CONTROLS ON STRATIFICATION IN THE RHINE ROFI SYSTEM

Stratification in the Rhine ROFI is very variable; the mean water colu mn stability is controlled by the combined effect of tidal, wind and w ave stirring which, at times, brings about complete vertical homogenei ty. Control by the mixing variables has been elucidated by a regressio n analysis of mean stratification on the components of the windstress and significant wave height. Then is strong partial correlation with a ll three variables which explains between 56% and 65% of the variance in two time series of observations in October 1990 and September 1992, respectively. During periods of low stirring the water column was obs erved to re-stratify over the whole inshore region through the relaxat ion of the horizontal gradients under gravity and with the influence o f rotation. Superimposed on the mean stratification there is strong se mi-diurnal variation, occurring throughout the stratified region at ti mes of reduced mixing. The amplitude of this semi-diurnal variation is of the same order as the mean stability and frequently results in con ditions being mixed or nearly mixed once per tide. This semi-diurnal v ariation results primarily from cross-shore tidal straining which inte racts with the main density gradient to induce stratification. The hyp othesis that water column stability is controlled by the combination o f these processes has been tested using a reduced physics model which has been successful in reproducing the main features of both the mean and semi-diurnal components of stratification.