5. Conclusions

In this chapter, we described specific features of structure and dynamics of small river plumes, which are significantly different from those typical for large plumes. Small river plumes are river plumes with small residence time of freshened water (hours and days), which mixing with ambient sea limitedly influences its salinity. Small plumes have sharp salinity gradients at their boundaries with ambient sea, which hinders vertical energy transfer between a small plume and subjacent sea. As a result, small plumes are mainly wind driven, while the role of circulation of ambient sea in their dynamics is negligible.

Small plumes are characterized by energetic temporal variability and inhomogeneous horizontal structure. Position, shape, and area of a small plume can significantly change during several hours in response to variability of wind forcing and river discharge rate. Small plumes have very small near-field part and do not form a recirculating bulge adjacent to river mouths due to efficient deceleration of inflowing river runoffs and quick decay of their initial inertia. The wind-induced Ekman transport within a small plume occurs at a wide range of angles to the wind direction during upwelling, onshore, and offshore wind-forcing periods with the largest values in the near-field part of a plume. Interaction between neighboring small plumes can strongly influence their structure and dynamics. Collision and coalescence of multiple small plumes in response to rain-induced flooding events results in the decrease of mixing intensity within small plumes and intensification of their alongshore spreading.

Finally, high-frequency internal waves are generated in certain small plumes formed by rivers with high flow velocity. Such a river inflows to coastal sea, abruptly decelerates, and forms a hydraulic jump in vicinity of a river mouth. Formation and relaxation of a hydraulic jump induces generation of high-frequency internal waves that propagate offshore. These internal waves strongly influence turbulence and mixing at a stratified layer between a buoyant river plume and subjacent ambient sea.
