kelsey fall*, carl friedrichs , and grace cartwright virginia institute of marine science
DESCRIPTION
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 Cartwright Virginia Institute of Marine Science . - PowerPoint PPT PresentationTRANSCRIPT
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
Tidal Velocity Phase(θ/π)
Increasing IuI Decreasing IuI
5/11
(a) Tidal Current Speed (cm/s)
15
30
45
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
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
Tidal Velocity Phase(θ/π)
Increasing IuI Decreasing IuI
5/11
(a) Tidal Current Speed (cm/s)
15
30
45
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
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
Tidal Velocity Phase(θ/π)
Increasing IuI Decreasing IuI
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
)
(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-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).
(a) Sediment Bulk Settling Velocity, WsBULK
(b)
Phase-Averaged Settling Velocity for Two Regimes
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
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
)
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)
Phase-Averaged Settling Velocity for Two Regimes
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
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
)
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)
Phase-Averaged Settling Velocity for Two Regimes
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
= ~ 1.5 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 .
Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2
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