Flooding Guidance

Flooding problems range from a mild nuisance to a serious threat to property and safety.

Why Does Flooding Occur?

When it rains, a stormwater system of inlets, storm drains, pipes, and ditches carries runoff to local water bodies.  Stormwater is not carried to a wastewater treatment plant.
 
As drainage areas are developed, the amount and flowrate of stormwater increases, sometimes beyond the capacity of the stormwater system and the receiving stream to carry it.
 
Stormwater systems are usually designed to carry the largest storm that would be expected over 10 or 25 years based on historical rainfall data. However, with the current frequency of Hurricanes and Tropical Storms, the historical data no longer accurately reflects the size of storm events going forward.
Source:  US Environmental Protection Agency
 

Natural areas typically allow 90% or more of runoff to infiltrate into the ground.  The roads, parking lots and roofs associated with development prevent infiltration.  In a highly developed area, typically only about 50% of runoff infiltrates. 

Sometimes the stormwater system needs time to "catch up" with the storm.  As an example, in Raleigh, the 10-year storm is reached if 1.25 inches of rain falls in 15 minutes.  If that storm intensity continues, the stormwater system will back up just as it would in a sink if the faucet exceeds the sink drain’s capacity.  However, the ponded water will drain as soon as the storm intensity diminishes and the system catches up.
 
Other potential causes of flooding are:
  • Obstacles clogging the stormwater system or the downstream drainageways.
  • High groundwater tables that cause ponding in low areas.

Who Should I Contact for Help with Flooding Issues?

Stormwater systems are maintained by a local government, NCDOT, private citizens, businesses, or Homeowners Associations, depending on where they are located.
  • For flooding on your property, contact your local government.  Questions your local government will likely ask you include:
    • Does flooding occur during abnormally heavy rains or almost every time it rains?
    • Has something upstream of your property changed the drainage or runoff pattern?
    • Have you maintained the stormwater system on your property?
  • For flooding on state-owned streets and rights-of-way, contact the N.C. Department of Transportation State Road Maintenance Division at (919) 733-2191.  NCDOT's State Road Map Viewer indicates the locations of state-owned roads.  If you are unsure whether a public street is state-owned, your local government can assist you.
  • For flooding adjacent to streams and other waters, contact your Regional Floodplain Manager.  You can find out if you live in a floodplain at the Federal Emergency Management Agency's web site Knowing Your Flood Risk.  If your property is within a floodplain, this was disclosed when you purchased the property.

DEQ's Role in Stormwater Management

DEQ's role in stormwater management is to reduce the pollutants that stormwater runoff can pick up as it flows over developed areas, roads, industrial sites, and landscaped areas. 
  • DEQ has statutory authority to manage the state’s water resources to maintain, protect and enhance water quality [N.C.G.S. Article 21, Chapter 143].  
  • DEQ does not have statutory authority to regulate or remedy flooding issues.

Overview of DEQ's Post-Construction Program

DEQ's Post-Construction Program requires that new development projects located in subject areas (see the Post-Construction Stormwater Map) obtain either a high or low density stormwater permit:
  • Low density projects must limit the amount and concentration of development to reduce pollution and to allow stormwater to naturally pond and infiltrate into the soil.
  • High density projects must collect stormwater runoff and use engineered stormwater control measures (SCMs) such as infiltration systems, wet ponds, wetlands, bioretention cells, etc. to remove pollutants before the runoff leaves the site.

The requirements for low and high density projects are designed specifically for water quality.  SCMs in high density development often capture and treat the “first flush” of runof from smaller storm events, often the 1" or 1.5" storm.  These smaller storms carry most of the pollutants, thus the name "first flush."  SCMs often bypass runoff from larger storms to maintain treatment capacity, although SCMs can be designed to achieve both state water quality and local government flooding requirements at the same time. 

Post-construction stormwater permits are issued by either DEQ or a local government entity. If you have a water quality complaint involving a development project, you can determine the appropriate permitting authority to contact by using the Post-Construction Stormwater Map. 

How to Minimize Flooding

A few tips on how to prevent and minimize flooding:

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