Groundwater response to extreme rainfall event – eastern Sussex County

Unless you have a wet basement, chances are you do not think about what is happening beneath the ground during an intense rainstorm.  DGS scientists are monitoring underground conditions 24 hours a day, 365 days a year, to document what is happening beneath the ground under all types of weather conditions.

On September 29-30, 2016 an extreme rainfall event occurred near Harbeson, Delaware.  A Delaware Environmental Observation System (DEOS) meteorological station (DE-REC) recorded over 13 inches of rainfall.  Just over 4 and a quarter inches of rain fell between 3:00 and 4:00 AM.  The storm caused localized flooding and closed roads (more at…).  The daily intensity is estimated to be just less than a 1 in 500 year event, and the maximum hourly intensity is a nearly 1 in 1000 year event (

The Delaware Geological Survey (DGS) operates a shallow monitoring well equipped with a water level data recorder next to the DEOS station as part of a study that is examining water demands by plants (  The well, open to the Columbia aquifer between 8 and 13 feet below land surface (bls), monitors water table fluctuations.  The water level record shows that the water table rose from about 6 feet bls to about 0.8 feet bls, just over 5 feet in less than a day and a half.

The water table receded almost 2 feet during the following week of dry weather.  During a subsequent storm, October 8 - 9, another 3.6 inches of rainfall was recorded at the DEOS station.  The water table rose nearly 2 feet in response and flooded ground surface for a period of about 8 hours on October 9.  

Flooding by groundwater has been previously observed in eastern Sussex County.  In early March 2010, the water table rose several feet in response to melting of accumulated snow and ice and flooded a number of homes in the area.

Evaluation of weather and groundwater conditions is part of the DGS’s mission to advise the people of Delaware and government agencies about earth and water resources and flooding hazards caused by storms.  Follow the link to Water Resources at our home page to learn more about the DGS water resources data offerings.  Explore the links to Projects and Staff to see stories about the wide variety of topics and activities that DGS works on.

For questions and information, contact DGS at, 302-831-2833