17 In Situ Settling Velocities and Concentrations of Suspended Sediment in Spill Plumes
183
very small, such sediment clouds can be traced for several kilometres in the Sound. The
sediment mainly accumulates in areas known as natural sinks for fine-grained material (Pejrup and Larsen 1994).
17.3
Methods
Median settling velocities of the spilled sediment were calculated on the basis of field
investigations using a Braystoke SK 110 settling tube to measure the in situ settling
velocity. The measured settling velocities were compared with settling velocities determined from sediment fluxes to the seabed (Valeur et ai. 1996).
Suspended sediment concentrations were measured during dedicated field surveys
and were also obtained from the constructor who determines concentrations on a routine basis in the course of construction. Only final quality-assured data were used. Concentrations were measured with a GMI type TUI50-IR turbidity sensor calibrated by
water samples collected in the field. The water samples were filtered through pre-weighed
Millipore 0.45 !lm CEM filters. The filters were dried at 60 DC for 2 h and the suspended
sediment concentration determined.
Sediment accumulation at specific sites in the 0resund was monitored using a Haps
core bottom sampler, which extracts samples down to 30 cm below the surface of the
seabed. Comparisons between measured and modelled sediment concentrations and
sediment accumulations were used to validate the sediment dispersal model MIKE 21
HD, PA/MT (DHI software). Grain sizes of the deposited sediment were analysed in the
laboratory using a Malvern Laser Particle Sizer, which covers a particle size range from
0.2 to 600 ~m.
17.4
Results
17.4.1
Settling Velocity
The suspended spill material in the 0resund has median particle sizes of about 6 !lm as
determined by the Malvern Laser Particle Sizer. As it consists of mainly fine chalky material it does not flocculate, and therefore has very low settling velocities. In situ settling velocities of the sediment suspended during dredging operations were computed
on the basis of several settling tube analyses using Stokes' law. Measurements were
carried out in sediment plumes from two different dredgers. One was a dipper dredger
(Chicago) and the other a cutter suction dredger (Castor). Although the two dredgers
have very different modes of operation the measured settling velocities from the sediment spill were quite similar. Thus, the median settling velocity of the sediment spilled
from Chicago at a temperature of 10 DC was 1 x 10-5 m S-1 compared to 6 x 10-5 m S-1
from Castor at a temperature of 15 DC (values extrapolated to the 50% percentile). The
average median in situ settling velocity was determined to be 3 x 10- 5 m S-1 (see Fig. 17.2).
Earlier investigations of vertical particle fluxes determined from sediment traps (Valeur
et ai. 1996) report a mean computed settling velocity of 4 x 10- 5 m S-I, which compares
well with the results of this study.
183
very small, such sediment clouds can be traced for several kilometres in the Sound. The
sediment mainly accumulates in areas known as natural sinks for fine-grained material (Pejrup and Larsen 1994).
17.3
Methods
Median settling velocities of the spilled sediment were calculated on the basis of field
investigations using a Braystoke SK 110 settling tube to measure the in situ settling
velocity. The measured settling velocities were compared with settling velocities determined from sediment fluxes to the seabed (Valeur et ai. 1996).
Suspended sediment concentrations were measured during dedicated field surveys
and were also obtained from the constructor who determines concentrations on a routine basis in the course of construction. Only final quality-assured data were used. Concentrations were measured with a GMI type TUI50-IR turbidity sensor calibrated by
water samples collected in the field. The water samples were filtered through pre-weighed
Millipore 0.45 !lm CEM filters. The filters were dried at 60 DC for 2 h and the suspended
sediment concentration determined.
Sediment accumulation at specific sites in the 0resund was monitored using a Haps
core bottom sampler, which extracts samples down to 30 cm below the surface of the
seabed. Comparisons between measured and modelled sediment concentrations and
sediment accumulations were used to validate the sediment dispersal model MIKE 21
HD, PA/MT (DHI software). Grain sizes of the deposited sediment were analysed in the
laboratory using a Malvern Laser Particle Sizer, which covers a particle size range from
0.2 to 600 ~m.
17.4
Results
17.4.1
Settling Velocity
The suspended spill material in the 0resund has median particle sizes of about 6 !lm as
determined by the Malvern Laser Particle Sizer. As it consists of mainly fine chalky material it does not flocculate, and therefore has very low settling velocities. In situ settling velocities of the sediment suspended during dredging operations were computed
on the basis of several settling tube analyses using Stokes' law. Measurements were
carried out in sediment plumes from two different dredgers. One was a dipper dredger
(Chicago) and the other a cutter suction dredger (Castor). Although the two dredgers
have very different modes of operation the measured settling velocities from the sediment spill were quite similar. Thus, the median settling velocity of the sediment spilled
from Chicago at a temperature of 10 DC was 1 x 10-5 m S-1 compared to 6 x 10-5 m S-1
from Castor at a temperature of 15 DC (values extrapolated to the 50% percentile). The
average median in situ settling velocity was determined to be 3 x 10- 5 m S-1 (see Fig. 17.2).
Earlier investigations of vertical particle fluxes determined from sediment traps (Valeur
et ai. 1996) report a mean computed settling velocity of 4 x 10- 5 m S-I, which compares
well with the results of this study.