Large-depth underground drainage facility and method of running same

In a large-depth underground drainage facility including an underground water channel of a large capacity which is provided in an underground location at a large depth, and which is slanting gently;

• a plurality of vertical shafts for leading drain water downwardly into said underground channel from flood control channels provided in and near to a ground surface;
• a pump well provided at a downstream end of said underground channel;
• and a pump station having a pump for pumping water which has flowed into said pump well to rivers or the sea;
• the improvement wherein said pump in said pump station includes an impeller disposed generally at a medium water level of said underground channel;
• and wherein said pump is of a two-stage impeller type, including a first impeller capable of providing a total head at which the water can be pumped from a closed channel water level at a rated flow rate and a second impeller capable of providing a total head at which the water can be pumped from an open channel water level at a minimum flow rate.

In a large-depth underground drainage facility including an underground water channel of a large capacity which is provided in an underground location at a large depth, and which is slanting gently;

• a plurality of vertical shafts for leading drain water downwardly into said underground channel from flood control channels provided in and near to a ground surface;
• a pump well provided at a downstream end of said underground channel;
• and a pump station having a pump for pumping water which has flowed into said pump well to rivers or the sea;
• the improvement wherein said pump is disposed in an underground location at a level higher than the upper end of said underground channel;
• and wherein a pumping tank of a large capacity is provided between said pump well and said pump, with the bottom of said pumping tank disposed generally at the same level as that of said pump.

In a method of selecting a large-depth underground drainage facility, which includes an underground water channel of a large capacity which is provided in an underground location at a large depth and which is slanting gently;

• a plurality of vertical shafts for leading drain water downwardly into said underground channel from flood control channels provided in and near to a ground surface;
• a pump well provided at a downstream end of said underground channel;
• and a pump station having a pump for pumping water which has flowed into said pump well to rivers or the sea, in such a manner that the construction cost of the large-depth underground drainage facility can be minimized;
• the improvement wherein when it is difficult to obtain a construction lot for said pump station, a type of large-depth underground drainage facility for the pump station is selected in which said pump is disposed generally at a medium water level of said underground channel;
• and when it is easy to obtain a construction lot, a type of large-depth underground drainage facility for the pump station is selected in which said pump is disposed at a level higher than the upper end of said underground channel, and a pumping tank of a large capacity is provided between said pump well and said pump.

A method of running a large-depth underground drainage facility comprising the steps of:

• calculating an anticipated amount of rainfall in accordance with rainfall information;
• calculating the rate of flow of water into vertical shafts in accordance with said calculated rainfall amount and an outflow factor defined in accordance with the interval between a first rainfall and a subsequent rainfall;
• calculating the rate of flow of water into a pump station in accordance with the rate of flow of water into said vertical shafts;
• and
• determining the number of pumps to be operated in the drainage facilities, as well as the ability thereof, in accordance with the calculated rate of flow of water into said pump station, so as to drain water from said pump station.