Controls on particle settling velocity and bed erodibilty in the presence of muddy flocs and pellets as inferred by ADVs, York

River estuary, Virginia, USA

Kelsey Fall*, Carl Friedrichs, and Grace CartwrightVirginia Institute of Marine Science

Controls on particle settling velocity and bed erodibilty in the presence of muddy flocs and pellets as inferred by ADVs, York

River estuary, Virginia, USA

Kelsey Fall*, Carl Friedrichs, and Grace CartwrightVirginia Institute of Marine Science

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

Physical-biological gradient found along the York estuary :

-- Physically Dominated Site-Upper Estuary : Dominated by physical processes (ETM)

-- Intermediate Site-Mid-estuary: Mixed Physical and Biological Influences (Seasonal STM)

-- Biological Site-Lower Estuary: Biological Influences Dominate

Study site: York River Estuary, VA(MUDBED Long-term

Observing System)

Dickhudt et al., 2009 ;Schaffner et al., 2001 1/9

ADV at deployment

-- ADVs provide continual long-term estimates of:

• Suspended mass concentration(c) from acoustic backscatter when calibrated by pump samples

• Bed Stress (τb): τb=ρ*<u’w’>

• Bulk Settling Velocity (WsBULK ): WsBULK=<w’c’>/c

• Erodibility (ε): ε = τb/M

(where M is depth-integrated c)

• Drag Coefficient (Cd ): Cd = <u’w’>/(u2)

ADVafter retrieval

Observations provided by an Acoustic Doppler VelocimeterSensing volume ~ 35 cmab

(Photos by C. Cartwright)

Fugate and Friedrichs ,2002; Friedrichs et al., 2009; Cartwright, et al. 2009 and Dickhudt et al., 2010 2/9

ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009)

Cartwright et al., 2009

-- Spatial variability in WsBULK and bed ε between Biological Site and Intermediate Site.-- Little seasonal variability in WsBULK and ε at the Biological Site.-- Two distinct regimes linked to seasonal variability in WsBULK and ε at the Intermediate Site.

3/9

Cartwright et al., 2009

Objective: Use tidal phase analysis on ADV data to investigate what is happening at the Intermediate site when Regime 1Regime 2.

Tidal Phase Average Analysis (Fall, 2012): Average ADV data (current speed, concentration, bed stress, drag coefficient, and settling velocity) over the tidal phases with the strongest bed stresses for each regime to obtain representative values of each parameter throughout a tidal phase.

3/9

(a) Tidal Current Speed (cm/s)

15

30

45

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

(b) Bed Stress (Pa)

(c) Concentration (mg/L)

0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25

(d) Drag Coefficient

0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

Regime 1

Regime 1

Regime 1

Regime 1

Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d))

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

4/9

Regime 2

Regime 2

Regime 2

Regime 2

(a) Tidal Current Speed (cm/s)

15

30

45

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

(b) Bed Stress (Pa)

(c) Concentration (mg/L)

0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25

(d) Drag Coefficient

0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

Regime 1

Regime 1

Regime 1

Regime 1

Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d))

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

Regime 1: Flocs/Fines -High C at relatively low τb (trapping of fines)

4/9

Regime 2: Pellets+Flocs

--Lower C at high τb (dispersal of fines, pellets suspended)

Regime 2

Regime 2

Regime 2

Regime 2

(a) Tidal Current Speed (cm/s)

15

30

45

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

(b) Bed Stress (Pa)

(c) Concentration (mg/L)

0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25

(d) Drag Coefficient

0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

Regime 1

Regime 1

Regime 1

Regime 1

Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d))

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

Regime 1: Flocs/Fines -High C at relatively low τb (trapping of fines)

- More stratified WC: Lower ADV derived Cd plus ΔS about 3 ppt (VECOS)

-Lower τb despite higher similar current speeds

4/9

Regime 2: Pellets+Flocs

--Lower C at high τb (dispersal of fines, pellets suspended)

- Less stratified WC: Higher ADV derived Cd plus ΔS about 1 ppt (VECOS)

Regime 2

Regime 2

Regime 2

Regime 2

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

(a) Sediment Bulk Settling Velocity, WsBULK

Regime 1

Regime 2

Increasing |u| and τb

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

Similar WsBULK at the beginning of tidal phase suggest presence of flocs during both regimes

(Note that Bulk Settling Velocity, wsBULK = <w’c’>/cset is considered reliable for mud only during accelerating half of tidal cycle.)

