Growth of Impervious Cover: Most Consistent Driver of Flooding Worldwide

3/30/31 – According to numerous engineering studies, the single most consistent, anthropogenic (human-created) driver of flooding worldwide is expansion of impervious cover related to urbanization.

How Impervious Cover Contributes to Flooding

Any area growing in population adds roads and rooftops. When stormwater falls on them, it runs off quickly, unlike when forests or grass covered the land. Engineers talk of friction coefficients. Less friction with concrete increases the speed of runoff.

But it’s not just the speed, it’s also the volume of runoff. Engineers also measure the permeability of different land surfaces and soil types. Is the surface clay or sand, for instance. One holds water; the other lets it sink in.

Regardless, replacing either with concrete and shingles increases the speed and volume of runoff. Think of water running off a roof and down a gutter during a torrential rain.

Note how volume peaks higher and faster after development.

The graph above assumed:

• Watershed area: 100 acres
• Storm depth: 5 inches
• Method: NRCS Curve Number runoff depth plus a synthetic hydrograph
• Pre-development: CN 68, Time of concentration 2.5 hr
• Post-development: CN 90, Time of concentration 0.8 hr

Results:

• Pre-development peak flow: about 81 cfs
• Post-development peak flow: about 402 cfs
• Pre-development time to peak: about 1.75 hr
• Post-development time to peak: about 0.73 hr
• Runoff volume: increases from about 21.6 ac-ft to 44.8 ac-ft

This example is not site specific. It is an example for illustrative purposes only. While the numbers would change depending on soil types, slope and native ground cover, something similar happens everywhere urban growth occurs. You see:

• Increased runoff volume
• Faster time of concentration (peaking)
• Higher peak discharges.

You see similar, though not identical, responses worldwide. For example…

Atlanta: Faster, Higher Peaks after Urbanization

A widely cited Georgia State University study of eight metro-Atlanta streams correlated population increases with flooding frequency and severity from 1986 to 2010. During that time, developed land and high-flow days (flood-like conditions) roughly doubled.

Urbanization led to a 26 percent increase in annual stream flow. The increase was not rainfall driven; the study controlled for precipitation.

It reflected: increased impervious cover, reduced infiltration, and faster concentration in channels.

“This means that during a storm event, you’ll now see more runoff, more extreme flows and more flooding than you would have seen for a similar storm event in 1986,” said Jeremy Diem, the study’s lead author and associate professor in the Department of Geosciences at Georgia State.

Another study, “The Influence of Urban Development Patterns on Streamflow Characteristics in the Charlanta Megaregion,” found similar results. “The statistical analysis revealed that increasing the extent of urban development enhanced high and low flow frequency as well as annual peak unit discharge,” said the authors. “Impervious surfaces in source areas distant from streams increased the frequency of high flows.”

“Flashiness” also increased. USGS found that urbanization increased flood magnitude most strongly for moderate storms (e.g., 2–10 year events). The difference narrowed for very large storms because everything becomes saturated and behaves as impervious cover anyway.

The American Meteorological Society found that flood severity was driven as much by runoff efficiency as rainfall magnitude. In other words, in urban areas, rainfalls that aren’t historically extreme can produce exceptional stream rises and flooding.

Dallas and the Trinity River

Studies by the U.S. Geological Survey and U.S. Army Corps of Engineers documented similar problems in Dallas. As the city expanded after World War II, an explosion of impervious cover dramatically increased the speed of runoff. Storm sewers carried water to the river much faster so the city saw steeper and earlier flood peaks.

Peak flows from tributaries stacked on top of each other rather than arriving at staggered intervals. This created higher peaks on the Trinity and more frequent “bank full” conditions. It also put greater stress on levees.

The levees enabled economic development in floodplains, but narrowed channel width, increasing water surface elevation and speed.

As a result, the system became efficient at passing moderate floods—but more vulnerable to extreme ones.

Conclusion

In city after city, hydrologists find that the growth of impervious cover creates more intense, faster, and higher peak flooding. Soil differences affect infiltration and runoff rates. But it is not uncommon to find pre-/post-development differences of approximately 2X.

The two pictures below taken within a few miles of each other in the north Houston Area speak volumes.

All those dots in the pavement above are storm drains that act as superhighways for rainfall. They channel it straight to the nearest stream.

According to a recent New York Times article, nine of the 20 counties in the U.S. that have experienced the most development the last decade are in Texas. 

Posted by Bob Rehak on 3/30/2026

3135 Days since Hurricane Harvey