Buoyant coastal currents over slopes
Steve Lentz and Karl Helfrich
Discharge on to the continental shelf of relatively fresh, buoyant
water from rivers and estuaries is a common feature of coastal
regions. The fate of this buoyant fluid is of crucial importance
because these currents transport constituents such as sediment,
marine organisms, nutrients, and chemical pollutants. As this
fresh water enters the coastal ocean, a fraction of the transport
will be directed by the earth's rotation (the Coriolis acceleration)
along the coast (see Figure 1) to form surface-trapped buoyancy
current that propogates with the coastline on its right-hand
side (in the northern hemisphere).
Most existing theories and numerical
modeling work have been based on rotating buoyancy currents
against a vertical wall. But as the figure shows, the continental
shelf is not a vertical wall and the continental slope can be
expected to influence the structure (width, depth and velocity
fields) and the speed of the nose of the gravity current.
To address deficiency, we have developed a scaling theory for
a rotating buoyancy current over a sloping bottom. The theory
gives the structure and nose speed of the current as a function
of the external parameters, the freshwater volume transport
Q, the bottom slope ALPHA, the density difference between the
fresh and saltier ambient water DELTA-RHO, and the Coriolis
The scaling theory was tested against laboratory experiments
on a 2-meter diameter rotating table (see Figure 2) in which
these external parameters could be accurately varied.
Figure 3 shows a top-view image of an experimental buoyant current
(dyed dark for visualization) from which nose speed and plume
width could be measured.
The laboratory measurements were in very good agreement with
the predictions of the scaling theory and our next steps are
to try to apply these results to oceanic observations of buoyant
plumes and broaden the laboratory work with further experiments
and numerical model calculations of the phenomena.