Cities are no strangers to urban flooding. Excessive runoff from clogged storm drains can be a common occurrence. 

But we may be hearing the term “sponge cities” more often. These creative cities are working to fight back and come up with ways to absorb rainwater rather than just letting it head to the drains.

The term "sponge cities" was coined by Chinese landscape architect Kongjian Yu to describe ways for urban areas to fight back against flooding.

The bottom line

Whether your city shrugs off a downpour or ends up flooded often boils down to one key factor: how much water its surfaces allow to pass through.

The Environmental Protection Agency (EPA) discovered that a parking lot is roughly 95% impervious. Rain basically hits and runs straight to a storm drain. 

Natural land cover is basically the opposite. Since it is close to 0% impervious, water soaks in and moves slowly through soil. 

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Watersheds are land areas that channel water into streams and rivers, eventually leading to the ocean. Paving over these areas leads to a 10 to 20% increase in runoff. 

Cities slurping up rain?

Minnesota began building rain gardens as early as the 2000s and has continued refining the approach, setting a goal of preventing any increase in runoff after new construction is completed. Since 2013, all new urban developments in the state have been required to manage just over an inch of rainfall, while projects farther from urban centers must retain at least an inch of rainwater. The hope is to change the environment as little as possible after developing the land, according to this study.

But cities are just starting to make a conscious effort to essentially track water conservation on a larger scale.

Kansas City is also working to modify all new developments to handle over an inch of rain.

The Los Angeles Department of Water and Power says its stormwater capture facilities can take in more than 27 billion gallons a year under average conditions, and the city is targeting 48.9 billion by 2035.

New York City is also working to meet goals for catching a certain amount of rainwater per year. It will take time to see just how much of a benefit continuing to catch water can be, and how we can repurpose it to create savings for city dwellers.

Here are some of the ways cities are improving: 

• Rain gardens: Man-made depressions that are designed to capture and reduce runoff from rain. You can often see these between parking lots, indicated by the rocks and combination of plants and grasses that excel at helping water stay in place.

• Green roofs: A layer of vegetation on top of a man-made structure. These green roofs usually feature vegetation, dirt, a root barrier, an irrigation layer and a waterproof membrane close to the structure of the roof. Not only does this help absorb rainwater, but it can also save on energy costs.
• Permeable pavement: This pavement is much more porous than standard asphalt, which allows rainwater to pass through and eventually reach groundwater. 
• Restored wetlands: When cities are built, sometimes wetlands are impacted. These wetlands are natural barriers to prevent runoff. Wetlands can be restored, bringing back a pathway for rainwater to reach groundwater, which a city can then tap into. 

Why focus on runoff?

Excessive runoff can cause more downstream problems than you may realize.

Stormwater runoff can carry pollutants that are detrimental to ecosystems. Excessive running water can carry fertilizers, dirt, debris and even waste to local bodies of water. This runoff can strain ecosystems and lead to shifts in the living conditions for certain species.

A dramatic example of this occurs at the base of the Mississippi River Delta, seen below. The lighter color of the water shows how too much runoff too quickly can actually harm species and hinder tourism in places that rely on it. 

Water gardens and attempts to catch rainwater have been done on a smaller scale across most cities for decades. We have all seen the dips between parking lots full of rocks and grasses.

With the past several years being among the hottest on record, we are seeing increased ocean evaporation, more atmospheric moisture and more frequent heavy rainfall events. Cities will need to continue to adapt and work to strengthen these ways to absorb rainfall.

What you can do

The average residential lawn sits around 40% permeability. That may seem low, but it is because soil compacted by construction equipment and foot traffic behaves a lot like pavement.

There is something small and potentially cost-effective you can do: build your own rain garden in your yard. The key is to build it downstream from your gutters so the water collects at its natural low point. A study has shown that a typical rain garden can cost $3 to $5 dollars per square foot, but this could change depending on your yard and where you live.

(MORE: How to use leaves to help your yard)