PURPOSE:To enhance workability and reduce a material cost by replacing a required weight of crushed stones partially or totally with a lightweight permeable material CONSTITUTION:Rainwater flow holes 14 are formed on the side surfaces of a concrete-made cylindrical body 12 buried in the central part of a basin 10. Permeable members 20 are laid out in the central part of crushed stones filled up in such a fashion that they may be separated vertically. Preferably, the permeable members 20 should be formed with lightweight fillers 16 enveloped with a permeable sheet. Especially, it is highly desirable that the lightweight fillers 16 should be made of a foam styrenes material while the permeable sheet should be made of an unwoven cloth. The rainwater which flows into the permeable basin 10, is temporarily stored inside the cylindrical body 12 and then scattered and penetrated underground gradually from the crushed stones by way of the permeable members 20. When the permeable basin 10 is filled up with the rainwater and the lightweight permeable members 20 are subjected to the action of buoyancy, no permeable members 20 will flat up due to the load of the upper crush stones 16.
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