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

Controls on particle settling velocity and bed erodibility 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 :

-- In the middle to upper York River estuary, disturbance by sediment transport reduces macrobenthic activity, and sediment layering is often preserved. (e.g., Clay Bank – “Intermediate Site”)

-- In the lower York and neighboring Chesapeake Bay, layering is often destroyed by bioturbation. (e.g., Gloucester Point – “Biological Site”)

-- NSF MUDBED project ADV tripods provide long-term observations within a strong physical-biological gradient.

Study site: York River Estuary, VA

(X-rayscourtesy of

L. Schaffner)

Schaffner et al., 2001 1/11

ADV at deployment

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

• Suspended mass concentration (c) from acoustic backscatter

• Bed Stress (τb): ρ*<u’w’>

• Bulk Settling Velocity (wsBULK ): <w’c’>/cset

• Erodibility (ε) given by ε = τb/M, where M is depth-integrated C

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 2/11

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 bulk settling velocity (WsBULK) and bed erodibility (ε) is observed at the Intermediate Site.

3-day mean of ε from fits to M = ε τb using ADVs

Biological siteWsBULK ~1 mm/s

Intermediate siteWsBULK varies from ~ 0.5 mm/s (Regime 1) to ~ 1

mm/s (Regime 2)

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

1.5

1.0

0.5

0

WsB

ULK

(m

m/s

)

Cartwright et al., 2009 3/11

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

)

3-day mean of ε from fits to M = ε τb using ADVs

Biological siteWsBULK ~1 mm/s

Intermediate siteWsBULK varies from ~ 0.5 mm/s (Regime 1) to ~ 1

mm/s (Regime 2)

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

1.5

1.0

0.5

0

WsB

ULK

(m

m/s

)

Cartwright et al., 2009

What is happening at Intermediate Site when Regime 1 Regime 2?

3/11

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

(a) Sediment Bulk Settling Velocity, WsBULK

Phase-Averaged Settling Velocity 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-Higher observed WsBULK at peak |u| and τb (~1.2 mm/s)-Influence of pellets on WsBULK

7/11

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

Tidal Analysis highlights differences in Regime 1 and Regime 2.

Tidal Velocity Phase(θ/π)

Increasing IuI Decreasing IuI

(b) Bed Stress (Pa)

(d) Concentration (mg/L)

0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25

(c) Drag Coefficient

0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

(a) Tidal Current Speed (cm/s)

15

30

45



5/11


15

30

45




(b) Bed Stress (Pa)


0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25


0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

Regime 1: Flocs -High C at relatively low τb

-Lower ADV derived Cd (more stratified water column)

-Lower τb despite higher similar current speeds

Regime 1

Regime 1

Regime 1

Regime 1



5/11


15

30

45




(b) Bed Stress (Pa)


0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25


0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

Regime 1: Flocs -High C at relatively low τb

-Lower ADV derived Cd (more stratified water column)

-Lower τb despite higher similar current speeds

Regime 2: Pellets+Flocs-Lower C at high τb

-Increase in Cd (Water column less stratified)

Regime 2

Regime 2

Regime 2

Regime 2



5/11

Conc

entr

ation

(mg/

L)

(a) (b)

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 .

τcDEP flocs = ~ 0.08 Pa

Washload (~20%)

Flocs (~80%)

Washload (~20%)

Flocs (~50%)

Pellets (~30%)

Bed Stress (Pa) Bed Stress (Pa)

Conc

entr

ation

(mg/

L)

τcDEP flocs = ~ 0.08 Pa

τcINT = ~ 0.05 Pa

τcINT = ~ 0.02 Pa

Regime 1 Regime 2

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

Erosion

-- 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

6/11

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)



Regime 1

Regime 2

Increasing |u| and τb


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

7/11

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

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).


(b)


Remove cwash

Regime 1

Regime 2


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

8/11

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

WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

WsD

EP =

(c/(

c-c w

ash))

*WsB

ULK

(m

m/s

)



(b)


Remove cwash

Regime 1

Regime 2


Regime 1

Regime 2

0.1 0.2 0.3 0.4 0.5



WsFLOC = ~ 0.85 mm/s

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

WsDEP = WsFLOCS

8/11


WsB

ULK

= <

w’c’

>/<c

> (m

m/s

)

WsD

EP =

(c/(

c-c w

ash))

*WsB

ULK

(m

m/s

)



(b)


Remove cwash

Regime 1

Regime 2


Regime 1

Regime 2

0.1 0.2 0.3 0.4 0.5



WsDEP = WsFLOCS

WsDEP = fFWsFLOCS + fFWsPELLETS

= ~ 1.5 mm/s at peak tb

Assume: fF = 5/8, fP = 3/8 This gives:WsPELLETS = ~ 2 mm/s

8/11

WsFLOC = ~ 0.85 mm/s

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


25 or 120 Hour Averaged Bed Stress (Pa)

25 H

our A

vera

ged

Erod

ibili

ty, (

kg/m

2 /Pa

)

Daily-averaged erodibility is correlated either to 5-Day-averaged tb (Regime 1) or to daily-averaged tb (Regime 2), revealing two distinct relationships between ε and 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.

Influence of Stress History on Bed Erodibility for Two Regimes

Regime 1

Regime 2

9/11

Summary and Future Work:

• 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 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 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.

10/11

AcknowledgementsMarjy FriedrichsTim GassWayne Reisner Funding:Julia MoriarityCarissa Wilkerson

Questions?

11/11

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

Documents

Transcript of Kelsey Fall*, Carl Friedrichs , and Grace Cartwright Virginia Institute of Marine Science