Flooding Linked to Development, Researcher Says

Penn State hydrologist Dr. David De Walle, studying how urbanization affects stream flow, says that communities built in watersheds, such as the Windsor - Santa Rosa - Rohnert Park sprawl, can cause dramatic jumps in downstream runoff.

"As the population increased, there was more flow into the streams and rivers," says De Walle.

"There have been a great many studies of stream flow, temperature and precipitation, but they have all been done on undeveloped watersheds that are minimally impacted by humans," he says. "No one, to our knowledge, has formally looked at urban watersheds in this way."

De Walle is particularly concerned about what will happen in these areas when climate changes bring even more rain. Although scientists agree that the greenhouse effect is real, no one knows how that will impact flooding.

"If the greenhouse effect causes global warming, everyone is pretty confident that temperatures will increase, but how much precipitation will change is hotly debated," he says.

Combining population census counts, precipitation statistics from the National Weather Service, and stream flow data, he is looking at four regions of the U.S. over the past 50 years. In each area, De Walle is comparing five urban watersheds with five rural watersheds. Only analysis of the northeastern sector is complete.

A completely urbanized area such as Saddle River, N.J., for example, has about 2,600 people per square mile -- still smaller than the population of the Santa Rosa area, which has roughly 3,200 per square mile. Over the last half-century, Saddle River's stream flow has jumped 70 percent. "It was a pretty large effect," De Walle says.

According to the General Plan, about twenty years from now Santa Rosa will have about 4,400 people per square mile -- almost twice as large as the New Jersey community in the study.

By contrast, the researcher found that areas with a small population increase had low runoffs. Says Dr. De Walle, "As population grew [in the area], the stream flow also increased. When slightly higher flows from each subdivision are added together, the effect downstream can be significant."

Reilly sees Colorado as a model. "A few years ago, Denver had a $300 million flood and a number of people died," he says. "Now planners work retention of water into their plans; there are rooftop gardens, and 18 inches of water is stored during storms. They've put parklands in low-lying areas," says Reilly.

Reilly also cites research already done by Rue Furch and Otto Teller, showing that Sonoma County has built its subdivisions in the most sensitive places possible.

Environmentalists Furch and Teller were curious about the county General Plan, which claimed that only 3 percent of the county was developed. "Three percent doesn't sound like much, but it's interesting where it is located," says Furch. "Most of the roads, parking lots, and buildings are in the major recharge areas for our watershed."