This is a really interesting topic, coz now in China; there are very similar ideas called “the sponges’ cities”.
Sponges’ cities have the ability to absorb water like the sponges. These cities can retain rainwater as much as possible. For instance, when we design a park, we try to use the water absorbent materials to build the ground. It is a good leisure place for citizens to stay. But during the rainy day, this place becomes a rainwater container. Not only the special materials, but also the mud, glasses, woods, lakes and rivers can absorb rainwater. There are fewer risks for the city flood. Moreover, this rainwater could be reused, such as irrigating flowers and car washing. It is also a good solution for water shortage.
Sponges’ cities have also reflected on the building aspects. The government tries to transform most of the buildings in cities as the rainwater collection buildings. Roof garden is one of the options for this strategy.