Turbulent dissipation in the Rhine ROFI forced by tidal flow and wind stress

Abstract

The Rhine ROFI (Region Of Freshwater Influence), which extends northwards along the Dutch coast from the estuary source, exhibits strong cross-shore salinity gradients which interact with straining by the tidal flow and stirring, by tides and wind stress, to produce a semi-diurnal cycle of stratification. Here we report new time-series observations of the rate of turbulent kinetic energy dissipation ε from the FLY Profiler, which were made in parallel with measurements of density structure and horizontal flow using moored instruments and bottom-mounted Acoustic Doppler Current Profilers. Under light wind conditions, ε in the lower half of the water column follows a predominantly M 4 cycle although with higher values during north–eastward flow associated with the residual flow in that direction. Following the strong peak in dissipation near the bed, enhanced dissipation is apparent extending up the water column. The time delay of the maximum in ε increases with height above the bed and peak values increase up to the highest level, 15 mab (metres above bed), at which measurements were possible. At this level, ε shows a predominantly M 2 variation with the maximum occurring at a time when vertical stability is negligible and the effect of tidal straining is to create instability in the water column as the cross-shore shear forces higher salinity water over fresher. The inference is that potential energy released by straining is responsible for convective motions and, hence, the enhanced turbulent activity observed. Under conditions of substantial wind stress (τ w ∼0.2 N m −2), turbulent activity in the upper layers is considerably increased with values up to 0.1 W m −3 at 15 mab which is enough to obscure the influence of convection. Near the bed, dissipation is still dominated by the M 4 cycle with asymmetry due to the residual flow. The cycle of dissipation in the Rhine ROFI is compared with that previously reported for Liverpool Bay. It is suggested that differences between the two cases may be accounted for in terms of the different relative phasing of tidal straining and stirring which result from the different forms of the tidal wave, i.e. a mainly progressive, Kelvin, wave in the Rhine system as opposed to a standing wave in the Liverpool Bay system.