When it rains in a city, the story is very different. Much of the land in urban areas is covered by pavement or asphalt. Because rain can’t soak into the soil underneath, these covered areas are referred to as impermeable surfaces. As the amount of impermeable surface increases with urbanization, so too does the amount of runoff (see figure at right).
Even when urban soil is not covered by houses, stores, parking lots, or roads, it’s often compacted. Compaction reduces the pore space in the soil, which drastically slows the rate at which water can infiltrate, or percolate, into the soil. Because compacted soils only let minimal amounts of water percolate through, they act more like asphalt than functional soils.
When only limited places exist where water can infiltrate into the ground, stormwater moves over the ground instead. This process is called “overland flow.” During overland flow, water picks up speed and objects that get in its way, including trash, sediment, and other contaminants such as motor oil, nutrients, and metals.
Many cities have also increased the speed of overland flow and the amount of runoff because gray infrastructure has been designed to move water off streets as quickly as possible through gutters, storm drains, and sewer pipes.
So where does all that runoff go?
The fate of urban stormwater runoff
Urban stormwater that does not percolate into urban soils often flows directly into streams, lakes, or the ocean either by overland flow or through storm drains that discharge directly into natural waters.
Most municipalities also have combined sewer systems, in which stormwater may be carried via pipes to water treatment facilities. This ends in one of two ways: 1) municipalities expend money and resources to clean the stormwater, or 2) large volumes of stormwater, combined with normal sewage levels, overload treatment facilities.
In the first case, dilute stormwater is energy-intensive to treat as wastewater plants are designed to treat more concentrated influent. The dilute stormwater reduces the operating efficiency of the plants and so wastes energy.
In the second case, large volumes of stormwater can overwhelm the capacity of a wastewater treatment plant, causing it to release a portion of the stormwater, combined with untreated sewage, into natural waters. This type of event is called a “combined sewer overflow,” or CSO. Municipalities are allowed a certain number of these events each year. But regulations are also tightening due to concerns over water quality.
So, to summarize, if a soil has been compacted or paved over it will have low to zero permeability, preventing water from infiltrating into the soil. This results in larger volumes of water moving across the surface, which in turn causes flooding, water pollution, increased erosion, and decreased storage of water in the ground for later use. In other words, there are two major problems with the methods most municipalities use to manage stormwater: issues of water quantity and water quality. We’ll explore these effects further in the next section.