Phase-averaged WsBULK for two regimes suggest different particles in are suspended during Regime 1 than Regime 2.

5/9

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

(a) Sediment Bulk Settling Velocity, WsBULK

Regime 1

Regime 2

Increasing |u| and τb

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

Similar WsBULK at the beginning of tidal phase suggest presence of flocs during both regimes

Regime 1: Flocs-Lower observed WsBULK at peak |u| and τb (<0.8 mm/s)

Regime 2: Pellets+Flocs-Higher observed WsBULK at peak |u| and τb (~1.2 mm/s)-Influence of pellets on WsBULK

(Note that Bulk Settling Velocity, wsBULK = <w’c’>/cset is considered reliable for mud only during accelerating half of tidal cycle.)

Phase-averaged WsBULK for two regimes suggest different particles in are suspended during Regime 1 than Regime 2.

5/9

Phase- Averaged Erosion and Deposition for Two Regimes

-- Once tb increases past a critical stress for initiation (tcINIT), C continually increases for both Regime 1 and for Regime 2

Erosion

Conc

entr

ation

(mg/

L)

WashloadWashload

Bed Stress (Pa) Bed Stress (Pa)

Conc

entr

ation

(mg/

L)

τcINT = ~ 0.05 Pa

τcINT = ~ 0.02 Pa

Regime 2

Hysteresis plots of C vs. tb for the top 20 % of tidal cycles with the strongest tb for (a) Regime 1 and (b) Regime 2 . 6/9

Regime 1

Phase- Averaged Erosion and Deposition for Two Regimes

WashloadWashload

Bed Stress (Pa) Bed Stress (Pa)

Conc

entr

ation

(mg/

L)

τcINT = ~ 0.05 Pa

τcINT = ~ 0.02 Pa

Regime 2 Regime 1

Conc

entr

ation

(mg/

L)

-- As tb decreases for Regime 1, C does not fall off quickly until tb ≤ 0.08 Pa, suggests that over individual tidal cycles, cohesion of settling flocs to the surface of the seabed is inhibited for τb larger than ~ 0.08 Pa. -- As tb decreases for Regime 2, C decreases more continually, suggesting pellets without as clear a tcDEP. But the decline in C accelerates for tb ≤ ~ 0.08 Pa, suggesting (i) a transition to floc deposition and (ii) that settling C component is ~ 3/8 pellets, ~ 5/8 flocs.

Deposition

τcDEP flocs = ~ 0.08 Pa

τcDEP flocs = ~ 0.08 Pa

Hysteresis plots of C vs. tb for the top 20 % of tidal cycles with the strongest tb for (a) Regime 1 and (b) Regime 2 . 6/9

Phase- Averaged Erosion and Deposition for Two Regimes

Washload (~20%)

Flocs (~80%)

Washload (~20%)

Flocs (~50%)

Pellets (~30%)

Bed Stress (Pa) Bed Stress (Pa)

Conc

entr

ation

(mg/

L)

τcINT = ~ 0.05 Pa

τcINT = ~ 0.02 Pa

Regime 2 Regime 1

Conc

entr

ation

(mg/

L)

-- As tb decreases for Regime 1, C does not fall off quickly until tb ≤ 0.08 Pa, suggests that over individual tidal cycles, cohesion of settling flocs to the surface of the seabed is inhibited for τb larger than ~ 0.08 Pa. -- As tb decreases for Regime 2, C decreases more continually, suggesting pellets without as clear a tcDEP. But the decline in C accelerates for tb ≤ ~ 0.08 Pa, suggesting (i) a transition to floc deposition and (ii) that settling C component is ~ 3/8 pellets, ~ 5/8 flocs.

