drain·age 
/ˈdreɪnɪʤ/ noun
/ˈdreɪnɪʤ/ noun [noncount] : the act or process of draining something : the act or process of removing water or liquid from a place or thing
▪ the drainage of swamps ▪ The soil has good drainage. [=extra water does not stay in the soil] ▪ The holes in the bottom of the flower pot are for drainage. [=are for the extra water to flow out] — often used before another noun ▪ a drainage system/basin/ditch/channel
▪ the drainage of swamps ▪ The soil has good drainage. [=extra water does not stay in the soil] ▪ The holes in the bottom of the flower pot are for drainage. [=are for the extra water to flow out] — often used before another noun ▪ a drainage system/basin/ditch/channel
