The Coastal Ground‐Water Boundary

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In many island and coastal ground‐water problems, the position of the ground‐water/free sea‐water interface is an essential, but highly dynamic boundary. The fluctuating boundary commonly is simplified to a fixed position in problem formulation. This fixed position is frequently taken as the location of local mean free‐water level (mean sea level) in the adjacent salt‐water body. However, tidal fluctuation and wave runup on a sloping beach cause mounding of sea water in the upper beach with a consequent effective mean sea level as much as two feet higher than local mean sea level. This phenomenon is particularly important in small islands and coastal situations where the ground‐water flow pattern is highly dependent on the position of the effective boundary both vertically and horizontally. In coastal hydrological situations, a good approximation for the effective vertical position, i.e. effective mean sea level, of the ground‐water/free sea‐water boundary is the mean equivalent salt‐water head in a deep discretely screened piezometer located in the beach near the upper swash line. The veracity of this technique is demonstrated by the results of field research in a New England barrier beach. Copyright © 1989, Wiley Blackwell. All rights reserved

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