OPTICS, ACOUSTICS, AND STRESS IN A NEARSHORE BOTTOM NEPHLOID LAYER The role of in situ particle size distribution Paul Hill 1 , Timothy Milligan 2 , and Kristian Curran 1 1 Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J1 2 Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada, B2Y 4A2
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OPTICS, ACOUSTICS, AND STRESS IN A NEARSHORE BOTTOM
NEPHLOID LAYER
The role of in situ particle size
distribution
Paul Hill1, Timothy Milligan2, and Kristian Curran1
1Department of Oceanography, Dalhousie University,
Halifax, Nova Scotia, Canada, B3H 4J1
2Fisheries and Oceans Canada, Bedford Institute of Oceanography,
Dartmouth, Nova Scotia, Canada, B2Y 4A2
Conventional wisdom
1. Small particles in suspension affect the optical and acoustical properties of the water column.
2. Large particles in suspension determine the mass flux of sediment to the seabed.
1 10 1001 10 1000.01
0.1
1
10
Diameter, Diameter, mm
Co
nce
ntr
ati
on
Co
nce
ntr
ati
on
The Effects of Flocculation
1 10 1001 10 1000.01
0.1
1
10
Chemically Dispersed Salt Water
< 0.2 Pa > 0.2 Pa
The Effects of Flocculation
Emerging views
1. Flocs interact strongly with small particles in suspension.
2. Under some circumstances flocs themselves affect the optical and acoustical properties of
the water column.
The Effects of Flocculation(from Flory et al., 2004, DSR)
Backscatter observed by the Hydroscat-6
vs.
the sum of the area per unit volume of particles
observed in corresponding in situ
images
Goals of the study
1. Measure the suspended size distribution of particles 1 µm to 1 cm in diameter in the bottom boundary layer.
2. Link the suspended particle size distribution
to optics, acoustics, and stress.
Preliminary results
-7
-3.5
0
3.5
7
Win
d (
m s
-1)
0.25
0.5
0.75
1
1.25
Wa
ves
(m)
14 15 16 17 18 19 20-1
-0.5
0
0.5
1
Tid
e (
m) D
25 ( m)
September 2004
100
300
500
Preliminary results
100
101
102
103
104
0
10
20
30
40
50
Diameter (m)
Vo
l Co
nc
(mm
3 L-1
)
September 16, 2004 at 0800 (UTC)
LISST
DFC
D25 = 435 µm
September 14, 2004 at 1900 (UTC)
Challenge to overcome
Plans for 2005
1. Alter the digital floc camera orientation to prevent accumulation of sediment on the camera housing.
2. Deploy optics, acoustics, particle sizing and flow velocity instruments simultaneously.
3. Deploy INSSECT?
INSSECT
Digital floc camera
LISST
Tilt/compass
Settling column
Digital video camera
Carousel sediment trap
Rotating base
a
Floc diameter (µm)
100 1000
Ws
(mm
s-1
)
0.1
1
10b
Floc diameter (µm)
100 1000
(from Mikkelsen et al., 2005, CSR)
Floc settling velocities
Transmission image Florescence image
300 µm
CLSM floc images
300 µm
EuroSTRATAFORM - Gulf of Lions, 2005
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