A trapezoidal channel of base width 30 ft and side of slope of 2H:1V is excavated in alluvial country, a sieve analysis of which gives a D- size of 2 in. The channel, which will be unlined, is to carry discharge of 1250 cfs under uniform flow condition and the velocity is not to be exceed 5 ft/sec. Design for the maximum allowable slope pf the channel bed. ( use the empirical formula for Manning's roughness n=0.031 (d) ^1/6 where sieve size d is measure in ft).