Find the time required to lower the water surface of a rectangular tank of cross sectional area 5000

Find the time required to lower the water surface of a
rectangular tank of cross sectional area 5000 m2 from an elevation
of 10.20 m to 9.20 m. A rectangular suppressed weir of length 2.0 m with its
crest at elevation 7.7 m above the datum is used for lowering the water surface
(Assume Cd = 0.62).