Great cities should be designed to accumulate water, as they do knowledge and gold - and the water should be recycled within city boundaries.
In the Roman Empire, fresh water was brought in by aqueduct and dirty water was discharged into streams, which became sewers, and then into the Tiber via the cloaca maxima. The Roman approach to water management remained the only possibility until modern times. Now, we can extract water from the foulest rivers and purify it for household use. We can also afford to lay different pipes for the supply and disposal of different types of water: roofwater, roadwater, sewage water (without solids). The water types can be described as bluewater, greywater and brownwater. Each should be used in a different way. Bluewater should be infiltrated where it falls. Greywater may have to be filtered to remove hydrocarbons. Brownwater should be treated in reedbeds or by conventional means
The new approach to water management has profound consequences for physical planning. Roofs should be designed to detain and evaporate water. Parks should have reed beds, storm detention ponds and infiltration ponds. Gardens should have large tanks, as they used to in the old days, to store rainwater and to supply water features. Cities should have more trees. Pedestrian surfaces should be porous, so that they do not produce surface water runoff. Urban valleys should be designed so that they can flood. New buildings in flood plains should be floodable. All buildings should be designed with rainwater storage and infiltration capacity, on the roof or in the grounds.
The catchment of the River Quaggy has been re-designed, in order to detain and infiltrate water.
Roman Aqueduct (in Istanbul)
Roman Cistern (in Istanbul)