outline of presentation : motivation: acoustic doppler velocimeter (adv) field o bservations

Dual Use of a Sediment Mixing Tank for Calibrating Acoustic Backscatter and Direct Doppler Measurement of Settling Velocity

(and Related Field Motivation and Observations)

Grace Cartwright, Carl Friedrichs, and Paul Panetta

Outline of Presentation:

• Motivation: Acoustic Doppler Velocimeter (ADV) Field Observations• Sediment Mixing Tank: Acoustic Backscatter Calibrations• Sediment Mixing Tank: Doppler Settling Measurements• Independent field-based test of ADV-measured settling velocity

Motivation: Determine fundamental controls on sediment settling velocity and bed erodibility in muddy estuaries

Physical-biological gradient found along the York estuary :

-- In the middle to upper York River estuary, macrobenthos are seasonally overwhelmed by the floc-rich estuarine turbidity maximum, and sediment layering is often preserved. (e.g., Clay Bank – “Intermediate Site”)

-- In the lower York and neighboring Chesapeake Bay, layering is generally destroyed by bioturbation, and abundant macrobenthos pelletize the mud as they feed (e.g., Gloucester Point – “Biological Site”)

-- Acoustic Doppler Velocimeter (ADV) tripods provide long-term observations within a strong physical-biological gradient.

Study site: York River Estuary, VA

(X-rayscourtesy of

L. Schaffner)

ADV at deployment

-- ADVs often provide quality long-term data sets despite extensive biofouling.-- ADVs can provide continual long-term estimates of:

• 3-D velocity (u,v,w) in ~ 1 cm3 sampling volume including ~ Hz turbulent fluctuations (u’,v’,w’) • Suspended mass concentration (c) from acoustic backscatter including turbulent fluctuations (c’)• Turbulent Reynolds Shear Stress, τ = ρ*<u’w’>

• Sediment Settling Velocity, ws = <w’c’>/c

• Elevation of seabed relative to tripod

ADVafter retrieval

Observations provided by a Sontek 5 MHz Acoustic Doppler Velocimeter (ADV)Sensing volume ~ 35 cmab

(Photos by C. Cartwright)

(Cartwright et al., 2009)

ConcentrationCalibration Curves

• In-situ pump samples analyzed for total suspended solids

• Concentrations used to calibrate acoustic backscatter from deployed tripods

• All observations utilize Sontek 5 MHz ADV“Ocean” Model

Significant scatter in suspended solids vs. acoustic backscatter relationship because of variations in suspended particle size, particle density, and response of individual ADVs

Days since December 4, 2006

Bed

ele

v (c

m)

TSS

(mg/

liter

)C

urre

nt (c

m/s

ec)

Lower Concentration Period at Biological (Gloucester Point) site (ADV height ~ 35 cm)

~ 40 cm/s

~ 50 mg/l

~ 4 cm change

(Cartwright et al. 2009)

Upwards turbulent sediment flux

Downwards gravitationalsettling=

<w'C'> = ws <C>

w' = vertical turbulent velocity, C' = turbulent concentration fluctuation< > = burst average, ws = sediment settling velocity, <C> = burst-average TSS

<C> (mg/liter)

<w'C'>

(mm/s)(mg/liter)Slope = ws = <w’C’>/<C>

= 1.5 mm/s

Biological site (GP) ADV Data:

Assume:

(Fugate & Friedrichs, 2003)

-- Insensitive to ADV calibration for C , 50% change in calibration = 10% change in ws.


