The Rhine outflow: A prototypical pulsed discharge plume in a high energy shallow sea

Over the past years, a wealth of data has been collected in the Rhine ROFI (Region Of Freshwater Influence) where freshwater from the Rhine is transported and dispersed along the Netherlands coast. The Rhine ROFI may be considered as a prototypical one, its spreading being relatively uncomplicated by coastal and bathymetric features. The discharge of the Rhine, and comparable rivers like the Connecticut, is heavily modulated by the tidal wave propagating into the estuary. As a result, this river water exits into the coastal zone as successive low salinity pulses. The processes and parameters involved in the development of such pulsed river plumes are qualitatively examined. The main criteria that lead to a pulsed discharge are that the net discharge is halted at some stage during the tidal cycle and that a pulse is sufficiently far removed from the river mouth after one tidal cycle so as not to interfere with the development of the successive pulses. From this we have derived necessary conditions for the formation of a pulsed discharge, namely (1) the maximal tidal current amplitude exceeds the river discharge velocity and (2) the inertial radius is larger than the river mouth width. Within this parameter regime rivers can thus be classified as falling within the pulsed plume regime or not. Mixing processes, particularly by winds and tides, are important at all times. Without wind and around neap tide (low mixing rates) the ''life expectancy'' of a typical Rhine pulse is of the order of a week, an upper bound. This order decreases quickly with increasing winds. However, then also velocities in the long shore ''Dutch Coastal Current'' go up. The observations show that under such extreme conditions a train of fresh water lenses can be transported along the Dutch coast. (C) 1997 Elsevier Science B.V.