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. Schaffner et al., 2001 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)

ADV at deployment -- ADVs provide continual long-term estimates of: Suspended mass concentration(c) from acoustic backscatter when calibrated by pump samples Bed Stress (τ b ): τ b =ρ* Bulk Settling Velocity (W sBULK ): W sBULK = /c Erodibility (ε): ε = τ b /M(where M is depth-integrated c) Drag Coefficient (C d ):C d = /(u 2 ) ADV after retrieval Observations provided by an Acoustic Doppler Velocimeter Sensing volume ~ 35 cmab (Photos by C. Cartwright) Fugate and Friedrichs,2002; Friedrichs et al., 2009; Cartwright, et al and Dickhudt et al., /9

ADV Observed Settling Velocity (W sBULK ) and Bed Erodibility (ε) ( ) Cartwright et al., Spatial variability in W sBULK and bed ε between Biological Site and Intermediate Site. -- Little seasonal variability in W sBULK and ε at the Biological Site. -- Two distinct regimes linked to seasonal variability in W sBULK and ε at the Intermediate Site. 3/9

ADV Observed Settling Velocity (W sBULK ) and Bed Erodibility (ε) ( ) Cartwright et al., Spatial variability in W sBULK and bed ε between Biological Site and Intermediate Site. -- Little seasonal variability in W sBULK and ε at the Biological Site. -- Two distinct regimes linked to seasonal variability in W sBULK and ε at the Intermediate Site. 3/9 Regime 1:Low w s, High ε

ADV Observed Settling Velocity (W sBULK ) and Bed Erodibility (ε) ( ) Cartwright et al., Spatial variability in W sBULK and bed ε between Biological Site and Intermediate Site. -- Little seasonal variability in W sBULK and ε at the Biological Site. -- Two distinct regimes linked to seasonal variability in W sBULK and ε at the Intermediate Site. 3/9 Regime 2: High w s, Low ε

Cartwright et al., 2009 Objective: Use tidal phase analysis on ADV data to investigate what is happening at the Intermediate site when Regime 1(Low w s, High ε)  Regime 2 (High w s, 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

W sBULK = / (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK Regime 1 Regime 2 Increasing |u| and τ b Tidal Velocity Phase (  ) Similar W sBULK at the beginning of tidal phase suggest presence of flocs during both regimes (Note that Bulk Settling Velocity, w sBULK = /c set is considered reliable for mud only during accelerating half of tidal cycle.) Phase-averaged W sBULK for two regimes suggest different particles in are suspended during Regime 1 (Low w s, High ε) than Regime 2 (High w s, Low ε). 4/9

W sBULK = / (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK Regime 1 Regime 2 Increasing |u| and τ b Tidal Velocity Phase (  ) Similar W sBULK at the beginning of tidal phase suggest presence of flocs during both regimes Regime 1: Flocs -Lower observed W sBULK at peak |u| and τ b (<0.8 mm/s) Regime 2: Pellets+Flocs -Higher observed W sBULK at peak |u| and τ b (~1.2 mm/s) -Influence of pellets on W sBULK (Note that Bulk Settling Velocity, w sBULK = /c set is considered reliable for mud only during accelerating half of tidal cycle.) 4/9 Phase-averaged W sBULK for two regimes suggest different particles in are suspended during Regime 1 (Low w s, High ε) than Regime 2 (High w s, Low ε).

(a) Tidal Current Speed (cm/s) Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI (b) Bed Stress (Pa) (c) Concentration (mg/L) (d) Drag Coefficient C WASH 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 5/9 Regime 2 Regime 1(Low w s, High ε) Regime 2 (High w s, Low ε) Regime 1:Flocs/Fines Regime 2:Pellets+Flocs

(a) Tidal Current Speed (cm/s) Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI (b) Bed Stress (Pa) (c) Concentration (mg/L) (d) Drag Coefficient C WASH Regime 1 Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI 5/9 Regime 2 Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d)) Regime 1(Low w s, High ε) Regime 2 (High w s, Low ε) Regime 1: Flocs/Fines - Lower τ b despite similar current speeds -High C at relatively low τ b -More stratified WC: Lower ADV derived C d plus ΔS about 3 ppt (VECOS) Regime 2: Pellets+Flocs -Lower C at high τ b -Less stratified WC: Higher ADV derived C d plus ΔS about 1 ppt (VECOS)