Days since February 27, 2007

Bed

ele

v (c

m)

TSS

(mg/

liter

)C

urre

nt (c

m/s

ec)

Higher Concentration Period at Intermediate (Clay Bank) site (ADV height ~ 35 cm)

~ 40 cm/s

~ 100 mg/l

~ 20 cm change


Days since February 27, 2007

Bed

ele

v (c

m)

TSS

(mg/

liter

)C

urre

nt (c

m/s

ec)

Cur

rent

(cm

/sec

) ws = 0.55 mm/s ws = 0.77 mm/s ws = 0.20 mm/s

ws = 0.80 mm/s

<w’C’> vs. <C>

<w'C'> vs. <C>

<w'C'> vs. <C> <w'C'> vs. <C>

Higher Concentration Period at Intermediate (Clay Bank) site (ADV height ~ 35 cm)


Biological siteGenerally < 1 kg/m2/Pa

Intermediate siteε varies from ~ 3 kg/m2/Pa

(Regime 1) to ~ 1 kg/m2/Pa (Regime 2)

1

2

3

4

5

6

ε (k

g/m

2 /Pa

)Seasonal Variability in sediment settling velocity (ws ) and erodibility (e) is observed at the Intermediate Site.

3-day mean of erodibility (e) using ADVs and Gust erosion chambers

Biological siteWs ~ 1.5 mm/s

Intermediate siteWs varies from

~ 0.5 mm/s to ~ 1 mm/s

3- day Mean Ws from fits to <w’C'> = ws<C> using ADVs2

1.5

1.0

0.5

0

Ws

(mm

/s)

Cartwright et al., 2009

Biological siteGenerally < 1 kg/m2/Pa

Intermediate siteε varies from ~ 3 kg/m2/Pa

(Regime 1) to ~ 1 kg/m2/Pa (Regime 2)

1

2

3

4

5

6

ε (k

g/m

2 /Pa

)Seasonal Variability in sediment settling velocity (ws ) and erodibility (e) is observed at the Intermediate Site.

3-day mean of erodibility (e) using ADVs and Gust erosion chambers

Biological siteWs ~ 1.5 mm/s

Intermediate siteWs varies from

~ 0.5 mm/s to ~ 1 mm/s

3- day Mean Ws from fits to <w’C'> = ws<C> using ADVs2

1.5

1.0

0.5

0

Ws

(mm

/s)

Cartwright et al., 2009

Questions:

-- Are these ADV-based estimates of sediment concentration and settling velocity accurate and reliable?

-- Can controlled lab experiments provide insight?

(a) VIMS Sediment Mixing Tank, with suspended sampling tubes highlighted, (b) example placement of ADV in chamber, with pump circulation outlets highlighted

Acrylic portion of tank is 32 cm x 32 cm x 1.5 m, with bottom 0.5 m tapering in toward the 44 liter/minute pump inlet. Flow is returned 25 cm below the top of the tank through 4 circulation outlets. Sediment concentration is sampled at 1 m/s through sampling tubes.


Example of 63 micron component Example of 125 micron component

Lab calibration with quartz sand:

-- Commercially available, predominantly quartz sand was divided into size classes using sieves with mesh diameters of 63, 75, 90, 106, 125, and 150 microns.


SamplingTubes

Quartz Sand Only calibrations (For each phi size)

• A successive series of sand was added to the chamber• Acoustic backscatter collected for 10 minutes (10Hz) and averaged.• For each concentration a water sample was pulled from mid-chamber• Samples were dried and weighed for suspended mass concentration.

(With ADV mounted mid-chamber)

Mixed Sediment and Mud Only calibrations (For three sites)(This section of experiment part of Newbill, 2010)

• Analysis methodology similar to Sand Only calibrations• Did calibrations with natural muddy sediment collected from 3 sites

• Claybank (CB) Channel (~1 % sand)• Claybank (CB) Shoal (~20 % sand)• Ferry Point ( FP) Shoal (~10% sand)

• Repeated calibrations with Mud Only portions <63 microns• Claybank (CB) Shoal • Ferry Point ( FP) Shoal

Sediment Mixing Tank: Acoustic Backscatter Calibrations

(Photo by C. Cartwright)

SamplingTubes

Quartz Sand Only calibrations (For each phi size)

• A successive series of sand was added to the chamber• Acoustic backscatter collected for 10 minutes (10Hz) and averaged.• For each concentration a water sample was pulled from mid-chamber• Samples were dried and weighed for suspended mass concentration.

