4.2 Interaction between small river plumes

The Mzymta River has a drought period from late summer to the end of winter and a freshet period in spring and early summer caused by snow melting. All other rivers of the study region are mainly rain-fed and are prone to regular flash floods that provide the majority of their total annual runoff. These flash floods are characterized by sharp rises and falls of discharge due to small sizes (<900 km<sup>2</sup> ) and steep relief of the drainage basins and their high drainage densities (0.85–1.05). It results in quick response of discharge of these rivers on precipitation events, which can significantly increase during several hours after a heavy rain [69].

Under average climatic discharge conditions, plumes formed by small rivers at RCBS are distinctly separated because their spatial scales do not exceed the distances between the river estuaries. However, during rain-induced floods, the areas of the river plumes significantly increase, and the plumes can collide and coalesce with neighboring plumes (Figure 4). As a result, the point-source spread of continental discharge dominated by several large rivers under average climatic conditions can change to the line-source discharge from numerous small rivers situated along the coast in response to heavy rains. We studied interaction between these river plumes using a nested combination of the INMOM [70, 71] and the STRiPE numerical models [25]. The Eulerian model INMOM reproduced general ocean

#### Figure 3.

Schematic of spreading patterns of a small river plume and the related locations of sharp frontal zones of a plume under (a) downwelling, (b) onshore, (c) upwelling, (d) offshore, and (e) low wind-forcing conditions.

#### Figure 4.

Satellite-derived surface TSM distribution at the Russian coast of the Black Sea before (a) and after (b) a heavy rain, illustrating collision and coalescence of multiple small plumes at RCBS in response to rain-induced flooding event.

circulation at the northeastern part of the Black Sea and provided boundary conditions for the Lagrangian model STRiPE, which was used for simulating the dynamics of river plumes.

Numerical experiments showed that short-term rain-induced flooding events significantly influence sediment transport and deposition patterns at RCBS. Under average climatic discharge conditions, the total runoff of fluvial water and terrigenous sediments in the study area is dominated by several largest rivers. Discharge of fresh water and terrigenous sediments from the small rivers is relatively low. As a result, plumes formed by small rivers have small sizes, small water residence time, and their influence on coastal sea is almost negligible. Thus, river discharges affect local water quality and sediment accumulation only near the estuaries of several largest rivers. Heavy rains can induce a rapid and substantial increase in discharge of fresh water and terrigenous sediments from the small rivers of the study region. During these flash flooding periods, areas of small plumes substantially increase, neighboring plumes coalesce, and strips of freshened water masses can be formed along large segments of the seashore.

Numerical modeling revealed that interaction between river plumes significantly influence their structure and dynamics. During flash flooding periods, alongshore strips of freshened and turbid water are formed in the study region and their total length can exceed 200 km. The resulting line-source discharge pattern induces switch in dynamics of river plumes and transport pathways of river-borne suspended and dissolved matters at RCBS. The mixing intensity between the plumes and the adjacent strips of freshened and turbid water is relatively low due to the decrease of salinity gradient. As a result, the river plumes exhibit slower dissipation, have larger spatial scales, and have larger water residence time, as compared to point-source discharge conditions. Moreover, line-source discharge conditions induce alongshore geostrophic currents of turbid and freshened water. These currents induce the intense alongshore transport of river-borne sediments in a northwestern direction. This process strongly affects local water quality and causes active sediment accumulation along large segments of the sea shore at the study region, as compared to point-source discharge conditions.
