Structure and Dynamics of Plumes Generated by Small Rivers

Abstract

The total share of small rivers in the influxes of fluvial water and suspended matter to the world ocean is estimated at between 25 and 40%. On a regional scale, this contribution can be even more significant for many coastal regions. In this chapter, we show that dynamics of small river plumes is significantly different from that of plumes generated by large rivers. Spatial structure of small plumes is generally characterized by sharper horizontal and vertical gradients. As a result, small plumes exhibit more energetic temporal variability in response to external forcing. In this chapter, we address several dynamical features typical for small plumes. We describe and discuss the response of small plumes to wind forcing and river discharge variability, the interaction between neighboring small plumes, and the generation of high-frequency internal waves in coastal ocean by small rivers. We also substantiate the Lagrangian approach to numerical modeling of small river plumes.

Spatial Structure, Short-temporal Variability, and Dynamical Features of Small River Plumes as Observed by Aerial Drones: Case Study of the Kodor and Bzyp River Plumes

“…However, small rivers play an important role in global land-ocean fluxes of fluvial water and suspended and dissolved sediments [54][55][56]. Small rivers form buoyant plumes that have small spatial scales and, therefore, small residence time of freshened water, which is equal to hours and days, due to relatively low volume of river discharge and its intense mixing with ambient sea [57]. Dissipation of freshened water as a result of mixing of a small plume with subjacent saline sea limitedly influences ambient sea and does not result in accumulation of freshwater in adjacent sea area.…”

“…This feature is not typical for large river plumes and results in significant differences in spreading and mixing between small plumes and large plumes. Sharp vertical density gradient at the bottom boundary of a small plume hinders vertical energy transfer between a small river plume and subjacent sea [57]. This feature strongly affects spreading dynamics of a small plume due to the following reasons.…”

“…Study of structure and variability of small river plumes at small spatial and temporal scales is essential for understanding the fate of freshwater discharge from small rivers to sea and the related transport of suspended and dissolved river-borne constituents. However, high short-temporal variability of small plumes and their small vertical sizes inhibit precise in situ measurements of their thermohaline and dynamical characteristics [57]. Satellite remote sensing also does not provide the necessary spatial resolution and temporal coverage for small river plumes.…”

Calculating lateral plume spreading with surface Lagrangian drifters

“…Numerical models have proven useful for the estimation of plume spreading rates and the understanding of plume structure (Garvine 1984;O'Donnell 1990;Xing and Davies 1999;Mestres et al 2003;Hetland and MacDonald 2008). A number of observational studies examining the structure of river plumes have been conducted but few of them focus explicitly on the identification for the mechanisms governing lateral plume spreading (Wright and Coleman 1971;MacDonald and Geyer 2004;MacDonald et al 2007;Chen et al 2009;McCabe et al 2009;Osadchiev and Zavialov 2019) or presenting techniques for calculating plume expansion. Most of the previous observational studies on lateral plume spreading have been focused explicitly in the plume's near-field, with extremely limited Fig.…”

“…One such dynamic process is the complicated and poorly understood phenomenon of lateral spreading in river plumes, which can be an important consideration for major oceanographic and environmental engineering applications concerning coastal pollutant transport, oil spills, sediment loads, and so on. The discharge of brackish water from estuaries typically forms distinct coastal plumes, often visible through their color signature due to sediment load and particulate matter, changes in sea surface roughness, and clearly defined frontal boundaries (Pritchard and Huntley 2006;Osadchiev and Zavialov 2019). Buoyant surface discharges can exhibit multiple complex physical processes, which range temporally and spatially from jet mixing (near-and mid-field) with passive ambient diffusion in the far field (Jones et al 2007).…”