On Tuesday, 11/15/22, the United Nations estimated that the Earth’s population topped 8 billion people. I promptly wondered about the impact on impervious cover, a notorious link to flooding. However, I discovered it’s not as simple as you might think.

Impervious cover directly links to flooding. But the growth of impervious cover (new homes, streets, parking lots, etc.) does not directly link to population. Two studies cited below found huge variations in the growth of impervious cover related to LOCATION and LIFESTYLE. It doesn’t all depend on population.

Increasing Rate of Population Growth

It took Earth 200 thousand years to reach 1 billion people in 1804. Since then, we’ve added 7 billion people in a little more than two centuries. The last billion took just 12 years!

UN World Population Milestones

Population in Billions	1	2	3	4	5	6	7	8
Year Reached	1804	1930	1960	1974	1987	1998	2010	2022
Years elapsed	200,000+	126	30	14	13	11	12	12

Clearly, growth has accelerated. Such numbers demand reflection. They prompt at least two questions: Are we living sustainably? And does the increase in impervious cover associated with population growth necessarily lead to a corresponding increase in flooding? I can’t answer the first. But based on these studies, I’ll answer the second with, “Not necessarily. It depends.”

2007 Study Shows Widely Varying Rates of Impervious Cover Worldwide

A 2007 study published in the journal Sensors estimated impervious surface area (ISA) in 100 counties. Called “Global Distribution and Density of Constructed Impervious Surfaces,” the authors included Christopher D. Elvidge, Benjamin T. Tuttle, Paul S. Sutton, Kimberly E. Baugh, Ara T. Howard, Cristina Milesi, Budhendra L. Bhaduri, and  Ramakrishna Nemani. Among other things, they examined the impacts of hydrological and ecological disturbances associated with the growth of impervious cover.

They note that:

• ISA alters the character of watersheds by increasing the frequency and magnitude of surface runoff pulses.
• Increased overland flow also alters the shape of stream channels, raising water temperatures, and sweeping urban pollutants into aquatic environments.
• Hydrologic consequences of ISA include:
  • Increased flooding
  • Reductions in ground water recharge
  • Reductions in surface water quality.

So Who Has the Most Impervious Surface?

The three countries with the most ISA are China, the U.S. and India. But our population varies dramatically from the other two. With less than a third of the population, we have roughly four times more impervious cover. That makes our ISA per person roughly 4-5X higher.

So there’s not a direct correlation between population and impervious cover.

While noting that the world’s most developed nations also have the highest percentage of impervious cover, the study does not go much beyond that. It does not quantify the relative rates of flooding in each country studied. The main objective was simply to offer a framework and methodology for measuring impervious cover that other researchers could build on.

An Urban Planning Perspective

A second study reviewed the study above from an urban-planning perspective and led off with these two images.

As you probably already guessed, the area on the left has the most pavement per person, despite appearing to have less concrete.

The area on the right is in Germany, which has about one third of the paved surface per capita of the U.S. Both countries are comparably wealthy and both famed for their highways. This article digs deeper into planning issues associated with:

• Distribution of impervious cover
• Infrastructure maintenance costs
• Urban planning strategies

Impervious Cover Related to Auto Culture

Daniel Herriges, the author, points out that impervious surfaces exist for three major reasons:

• Buildings
• Streets/Roads
• Parking

He adds, “Two of those three have everything to do with cars. And on nearly every measure to do with car usage, well, America is #1, Baby.”

Using the EPA’s interactive EnviroAtlas, Herriges created heat maps of several major cities. They consistently revealed that the highest impervious surface per capita is in suburbs, not central cities.

He continues, “The paradox this data reveals is stark: New York City is dominated by brick and glass and concrete and steel. But NYC residents have just about the least amount of pavement to their name of any Americans. Meanwhile, our greenest places are in one sense the least ‘green,’ when you account for the parking lots and six-lane stroads that come with large grassy lawns.”

What Appears to Be Green Can Be Deceiving

Herriges concludes: “…what appears green can be deceiving.”

He argues to “Let cities be cities and rural be rural.” In productive places that generate wealth…we can afford to deal with stormwater through more sophisticated technological means: pipes, pumps, levees, as well as newer technologies like green roofs and permeable pavement.”

But he argues, “Places that produce comparatively lower revenue warrant a different approach, a more natural and low-tech one. It’s not that verdant suburbs are always bad: it’s that we should deal with drainage in those places by keeping our paved footprint to a minimum, and absorbing as much stormwater back into the ground as possible.”

Unfortunately, he doesn’t delve into the factors that drive suburban migration, such as school quality and crime rates. Nor does he hint at what to do with the auto-oriented suburbs and commuting culture we already have. Still, he’s a brilliant writer who offers much to think about.

If he proves one thing, it’s that population growth doesn’t automatically lead to more impervious cover per capita and increased flooding.

But is it possible to wean Americans off automobiles? It seems that’s an even bigger ask than preserving natural floodplains.

Posted by Bob Rehak on 11/16/22

1905 Days since Hurricane Harvey