Flushing of embayments

This thesis examines the flow of sea water in an embayment or coastal bay. The embayment is a relatively quiescent environment, separated from the external flow by a mixing or shear layer. The external flow is tidal and therefore flows back and forth across the embayment mouth. The exchange between the embayment and external flow is an important mechanism for the transport and dispersion of pollutants, and also substances such as nutrients and sediments. Entrapment, where pollutants are retained in the embayment for some time, is of particular interest. Understanding the flow patterns in and around an embayment may enable the prediction of substance transport and dispersion in its locality. The flow in an embayment was investigated in three ways. Fieldwork was undertaken in a coastal bay at Blackpool Sands (Devon, UK) in which measurements of the flow velocities were taken using an acoustic Doppler current profiler. Laboratory experiments were designed and conducted in a bi-directional flume tank where the effects of varying embayment and tidal parameters were investigated. Finally, a numerical streamfunction-vorticity model with a second-order advection scheme was constructed to attempt to replicate the laboratory experiments. Measurements of the flow velocities in Blackpool Sands embayment indicated the presence of a gyre caused by the tidal streams which separate at the upstream headland. This gyre was present on both the ebb and flood tides and was found to be approximately two-dimensional. Laboratory experiments demonstrated the life-cycle of a gyre in a rectangular embayment with an oscillating freestream flow across the embayment mouth. The gyre was observed to be swept out of the bay when the tide changed direction and a dipole was formed as the gyre was swept past the downstream headland, where the flow separated. The numerical simulation of the laboratory experiments showed good agreement with the laboratory experiment and fieldwork measurements.EThOS - Electronic Theses Online ServiceGBUnited Kingdo