1992

Stormwater Management

Think about what happens as a city or suburb develops. Prior to the arrival of people, the land is likely forested and absorbs much of the rain that falls. Between the leaf litter, the friable soil under it, the dips in the terrain, etc., runoff is minimal. Creeks and rivers easily handle the runoff that does occur.

To build the city, developers cut down the trees and construct buildings. They also build parking lots, sidewalks, and roadways. All of these are impermeable surfaces like asphalt or concrete. When rainfall occurs on an impermeable surface, none of it gets absorbed or even slowed—it all runs off immediately.

In a dense urban area filled with impermeable surfaces, the effect can be devastating if there is a burst of rainfall. Imagine a city covering 100 square miles (260 square km). If an inch of rain falls and the entire area is impermeable, that’s 401,448,960,000 cubic inches of water, or 1.7 billion gallons, or 6.6 billion liters. If engineers don’t do anything about it, that water all immediately flows into the nearby river in an hour or two. If six inches of rain falls, as in a hurricane, 10 billion gallons (38 billion liters) ends up in the river, and there is no way the river can handle the pulse.

Therefore, in most urban and suburban areas, the idea of stormwater management has become a major aspect of engineering during development. For example, every subdivision, shopping center, and mall in a suburban area will typically now have a pond area that takes the runoff from roofs and parking lots, and stores it for slow release into local streams and rivers, or absorption into the ground. This approach mimics what used to happen when the land was forested.

In cities the same kind of thing happens, but often in massive underground vaults that catch and hold the stormwater. One vault in Saitama, Japan, built in 1992, measures 225 feet (78 meters) wide by 580 feet (177 meters) long by 83 feet (25 meters) tall.

Without stormwater management, flooding becomes a huge problem. With it, engineers have a great deal more control over flooding.

SEE ALSO Concrete (1440 AAA), Asphalt (625 AAA), Pompeii (79), Bath County Pumped Storage (1985), Venice Flood System (2016).

The underground stormwater system in Japan is awe-inspiringly huge.