Tidal Boundary Conditions in SEAWAT

Ground Water
By: , and 

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Abstract

SEAWAT, a U.S. Geological Survey groundwater flow and transport code, is increasingly used to model the effects of tidal motion on coastal aquifers. Different options are available to simulate tidal boundaries but no guidelines exist nor have comparisons been made to identify the most effective approach. We test seven methods to simulate a sloping beach and a tidal flat. The ocean is represented in one of the three ways: directly using a high hydraulic conductivity (high-K) zone and indirect simulation via specified head boundaries using either the General Head Boundary (GHB) or the new Periodic Boundary Condition (PBC) package. All beach models simulate similar water fluxes across the upland boundary and across the sediment-water interface although the ratio of intertidal to subtidal flow is different at low tide. Simulating a seepage face results in larger intertidal fluxes and influences near-shore heads and salinity. Major differences in flow occur in the tidal flat simulations. Because SEAWAT does not simulate unsaturated flow the water table only rises via flow through the saturated zone. This results in delayed propagation of the rising tidal signal inland. Inundation of the tidal flat is delayed as is flow into the aquifer across the flat. This is severe in the high-K and PBC models but mild in the GHB models. Results indicate that any of the tidal boundary options are fine if the ocean-aquifer interface is steep. However, as the slope of that interface decreases, the high-K and PBC approaches perform poorly and the GHB boundary is preferable.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Tidal Boundary Conditions in SEAWAT
Series title Ground Water
DOI 10.1111/j.1745-6584.2010.00788.x
Volume 49
Issue 6
Year Published 2011
Language English
Publisher National Ground Water Association
Publisher location Westerville, OH
Contributing office(s) Office of Groundwater
Description 14 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Ground Water
First page 866
Last page 879