188
Fig. 17.6. Grain size distributions of surface samples from
cores HAP 06 (a) and HAP 12 (b)
%50
%50
a
1000
b
1000
K. Edelvang
100
10
.1
Particle size (~m)
100
10
.1
Particle size (~m)
ing, the results document that, with the information used, the model actually simulates
areas of accumulation rather than calculating actual accumulation rates. Possible accumulations of spill outside the predicted areas were not tested.
Surface samples from HAP 06 and HAP 12 were analysed in the laboratory. The
median particle size was 70 flm for sample HAP 06 (Fig. 17.6a), being equivalent to a
settling velocity of about 4 x 10- 3 m S-I. The median diameter for HAP 12 (Fig. 17.6b)
was 2511m, which corresponds to a fall velocity of 6 x 10- 4 m S-I. In both cases the median particle sizes are much larger than those measured for the spill material from the
dredgers. The positive skewness for both analyses indicates a population of fine material of about 6 llm, which may very well be derived from spill. Mineralogical analyses
have shown that only about 10% of the material deposited at HAP 06 was derived from
spill plumes (Knudsen and Lomholt 1997). The much larger particle sizes thus indicate
that coarser natural sediments were also deposited.
17.5
Discussion and Conclusions
The settling velocities measured in the areas surrounding the dredgers are the same for
the different types of dredgers. The average median settling velocity is computed to