Skip to main content
U.S. flag

An official website of the United States government

Determining the mean hydraulic gradient of ground water affected by tidal fluctuations

January 1, 1991

Tidal fluctuations in surface-water bodies produce progressive pressure waves in adjacent aquifers. As these pressure waves propagate inland, ground-water levels and hydraulic gradients continuously fluctuate, creating a situation where a single set of water-level measurements cannot be used to accurately characterize ground-water flow. For example, a time series of water levels measured in a confined aquifer in Atlantic City, New Jersey, showed that the hydraulic gradient ranged from .01 to .001 with a 22-degree change in direction during a tidal day of approximately 25 hours. At any point where ground water tidally fluctuates, the magnitude and direction of the hydraulic gradient fluctuates about the mean or regional hydraulic gradient. The net effect of these fluctuations on ground-water flow can be determined using the mean hydraulic gradient, which can be calculated by comparing mean ground- and surface-water elevations. Filtering methods traditionally used to determine daily mean sea level can be similarly applied to ground water to determine mean levels. Method (1) uses 71 consecutive hourly water-level observations to accurately determine the mean level. Method (2) approximates the mean level using only 25 consecutive hourly observations; however, there is a small error associated with this method.

Publication Year 1991
Title Determining the mean hydraulic gradient of ground water affected by tidal fluctuations
DOI 10.1111/j.1745-6584.1991.tb00546.x
Authors Michael E. Serfes
Publication Type Article
Publication Subtype Journal Article
Series Title Ground Water
Index ID 70016699
Record Source USGS Publications Warehouse