Deposition

τcDEP flocs = ~ 0.08 Pa

τcDEP flocs = ~ 0.08 Pa

Hysteresis plots of C vs. tb for the top 20 % of tidal cycles with the strongest tb for (a) Regime 1 and (b) Regime 2 . 6/9

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

(a) Sediment Bulk Settling Velocity, WsBULK

Phase-Averaged WsBULK for Two Regimes

Regime 1

Regime 2

Increasing |u| and τb

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

Similar WsBULK at the beginning of tidal phase suggest presence of flocs during both regimes

Regime 1: Flocs-Lower observed WsBULK at peak |u| and τb (<0.8 mm/s)

Regime 2: Pellets+Flocs-Lower observed WsBULK at peak |u| and τb (~1.2 mm/s)-Influence of pellets on WsBULK

(Note that Bulk Settling Velocity, wsBULK = <w’c’>/cset is considered reliable for mud only during accelerating half of tidal cycle.)

7/9

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

WsD

EP =

(c/(

c-c w

ash))

*WsB

ULK

(m

m/s

)

Analysis of WsBULK by removing CWASH and solving for settling velocity of the depositing component (WsDEP) during increasing tb allows separate estimates for settling velocities of flocs (WsFLOCS) and pellets (WsPELLETS).

(a) Sediment Bulk Settling Velocity, WsBULK

(b)Remove cwash

Regime 1

Regime 2

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

Regime 1

Regime 2

0.1 0.2 0.3 0.4 0.5

(b) Depositing component of Settling Velocity, WsDEP

Increasing |u| and τb Increasing |u| and τb

Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2

Phase-Averaged WsBULK for Two Regimes

8/9

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

WsD

EP =

(c/(

c-c w

ash))

*WsB

ULK

(m

m/s

)

(a) Sediment Bulk Settling Velocity, WsBULK

(b)Remove cwash

Regime 1

Regime 2

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

Regime 1

Regime 2

0.1 0.2 0.3 0.4 0.5

(b) Depositing component of Settling Velocity, WsDEP

Increasing |u| and τb Increasing |u| and τb

WsFLOC = ~ 0.85 mm/s

Implies floc size is limited by settling-induced shear rather than tb .

WsDEP = WsFLOCS

Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2

Analysis of WsBULK by removing CWASH and solving for settling velocity of the depositing component (WsDEP) during increasing tb allows separate estimates for settling velocities of flocs (WsFLOCS) and pellets (WsPELLETS).

Phase-Averaged WsBULK for Two Regimes

8/9

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

WsD

EP =

(c/(

c-c w

ash))

*WsB

ULK

(m

m/s

)

(a) Sediment Bulk Settling Velocity, WsBULK

(b)Remove cwash

Regime 1

Regime 2

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

Regime 1

Regime 2

0.1 0.2 0.3 0.4 0.5

(b) Depositing component of Settling Velocity, WsDEP

Increasing |u| and τb Increasing |u| and τb

WsDEP = WsFLOCS

WsDEP = fFWsFLOCS + fFWsPELLETS

= ~ 1.43 mm/s at peak tb

Assume: fF = 5/8, fP = 3/8 This gives:WsPELLETS = ~ 2.4 mm/s

WsFLOC = ~ 0.85 mm/s

Implies floc size is limited by settling-induced shear rather than tb .

Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2

Analysis of WsBULK by removing CWASH and solving for settling velocity of the depositing component (WsDEP) during increasing tb allows separate estimates for settling velocities of flocs (WsFLOCS) and pellets (WsPELLETS).

Phase-Averaged WsBULK for Two Regimes

8/9

• York River sediment settling velocity (Ws) and erodibility (ε) are described by two contrasting regimes:

• (i) Regime 1: a period dominated by muddy flocs [lower Ws, higher ε].

• (ii) Regime 2: a period characterized by pellets mixed with flocs [higher Ws, lower ε].

• Tidal phase-averaging of ADV records for the strongest 20% of tides for June to August 2007 reveals:

• The presence and departure of the STM (changes in water column stratification) may control transition from Regime 1 to Regime 2

• Deposition patterns allow for a rough estimate of the proportions of the three main particle types (washload, flocs, pellets) in suspension during Regime 1 and Regime 2

• Subtraction of CWASH from WSBULK for Regime 1 results in a stable floc settling velocity of WsFLOC ≈ 0.85 mm/s. The constant floc settling velocity implies that at lower beds stresses floc size is limited by settling-induced shear rather than turbulence associated with bed stress.

