Transport and dispersion of pollutants, nutrients, tracers in mixed nearshore water

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The mixing-induced transport that occurs in nearshore waters is a fundamental mechanism for all physical/biological/chemical phenomena that occur in the marine environment. The transport and dispersion of pollutants, nutrients and tracers in mixed nearshore water can have direct and indirect consequences for the marine ecosystems.


The release of pollutants from coastal and atmospheric sources heavily influences our lives in many different ways, from direct contamination of edible organisms to more indirect processes of impact and endangerment of the nearshore ecosystems. Such influences are felt not only in proximity to the source, but also in wider regions, determined primarily by the direct physical transport in marine waters. [1] High concentration zones of pollutants have been found along coastal regions far from the coastal source, and noticeable impacts have been detected even in deep sea regions.

Water mixing, sediment and pollutant transport

It is also important to acknowledge the close link between water mixing, sediment and pollutant transport. [2] For example, if the morphological evolution of the nearshore bottom is forced by wave-induced phenomena, including mixing, then the bondage between pollutants (e.g. trace metals, among which mercury, major elements, various organic and inorganic chemicals, etc.) and sediments (e.g. coastal inlets run-off of polluted sands, disposal of dredged materials, etc.) closes a circle in which a feedback is established between hydro- and morpho-dynamics, the pollutants only playing a passive role. The spatial extension of contaminated sediments and the pollutants concentration/toxicity/bioaccumulation strongly depend on the type of injection into the marine environment: while dumping sites are, usually, well protected from the hydrodynamics actions by means of a specific coverage (e.g. Ocean Dumping Act, 1972) this is not the case for the widespread run-off of water-sand mixtures from coastal inlets, the latter being largely influenced by transport phenomena. [3] [4]

Many ecological issues strongly depend on the action of large-scale eddies. For example, these are responsible for the complex but predictable patterns of dispersion of phytoplankton [5] and a number of marine species during their larval stage. Hence, the recent and growing attention to Lagrangian transport of marine species. [6] [7] It is particularly interesting that increasing acknowledgement is given of the fundamental action of complex topographic features on the flow mixing and the related transport mechanisms of organic matter. Moving fronts are found to transport plankton communities in the cross-shore direction and cause differentiation between coastal and oceanic communities. [8]

Rip currents

Recreational activities are also strongly influenced by water transport phenomena. More specifically swimming safety is seriously hampered by the horizontal water mixing induced by rip currents. [9] Rip currents are powerful, channelled currents of water flowing away from shore. They typically extend from the shoreline, through the surf zone, and past the line of breaking waves. Rip currents can occur at any beach with breaking waves but they become especially strong in the presence of bathymetric changes of the seabed. [10] Typically people who are wading or swimming accidentally venture into the rip currents, and are pulled out to sea. It is impossible to swim or walk against these currents (if you try, you will end up swimming or walking backwards out to deep water). The United States Lifesaving Association, one of the few with detailed and available statistics, estimates that the annual number of deaths due to rip currents on U.S beaches exceeds 100. Rip currents account for over 80% of rescues performed by surf beach lifeguards. In Australia, 35% of rescues and 18.5% of resuscitation cases, over a ten year period, from surf beaches were due to rip currents. [11] Rip currents are also recognised as a hazard for European beaches. Particularly where submerged breakwaters have been put in place in order to protect against erosion, but in fact are increasing the potential for localised rip currents(e.g. EU DELOS Project, EVK-2000-22038). Coastal engineers recognize rip currents as an important element in the nearshore circulation balance, particularly during storms. Nevertheless, little information is as yet available that describes where and when these currents occur. Hence, not only fundamental insight is needed into the conditions for the generation and development of rip currents and methodology for the related risk assessment, but strategies are also required for minimizing the hazard-related socio-economical costs.

Modelling the physics of nearshore water mixing

The mixing features of oceanic flows are well documented and fairly well understood be they related to the motion of abyssal streams (e.g. the Ocean Conveyor Belt) or to the horizontal circulation at the both global and meso-scale level (e.g. oceanic gyres and boundary currents). For historical reasons less effort has been put in the understanding/modelling of the mixing of the continental shelf shallow waters. However, this is the complex environment which provides a boundary between the inland, where most of the anthropogenic activities occur, and the deep water flow forcing. The most active agents of horizontal mixing are the large-scale eddies of the nearshore turbulence also known as “macrovortices”.