(With ADV mounted mid-chamber)

Mixed Sediment and Mud Only calibrations (For three sites) (This section of experiment also part of Newbill, 2010)

• Analysis methodology similar to Sand Only calibrations• Did calibrations with natural muddy sediment collected from 3 sites

• Claybank (CB) Channel (~1 % sand)• Claybank (CB) Shoal (~20 % sand)• Ferry Point ( FP) Shoal (~10% sand)

• Repeated calibrations with Mud Only portions <63 microns• Claybank (CB) Shoal • Ferry Point ( FP) Shoal

Sediment Mixing Tank: Acoustic Backscatter Calibrations


Mud only

MixedMud & Sand

Solid LinesSand OnlySe

dim

ent M

ass C

once

ntra

tion

(log 10

mg/

liter

)

ADV Backscatter (counts)

-- ADV backscatter systematically increases with concentration for any one sediment type.-- For quartz sand, ADV backscatter systematically increases with sand grain diameter.-- Natural mud responds less strongly than sand, but response is site specific.-- Mud + Sand is also site specific and doesn’t respond as a sum or average of the two.

ADV mounted above Circulation outlets 2-D map of vertical Doppler velocity(+ = upward, - = downward)

(Example for 125 micron case)

cm/s

Sediment Mixing Tank: Doppler Settling Measurements-- For each sand size, a grid of vertical velocity ADV measurements were collected with 10 min at each point

-- Vertical velocity measurements are total velocity of sand + water, i.e., ws + <w>.-- Horizontally-integrated flow associated with water along, <w>, must add up to zero.-- So the spatially averaged sum of ws + <w> must be the sediment settling velocity, ws .-- However, the spatial coverage of the tank by the ADV is incomplete.


cm/s

-- Spatially averaged sum of ws + <w> must be the sediment settling velocity, ws .-- However, the spatial coverage of the tank by the ADV is incomplete.-- Radially symmetry of tank can be used to extrapolate and interpolate flow before averaging to solve for settling velocity, ws .

Map of vertical Doppler velocity(+ = upward, - = downward)

(Example for 125 micron case)

c) Interpolated flow

a) Measured flow

b) Measured flow

Answer: Rapid Sediment Analyzer (RSA)

Balance connected to computer

Settling tube filled with water

Sediment drop and start button

Metal plate connected to balance(~150 cm from sediment top)

Computer records weight and settling time

Thermometer to measure water temp.

What independent “true” sand settling velocity can we compare Doppler measurements to?


Mean Ws = 1.310 ±0.063 cm/sec

Rapid Sediment Analyzer (RSA)

Example output from 106 micron sieve


RSA Ws and Individual Flow Fit Ws comparison RSA Ws and Global Flow Fit Ws comparison

Ws from Rapid Sand Analyzer (cm/s)Ws from Rapid Sand Analyzer (cm/s)

Ws f

rom

indi

vidu

al

Dopp

ler v

eloc

ity fi

ts (c

m/s

)

Ws f

rom

glo

bal

Dopp

ler v

eloc

ity fi

ts (c

m/s

)

Sediment Mixing Tank: Doppler Settling Measurements

Global Flow Fit Use result from average of all best-fit slopes of velocity vs. radial distance for all sizes, since slope (i.e., spatial distribution of flow) is entirely due to water, not sand.

Individual Flow Fita) Regression of distance from center vs. <w>b) Circular fit of regression



<w'C'> = ws <C>


<C> (mg/liter)

<w'C'>


= 1.5 mm/s


Assume:


This field method was the motivation – But it couldn’t be used in the mixing tank!

(The spatial variation in concentration couldn’t be resolved in the tank)



<w'C'> = ws <C>


<C> (mg/liter)

<w'C'>


= 1.5 mm/s


Assume:


This field method was the motivation – But it couldn’t be used in the mixing tank!

(The spatial variation in concentration couldn’t be resolved in the tank)

Question:

-- It there a reliable, independent measure of ws we can use in the field?

Comparison of ADV measurements to camera measurements of from York River estuary from October 6th 2012 (just last week):

Promising!

Period of strongest flow

outline of presentation : motivation: acoustic doppler velocimeter (adv) field o bservations

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