Paving paradise: the peril of impervious surfaces

Environmental Health Perspectives, July, 2005 by Lance Frazer

Bruce Wilson, a research scientist with the Minnesota Pollution Control Division, is midway through a satellite survey of impervious surface area in that state. What Wilson has seen thus far is enough to cause significant concern about the state's growth and development, and the impact of impervious surfaces on the water system.

"Impervious surfaces are impacting the lakes and streams on a number of fronts," he says. "Velocity of runoff is a big one. Water runs off of these surfaces so rapidly, it creates mini-tsunamis that can cause serious, even irreparable, harm to the stream ecosystem.... And of course, the ability to recharge the groundwater system is being impacted. If you get into a twenty- to thirty-percent drop in infiltration [into the aquifer], which means a loss of base flow, the impact on streams being fed by surface water gets magnified still further."

Another big problem for urban areas is the flash flooding that can occur when heavy rains fall over a city; according to hydrometeorologist Matt Kelsch, an authority on urban flash flooding with The University Corporation for Atmospheric Research in Boulder. Since runoff from an acre of pavement is about 10-20 times greater than the runoff from an acre of grass, Kelsch says impervious surfaces can quickly trigger devastating floods that can produce a host of their own environmental health hazards.

"In urban areas, anywhere from thirty to forty percent of the rainfall runs right into whatever stream is in the area, and in heavily urbanized areas it can be more than fifty percent," he explains (by comparison, he says, the amount of runoff in subsaturated woodlands is often less than 5%). "If the water overflows the stream banks, it's going to seek the path of least resistance. In most cases, that's going to be the roadways."

In many desert areas, Kelsch says, engineers take advantage of the natural topography, building houses at higher elevations and installing roads that lead up to residential areas. What this does is make the roads far more dangerous. More than 50% of the fatalities in flash floods occur on roads, according to Kelsch.

Floods are often given numerical designations such as "hundred year flood," meaning such a flood happens once every 100 years (or has a 1% chance of occurring in any given year). The Federal Emergency Management Agency maintains a national list of flood zones and maps of impacted areas. The problem, says Kelsch, is that we've changed the playing field. "A couple of factors come into play," he says. "First, this is still a pretty new country, so most places haven't been developed long enough to know about the historical risk of a devastating flood. Secondly, when we urbanize an area, we alter the historical frequency of these events. The more we develop an area, the more rainfall we put as runoff directly into streams that have evolved to handle only a fraction of that runoff, and the more that happens, the greater the likelihood of a catastrophic flood." Several such floods hit New Orleans in the 1980s, and three hit St. Paul-Minneapolis between 1990 and 2001.