MIXING AT THE FRONT OF GRAVITY CURRENTS

Observations made with the help of a movable-bed tank designed and ope rated to freeze the motion of gravity current fronts indicate that bot h the scale of the apparatus and the magnitude of the current Reynolds number have a strong influence on mixing rates. Low mixing rates are associated with fronts in relatively shallow fresh water depths and lo w Reynolds numbers. Mixing rates increase more rapidly as the ratio of current thickness to fresh water depth increases. For the deepest fre sh water flow conditions, when scale effects can be expected to be neg ligible, the dimensionless mixing rate is approximately equal to three times the relative current thickness and has an upper limit of about 0.3. Lower mixing rates are observed as the current Reynolds number de creases, suggesting that viscous effects can still exist even for deep fresh water conditions. Application of the experimental results to es timate some parameters of a hypothetical gravity current on the contin ental shelf yield reasonable values. However, it is clear that, owing to the scale and Reynolds number effects that might be present in labo ratory experiments, particular care should be exercised when trying to extrapolate results to gravity currents in nature.