For animals, the entire universe has been neatly divided into things to (a) mate with, (b) eat, (c) run away from, and (d) rocks.Terry Pratchett, Equal Rites
Draining is not safe
Walkable drains are not designed to be walkable–they are designed to move large volumes of water quickly. A large drain typically receives water from tributary drains that cover a large area. This means even small amounts of precipitation will get concentrated quickly. You will not be able to maintain footing against the water flow, and eventually you will get pushed down the drain, scraping against concrete and exposed pipes and rebar. Even if you survive that, you must still fight off drowning and hypothermia.
Abandoned buildings are not safe
Abandoned buildings are often stripped down and partially deconstructed – exposing elevator shafts, holes in the floor, asbestos, exposed chemicals, unprotected electrical wires, etc. Abandoned buildings are by definition not maintained, so structures like steel stairwells may be sufficiently eroded to collapse while your group is climbing on them.
For animals, the entire universe has been neatly divided into things to (a) mate with, (b) eat, (c) run away from, and (d) rocks.Terry Pratchett, Equal Rites