Structure and Dynamics of Plumes Generated by Small Rivers DOI: http://dx.doi.org/10.5772/intechopen.87843

We presume the following physical interpretation of dynamical features of a small plume described above. The Mzymta River has a rapid flow (1–2 m/s), but is relatively shallow (1–1.5 m) in its mouth. Thus, relatively small volume of fresh water inflows to sea from the Mzymta River mouth at a relatively high speed. This jet is then abruptly decelerated by the vertical friction with the subjacent sea and the initial inertia of the jet decays in vicinity of the river mouth. Thus, according to the reconstructed surface velocity fields, size of the inertia-governed near-field part of the Mzymta plume is relatively small (1–2 km). It is of one order of magnitude less than, first, was reported by in situ measurements for river plumes formed by rivers with similar discharge rates but lower river inflow velocities [24, 38, 59, 60], and, second, theoretical values of near-field part of a plume numerically estimated by formulae described by [12, 54].

The near-field jet abruptly decelerates and forms a sharp pressure gradient in vicinity of the river mouth, which is directed seaward. As a result, anticyclonic recirculation flow directed to the river mouth is hindered by the pressure gradient force. Thus, the large river inflow velocity and low river discharge volume are the limiting factors for formation of an anticyclonic bulge under low wind-forcing conditions. On the other hand, in case of low velocity and/or a large volume of river inflow, it is not abruptly decelerated in vicinity of the river mouth, and strong velocity and pressure gradients are not formed.

Strong nonuniformity of motion patterns of different parts of the far-field plume in response to wind forcing are revealed by the reconstructed surface velocity fields. Upwelling, onshore, and offshore winds induce spreading of the most stratified parts of the plume adjacent to the Mzymta mouth at an angle of up to 80° to the direction of wind forcing. On the other hand, this angle diminishes to 30–40° at the less stratified outer parts of the plume. This effect is presumed to be caused by inhomogeneity of Ekman layer depth due to strong variability of stratification of the Mzymta plume. These results are supported by numerical experiments focused on relation between parameters of Ekman transport and river plume stratification [61].

Dynamical features of the Mzymta plume described above significantly influence its structure, spreading patterns, and the associated transport of suspended and dissolved river-borne constituents. First, freshwater discharge does not accumulate at the small near-field part of the Mzymta plume, which is not the case for large rivers [9, 15, 55]. As a result, freshwater discharge is mainly accumulated at the far-field part of the Mzymta plume. Winds cause spreading of a far-field plume along the direction of Ekman transport till it is limited by a coastline. Thus, location of a restraining coastline defines two stable states of a plume, which are generally indicated by downstream/upstream location of a sharp plume front. First, an alongshore downstream current is formed if spreading of a small plume is limited by a downstream coastline. Second, a small plume is arrested near its estuary if its spreading is restrained by an upstream coastline.

The observed large angles between surface flow and wind-forcing directions at the strongly stratified part of the Mzymta plume causes significantly different wind-govern spreading patterns of a small plume, as compared to large plumes (Figure 3). Upstream spreading of large river plumes is caused by upwelling wind forcing [22, 62, 63], while upstream spreading and accumulation of a small plume was observed only during onshore winds. On the opposite, upwelling wind forcing induced intense offshore spreading of a small plume, while largest cross-shore scales of large plumes were registered during offshore wind-forcing conditions [64, 65]. Downstream spreading of a small plume as an alongshore coastal current during downwelling wind-forcing conditions is similar to spreading patterns observed for large plumes [22, 62, 66–68].
