及び木本平均樹高と表層土層厚とは,高い相関関係が認められた。草本植物の種類数は,山腹工事の経過年数に従って増加しているが,被度及び地上部現存量は減少していた。工事の際に導入したケンタッキー31フェスクは,施肥効果の減少および他の植物との競争で,地上部現存量は著しく減少していた。木本植物の胸高直径,平均樹高は山腹工事の経過年数に従って増加しているが,立木密度は12年を経過した時点で最大値が現われた。上層木は導入したヤマハンノキに占められているが,下層木は侵入したヤナギ類に占められ,他の種類の侵入は認められなかった。

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To increase a function of sabo dam surely, the study about a sabo dam with a sediment trap gate was carried out. Hydraulic model experiments were carried out to get the information necessary to design a sediment trap gate and behavior of grains near the sediment trap gate was observed. Following matters were found through this study.
(1) Collision speed of grains to the trap gate became slow with increase of grain size.
(2) Collision angles of grains to the trap gate was generally 90°.
(3) Collision energy increased with grain size.

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This study aimed to examine the potential for using Cs-137 and Be-7 to identify the sources of sediment based on the different penetration depth of both radio nuclides. A large scale sprinkling experiments (94m²) has been undertaken to obtain the sediment generated from overlandflow and pipe flow. At the initial part of the experiment, both Cs-137 and Be-7 activities were high in the sediment from overland flow, but as the time passing Be-7 activity become much lower than the Cs-137 activity. Such differing responses suggested that the mode of erosion became from sheet erosion to rill erosion. The Be-7 and Cs-137 activity of pipe outflow sediment was low, suggesting that the sediment was derived from deeper part of the soil. Such information may allow the development of a practical technique for estimation the sources and predominant production processes of sediment using these isotopes.

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The debris flow disaster by the heavy rainfall of Bai-u front occurred in July 10th, 1997, on the Harihara river, Izumi city, Kagoshima prefecture. We examined about the characteristic of this heavy rainfall by AMeDAS data (1979-1997). There was not especially much precipitation in 1997 near the Harihara river compared with the 19 years average. Also, there was not especially much, too, antecedent precipitation. The total precipitation of this rainfall event (Jul., 6 to 9) which brought about a debris flow was 518 mm in Minamata, 401 mm in Izumi. These records were the record of in the higher rank in 19 years, but these were not a maximum. However, as for the daily amount of precipitation on July 9th in the debris flow occurrence previous day, both Minamata and Izumi were a maximum. This heavy rainfall was not the type of which especially strong rain concentrates on short time and was the type of which rather strong rain crosses to several hours of 10 and it continues.

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