j.m. abril department of applied physics (i); university of seville (spain) iaea regional training...

J.M. AbrilDepartment of Applied Physics (I); University of Seville (Spain)

IAEA Regional Training Course on Sediment Core Dating Techniques. RAF7/008 Project

J.M. Abril, University of Seville1

Lecture 9:

Case studies

Spatial variability in sedimentation rates and radionuclide inventories. An approach from Computational Fluid Dynamics Models

Irish Sea and Härsvatten lake

Horizontal movement , with water velocity

Settling velocity

Resuspension

BASIC PROCESSES FOR SPM DYNAMICS AND SEDIMENTATION RATES

BASIC PROCESSES FOR SPM DYNAMICS AND SEDIMENTATION RATES


Bathimetry

The Irish Sea


Distribution (in % dry weight) of small particles (ф ≤ 62.5 µm) in the top sedimentlayer for the Irish Sea.


Residual circulation

Stream function (km3 y-1) calibrated by modelling the salinity distribution


SOURCE TERM:Proportional to freshwater discharges along the shoreline


SEDIMENTATION RATES arise as net balance of deposition and resuspension or, alternatively, as a mass balance at each grid-cell

SEDIMENTATION RATES arise as net balance of deposition and resuspension or, alternatively, as a mass balance at each grid-cell


The grid used in our numerical model, with grid lengths Δx = Δy = 5.16 km. Numbers in the X and Y directions are the index used to label each grid-cell. Black boxes represent land and the white ones water. Eq. 1 is solved in the center of each grid-cell at every time step.


Modelling the mean annual SALINITY distribution served to calibrate the diffusion term in the transport equation

Observed

Model


Estimated river discharges (km3 y-1) for each coastal sector (defined between arrows) taken from MAFF (MAFF, 1987). Contours (x 103 km3 over each compartment) represent computed steady-state distribution of volumes from these river discharges.


Computed steady-state spatial distributions of SPM and sedimentation rates


VALIDATIONSPM (a) and net deposition (b) vs. distance from the Cumbrian coast-line. Circles are measured values.

SENSITIVY TEST: The continuous line is for our selected set of parameter values . The dashed and point dashed lines corresponds to double and a half value of settling velocity.


All these results for w confirm the general accepted impression about the sedimentary processes taking place at the Irish Sea. IOS and MAFF (Kirby et al., 1983) did not found evidences of net erosion nor sedimentation in the mud bank of Cumbria, concluding that the sedimentary regime in this area is relatively stable.


With the permission of the author


The model solves the three dimensional hydrodynamic equations, using normalized σ coordinates in the vertical

It includes wave–current interaction through by an increase in the current friction factor,


Suspended matter equations

ms is the SPM concentration, and ws is their settling velocity

Deposition and erosion of the sediment are incorporated into the sea bed boundary condition. Both processes have threshold velocities.

E is the erodability, M (2-5) is some power of the water velocity, and f gives the fraction of particles with a diameter <62.5 lm in the sediment

Includes a source term, as in Abril and García-León, 1992. J.M. Abril, University of Seville18

Lake Härsvatten


Large-scale and Long-term Environmental Behaviour of Transuranic Elements as Modelled

through European Surface Water Systems.

This work is part of a research project financially supported by the European Commission, Nuclear Fission Safety Programme, Contract No. FI4P-CT96-0046 (DG12-WSMN).

Coordinator: Dr. F. El-Daoushy (Uppsala University, Sweden).

Partners:Dr. F. El-Daoushy (Uppsala University, Sweden)Prof. P. G. Appleby (University of Liverpool, the United Kingdom);Prof. M. García-León (Universidad de Sevilla, Spain);Dr. P. Casper (Forschungsverbund Berlin e.V.-Gemeinsame Verwaltung, Germany);Prof. G. Ardisson (Université de Nice, France).


In radioecology, lakes have often been considered as well mixed water bodies

From CFD studies we know that the magnitude of the wind forced water currents is of the order of some cm/min to few cm/s over spatial scales of the order of few kilometers.

Consequently, the time scale involved in the dispersion and mixing of any input of pollutants will be, typically, of several days or weeks, being the lacustrine system comparable to an estuarine or a coastal areas with length-scales of ten or hundred kilometers and with water currents of the order of 1 m/s.

Thus, the spatial distribution of concentrations and inventories or the focussing, cannot be properly understood without considering the hydrodynamics of the lacustrine water body.


Focusing ?

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Bathymetric map and grid for numerical model

J.M. Abril, University of Seville

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Advective tracks


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Spheroidal carbonaceous fly-ash particles

R. Bindler et al.

Spatial distribution of SPC inventories by quartiles


http://www.fysik.uu.se/isotopgeo/NewFiles/trans.html


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Spatial distribution of SPC inventories by quartiles


The present model development has to be understood as an effort to produce a suitable virtual laboratory for testing our knowledge on radionuclide behaviour in lake systems and lacustrine sediments, being at time like a map to assist future research in this field


j.m. abril department of applied physics (i); university of seville (spain) iaea regional training...

Documents

hrsvatten lake slide

sedimentation rates

grid cell

freshwater discharges

grid lengths x

maff maff

spm dynamics

water velocity