The importance for shallow flows of horizontal, large-scale eddies (macrovortices hereinafter) has been widely reported for coastal flows. [12] [13] [14] Large-scale, horizontal mixing of coastal flows is mostly promoted by macrovortices which are generated because of a spatially-non-uniform breaking of the incoming waves. [13] [15] Although such differential breaking may be induced by various reasons (irregularity of the incoming field, wave-wave interaction, etc.) the major cause of persistent breaking unevenness is due to topography. This is often characterized by longshore, isolated (natural bumps or manmade submerged breakwaters) or almost-continuous features (bars or arrays of submerged breakwaters) over which uniform wave fronts break with large lateral gradients. Hence, macrovortices can alter both the hydrodynamics and the morphodynamics. [16] [15] A recent classification of vortex generation in shallow coastal environments distinguished three types: [17] [18]

  1. Topographic forcing (from islands, headlands, jetties or groynes),
  2. Transverse shear instabilities (jet flows from lagoons or rivers, mixing layers, wakes), and
  3. Secondary instabilities of the base flow (internal vortex interactions).
Fig. 1a Modelled water surface elevation of storm event  
Fig. 1b Strong beach erosion as a result of storm event  
Fig. 1c Quantitative description of macrovortices  
Fig. 1d Transport of passive tracers, as a result of macrovortices  

Brocchini et al. (2004[15]) proposed an analytical approach for vorticity generation mechanisms induced by isolated topographic features and the related general hydrodynamic behaviour, with particular attention to vortex trajectories and shedding periods (see figure 1). Subsequently Kennedy et al. (2006[10]) analysed the transition of startup macrovortices from isolated topographic features to nearby obstacles (rip current topographies, see figure 1) using computations and laboratory experiments. Both studies provide insight into the fundamental deterministic features of macrovortex evolution. The series of studies on shallow-water mixing is completed by a third study by Piattella et al. (2006[19]) which characterizes the mixing features of macrovortices in terms of the statistical properties of the flow they induce, in conjunction with waves, in the nearshore region. The analysis gives both important theoretical results on the mixing spatial patterns and temporal regimes and practical evaluations of eddy diffusivities to be used in Fickian-type closures. Convection-diffusion equations for scalars all need turbulent diffusivities, generally known through a constitutive relationship of Fickian-type. Such a closure is largely dominated by the presence of large-scale coherent features like macrovortices and is typical of the flow conditions at hand. Examples of closures for coastal flows can be found in Inman et al. (1971[20]), Larson & Kraus (1991[21]) and in Takewaka et al. (2003[22]).

2D turbulence

The chosen approach of deriving methods from the analysis of 2D turbulence is justified by the fact that results coming from recent experimental studies of shallow-water turbulence suggest that such turbulence, generated in shallow jets [23], wakes [24] and mixing layers [25], is characterized by spectral properties typical of 2D turbulence. In this respect it is also auspicable to model the transport properties of shallow-water macrovortices in analogy to those due to coherent barotropic vortices of 2D turbulence. [26]

In a 2D turbulent flow characterized by large-scale coherent structures the evolution of tracers and the flow dynamics are so intimately connected that knowledge of the former (e.g. diffusivity) may give a predictive key for the latter (e.g. energy spectrum), and vice versa. This approach, has been employed to investigate atmospheric [27] [28] and oceanic [29] [30] flows and is now becoming of interest for nearshore dynamics. [31] This is also connected with the recent developments made in the monitoring of coastal waters by means of video techniques.[32] With such equipment floats/dye released near the shore can be monitored for times/area large enough to provide the fundamental data for any dispersion analysis. For example, the recent work of Takewaka et al. (2003[22]) shows how it is possible to apply the mentioned approach to compute dispersive parameters of dye patches released near the breaking region. In this perspective, and with the aim of using information coming from prototype-scale and laboratory-scale experiments, we attempt at creating a theoretical framework useful for the interpretation of statistics of passive tracers released in coastal areas.


Finally, inspection of ongoing research shows that promising practical results seem to come from the recent adaptation to unsteady flow conditions of techniques which make use of residence-time maps. As shown by Lipphardt et al. (2006[33]) such adaptation leads to synoptic Lagrangian maps which provide a detailed and cheap approach for the horizontal transport of particles and the consequent residence times computation. Approaches of a more basic nature are those which inspect the spatial variation of mixing through the use of Direct Lyapunov Exponents (DLE). Such method have been recently used, with interesting characterizations of the flow inhomogeinities in the Norwegian Trondheim fjord, by Orre, Gjevik & LaCasce (2006[34]). These recent studies clearly show the potentials and interest in Lagrangian methods for an increasingly detailed description of the horizontal mixing of nearshore waters and consequently a more accurate evaluation of the related transport phenomena.

Related articles

Seawater intrusion and mixing in estuaries

See also

McGillicuddy, Jr., D.J., Robinson, A.R., Siegel, D.A., Jannasch, H.W., Johnson, R., Dickey, T.D. McNeil, J., Michaels A.F. & Knap, A.H. (1998). Influence of mesoscale eddies on new production in the Sargasso Sea. Nature 394, 263–265.


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The main author of this article is Maurizio Brocchini
Please note that others may also have edited the contents of this article.

Citation: Maurizio Brocchini (2021): Transport and dispersion of pollutants, nutrients, tracers in mixed nearshore water. Available from,_nutrients,_tracers_in_mixed_nearshore_water [accessed on 24-07-2024]