Transcript ppt_part1

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 Cartwright
Virginia 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 Cartwright
Virginia Institute of Marine Science
Motivation: Determine fundamental controls on sediment settling velocity and bed erodibility
in muddy estuaries
Study Site: York River Estuary ,VA (MUDBED Long-term Observing System)
-- NSF MUDBED project benthic ADV tripods (1) and monthly bed sampling cruises (2) provide long-term
observations within a strong physical-biological gradient.
Physical-biological gradient found along the York estuary :
-- Upper York Physically Dominated Site: Dominated by physical processes (ETM)
--Mid York Intermediate site: Seasonal STM
--Lower York Biological site: Biological Influences Dominate (No ETM)
Schaffner et al., 2001
Observations provided by an Acoustic Doppler Velocimeter
Sensing volume ~ 35 cmab
ADV at
deployment
ADV
after retrieval
(Photos by C. Cartwright)
-- 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
•
Drag Coefficient (Cd ):
Cd = <u’w’>/(u2)
Fugate and Friedrichs ,2002; Friedrichs et al., 2009; Cartwright, et al. 2009 and Dickhudt et al., 2010
(where M is depth-integrated c)
2/9
ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009)
-- 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.
Cartwright et al., 2009
3/9
ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009)
Regime 1:Low ws, High ε
-- 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.
Cartwright et al., 2009
3/9
ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009)
Regime 2: High ws, Low ε
-- 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.
Cartwright et al., 2009
3/9
Objective: Use tidal phase analysis on ADV data to investigate what is happening at
the Intermediate site when Regime 1(Low ws, High ε)Regime 2 (High ws, Low ε).
Tidal Phase Average Analysis (Fall, 2012):
Average ADV data (settling velocity, current speed, concentration, bed stress, and drag coefficient) over the
tidal phases with the strongest bed stresses for each regime to obtain representative values of each
parameter throughout a tidal phase.
Cartwright et al., 2009
3/9
1.6
Phase-averaged WsBULK for two regimes suggest different particles in are
suspended during Regime 1 (Low ws, High ε) than Regime 2 (High ws, Low ε)
.
1.4
(a) Sediment Bulk
Settling Velocity, WsBULK
WsBULK = <w’c’>/<c> (mm/s)
1.2
1
Regime 2
0.8
0.6
Regime 1
0.4
Similar WsBULK at the beginning of
tidal phase suggest presence of
flocs during both regimes
0.2
0
0
0.1
0.2
0.2
0.4
0.3
0.6
0.4
0.8
Tidal Velocity Phase (q/p)
Increasing |u| and τb
0.5
1
(Note that Bulk Settling Velocity,
wsBULK = <w’c’>/cset is considered reliable
for mud only during accelerating half of
tidal cycle.)
4/9
Phase-averaged WsBULK for two regimes suggest different particles in are
suspended during Regime 1 (Low ws, High ε) than Regime 2 (High ws, Low ε).
1.6
1.4
(a) Sediment Bulk
Settling Velocity, WsBULK
Regime 2: Pellets+Flocs
-Higher observed WsBULK at peak |u| and τb
(~1.2 mm/s)
-Influence of pellets on WsBULK
WsBULK = <w’c’>/<c> (mm/s)
1.2
1
Regime 2
0.8
Regime 1: Flocs
0.6
-Lower observed WsBULK at peak |u| and τb
(<0.8 mm/s)
Regime 1
0.4
Similar WsBULK at the beginning of
tidal phase suggest presence of
flocs during both regimes
0.2
0
0
0.1
0.2
0.2
0.4
0.3
0.6
0.4
0.8
Tidal Velocity Phase (q/p)
Increasing |u| and τb
0.5
1
(Note that Bulk Settling Velocity,
wsBULK = <w’c’>/cset is considered reliable
for mud only during accelerating half of
tidal cycle.)
