Estimating dispersion and flushing time-scales in a coastal zone: Application to the Plymouth area

Salinity measurements and modelled salinity and solute-tracer distributions are presented that are used to compute flushing time-scales for fresh water and for point-source tracer solute in the Plymouth coastal zone. A depth-averaged hydrodynamic model is used to compute tidal currents, and salinity is simulated in order to simultaneously provide depth-averaged density-driven currents. Flushing time-scales are initially computed for fresh water in the Plymouth Sound using measured salinity data (2.5 days) and are subsequently also estimated using the model (2.0 days). Model results illustrate the complexity of simulated point-source tracer distributions, which is largely a consequence of the spatial complexity of tidal and density-driven current patterns. Flushing time-scales for tracer and fresh water depend on location and the sea sub-region considered. Freshwater flushing time-scales, simulated for constant neap tide, mean tide and spring tide conditions and subject to a constant, annual-mean rate of freshwater input, vary from 6.5 to 13.5 days and are larger with increasing volumes of sea sub-region. Greater quantities of fresh waters are retained during neap tides and flushing time-scales are longer than during spring or mean tides. Solute tracer flushing time-scales, which are proportional to retained mass of solute and vary from 3 to 6.5 days when salinity (density-driven flow) is incorporated, and 2.5 to >15 days when it is not, also show longer and shorter flushing time-scales during neap and spring tides, respectively. The influence of salinity is dependent on point-source location.

► We model point-source tracer and salinity distributions in the Plymouth coastal zone.
► Freshwater flushing times vary from 6.5 to 13.5 days for the various sub-regions.
► At neap tides, freshwater flushing times can be twice as long as those at springs.
► Solute tracer flushing time-scales vary from 3 to 6.5 days when salinity is included.