No More Pavement! The Problem of Impervious Surfaces

by Lakis Polycarpou | July 13, 2010

Recent research, according to the New York Times, indicates that urban areas are about to get hotter — much hotter. Not exactly what blistering New Yorkers want to hear after one of the more brutal, record-breaking heat waves in memory.

Of course climatologists (and most of the rest of us) have known for a long time that cities were hotter than the surrounding countryside. It’s called the urban heat island effect.

In short, unlike the rural countryside, cities are largely paved-over or built on, so there is no vegetation or moisture to absorb heat and cool the landscape; asphalt, concrete and rooftops simply absorb the sun’s energy during the day and re-release it at night. According to a 2009 American Meteorological Study, nighttime temperatures can be as much as 14 degrees hotter in New York City than in rural areas 60 miles away.

According to the New York Times article, new modeling from climate researchers in the U.K. indicates that the urban/rural heat disparity is about to get much worse —equivalent to the doubling of CO2 in some areas.

What to do? Other studies have suggested that something as simple as painting building roofs white could reduce the heat island effect by as much as 33 percent. Unfortunately, white roofs don’t do anything to combat the many other serious problems of impervious surfaces — most notably their effect on water.

Essentially, in built areas with extensive impervious surfaces, water can’t soak naturally into the earth — instead it rushes across the landscape, carrying pollutants and biological contaminants into our waterways, poisoning fish, wildlife, and us.

Studies that have looked at the connection between water quality and the percentage of land cover in a watershed have shown that high stream concentrations of inorganic nitrogen and phosphorus can be correlated with both urban and agricultural land use. (Nitrogen and phosphorus are two of the three main ingredients in artificial fertilizer spread on both lawns and crops).

Unfortunately, the pollution of water from this fertilizer is the major cause of dead zones in the Chesapeake Bay and the Gulf of Mexico (recent oil spill notwithstanding).

It’s also a likely culprit in the increasing phenomena of “red tides” in which a certain species of algae blooms in the ocean, causing, according to the NRDC, “serious, and potentially life-threatening human illnesses . . . including diarrhea, nausea, vomiting, abdominal cramping and chills, among many others.”

None of this should be a surprise, especially in places with combined sewer systems. As the EPA describes it,

Combined sewer systems are sewers that are designed to collect rainwater runoff, domestic sewage, and industrial wastewater in the same pipe . . . during periods of heavy rainfall or snowmelt . . . combined sewer systems are designed to overflow occasionally and discharge excess wastewater directly to nearby streams, rivers, or other water bodies.”

In other words, don’t swim in the ocean after it rains — you might not like what’s in the water. According to the EPA, combined sewer overflows are “a major water pollution concern for the approximately 772 cities in the U.S.”

A 2003 study from the University of Connecticut indicated that the percent of impervious land in a watershed was significantly related to all water characteristics his team examined. Some studies suggest that paving over anything above 10 to 20 percent of the landscape is bad for the water; others put the concentration much lower for fish populations, for example.

Estimates of the percentage of impervious surface in urban areas range from 50 percent of moderately dense suburban dwellings to over 94 percent in Mid-Manhattan West.

So what is to be done? Fortunately, there are many potential solutions.

Green roofs. In dense urban areas with flat roofs, green roofs — planting the roof with either low-maintenance native plants or growing food — can be a key strategy.

Research by Eleftheria Alexandri and Phil Jones at the at the University of Cardiff in the UK modeled the effect of green roofs in nine cities around the world — including Montreal, London, Mumbai and Brazilia — and indicated that green roofs would cool the climate in all of the cities, with the greatest effects in the hottest cities. (According to the model, if the buildings in Riyad, Saudi Arabia had green roofs, the temperature during the hottest month would drop a shocking 9.1 Celsius or about 16 degrees Fahrenheit)

As for water, in one EPA study, green roofs retained over 50 percent of total precipitation over the course of a year, and over 95 percent in summer months. Comparing runoff between a typical asphalt roof and green roofs found that green roofs raised ph (a benefit to counteract acid rain) and appeared to remove atmospheric nitrate.

Rain gardens. Rain gardens are a portion of lawn or landscape planted to hold, filter and slowly release surface water runoff from roads, roofs, parking lots or other impervious structures into the landscape. According to the EPA, rain gardens reduce peak water flows that stress the system, and improve “the physical and biological integrity of receiving streams by reducing stream bank erosion and negative effects on aquatic communities.”

Permeable paving. New technologies allow for porous pavement surfaces that let water to percolate below the surface. Examples include pervious concrete, porous asphalt, and porous turf.

Better Urban Design. Conventional sprawl zoning requires a minimum amount of parking for businesses and residences. Not only does this requirement increase auto dependence and sprawl, it creates vast acres of (often unused) asphalt parking lots. The New Urbanism movement is one widely known planning approach that attempt to design for more compact land use, better access to pubic transit, and less space devoted to parking.

Depave! The Depave project, run by the community activist group City Repair in Portland, Oregon, provides information and technical assistance to citizens who have places they wish to depave.

If we want to protect our watersheds, we could do worse than starting with the ground under our feet.

Resources:

From the Environmental Protection Agency:

“Green Infrastructure.”

“Green Infrastructure Research Program: Providing Solutions to Manage Wet Weather Flow.”

“Green Streets.”

“Green Roofs for Stormwater Runoff Control.”

“How Urbanization Affects the Water Cycle.” California Water & Land Use Partnership.

“Long-Term Stormwater Quantity and Quality Performance of Permeable Pavement Systems.” Center for Water and Watershed Studies Department of Civil and Environmental Engineering, University of Washington.

“Impervious Surfaces,” NEMO (Nonpoint Education for Municipal Officials), University of Connecticut .

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