Tidal exchange between a coastal lagoon and offshore waters

A field program was performed to determine the exchange of water between Ninigret Pond and Block Island Sound (BIS) at tidal and subtidal frequencies. Time series on sea level variations in BIS and Ninigret Pond, spatially integrated velocities across the breachway connecting the two (obtained by a GEK) and wind speed and direction were collected from April 21–June 3, 1980. A hybrid hydrodynamic model incorporating a simplified one-dimensional approximation for the breachway channel systems and a two-dimensional vertically-averaged finite element solution on triangular grids for the pond proper was used to model the ponds’ response to ocean forcing. Model predictions were in good agreement with the field data. Both indicated a factor of 5.5 reduction in the semidiurnal tidal amplitude, and a high water shift of 2.5 hr, relative to BIS. Model and field data also show that as the frequency of sea level forcing becomes lower the pond sea level response is in phase and of the same magnitude as that in BIS.