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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 .

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

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  1. 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

  2. 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

  3. 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

  4. Observations provided by an Acoustic Doppler Velocimeter Sensing volume ~ 35 cmab ADV after retrieval ADV at deployment (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 (where M is depth-integrated c) • Drag Coefficient (Cd ): Cd= <u’w’>/(u2) 2/9 Fugate and Friedrichs ,2002; Friedrichs et al., 2009; Cartwright, et al. 2009 and Dickhudt et al., 2010

  5. ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009) -- Spatial variability in WsBULK and bed εbetween 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

  6. ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009) Regime 1:Low ws, High ε -- Spatial variability in WsBULK and bed εbetween 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

  7. ADV Observed Settling Velocity (WsBULK) and Bed Erodibility (ε) (2006-2009) Regime 2: Highws, Low ε -- Spatial variability in WsBULK and bed εbetween 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

  8. 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. 3/9 Cartwright et al., 2009

  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) . (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 WsBULK = <w’c’>/<c> (mm/s) Regime 1 Similar WsBULK at the beginning of tidal phase suggest presence of flocs during both regimes 0.1 0.2 0.3 0.4 0.5 Tidal Velocity Phase (q/p) • (Note that Bulk Settling Velocity, • wsBULK = <w’c’>/csetis considered reliable for mud only during accelerating half of tidal cycle.) Increasing |u| and τb 4/9

  10. 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). (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 Regime 2 Regime 1: Flocs -Lower observed WsBULK at peak |u| and τb (<0.8 mm/s) WsBULK = <w’c’>/<c> (mm/s) Regime 1 Similar WsBULK at the beginning of tidal phase suggest presence of flocs during both regimes 0.1 0.2 0.3 0.4 0.5 Tidal Velocity Phase (q/p) • (Note that Bulk Settling Velocity, • wsBULK = <w’c’>/csetis considered reliable for mud only during accelerating half of tidal cycle.) Increasing |u| and τb 4/9

  11. Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d)) Regime 1(Low ws, High ε) Regime 2 (High ws, Low ε) (a) Tidal Current Speed (cm/s) 45 (b) Bed Stress (Pa) 0.25 Regime 1:Flocs/Fines Regime 1 30 0.2 Regime 2 0.15 Regime 2 15 0.1 Regime 1 0.05 (d) Drag Coefficient (c) Concentration (mg/L) 0.0016 200 Regime 2:Pellets+Flocs 0.0012 150 Regime 2 Regime 1 0.00008 100 CWASH 0.00004 50 CWASH Regime 2 Regime 1 1 0 0.5 0.5 1 0 Tidal Velocity Phase (θ/π) Tidal Velocity Phase (θ/π) Decreasing IuI Decreasing IuI Increasing IuI Increasing IuI 5/9

  12. Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d)) Regime 1(Low ws, High ε) Regime 2 (High ws, Low ε) (a) Tidal Current Speed (cm/s) 45 (b) Bed Stress (Pa) 0.25 Regime 1: Flocs/Fines -Lower τb despite similar current speeds -High C at relatively low τb -More stratified WC: Lower ADV derived Cd plus ΔS about 3 ppt (VECOS) Regime 1 30 0.2 Regime 2 0.15 Regime 2 15 0.1 Regime 1 0.05 (d) Drag Coefficient (c) Concentration (mg/L) 0.0016 200 • Regime 2: Pellets+Flocs • -Lower C at high τb • -Less stratified WC: Higher ADV derived Cd plus ΔS about 1 ppt (VECOS) 0.0012 150 Regime 2 Regime 1 0.00008 100 CWASH 0.00004 50 CWASH Regime 2 Regime 1 1 0 0.5 0.5 1 0 Tidal Velocity Phase (θ/π) Tidal Velocity Phase (θ/π) Decreasing IuI Decreasing IuI Increasing IuI Increasing IuI 5/9

