same and also the storage capacity is the same in the embodiment of the present invention and the comparative example of fig3 as apparent the graph of fig4 the drainage power for a time 2hr t1 can be reduced according to the difference hwl2 hwl1 the flow rate head characteristics of the drain pump 8 according to the invention can be reduced as designated by solid line 23 of fig5 as compared with those designated by an alternate long and short dash line 24 according to the comparative example of fig3 additionally as shown in fig5 the flow rate power characteristics of the present invention and the comparative example of fig3 have the relationship designated by a solid line 25 and an alternate long and short dash line 26 a specific example of the power reduction effect is shown in table 1 in contrast with the comparative example table 1______________________________________items embodiment comparative example______________________________________main inflow diameter 10 m diameter 12 5 mconduit length 4500 m length 4500 m storage quantity storage quantity about 350 000 m sup 3 about 550 000 m sup 3underground storage quantity reservoir about 200 000 m sup 3total 300 m sup 3 s 300 m sup 3 sdischargefrom drainpumprated total h1 40 m h2 50 mhead ofdrain pumprequired about 145 000 kw about 181 000 kwpower hwlwas madereference t1 0 32 hr 0 32 hrt2 t2 39 0 46 hr 0 35 hrkwh 145 000 u00d7 2 0 32 181 000 u00d7 2 0 32 135 000 u00d7 0 46 178 000 u00d7 0 35 305 000 kwh 366 380 kwh______________________________________ this embodiment referenced in table 1 is constructed in the manner shown and described in connection with fig2 while the comparative.