A straight 100 m long raw water gravity main is to carry water from an intake structure to the jack well of a water treatment plant. The required flow through this water main is 0.21 $$m^3/s$$. Allowable velocity through the main is 0.75 m/s. Assume f = 0.01, g = 9.81 $$m/s^2$$. The minimum gradient (in cm/100m length) to be given to this gravity main so that the required amount of water flows without any difficulty is _____.