Appendices 5-13 - Nautilus Cares - Nautilus Minerals
Appendices 5-13 - Nautilus Cares - Nautilus Minerals
Appendices 5-13 - Nautilus Cares - Nautilus Minerals
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
dispersion is modelled using a random walk scheme (Bear and Verruijt, 1987), with the<br />
magnitudes scaled by horizontal and vertical diffusion coefficients (Okubo, 1971). The mixing<br />
parameters were conservative estimates of deep water conditions.<br />
2.2 Stochastic Modelling<br />
Originally CHEMMAP was designed to simulate specific spill incidents and/or ongoing discharges<br />
for evaluating impacts and damages (French et al. 1996). More recently, the model has been set<br />
up in a probabilistic stochastic configuration, allowing evaluation of risks of consequences and<br />
statistical computations (French McCay and Isaji, 2004). While a few chemical spill models exist<br />
that can simulate transport and physical fate of single events (Lunel, 1991; Shen et al., 1995; Rusin<br />
et al., 1996), CHEMMAP is unique in being able to evaluate biological impacts, in its stochastic<br />
implementation, and in its interconnection with hydrodynamic models, geographical information<br />
systems, and its graphical user interface. In the stochastic mode, CHEMMAP can be used to<br />
predict the fate of multiple or continuous releases that occur under a random selection of prevailing<br />
conditions (also known as stochastic modelling). The stochastic model performs a large number of<br />
sample simulations for a given release site, randomly varying the sample time frame, so that the<br />
transport, concentration and dilution of each particle representing the plume mass and<br />
concentration are subject to a different set of prevailing current conditions and water properties.<br />
During each simulation, the model records the grid cells that were contacted by the plume, as well<br />
as the amount of time that had elapsed prior to the contact or exposure.<br />
Once the stochastic modelling is complete, the results are compiled from each of the sample<br />
trajectories to provide a statistical weighting to the likelihood of exposure of grid cells. Stochastic<br />
results can be summarised as:<br />
1. The probability, frequency or risk that a grid cell may be exposed to the<br />
plume; and<br />
2. The maximum expected (averaged) concentrations of the plume in each grid<br />
cell.<br />
The stochastic modelling approach provides an objective measure of the possible outcomes of a<br />
release, as well as the means of quantifying the likelihood of a given outcome. The most commonly<br />
occurring conditions would be selected most often while conditions that are more unusual can also<br />
be represented.<br />
Page 6 of 24