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 $cm/100$ m length) to be given to this gravity main so that the required amount of water flows without any difficulty is ___________