You really can't assume a bike has been abandoned. Probably contact the owner of the property where the bike is. In the OP's case you would contact the city.
I see bikes all the time on my campus. Last year their was a nice Specialized Hard rock. The tires were flat, the chain warped and rusted, and a bunch of cables non operational. I believe after 6 -8 months campus police put an orange sticker on it (probably because they were removing the rack). About 2 months later the bike and rack were gone.
Our campus built two new racks with a see through covering and side (sort of like half a greenhouse). So far there are 3 bikes that have not moved since May. I guess we'll wait and see what happens when school starts back up again and people need a place for their bike.