Government Science & Industry Principle Animation:Sponge City Flood Lab

Roofs, roads and plazas are hard, so nearly all rain becomes runoff and reaches the sewer within minutes. At the storm peak the flow exceeds sewer capacity, water backs up out of the inlets and ponds at the low point; above 15 cm it counts as urban flooding.

Permeable paving lets rain pass into the gravel bed and soil; sunken green and rain gardens sit below the street, catch runoff from roofs and roads and overflow only when full; green roofs absorb some first. When flow exceeds about a third of sewer capacity, the excess goes into the underground tank, shaving the peak.

Left is the conventional block, right the sponge block, under the same storm. Sponge measures absorb part of the rain and lengthen flow paths, so the peak is both lower and later; the sewer copes and the street stays dry. Drag the rainfall above 120 mm: sponge measures have limits, and extreme storms still need pipes, pumps and flood routes.

Stored rainwater is used at about 250 m³ a day to irrigate green space and wash streets, emptying the tank within days for the next storm. Over a year most small and medium rains are absorbed on site; the usual sponge-city target for annual runoff volume control is 75–85%.