Modelling the cohesive sediment transport in the marine environment: the case of Thermaikos Gulf

[摘要] The transport of fine-grained sediments in the marine environment entailsrisks of pollutant intrusions from substances absorbed onto the cohesiveflocks' surface, gradually released to the aquatic field. These substancesinclude nutrients such as nitrate, phosphate and silicate compounds fromdrainage from fertilization of adjacent cultivated areas that enter thecoastal areas through rivers and streams, or trace metals as remainders fromurban and industrial activities. As a consequence, knowledge on the motionand distribution of sediment particles coming from a given pollutant sourceis expected to provide the 'bulk' information on pollutant distribution,necessary for determining the region of influence of the source and toestimate probable trophic levels of the seawater and potential environmentalrisks. In that aim a numerical model has been developed to predict the fateof the sediments introduced to the marine environment from differentpollution sources, such as river outflows, erosion of the seabed, aeoliantransported material and drainage systems.

The proposed three-dimensional mathematical model is based on the particletracking method, according to which matter concentration is expressed byparticles, each representing a particular amount of sedimentary mass,passively advected and dispersed by the currents. The processes affectingcharacteristics and propagation of sedimentary material in the marineenvironment, incorporated in the parameterization, apart from advection anddispersion, include cohesive sediment and near-bed processes. The movementof the particles along with variations in sedimentary characteristics andstate, carried by each particle as personal information, are traced withtime. Specifically, concerning transport processes, the local seawatervelocity and the particle's settling control advection, whereas the randomBrownian motion due to turbulence simulates turbulent diffusion. Thevertical stratification of the water-column is taken into consideration byappropriate damping of the vertical diffusion term. Variations in cohesivesediment properties during the abidance in the aquatic environment includecoagulation and flock break-up processes, quantification of the effects ofambient density to the density of the cohesive aggregate and the associatedalterations to the falling speed of the particle. In the vicinity of theseabed, particles may deposit and gradually consolidate with time, theparticles remain settled onto the bed, re-enter the flow at a later temporalpoint or may enter the water column for the first time, originating from theerosion of the bed. The occurrence of each of the aforementioned near-bedprocesses is defined according to the prevailing benthic shear stressconditions.

The mathematical model has been applied to the Thermaikos Gulf, an area ofhigh environmental and socioeconomic importance but also a region ofsignificant pollutant forcing from various anthropogenic activities takingplace in the adjoining land. Various kinds of outputs can be extracted, suchas trajectories of the overall movement of specific particles and relatedalterations of their characteristics with time, snapshots of the domain withrespect to suspended or deposited matter and natural concentrations ofsediments at every required temporal and spatial point. Indicative resultsfrom yearly and monthly simulations, using input baroclinic circulation datafrom the North Aegean Sea model and river discharges are presented anddiscussed, including outputs from a Typical One-Year Simulation (TOYS), thesimulation of the period from 3 September 2001 to 31 August2002 (S1A2) and the January 2003 experiment (J03).

The description of the processes that have been incorporated in theparameterization covers the most significant factors controlling transportand mixing of fine grained sediments in the marine environment, thusvalidating the accuracy and completeness of the model. One of the majoradvantages, apart

[学科分类] 海洋学与技术