A trapazoidal channel, of base width 30ft and a side slope @h:1V, has a bed slope of 0.001, mannings coefficent of 0.027, and is to deliver 2,500 cfs to a lake, whsoe level may fluctuate. the channel bed at its outflow into the lake is 10 ft above datum; determine the water level at a section one mile upstream from outflow when the lake level is (a) 14 ft, (b) 21 ft.