(a) Tidal Current Speed (cm/s) Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI (b) Bed Stress (Pa) (c) Concentration (mg/L) (d) Drag Coefficient C WASH Regime 1 Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI Regime 1: Flocs/Fines -High C at relatively low τ b - Lower τ b despite similar current speeds -More stratified WC: Lower ADV derived C d plus ΔS about 3 ppt (VECOS) -Trapping of fines (STM) 5/9 Regime 2: Pellets+Flocs -Lower C at high τ b -Less stratified WC: Higher ADV derived C d plus ΔS about 1 ppt (VECOS) -Dispersal of fines, pellets suspended (No STM) Regime 2 Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), Concentration(c)and Drag Coeff. (d)) Regime 1(Low w s, High ε) Regime 2 (High w s, Low ε)

Phase- Averaged Erosion and Deposition for Two Regimes -- Once  b increases past a critical stress for initiation (  cINIT ), C continually increases for both Regime 1 and for Regime 2 Erosion Concentration (mg/L) Washload (~20%) Bed Stress (Pa) Concentration (mg/L) τ cINT = ~ 0.05 Pa τ cINT = ~ 0.02 Pa Regime 2 (Pellets + Flocs) Hysteresis plots of C vs.  b for the top 20 % of tidal cycles with the strongest  b for (a) Regime 1 and (b) Regime 2. 6/9 Regime 1 (Flocs)

Phase- Averaged Erosion and Deposition for Two Regimes Washload (~20%) Bed Stress (Pa) Concentration (mg/L) τ cINT = ~ 0.05 Pa τ cINT = ~ 0.02 Pa Concentration (mg/L) -- As  b decreases for Regime 1, C does not fall off quickly until  b ≤ 0.08 Pa, suggests that over individual tidal cycles, cohesion of settling flocs to the surface of the seabed is inhibited for τ b larger than ~ 0.08 Pa. -- As  b decreases for Regime 2, C decreases more continually, suggesting pellets without as clear a  cDEP. But the decline in C accelerates for  b ≤ ~ 0.08 Pa, suggesting (i) a transition to floc deposition and (ii) that settling C component is ~ 3/8 pellets, ~ 5/8 flocs. Depositio n τ cDEP flocs = ~ 0.08 Pa Hysteresis plots of C vs.  b for the top 20 % of tidal cycles with the strongest  b for (a) Regime 1 and (b) Regime 2. 6/9 Regime 2 (Pellets + Flocs) Regime 1 (Flocs)

Phase- Averaged Erosion and Deposition for Two Regimes Washload (~20%) Flocs (~80%) Washload (~20%) Flocs (~50%) Pellets (~30%) Bed Stress (Pa) Concentration (mg/L) τ cINT = ~ 0.05 Pa τ cINT = ~ 0.02 Pa Concentration (mg/L) -- As  b decreases for Regime 1, C does not fall off quickly until  b ≤ 0.08 Pa, suggests that over individual tidal cycles, cohesion of settling flocs to the surface of the seabed is inhibited for τ b larger than ~ 0.08 Pa. -- As  b decreases for Regime 2, C decreases more continually, suggesting pellets without as clear a  cDEP. But the decline in C accelerates for  b ≤ ~ 0.08 Pa, suggesting (i) a transition to floc deposition and (ii) that settling C component is ~ 3/8 pellets, ~ 5/8 flocs. Depositio n τ cDEP flocs = ~ 0.08 Pa Hysteresis plots of C vs.  b for the top 20 % of tidal cycles with the strongest  b for (a) Regime 1 and (b) Regime 2. 6/9 Regime 2 (Pellets + Flocs) Regime 1 (Flocs)