  13. Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d)) Regime 1(Low ws, High ε) Regime 2 (High ws, Low ε) (a) Tidal Current Speed (cm/s) 45 (b) Bed Stress (Pa) 0.25 Regime 1: Flocs/Fines -High C at relatively low τb -Lower τb despite similar current speeds -More stratified WC: Lower ADV derived Cd plus ΔS about 3 ppt (VECOS) -Trapping of fines (STM) Regime 1 30 0.2 Regime 2 0.15 Regime 2 15 0.1 Regime 1 0.05 (d) Drag Coefficient (c) Concentration (mg/L) 0.0016 200 • Regime 2: Pellets+Flocs • -Lower C at high τb • -Less stratified WC: Higher ADV derived Cd plus ΔS about 1 ppt (VECOS) • -Dispersal of fines, pellets suspended (No STM) 0.0012 150 Regime 2 Regime 1 0.00008 100 CWASH 0.00004 50 CWASH Regime 2 Regime 1 1 0 0.5 0.5 1 0 Tidal Velocity Phase (θ/π) Tidal Velocity Phase (θ/π) Decreasing IuI Decreasing IuI Increasing IuI Increasing IuI 5/9

  14. Phase- Averaged Erosion and Deposition for Two Regimes Erosion -- Once tbincreases 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) Concentration (mg/L) Concentration (mg/L) τcINT = ~ 0.02 Pa Washload (~20%) Washload (~20%) τcINT = ~ 0.05 Pa Bed Stress (Pa) Bed Stress (Pa) Hysteresis plots of C vs. tb for the top 20 % of tidal cycles with the strongest tb for (a) Regime 1and (b) Regime 2. 6/9

  15. Phase- Averaged Erosion and Deposition for Two Regimes -- As tbdecreases 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 τblarger than ~ 0.08 Pa. -- As tbdecreases 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 Regime 1 (Flocs) Regime 2 (Pellets + Flocs) τcDEP flocs = ~ 0.08 Pa τcDEP flocs = ~ 0.08 Pa Concentration (mg/L) Concentration (mg/L) τcINT = ~ 0.02 Pa Washload (~20%) Washload(~20%) τcINT = ~ 0.05 Pa Bed Stress (Pa) Bed Stress (Pa) Hysteresis plots of C vs. tb for the top 20 % of tidal cycles with the strongest tb for (a) Regime 1and (b) Regime 2. 6/9

  16. Phase- Averaged Erosion and Deposition for Two Regimes -- As tbdecreases 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 τblarger than ~ 0.08 Pa. -- As tbdecreases 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 Regime 1 (Flocs) Regime 2 (Pellets + Flocs) τcDEP flocs = ~ 0.08 Pa τcDEP flocs = ~ 0.08 Pa Concentration (mg/L) Concentration (mg/L) Pellets (~30%) Flocs (~80%) Flocs (~50%) τcINT = ~ 0.02 Pa Washload (~20%) Washload (~20%) τcINT = ~ 0.05 Pa Bed Stress (Pa) Bed Stress (Pa) Hysteresis plots of C vs. tb for the top 20 % of tidal cycles with the strongest tb for (a) Regime 1and (b) Regime 2. 6/9

  17. Phase-Averaged WsBULK for Two Regimes (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 Regime 2 Regime 1: Flocs -Lower observed WsBULK at peak |u| and τb (<0.8 mm/s) WsBULK = <w’c’>/<c> (mm/s) Regime 1 Similar WsBULK at the beginning of tidal phase suggest presence of flocs during both regimes 0.1 0.2 0.3 0.4 0.5 Tidal Velocity Phase (q/p) • (Note that Bulk Settling Velocity, • wsBULK = <w’c’>/csetis considered reliable for mud only during accelerating half of tidal cycle.) Increasing |u| and τb 7/9

