Here's an urban farm we'll still be able to use when rising sea levels flood all our cities! Science Barge is a floating organic farm set aboard a barge in the Hudson river.
The Science Barge crops are grown hydroponically, delivering nutrients through irrigation water — so there's no soil, which means no waste runoff. Even the water is collected from rain barrels or from the river itself, and it can be used over and over again, so the water usage is sharply lower than on a conventional farm.
Suppose you don't have a boat? Well, the recirculating hydroponic system was actually devised for rooftop gardening — the Science Barge crew just put it on a boat because it's hard to drive a roof around and show it to people.