4/9
Velocity Tidal Phase Averaged Analysis
(Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d))
(a) Tidal Current Speed (cm/s)
45
Regime 1(Low ws, High ε)
Regime 2 (High ws, Low
ε)
(b) Bed Stress (Pa)
0.25
Regime 1:Flocs/Fines
Regime 1
0.2
Regime 2
30
0.15
Regime 2
0.1
15
Regime 1
0.05
200
(d) Drag Coefficient
(c) Concentration (mg/L)
Regime 2:Pellets+Flocs
0.0016
Regime 2
150
0.0012
Regime 1
0.00008
100
CWASH
CWASH 50
0
0.00004
Regime 2
Regime 1
0.5
Tidal Velocity Phase
(θ/π) Decreasing IuI
Increasing IuI
1
0
0.5
Tidal Velocity Phase
(θ/π) Decreasing IuI
Increasing IuI
1
5/9
Velocity Tidal Phase Averaged Analysis
(Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d))
(a) Tidal Current Speed (cm/s)
45
(b) Bed Stress (Pa)
0.25
Regime 1: Flocs/Fines
Regime 1
0.2
- Lower τb despite similar current
speeds
Regime 2
30
Regime 1(Low ws, High ε)
Regime 2 (High ws, Low
ε)
0.15
-High C at relatively low τb
Regime 2
0.1
-More stratified WC: Lower ADV
derived Cd plus ΔS about 3 ppt
(VECOS)
15
Regime 1
0.05
200
(c) Concentration (mg/L)
(d) Drag Coefficient
Regime 2: Pellets+Flocs
0.0016
-Lower C at high τb
Regime 2
150
0.0012
Regime 1
100
-Less stratified WC: Higher ADV
derived Cd plus ΔS about 1 ppt
(VECOS)
0.00008
CWASH
CWASH 50
0
0.00004
Regime 2
Regime 1
0.5
Tidal Velocity Phase
(θ/π) Decreasing IuI
Increasing IuI
1
0
0.5
Tidal Velocity Phase
(θ/π) Decreasing IuI
Increasing IuI
1
5/9
Velocity Tidal Phase Averaged Analysis
(Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d))
(a) Tidal Current Speed (cm/s)
45
Regime 1(Low ws, High ε)
Regime 2 (High ws, Low
ε)
(b) Bed Stress (Pa)
0.25
Regime 1: Flocs/Fines
Regime 1
-High C at relatively low τb
0.2
- Lower τb despite similar current
speeds
Regime 2
30
0.15
Regime 2
-More stratified WC: Lower ADV
derived Cd plus ΔS about 3 ppt
(VECOS)
0.1
15
Regime 1
0.05
-Trapping of fines (STM)
200
(c) Concentration (mg/L)
(d) Drag Coefficient
Regime 2: Pellets+Flocs
0.0016
-Lower C at high τb
Regime 2
150
0.0012
Regime 1
100
0.00008
-Less stratified WC: Higher ADV
derived Cd plus ΔS about 1 ppt
(VECOS)
0.00004
-Dispersal of fines, pellets
suspended (No STM)
CWASH
CWASH 50
0
Regime 2
Regime 1
0.5
Tidal Velocity Phase
(θ/π) Decreasing IuI
Increasing IuI
1
0
0.5
Tidal Velocity Phase
(θ/π) Decreasing IuI
Increasing IuI
1
5/9
Phase- Averaged Erosion and Deposition for Two Regimes
Erosion
-- Once tb increases past a critical stress for initiation (tcINIT), C continually increases for both
Regime 1 and for Regime 2
Regime 1 (Flocs)
Regime 2 (Pellets + Flocs)
From 31 to 60
180
180
160
160
140
140
(mg/L)
Concentration
Conc.
(mg/L)
Concentration
Conc.
From 31 to 60
120
100
80
60
0
0
100
80
60
40
40
20
120
τcINT = ~ 0.02 Pa
0.05
0.1
20
Washload (~20%)
0.15
0.2
0.25
0.3
0
0
τcINT = ~ 0.05 Pa
0.05
0.1
0.15
Washload (~20%)
0.2
0.25
stress(Pa)
stress(Pa)
BedBed
Stress
Bed Bed
Stress
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 .
0.3
6/9
Phase- Averaged Erosion and Deposition for Two Regimes
-- 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.
Depositio
n
-- 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.
Regime 1 (Flocs)
Regime 2 (Pellets + Flocs)
From 31 to 60
180
160
160
140
140
(mg/L)
Concentration
Conc.
(mg/L)
Concentration
Conc.
From 31 to 60
τcDEP flocs = ~ 0.08 Pa
180
120
100
80
60
0
0
τcDEP flocs = ~ 0.08 Pa
100
80
60
40
40
20
120
τcINT = ~ 0.02 Pa
0.05
0.1
20
Washload (~20%)
0.15
0.2
0.25
0.3
0
0
τcINT = ~ 0.05 Pa
0.05
0.1
0.15
Washload(~20%)
0.2
0.25
stress(Pa)
stress(Pa)
BedBed
Stress
Bed Bed
Stress
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 .
