Tidal Effects

by Glenn M. Duffield, President, HydroSOLVE, Inc.

Confined Aquifer

Assuming one-dimensional flow in a nonleaky confined aquifer which directly abuts a tidal body of water, the amplitude of groundwater fluctuation, hx, at distance x from the outcrop (bank or shore) is given by (Ferris et al. 1962; Todd 1980):

Tidal Equation 1

where h0 is the amplitude of the tide, S is aquifer storativity, T is transmissivity, and t0 is the tidal period.

An expression for the time lag, tL, of a given peak or trough after it occurs in the tidal body of water is as follows

Tidal Equation 2

When tL, x and t0 are known from observed data, we can rearrange the previous equation to estimate the aquifer diffusivity:

Tidal Equation 3

Typically, an estimate of S is used to determine T from the above equation.

See also: Aquifer Testing 101