Nj. Grigg et Gn. Ivey, A LABORATORY INVESTIGATION INTO SHEAR-GENERATED MIXING IN A SALT WEDGE ESTUARY, Geophysical and astrophysical fluid dynamics, 85(1-2), 1997, pp. 65-95
The combination of strong density stratification and energetic forcing
from tide and mean hows leads to complex circulation and exchange pro
cesses in estuaries. The present study was motivated by field results
in the Valdivia Estuary, Chile, and others where a dramatic difference
in salinity stratification was observed between low and high tides, b
elieved to be attributable to the interacting roles of tidal forcing,
topography and velocity shear effects. We describe results from a labo
ratory experiment which models an important aspect of the dynamics in
such estuaries: the role of a changing cross-section on the stratified
shear-generated internal mixing. Our experiments were conducted in a
long, laboratory tank fitted with a contracting central section. Fresh
water ran from one end of the tank, overflowing al an adjustable weir
al the other end. A slower salt water flow was introduced at the base
of the weir, and intruded upstream as a gravity current. The increase
d velocity shear which occurred as the salt water flow moved into the
contraction caused significant overturn events at the salt/freshwater
interface, so changing the density field in a manner similar to the fi
eld observations. Under certain conditions, this mixing was sufficient
to arrest the wedge in the contraction, so allowing measurements of t
he mean density and velocity fields, as well as extensive turbulent mi
crostructure records. This enabled us to describe the intensity and sp
atial variation of the turbulent mixing between the saline underflow a
nd the fresh water overflow for a saline intrusion under the influence
of topographic effects.