Floodplain / Stormwater Mapping

Introduction

Situated on the Delmarva Peninsula between the Delaware Bay and Chesapeake Bay, Delaware is the lowest state in the United States, with a mean elevation of just 60 feet above sea level.  Vulnerable to flooding from rising sea levels and ocean-fueled tropical storm systems, coastal Delaware is one of only three states located on a peninsula.  Delaware floods originate along the hilly, rocky Piedmont streams in northern New Castle County and from the tidal bay and Atlantic Ocean.

The 100-year Floodplain

More than 331 square miles, or 17 percent of Delaware’s landmass, lie within a mapped 100-year floodplain.  The distribution of floodplains in the three Delaware counties is similar, ranging from 16 percent to 18 percent of the land.

Area of the 100-year Floodplain in Delaware

Road Miles in Floodplain

Approximately 621 road miles are in the 100-year floodplain in Delaware.  New Castle, Kent, and Sussex Counties contain 128, 75, and 418 road miles in the 100-year floodplain, respectively.  Watersheds with the largest mileage of floodplain roads include the Christina River in New Castle County (44 miles), Murderkill in Kent County (16 miles), and Indian River Bay (106 miles).

Structures in Floodplain

More than 18,000 structures exist in the 100-year floodplain in Delaware—2,431 in New Castle County, 1,853 in Kent County, and 13,760 in Sussex County.  Watersheds with the most structures in the 100-year floodplain include the Christina River in New Castle County (1,007 structures), St. Jones River in Kent County (567 structures), and Indian River Bay in Sussex County (3,856 structures).

Flood Discharge

Watersheds with the largest FEMA 100-year-flood flow per drainage area include Shellpot Creek in New Castle County (1,161 cfs/sq mi), Duck Creek in Kent County (327 cfs/sq mi), and Indian River in Sussex County (52 cfs/sq mi).  Some of the largest recorded floods occurred on July 8, 1989 (July 4th storm), September 16, 1999 (Hurricane Floyd), Sept. 15, 2003 (Tropical Storm Henri), and Hurricane Irene (August 28-29, 2011).

Floodplain Mapping

• Using ArcMap GIS, the University of Delaware’s Water Resources Agency prepared interactive floodplain mapping: Coastal Inundation and Riverine Flooding.  The map shows coastal inundation potential from a Category 1 hurricane strike, as well as FEMA’s 100- and 5–year floodplains. The modeled depth of water on the roads of Delaware is also estimated (for illustration only; should not be used for planning purposes).