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

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by Maurizio Brocchini

This contribution finds its motivation in the awareness that the mixing-induced transport occurring in the nearshore waters is a fundamental mechanism underlying/forcing all the physical/chemical/biological phenomena occurring in the marine environment. It is also aimed at better highlighting/exploiting the wealth of knowledge built by the EU on the marine environment (about 20 years have passed since the first EU MAST Projects). Such knowledge is not yet being used at full because of the still i) scarce understanding of complex physical phenomena like mixing-induced transport; ii) fragmented view of the specialists on the marine processes.

Specific EU Projects have been funded (e.g. TRACMASS Project, MAS3-CT97-0142) and have been submitted (e.g. TRAPPS Project, EVK3-2000-22032) towards a more detailed understanding of the physical phenomena and a stronger exploitation of the specific knowledge on mixing for the sustainable management of the marine resources. However, still few experts in pollution management and remediation are fully aware of the fundamental role played by mixing in transporting chemicals, pollutants, nutrients and tracers in general.

The motivations

The release of pollutants from coastal distributed/localized 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 ecosystem. Such influences are felt not only in proximity of the sources, but also in wider regions, determined primarily by the direct physical transport due to marine waters (Lekien et al., 2005). High concentration zones of pollutants have been found along coastal regions far from coastal sources, and noticeable impact of pollutants on organisms has been detected even in deep sea regions.

It is also important to acknowledge the close link between water mixing, sediment and pollutant transport (Förstner, 1987). For example, if, on the one hand, the morphological evolution of the nearshore bottom is forced by wave-induced phenomena, including mixing, then on the other hand, 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 (e.g. Williamson et al., 2000; Bloom et al., 2004).

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 (Martin, 2003) and of most of marine species during their larval stage. Hence, it should not surprise the recent and growing attention to Lagrangian transport of marine species (e.g. Cowen et al., 2000, Siegel et al., 2003). 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. (e.g. Caldeira et al., 2002).

Recreational activities are also strongly influenced by water transport phenomena. More specifically swimming safety is seriously hampered by the horizontal water mixing induced by the so-called “rip currents” (Lushine, 1991). 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 (Kennedy et al., 2006). Typically people who are wading or swimming accidentally venture into the rip currents, and they 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.A. beaches exceeds 100. Rip currents account for over 80% of rescues performed by surf beach lifeguards (http://www.srh.noaa.gov/ripcurrents/index.shtml). In Australia, 35% of rescues and 18.5% of resuscitation cases, over a ten year period, from surf beaches were due to rip currents (Fenner, 1999). The role of rip current related hazard is becoming also recognized for European beaches. These are often protected against erosion by submerged breakwaters which, however, highly increase the potential of localized 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

Once clarified the fundamental role of the horizontal mixing of marine waters, a brief description is proposed of the phenomenon and of the recent advances in its representation/modelling. 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 both global and meso-scale level (e.g. oceanic gyres and boundary currents). For historical reasons less effort has been put in the understanding/modeling of the mixing of the continental shelf shallower 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. We here focus in particular on the most active agents of horizontal mixing i.e. 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 (e.g. Oltman-Shay et al., 1989; Peregrine, 1998; Brocchini et al. 2002). 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 (e.g. Peregrine 1998; Brocchini et al.,  2004). 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 be shed which alter both the hydrodynamics and the morphodynamics (Steijn et al., 1998; Brocchini et al., 2004). A recent classification of vortex generation in shallow coastal environments distinguished among three types (Jirka, 2001; Jirka & Uijttewal, 2004):

  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).


Broc mixing1.jpg Broc mixing2.jpg
Broc mixing3.jpg Broc mixing4.jpg
Figure 1 - Illustration of the influence of macrovortices in the nearshore. The top-left panel shows the modelled water surface elevation of the storm event which led to the strong beach erosion of the bottom-left panel. The top-right panel gives a quantitative description of the macrovortices forcing the transport of passive tracers of the bottom-right panel.

Very recently Brocchini et al. (2004) 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 also figure 1). Subsequently Kennedy et al. (2006) analysed the transition of startup macrovortices from isolated topographic features to nearby obstacles (rip current topographies, see also 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 work (Piattella et al., 2006) 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), Larson & Kraus (1991) and in Takewaka et al. (2003).

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 (Dracos et al. 1992), wakes (Chen & Jirka 1995) and mixing layers (Uijttewaal & Booij 2000), 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 (Provenzale 1999).

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, obviously, viceversa. This approach, which has been usefully employed to investigate atmospheric (e.g. Richardson 1926; Er-El & Peskin 1981) and oceanic (e.g. LaCasce & Bower 2000; LaCasce & Ohlmann, 2003) flows, is now becoming of interest also for nearshore dynamics (Fong & Stacey 2003). This is also connected with the recent developments made in the monitoring of coastal waters by means of video techniques (e.g. Lippmann & Holman 1989). 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) 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) 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 more basic nature are those which inspect the spatial variation of mixing through use of Direct Lyapunov Exponents (DLE). Such method has been recently used, with interesting characterizations of the flow inhomogeinities in the Norwegian Trondheim fjord, by Orre, Gjevik & LaCasce (2006). 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 a consequent more accurate evaluation of the related transport phenomena.

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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 (2007): Transport and dispersion of pollutants, nutrients, tracers in mixed nearshore water. Available from http://www.coastalwiki.org/wiki/Transport_and_dispersion_of_pollutants,_nutrients,_tracers_in_mixed_nearshore_water [accessed on 28-03-2024]