# 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, **h _{x}**, at distance

**x**from the outcrop (bank or shore) is given by (Ferris et al. 1962; Todd 1980):

where **h _{0}** is the amplitude of the tide,

**S**is aquifer storativity,

**T**is transmissivity, and

**t**is the tidal period.

_{0}An expression for the time lag, **t _{L}**, of a given peak or trough after it occurs in the tidal body of water is as follows

When t_{L}, x and t_{0} are known from observed data, we can rearrange the previous equation to estimate the aquifer diffusivity:

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

** See also:** Aquifer Testing 101