What is the function of waterstops in joints of box culverts and drainage channels?

The principal function of waterstops is to prevent liquids (e.g. water), water-borne
materials and solids to pass through concrete joints. In essence, it aims at providing
watertightness to the drainage channel.
Besides, waterstops in drainage channels or box culverts can also serve two other purposes:
(i) to avoid water contacting joints’ dowel bars and causing corrosion. (ii) to avoid water
seeping in from the underside of drainage channels or box culverts, thereby washing in soil
particles and causing voids underneath these structures and finally leading to their failure.
To serve the second purpose, obviously only one waterstop is required at any depth
location.
To serve the first purpose, a waterstop has to be installed on top of dowel bars to prevent
water from drainage channels from leaking through. On the other hand, a waterstop has to
be provided below dowel bars to avoid underground water from surging upwards.
In fact, the other way out to serve the first purpose is by using corrosion resistant bars