Laboratory experiments simulating a coastal river inflow

The dynamics of buoyant water entering a rotating basin are studied using a series of laboratory experiments designed to elucidate the alongshore transport mechanisms in river plumes. Inflowing water, which is discharged perpendicular to the tank wall, is observed to form a growing anticyclonic bulge and a coastal current downstream of the bulge. Detailed simultaneous measurements of the velocity and buoyancy fields in the plume confirm that the bulge momentum is in a gradient–wind balance and the coastal current is geostrophic. The growth of the bulge and accumulation of fluid within it coincides with a reduction in coastal current transport to approximately 50% of the inflow discharge. The bulge is characterized by a depth scale, h, which is proportional to the geostrophic depth, hg , and two time-dependent horizontal length scales, yc, the displacement of the bulge centre from the wall, and rb, the effective radius of the bulge. These two length scales are proportional to the inertial radius, Li , and the local Rossby radius, Lb, respectively. When rb yc, the bulge is held tightly to the wall, and a relatively large fraction of the inflow discharge is forced into the coastal current. For plumes with yc approaching rb, the bulge is further from the wall, and the coastal current flux is reduced. Once yc/rb > 0.7, the bulge separates from the wall causing flow into the coastal current to cease and the bulge to become unstable. In this state, the bulge periodically detaches from and re-attaches to the wall, resulting in pulsing transport in the coastal current. Scaling of the bulge growth based on hg , Li and Lb predicts that it will increase as Ro , where Ro is the inflow Rossby number. The bulge growth, inferred from direct measurements of the coastal current transport, is proportional to Ro and agrees with the predicted dependence within the experimental error.