  18. Phase-Averaged WsBULK for Two Regimes Analysis of WsBULK by removing CWASH and solving for settling velocity of the depositing component (WsDEP) during increasing tballows separate estimates for settling velocities of flocs (WsFLOCS) and pellets (WsPELLETS). Remove cwash (b) Regime 2 (Pellets +Flocs) (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 WsBULK = <w’c’>/<c> (mm/s) WsDEP = (c/(c-cwash))*WsBULK (mm/s) Regime 1 (Flocs) Regime 1 (b) Depositing component of Settling Velocity, WsDEP 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 Tidal Velocity Phase (q/p) 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 8/9

  19. Phase-Averaged WsBULK for Two Regimes Analysis of WsBULK by removing CWASH and solving for settling velocity of the depositing component (WsDEP) during increasing tballows separate estimates for settling velocities of flocs (WsFLOCS) and pellets (WsPELLETS). Remove cwash (b) Regime 2 (Pellets +Flocs) (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 WsDEP= WsFLOCS WsFLOC = ~ 0.85 mm/s Implies floc size is limited by settling-induced shear rather than tb . WsBULK = <w’c’>/<c> (mm/s) WsDEP = (c/(c-cwash))*WsBULK (mm/s) Regime 1 (Flocs) Regime 1 (b) Depositing component of Settling Velocity, WsDEP 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 Tidal Velocity Phase (q/p) 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 8/9

  20. Phase-Averaged WsBULK for Two Regimes Analysis of WsBULK by removing CWASH and solving for settling velocity of the depositing component (WsDEP) during increasing tballows separate estimates for settling velocities of flocs (WsFLOCS) and pellets (WsPELLETS). Remove cwash WsDEP= fFWsFLOCS+ fFWsPELLETS = ~ 1.43 mm/s at peak tb Assume: fF = 5/8, fP = 3/8 This gives: WsPELLETS= ~ 2.4 mm/s (b) Regime 2 (Pellets +Flocs) (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 WsDEP= WsFLOCS WsFLOC = ~ 0.85 mm/s Implies floc size is limited by settling-induced shear rather than tb . WsBULK = <w’c’>/<c> (mm/s) WsDEP = (c/(c-cwash))*WsBULK (mm/s) Regime 1 (Flocs) Regime 1 (b) Depositing component of Settling Velocity, WsDEP 0.1 0.2 0.3 0.4 0.5 0.1 0.2 0.3 0.4 0.5 Tidal Velocity Phase (q/p) 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 8/9

  21. 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 pelletsmixed 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

  22. Acknowledgements MarjyFriedrichs Tim Gass Wayne Reisner Funding: Julia Moriarity Carissa Wilkerson 10/10

  23. 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 1/9 Dickhudt et al., 2009 ;Schaffner et al., 2001

  24. 22 22 400 20 300 Salinity 0.5 mab (ppt) Pamunkey River Discharge (m3/s) 200 18 100 16 0 14 June 12- August 31, 2007 09/01 08/01 07/01 06/01

  25. 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 pelletsmixed 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 bothRegimes. • Once stress (τb) exceeds an initial critical value (τcINIT) of ~ 0.02 to 0.05 Pa, sediment concentration (C) continually increases with τbfor both Regimes. • As τbdecreases, 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 tbaveraged 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

  26. 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 b. Daily-averaged ε vs. 5-day-averaged tb 25 Hour Averaged Erodibility, (kg/m2/Pa) 25 Hour Averaged Erodibility, (kg/m2/Pa) 25 Hour Averaged Bed Stress (Pa) 120 Hour Averaged Bed Stress (Pa) 9/10

  27. 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 b. Daily-averaged ε vs. 5-day-averaged tb R=0.6042 R=0.7395 25 Hour Averaged Erodibility, (kg/m2/Pa) 25 Hour Averaged Erodibility, (kg/m2/Pa) 25 Hour Averaged Bed Stress (Pa) 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

  28. 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 b. Daily-averaged ε vs. 5-day-averaged tb R=0.6042 R=-0.7759 R=0.7395 R=-0.6774 25 Hour Averaged Erodibility, (kg/m2/Pa) 25 Hour Averaged Erodibility, (kg/m2/Pa) 25 Hour Averaged Bed Stress (Pa) 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

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