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Buoyant coastal currents over slopes
Steve Lentz and Karl Helfrich

Figure 1.

Click to enlarge

Figure 2.

Figure 3.
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 frequency f.

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.
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