0.3
6/9
Phase- Averaged Erosion and Deposition for Two Regimes
-- 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.
Depositio
n
-- 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.
Regime 1 (Flocs)
Regime 2 (Pellets + Flocs)
From 31 to 60
180
160
160
140
140
(mg/L)
Concentration
Conc.
(mg/L)
Concentration
Conc.
From 31 to 60
τcDEP flocs = ~ 0.08 Pa
180
120
100
80
Flocs (~80%)
60
0
0
τcDEP flocs = ~ 0.08 Pa
100
Pellets
(~30%)
80
60
Flocs (~50%)
40
40
20
120
τcINT = ~ 0.02 Pa
0.05
0.1
20
Washload (~20%)
0.15
0.2
0.25
0.3
0
0
τcINT = ~ 0.05 Pa
0.05
0.1
0.15
Washload (~20%)
0.2
0.25
stress(Pa)
stress(Pa)
BedBed
Stress
Bed Bed
Stress
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 .
0.3
6/9
Phase-Averaged WsBULK for Two Regimes
1.6
1.4
(a) Sediment Bulk
Settling Velocity, WsBULK
Regime 2: Pellets+Flocs
-Lower observed WsBULK at peak |u| and τb
(~1.2 mm/s)
-Influence of pellets on WsBULK
WsBULK = <w’c’>/<c> (mm/s)
1.2
1
Regime 2
0.8
Regime 1: Flocs
0.6
-Lower observed WsBULK at peak |u| and τb
(<0.8 mm/s)
Regime 1
0.4
Similar WsBULK at the beginning of
tidal phase suggest presence of
flocs during both regimes
0.2
0
0
0.1
0.2
0.2
0.4
0.3
0.6
0.4
0.8
Tidal Velocity Phase (q/p)
Increasing |u| and τb
0.5
1
(Note that Bulk Settling Velocity,
wsBULK = <w’c’>/cset is considered reliable
for mud only during accelerating half of
tidal cycle.)
7/9
Phase-Averaged WsBULK for Two Regimes
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).
1.6
cwash
(a) Sediment Bulk
Settling Velocity, WsBULK
WsDEP = (c/(c-cwash))*WsBULK (mm/s)
1.4
Remove
WsBULK = <w’c’>/<c> (mm/s)
1.2
1
Regime 2
0.8
0.6
Regime 1
0.4
0.1
0.2
0.2
0.4
0.3
0.6
Increasing |u| and τb
0.4
0.8
0.5
1
(b)Regime 2 (Pellets +Flocs)
1.4
1.4
1.2
1.2
11
0.8
0.8
Regime 1 (Flocs)
0.6
0.6
0.4
0.4
0.2
0.2
0.2
0
0
1.6
1.6
00
0
Tidal Velocity Phase (q/p)
(b) Depositing component of
Settling Velocity, WsDEP
0.2
0.1
0.2
0.4
0.2
0.4
0.6
0.3
0.6
0.8
0.4
0.8
0.511
Increasing |u| and τb
Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2
8/9
Phase-Averaged WsBULK for Two Regimes
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).
1.6
cwash
(a) Sediment Bulk
Settling Velocity, WsBULK
WsDEP = (c/(c-cwash))*WsBULK (mm/s)
1.4
Remove
WsBULK = <w’c’>/<c> (mm/s)
1.2
1
Regime 2
0.8
0.6
Regime 1
0.4
0.1
0.2
0.2
0.4
0.3
0.6
Increasing |u| and τb
0.4
0.8
0.5
1
(b)Regime 2 (Pellets +Flocs)
1.4
1.4
1.2
1.2
11
WsDEP = WsFLOCS
0.8
0.8
WsFLOC = ~ 0.85 mm/s
Implies floc size is limited
by settling-induced shear
rather than tb .
Regime 1 (Flocs)
0.6
0.6
0.4
0.4
0.2
0.2
0.2
0
0
1.6
1.6
00
0
Tidal Velocity Phase (q/p)
(b) Depositing component of
Settling Velocity, WsDEP
0.2
0.1
0.2
0.4
0.2
0.4
0.6
0.3
0.6
0.8
0.4
0.8
0.511
Increasing |u| and τb
Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2
8/9
Phase-Averaged WsBULK for Two Regimes
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).
