Impervious Surface

An impervious surface is a hard area that doesn’t allow water to seep into the ground. Instead, the water runs off the impervious surface, picking up many types of pollution in the process, and then flows into a storm drain or a nearby body of water.

How Impervious Areas Affect the Environment


Large amounts of impervious surfaces change the water cycle. The graphic below shows how impervious surfaces change the way rain moves through the water cycle. This and more information is available from the Federal Interagency Stream Corridor Restoration Working Group.
In a place with little impervious area, such as a forest, rain water is able to seep into the ground. It will then travel slowly through the ground, where it is filtered by natural processes, before reaching streams and creeks. This process helps keep the amount of water in streams and creeks from changing too much or too quickly. Research has shown that as the amount of impervious surface in an area increases, the amount of polluted runoff also increases. Since water cannot seep into the ground in areas with large amounts of impervious surface, more than 5 times as much water can quickly run off the land into nearby water bodies.
Illustration of Impervious Surfaces

Examples


An impervious surface is a hard surface that does not let water soak into the ground or greatly reduces the amount of water that soaks into the ground. Examples include:
  • roofs
  • solid decks
  • patios
  • sidewalks
  • driveways
  • parking areas
  • roads
  • compacted gravel
Impervious surfaces are the single most important factor affecting the amount of water flowing off a property, how quickly that water will flow, and the amount of pollution that will be picked up and carried with that water.

Impact of Added Water


Since more stormwater is quickly reaching streams and rivers, there is an increased risk of floods occurring. The large amount of water also leads to streams flowing faster. This causes erosion and changes the shape of the stream. Both of these can be a serious problem as they lead to worse flooding and serious damage to wildlife habitats. Another problem associated with these changes is that there is less moisture in the ground because impervious surfaces do not allow water to filter into the soil. This means that plants (such as grass in lawns and ornamental plants) may die or need extra water from irrigation to survive.

Studies have shown that local water bodies are less healthy when as little as 10% of an area is covered in impervious surfaces. If more than 30% of an area is covered in impervious surfaces this can severely damage nearby streams, rivers, and lakes.

Law Revision


A recent change to state law redefines “built-upon area” so that it does not include gravel, the water area of a swimming pool, or wooden slatted decks. Note: Scientists generally agree that compacted gravel is an impervious surface. This means water does not travel down through the gravel material into the ground.

The revised law sets a new state minimum standard for built-upon area. As in most parts of environmental regulation, cities and towns may pass local laws with higher standards than the state minimum. The section of the state laws changed by this provision clearly allows cities and towns to exceed state minimum standards to implement stormwater programs (G.S. §143-214.7(d)).

Calculating Stormwater Measures


Cities and towns generally study whether a surface is impervious when calculating needed stormwater measures for new development and redevelopment projects. Classifying gravel as impervious generally results in additional stormwater control devices for a property. This classification is a higher standard than the new state minimum created by this law. A higher standard such as this is allowed for a local program. This means that any existing municipal ordinances that label gravel as impervious do not conflict with the new state law. Therefore, they do not need to be changed.

Another context in which cities and towns consider whether gravel is impervious is when a city or town calculates the stormwater fee for a property. The calculation is generally based on the amount of impervious surface on the property. The new law applies only to the implementation of state stormwater quality programs authorized by G.S. § 143-214.7. Therefore, it does not affect local stormwater programs. Local stormwater programs are authorized under a different statutory section (G.S. § 160A-314). This means that a city or town with a stormwater fee that counts gravel as impervious surface does not need to change that practice.