Understanding Flooding

There are four different types of floodplains—Valley, Major River, Shallow, and Coastal. 

  • Valley:  the ground in this type of area is more “defined,” by creek valleys.  Flooding can be very deep and usually lasts a few days.
  • Major River:  the floodplain along major rivers can be large, deep, and swift, and flooding can last a week or more.
  • Shallow:  these floodplains affect thousands of residences and businesses.  When a channel capacity is exceeded, flooding begins and lasts only a few hours.
  • Coastal:  this flooding occurs when storm events coincide with unusually high tides or a hurricane surge floods low-lying areas. 


Ponding/Overland flooding is caused by intense rainfall where no identifiable floodplain exists.  This occurs when street drainage systems cannot move the storm water to the nearest channel because too much rain fell in a short period of time.  This type of flooding is not restricted to any one area of the U.S. or to any one area of a community.  It can occur almost anywhere.  When intense rainfall exceeds storm sewer or roadside ditch capacity, the water can “pond” in the streets deep enough to flood residences that are not even near a creek or wetland.  The water will seek a path to the channel by flowing overland.


Federal Emergency Management Association (FEMA)’s Flood Map Service Center

The FEMA Flood Map Service Center (MSC) is the official public source for flood hazard information produced in support of the National Flood Insurance Program (NFIP). Use the MSC to find your official flood map, access a range of other flood hazard products, and take advantage of tools for better understanding flood risk by entering your address in the search field.

New flood maps will be going into effect September 29, 2017.


FEMA to Hold Open House July 20 for Review of New Flood Insurance Rate Maps (FIRM)

The Federal Emergency Management Agency (FEMA) has updated the Chester County Flood Insurance Rate Maps to revise floodplain boundaries for the Octoraro, Brandywine, Red Clay and White Clay Creek watersheds.  These changes to the Flood Insurance Rate Maps (FIRM) are final and go into effect September 29, 2017. Stop by the FEMA Open House at the Chester County Government Services Center’s Room 171, 601 Westtown Road, West Chester, 19380, on Thursday, July 20, anytime between 4:30 and 7:30 pm, and FEMA staff will show property owners the revised floodplain boundary mapping, explain what it means, and answer questions. This event is being coordinated by the Chester County Water Resources Authority and the Chester County Planning Commission. Property owners can also review the pending FEMA floodplain mapping posted on ChescoViews by clicking hereFor more information, click here or contact Dani-Ella Betz of the Chester County Water Resources Authority at, or call (610) 344-5400.


Last update:  July 5, 2017