Pipe flow and pipe distribution at Toinotani Basin in Ashiu Experimental Forest

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  • 芦生演習林トヒノ谷におけるパイプ流出とパイプ網に関する研究
  • アシウ エンシュウリン トイノタニ ニ オケル パイプ リュウシュツ ト パイ

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Two-year observation of the discharge from underground pipes made following things clear. First, ten pipes were found in a small hollow. Three pipes of the seven, that locate at the downstream end of the hollow always dischrge water. Others drain during only heavy rains. Most volume of direct discharge is covered by the pipe flow. Second, the pipe flow can only be seen when antecedent rain is enough and rainfall intensity is enough. Third, the patterns of pipe flow rate well correspond to rinfall patterns. Forth, the discharge rate from pipes has the upper limits in volume. The limit may be controlled by the dimensions of the pipes. Fifth, digging out pipes revealed that pipes long only 50 cm and they lead to the layer of gravel. It means that the flow comes out from pipes should be called not pipe flow but gravel layer flow in this case. Sixth, the outflow rates from pipes are well explained with a tank model. These facts have possibility to explain the coincidence between the peak of rainfalls and the occurrence time of debris flows and/or landslides that have not been able to be explained by the theories developed assuming homogeneous soil layers. Smooth outflow through pipes, however, does not explain the occurrence of debris flow. Collapse happened inside of pipes, which was once observed during the observation period, increases hydraulic pressure and may trigger landslides.

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