• Separation of WsFLOC and CWASH from WSBULK for Regime 2 finally yields WSPELLET ≈ 2.4 mm/s.

• Future work will include (i) vertically stacked ADVs and (ii) deployment of a high-definition particle settling video camera.

Summary and Future Work:

9/9

10/10

AcknowledgementsMarjy FriedrichsTim GassWayne Reisner Funding:Julia MoriarityCarissa Wilkerson

400

300

200

100

0

22

20

22

18

16

1406/01 07/01 08/01 09/01

Pam

unke

y R

iver

Dis

char

ge (m

3 /s) Salinity 0.5 m

ab (ppt)

June 12- August 31, 2007

• York River sediment settling velocity (Ws) and erodibility (ε) are described by two contrasting regimes:

• (i) Regime 1: a period dominated by muddy flocs [lower Ws, higher ε].

• (ii) Regime 2: a period characterized by pellets mixed with flocs [higher Ws, lower ε].

• Tidal phase-averaging of ADV records for the strongest 20% of tides for June to August 2007 reveals:

• A non-settling wash load (CWASH) is always present during both Regimes.

• Once stress (τb) exceeds an initial critical value (τcINIT) of ~ 0.02 to 0.05 Pa, sediment concentration (C) continually increases with τb for both Regimes.

• As τb decreases, cohesion of settling flocs to the surface of the seabed is inhibited for τb larger than ~ 0.08 Pa for both Regimes.

• Subtraction of CWASH from WSBULK for Regime 1 results in a stable floc settling velocity of WsFLOC ≈ 0.85 mm/s. The constant floc settling velocity implies that at lower beds stresses floc size is limited by settling-induced shear rather than turbulence associated with bed stress.

• Separation of WsFLOC and CWASH from WSBULK for Regime 2 finally yields WSPELLET ≈ 2.4 mm/s.

• During Regime 1, ε increases with tb averaged over the previous 5 days, consistent with cohesive bed evolution; while for Regime 2, ε decreases with daily tb, perhaps consistent with bed armoring.

• Future work will include (i) vertically stacked ADVs and (ii) deployment of a high-definition particle settling video camera.

Summary and Future Work:

10/10

Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2

25 Hour Averaged Bed Stress (Pa)

25 H

our A

vera

ged

Erod

ibili

ty, (

kg/m

2 /Pa

)

120 Hour Averaged Bed Stress (Pa)25

Hou

r Ave

rage

d Er

odib

ility

, (kg

/m2 /

Pa)

Reveals two distinct relationships between ε and tb.

9/10

b. Daily-averaged ε vs. 5-day-averaged tb a. Daily-averaged ε vs. daily averaged tb

Regime 1: Erodibility (ε) increases proportional to the average stress over the last 5 days, consistent with cohesive bed evolution dominated by the consolidation state of flocs.

25 Hour Averaged Bed Stress (Pa)

25 H

our A

vera

ged

Erod

ibili

ty, (

kg/m

2 /Pa

)

120 Hour Averaged Bed Stress (Pa)25

Hou

r Ave

rage

d Er

odib

ility

, (kg

/m2 /

Pa)R=0.6042 R=0.7395

Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2

Reveals two distinct relationships between ε and tb.

9/10

b. Daily-averaged ε vs. 5-day-averaged tb a. Daily-averaged ε vs. daily averaged tb

Regime 1: Erodibility (ε) increases proportional to the average stress over the last 5 days, consistent with cohesive bed evolution dominated by the consolidation state of flocs.

Regime 2: Erodibility (ε) decreases with greater stress, possibly associated with the effects of bed armoring by the pellet component.

25 Hour Averaged Bed Stress (Pa)

25 H

our A

vera

ged

Erod

ibili

ty, (

kg/m

2 /Pa

)

120 Hour Averaged Bed Stress (Pa)25

Hou

r Ave

rage

d Er

odib

ility

, (kg

/m2 /

Pa)R=0.6042

R=-0.7759R=0.7395R=-0.6774

Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2

Reveals two distinct relationships between ε and tb.

9/10

b. Daily-averaged ε vs. 5-day-averaged tb a. Daily-averaged ε vs. daily averaged tb