Hydrodynamic timescales in a hyper-tidal region of freshwater influence

• A modelling study is conducted to investigate transport timescales in Liverpool Bay.
• Flushing time and residence time are calculated.
• The mean age distribution is presented.
• Idealised models are used to explain temporal variability in age.
• A method to approximate age from salinity is presented.

This study uses a three-dimensional hydrodynamic model to investigate transport timescales in Liverpool Bay, a shallow hyper-tidal Region of Freshwater Influence (ROFI) with a density-driven baroclinic residual circulation. Flushing time, residence time and age are evaluated, providing rigorously defined parameters to describe the rate of offshore freshwater transport and basin replenishment. Additional challenges encountered when assessing these timescales in a tidally mixed regime are highlighted by idealised models.Climatological river gauge data reveals that the numerous local rivers contribute an average of 203 m3  s−1 of freshwater to Liverpool Bay. Based upon the mean salinity distribution, this would suggest a flushing time of approximately 136 days. The mean residence time of the region is approximately 103 days although small concentrations of water are retained over several years due to vigorous tidal mixing.Age in the region is highly variable with regular oscillations caused by tidal advection, whilst long term fluctuations are governed by river flow rates. The mean age gradient is directed offshore, approximately parallel to both the salinity gradient and the major axis of the tidal ellipse, with basin wide average magnitude of 6 days km−1. It is shown that salinity may be used to estimate the age of freshwater, which is not directly observable in practice.