Why cities flood<\/h2>\n\n\n\n
Most modern cities were built to move water away as fast as possible. Roads, sidewalks, parking lots, and rooftops are usually made of concrete, asphalt, and other hard surfaces that do not let much water sink into the ground.<\/p>\n\n\n\n
That is why a heavy storm can turn a street into a stream in minutes. Drexel University civil engineer Franco Montalto has described the problem as cities having \u201cessentially sealed the surface of the landscape,\u201d leaving rain with fewer places to go.<\/p>\n\n\n\n
Older sewer systems add another layer of risk. In New York City, about 60% of the city uses a combined sewer system<\/a>, meaning stormwater and sewage flow through the same pipes toward treatment plants. During heavy rain, that mix can overflow into local waterways.<\/p>\n\n\n\n A sponge city tries to slow water down instead of pushing all of it into pipes at once. This means rain gardens along curbs, green roofs on buildings, artificial wetlands, tree-lined medians, and basins that temporarily hold water.<\/p>\n\n\n\n Can a park really help during a storm? Yes, to a point. Soil, roots, and vegetation can absorb water, filter pollution, and reduce the amount of runoff rushing into streets and sewers.<\/p>\n\n\n\n The U.S. Environmental Protection Agency<\/a> says green infrastructure can capture and absorb stormwater while filtering pollutants that would otherwise wash into streams, lakes, and beaches. Urban trees can also reduce stormwater runoff by absorbing a share of annual rainfall.<\/p>\n\n\n\n Copenhagen became one of the best-known examples after a devastating cloudburst in July 2011. More than 5 inches of rain fell in parts of the Danish capital, causing damage estimated at more than $1 billion and pushing officials to rethink flood planning.<\/p>\n\n\n\n The city responded with a cloudburst management plan<\/a> built around a mix of gray and green infrastructure. That includes engineered drainage, redesigned streets, and public spaces that can store water when storms arrive.<\/p>\n\n\n\n The lesson was not that pipes no longer matter. It was that pipes alone are often not enough, especially when a city also needs safer streets, cooler neighborhoods, and public spaces that work on dry days too.<\/p>\n\n\n\n In the United States, many sponge-style projects are still scattered rather than fully connected. One neighborhood may have rain gardens, another may get porous pavement, and another may still depend mostly on old drains and hard surfaces.<\/p>\n\n\n\n Los Angeles<\/a> shows both the promise and the challenge. City officials said they captured more than 13.5 billion gallons of stormwater during historic February 2024 rainstorms, enough to serve nearly 165,000 households for a year.<\/p>\n\n\n\n That is a serious amount of water. Still, scattered projects cannot always protect an entire city from a storm that drops rain faster than the ground, basins, and sewers can handle it.<\/p>\n\n\n\nWhat sponge cities do<\/h2>\n\n\n\n
Read More: Sponge-city tech is teaching concrete to drink stormwater like a thirsty park \u2014 and it can\u2019t happen fast enough<\/a><\/h3>\n<\/div>\n<\/div><\/div>\n<\/div>\n\n\n\n
![\"Rain](\"https:\/\/www.ecoticias.com\/en\/wp-content\/uploads\/2026\/06\/urban-flooded-street-extreme-rain-sponge-city-limits.jpg\" "\\"\\"")
Copenhagen\u2019s warning<\/h2>\n\n\n\n
A patchwork problem<\/h2>\n\n\n\n
Nature has limits<\/h2>\n\n\n\n