This idea provides two benefits simultaneously - it produces free electricity and reduces flooding and erosion.
The idea is to put large rain-storage tanks on the roofs of skyscrapers. Then hours or days after a rain storm, the water would flow to ground level through a pipe with a water turbine to generate electricity. The power is greatly increased because of the height of the building.
The storage of water for a few days reduces the problems of flooding and erosion in the same way that traditional detention ponds work.