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| Titel |
Modelling the cohesive sediment transport in the marine environment: the case of Thermaikos Gulf |
| VerfasserIn |
Y. N. Krestenitis, K. D. Kombiadou, Y. G. Savvidis |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1812-0784
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| Digitales Dokument |
URL |
| Erschienen |
In: Ocean Science ; 3, no. 1 ; Nr. 3, no. 1 (2007-02-13), S.91-104 |
| Datensatznummer |
250000769
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| Publikation (Nr.) |
 copernicus.org/os-3-91-2007.pdf |
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| Zusammenfassung |
The transport of fine-grained sediments in the marine environment entails
risks of pollutant intrusions from substances absorbed onto the cohesive
flocks' surface, gradually released to the aquatic field. These substances
include nutrients such as nitrate, phosphate and silicate compounds from
drainage from fertilization of adjacent cultivated areas that enter the
coastal areas through rivers and streams, or trace metals as remainders from
urban and industrial activities. As a consequence, knowledge on the motion
and distribution of sediment particles coming from a given pollutant source
is expected to provide the 'bulk' information on pollutant distribution,
necessary for determining the region of influence of the source and to
estimate probable trophic levels of the seawater and potential environmental
risks. In that aim a numerical model has been developed to predict the fate
of the sediments introduced to the marine environment from different
pollution sources, such as river outflows, erosion of the seabed, aeolian
transported material and drainage systems.
The proposed three-dimensional mathematical model is based on the particle
tracking method, according to which matter concentration is expressed by
particles, each representing a particular amount of sedimentary mass,
passively advected and dispersed by the currents. The processes affecting
characteristics and propagation of sedimentary material in the marine
environment, incorporated in the parameterization, apart from advection and
dispersion, include cohesive sediment and near-bed processes. The movement
of the particles along with variations in sedimentary characteristics and
state, carried by each particle as personal information, are traced with
time. Specifically, concerning transport processes, the local seawater
velocity and the particle's settling control advection, whereas the random
Brownian motion due to turbulence simulates turbulent diffusion. The
vertical stratification of the water-column is taken into consideration by
appropriate damping of the vertical diffusion term. Variations in cohesive
sediment properties during the abidance in the aquatic environment include
coagulation and flock break-up processes, quantification of the effects of
ambient density to the density of the cohesive aggregate and the associated
alterations to the falling speed of the particle. In the vicinity of the
seabed, particles may deposit and gradually consolidate with time, the
particles remain settled onto the bed, re-enter the flow at a later temporal
point or may enter the water column for the first time, originating from the
erosion of the bed. The occurrence of each of the aforementioned near-bed
processes is defined according to the prevailing benthic shear stress
conditions.
The mathematical model has been applied to the Thermaikos Gulf, an area of
high environmental and socioeconomic importance but also a region of
significant pollutant forcing from various anthropogenic activities taking
place in the adjoining land. Various kinds of outputs can be extracted, such
as trajectories of the overall movement of specific particles and related
alterations of their characteristics with time, snapshots of the domain with
respect to suspended or deposited matter and natural concentrations of
sediments at every required temporal and spatial point. Indicative results
from yearly and monthly simulations, using input baroclinic circulation data
from the North Aegean Sea model and river discharges are presented and
discussed, including outputs from a Typical One-Year Simulation (TOYS), the
simulation of the period from 3 September 2001 to 31 August
2002 (S1A2) and the January 2003 experiment (J03).
The description of the processes that have been incorporated in the
parameterization covers the most significant factors controlling transport
and mixing of fine grained sediments in the marine environment, thus
validating the accuracy and completeness of the model. One of the major
advantages, apart from the observation of the phenomena in scales smaller
than the grid size, describing the natural processes more accurately, is the
flexibility in accepting various pollutant sources and the applicability to
different domains with minor modifications. The model has been incorporated
in the MFSTEP project, as part of the developed operational forecasting
system for the Mediterranean Sea. The application can be used for the
prognosis of the seawater quality for current and for future conditions,
enabling employment as part of a near-real time observation system or to
formulate decisions for the protection of the seawater environment. |
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