A modeling study of the circulation of East Lagoon, Galveston, Texas

Abstract

A three-dimensional time-dependent primitive equation model has been constructed to investigate the processes that control the circulation in East Lagoon, a narrow, shallow lagoon on Galveston Island, Texas. The primarily tidal circulation in East Lagoon is affected by wind mixing and density stratification. A previous modeling study of East Lagoon (Show, 1977) suggested that tidal circulation in the lagoon is further altered by the presence of an ore dike which separates the shallow seaward channel from the deeper landward basin. The simulated distributions obtained with the circulation model indicate that the flow in East Lagoon is primarily that of a flat-bottomed, tidally forced canal. Inclusion of realistic bathymetry results in maximum flow in the shallow seaward channel, and minimal flow in the deep landward basin. The addition of wind stress results in acceleration of the surface flow in the windward direction and return flow near the bottom, with upwelling on the windward side and downwelling on the leeward side of the lagoon. Initial stratification with tidal mixing creates density advection instabilities which, with the addition of seaward wind stress, mix both the shallow channel and the deep basin. Contrary to Show's (1977) study, present results indicate that the ore dike has a negligible effect on the circulation. Residence time of the waters in the deep basin of East Lagoon, as estimated from the volume transport, is approximately 31 h. The simulated circulation patterns were also used in conjunction with a Lagrangian model to examine the transport patterns and transit times of particles released throughout the lagoon. Results from the particle tracing model show that particles initially in the deep landward basin of East Lagoon are not flushed out of the basin during one 12.4 h tidal cycle; whereas, particles released in the channel are, with few exceptions, flushed from the lagoon in one tidal cycle. It is only with the addition of an along-channel wind stress that particles are removed from the deep basin. Allowing the particle to migrate with a diel periodicity such that it is near the surface during flood tide and at the bottom during ebb tide reduces the loss of particles from the deep basin.