New York City wants to catch and store rainwater temporarily in new roof systems to stop heavy storms sending sewage spilling into city waterways. The catchment systems would consist of "blue" roofs that have a series of drainage pools and "green" or grass- or ivy-covered roofs, under a plan unveiled by Mayor Michael Bloomberg. Bloomberg estimates the city could save $2.4 billion over 20 years if the state allows it to use this kind of green technology instead of relying on so-called grey infrastructure, such as storage tanks and tunnels.