UNDERDRAINAGE SYSTEM AND OPERATING METHOD THEREOF

PURPOSE:To prevent any flooding damage from occurring even in the case where a sudden large scale inflow into an underground tailrace from respective rivers and drainageways is predicted, by installing a movable inflow water rate control device in a conduit leading to a vertical shaft from the rivers and drainageways. CONSTITUTION:Draining taken into three inflow shafts 1, 2, 3 from a river and a drainageway is conducted to an underground tailrace 4, and drainage is made so as to be done into rivers and the sea on the surface of the earth by a pump 7 from a pump #6 installed in a part of an underground tailrace, thus an underdrainage system is constituted. In succession, a movable inflow adjusting device 8 is installed in a conduit leading to inflow shafts 1, 2, 3 from rivers and drainageways. Then an amount of inflow to the underground tailrace 4 from respective rivers and drainageways is adjusted, and the inflow to these inflow shafts 1, 2, 3 from rivers and drainageways is separately adjusted according to the water level and inflow of the rivers and the drainageways.