In Engineering (Drainage Works), What is the function of water stops in joints of box culverts and drainage channels?

The principal function of water stops is to prevent liquids (e.g. water), water-borne materials and solids to pass through concrete joints. In essence, it aims at providing water tightness to the drainage channel.   Besides, water stops in drainage channels or box culverts can also serve other purpose is to avoid water contacting joints’ dowel bars and causing corrosion. 

to saveguard leaks into surrounding areas 

Water Stops installed on construction joints of structures prevents ground water infiltration.

Function of Water stops in drainage works is Water proofing.