1.6
cwash
(a) Sediment Bulk
Settling Velocity, WsBULK
WsDEP = (c/(c-cwash))*WsBULK (mm/s)
1.4
Remove
WsBULK = <w’c’>/<c> (mm/s)
1.2
1
Regime 2
0.8
0.6
Regime 1
0.4
0.1
0.2
0.2
0.4
0.3
0.6
Increasing |u| and τb
0.4
0.8
0.5
1
WsDEP = fFWsFLOCS + fFWsPELLETS
= ~ 1.43 mm/s at peak tb
Assume: fF = 5/8, fP = 3/8
This gives:
(b)Regime 2 (Pellets +Flocs)
1.4
1.4
1.2
1.2
WsPELLETS = ~ 2.4 mm/s
11
WsDEP = WsFLOCS
0.8
0.8
WsFLOC = ~ 0.85 mm/s
Implies floc size is limited
by settling-induced shear
rather than tb .
Regime 1 (Flocs)
0.6
0.6
0.4
0.4
0.2
0.2
0.2
0
0
1.6
1.6
00
0
Tidal Velocity Phase (q/p)
(b) Depositing component of
Settling Velocity, WsDEP
0.2
0.1
0.2
0.4
0.2
0.4
0.6
0.3
0.6
0.8
0.4
0.8
0.511
Increasing |u| and τb
Recall: peak τb ~ 0.15 Pa for Regime 1, and peak τb ~ 0.22 Pa for Regime 2
8/9
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:
•
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.
9/9
Acknowledgements
Marjy Friedrichs
Tim Gass
Wayne Reisner
Julia Moriarity
Carissa Wilkerson
Funding:
10/10
Motivation: Determine fundamental controls on sediment settling velocity and bed
erodibility in muddy estuaries
Study site: York River Estuary, VA
(MUDBED Long-term
Observing System)
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
Dickhudt et al., 2009 ;Schaffner et al., 2001
1/9
Pamunkey River Discharge (m3/s)
22
300
20
200
18
100
16
0
06/01
07/01
08/01
June 12- August 31, 2007
14
09/01
Salinity 0.5 mab (ppt)
400
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 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.
10/10
Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2
Reveals two distinct relationships between ε and tb.
a. Daily-averaged ε vs. daily averaged tb
4
25 Hour Averaged Erodibility, (kg/m2/Pa)
25 Hour Averaged Erodibility, (kg/m2/Pa)
4
3.5
3
2.5
2
1.5
1
0.5
0
0
0.02
0.04
0.06
0.08
0.1
25 Hour Averaged Bed Stress (Pa)
0.12
b. Daily-averaged ε vs. 5-day-averaged tb
3.5
3
2.5
2
1.5
1
0.5
0
0
0.02
0.04
0.06
0.08
0.1
0.12
120 Hour Averaged Bed Stress (Pa)
9/10
Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2
Reveals two distinct relationships between ε and tb.
a. Daily-averaged ε vs. daily averaged tb
4
25 Hour Averaged Erodibility, (kg/m2/Pa)
25 Hour Averaged Erodibility, (kg/m2/Pa)
4
R=0.6042
3.5
3
2.5
2
1.5
1
0.5
0
0
0.02
0.04
0.06
0.08
0.1
25 Hour Averaged Bed Stress (Pa)
0.12
b. Daily-averaged ε vs. 5-day-averaged tb
R=0.7395
3.5
3
2.5
2
1.5
1
0.5
0
0
0.02
0.04
0.06
0.08
0.1
0.12
120 Hour Averaged Bed Stress (Pa)
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.
9/10
Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2
Reveals two distinct relationships between ε and tb.
a. Daily-averaged ε vs. daily averaged tb
4
25 Hour Averaged Erodibility, (kg/m2/Pa)
25 Hour Averaged Erodibility, (kg/m2/Pa)
4
R=0.6042
R=-0.7759
3.5
3
2.5
2
1.5
1
0.5
0
0
0.02
0.04
0.06
0.08
0.1
25 Hour Averaged Bed Stress (Pa)
0.12
b. Daily-averaged ε vs. 5-day-averaged tb
R=0.7395
R=-0.6774
3.5
3
2.5
2
1.5
1
0.5
0
0
0.02
0.04
0.06
0.08
0.1
0.12
120 Hour Averaged Bed Stress (Pa)
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.
9/10