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.

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