Circulation dynamics in the Derwent Estuary

Abstract

For moderate river discharges the Derwent estuary is strongly stratified in its upstream reaches; however, salt walter is flushed completely as far downstream as Bridgewater, for river discharges greater than about 150 m³ s^-1. Salinity returns to normal in this section within about 10-20 days of a rainstorm. The main mixing mechanism appears to be surface stirring by the wind; a semi-empirical formula for wind-driven entrainment velocity gives values a factor of about 1 .5 too small, but this may be due to a number of sources of observational error. Tidal mixing is detectable in the upstream Derwent, but is small compared to wind mixing in terms of induced vertical flows.