Impervious Surfaces and Flooding

Science Center Objects

The banner picture shows it all — Superhighways! Streets and pavement! Driveways! House roofs! These are all "impervious surfaces"; impervious to the water from precipitation. When it rains in this locale, water no longer seeps into the ground, but now runs off into storm sewers and then quickly into local creeks. Localized flooding is too often the result.

A house raised up on stilts to prevent flood damage.

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Why is this house wearing stilts?

This picture shows a house across the street from Peachtree Creek, near downtown Atlanta, Georgia, USA.
Question:  "Why is the front door 10 feet off the ground?"
Answer:  "Because the first floor of the house is 10 feet up."
 

Question:  "Why would someone build a house with the first floor so high?"
Answer:  "They didn't. In 1977 the house was raised 10 feet because Peachtree Creek flooded the first floor in 1975 and 1976."
 

Question:  "Why would someone build a house where it floods?"
Answer:  Well, this is harder to answer. One possibility is that when these houses were built in the late 1940's and early 1950's that Peachtree Creek did not flood as often and as severely as it does now. Studies have shown that as development and the amount of impervious surfaces increases in a watershed, severe flood events happen more often.

 

What a typical flood on Peachtree Creek looks like is shown below in a "before and after" picture from the homeowner's now 10-foot high entryway. The flood picture was taken on May 6, 2003 when stream stage was about 17 feet. Peak stage that day occurred in the evening, when the stream stage reached 17.77 feet with a corresponding instantaneous streamflow of 6,960 cubic feet per second.

 

Impervious surfaces and flooding

If you are not familiar with the term "impervious surface," this picture will help explain it. As cities grow and more development occurs, the natural landscape is replaced by roads, buildings, housing developments, and parking lots. The metro Atlanta region has experienced explosive growth over the last 50 years, and, along with it, large amounts of impervious surfaces have replaced the natural landscape.

Impervious surfaces can have an effect on local streams, both in water quality and streamflow and flooding characteristics. The picture to the right illustrates how water-quality problems can occur from development. Sediment-laden water from a tributary where construction is taking place is shown entering the Chattahoochee River, just west of Atlanta.

Effects of impervious surfaces on streamflow

A significant portion of rainfall in forested watersheds is absorbed into soils (infiltration), is stored as groundwater, and is slowly discharged to streams through seeps and springs. Flooding is less significant in these conditions because some of the runoff during a storm is absorbed into the ground, thus lessening the amount of runoff into a stream during the storm.

As watersheds are urbanized, much of the vegetation is replaced by impervious surfaces, thus reducing the area where infiltration to groundwater can occur. Thus, more stormwater runoff occurs - runoff that must be collected by extensive drainage systems that combine curbs, storm sewers, and ditches to carry stormwater runoff directly to streams. More simply, in a developed watershed, much more water arrives into a stream much more quickly, resulting in an increased likelihood of more frequent and more severe flooding. As this picture of Woodward Way, which runs alongside Peachtree Creek shows, frequent flooding causes problems for residents and also the local government which has to clean up the sand deposited on the road, and also had to install the drainage pipe to move water off the roadway back into Peachtree Creek.

Find out how much water falls during a storm.
Visit our Activity Center and find out.