Last winter, we at Rain Brothers, LLC designed, developed, and installed a simple, cost-effective "blue roof" system for managing stormwater for the City of Grove City here in Ohio. A "blue roof" is a new best management practice (or BMP) for stormwater -- specifically, it is a way to slow a rain event by creating localized absorption on the roof itself, as well as by creating filtration layers to restrict the flow of the rainwater through the gutter/drainage (and, yes, there are lots of structural analysis calculations that are involved in ensuring the roof can support a slowed rain event).
The more we can spread out a rain event, the less immediate pressure we put on a municipality's storm system. This, in turn, means that the direct discharge in our rivers and streams (storm sewers often discharge directly into our watershed) is slowed, which lessens the chance for erosion, river turbidity, silting, etc.
In isolation, the act seems small. Adapting these stormwater BMPs on a large scale, however, will have a dramatic impact in not only preserving our eco-system but also in maintaining high water quality in our watershed regions.
Here is a link to a recent article about our blue roof system.
Below are pictures (before and after) of the installation of the blue roof at the Grove City Municipal Building.