W sBULK = / (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK Phase-Averaged W sBULK for Two Regimes Regime 1 Regime 2 Increasing |u| and τ b Tidal Velocity Phase (  ) Similar W sBULK at the beginning of tidal phase suggest presence of flocs during both regimes Regime 1: Flocs -Lower observed W sBULK at peak |u| and τ b (<0.8 mm/s) Regime 2: Pellets+Flocs -Lower observed W sBULK at peak |u| and τ b (~1.2 mm/s) -Influence of pellets on W sBULK (Note that Bulk Settling Velocity, w sBULK = /c set is considered reliable for mud only during accelerating half of tidal cycle.) 7/9

W sBULK = / (mm/s) W sDEP = (c/(c-c wash ))*W sBULK (mm/s) Analysis of W sBULK by removing C WASH and solving for settling velocity of the depositing component (W sDEP ) during increasing  b allows separate estimates for settling velocities of flocs (W sFLOCS ) and pellets (W sPELLETS ). (a) Sediment Bulk Settling Velocity, W sBULK (b) Remove c wash Regime 1 Regime 2 Tidal Velocity Phase (  ) Regime 1 (Flocs) Regime 2 (Pellets +Flocs) (b) Depositing component of Settling Velocity, W sDEP Increasing |u| and τ b Recall: peak τ b ~ 0.15 Pa for Regime 1, and peak τ b ~ 0.22 Pa for Regime 2 Phase-Averaged W sBULK for Two Regimes 8/9

W sBULK = / (mm/s) W sDEP = (c/(c-c wash ))*W sBULK (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK (b) Remove c wash Regime 1 Regime 2 Tidal Velocity Phase (  ) (b) Depositing component of Settling Velocity, W sDEP Increasing |u| and τ b W sFLOC = ~ 0.85 mm/s Implies floc size is limited by settling-induced shear rather than  b. W sDEP = W sFLOCS Recall: peak τ b ~ 0.15 Pa for Regime 1, and peak τ b ~ 0.22 Pa for Regime 2 Analysis of W sBULK by removing C WASH and solving for settling velocity of the depositing component (W sDEP ) during increasing  b allows separate estimates for settling velocities of flocs (W sFLOCS ) and pellets (W sPELLETS ). Phase-Averaged W sBULK for Two Regimes 8/9 Regime 1 (Flocs) Regime 2 (Pellets +Flocs)

W sBULK = / (mm/s) W sDEP = (c/(c-c wash ))*W sBULK (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK (b) Remove c wash Regime 1 Regime 2 Tidal Velocity Phase (  ) (b) Depositing component of Settling Velocity, W sDEP Increasing |u| and τ b W sDEP = W sFLOCS W sDEP = f F W sFLOCS + f F W sPELLETS = ~ 1.43 mm/s at peak  b Assume: f F = 5/8, f P = 3/8 This gives: W sPELLETS = ~ 2.4 mm/s W sFLOC = ~ 0.85 mm/s Implies floc size is limited by settling-induced shear rather than  b. Recall: peak τ b ~ 0.15 Pa for Regime 1, and peak τ b ~ 0.22 Pa for Regime 2 Analysis of W sBULK by removing C WASH and solving for settling velocity of the depositing component (W sDEP ) during increasing  b allows separate estimates for settling velocities of flocs (W sFLOCS ) and pellets (W sPELLETS ). Phase-Averaged W sBULK for Two Regimes 8/9 Regime 1 (Flocs) Regime 2 (Pellets +Flocs)

York River sediment settling velocity (W s ) and erodibility (ε) are described by two contrasting regimes: (i) Regime 1: a period dominated by muddy flocs [lower W s, higher ε]. (ii) Regime 2: a period characterized by pellets mixed with flocs [higher W s, 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 C WASH from W SBULK for Regime 1 results in a stable floc settling velocity of W sFLOC ≈ 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 W sFLOC and C WASH from W SBULK for Regime 2 finally yields W SPELLET ≈ 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 Acknowledgements Marjy Friedrichs Tim Gass Wayne Reisner Funding: Julia Moriarity Carissa Wilkerson

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., /9

/01 07/01 08/01 09/01 Pamunkey River Discharge (m 3 /s) Salinity 0.5 mab (ppt) June 12- August 31, 2007

York River sediment settling velocity (W s ) and erodibility (ε) are described by two contrasting regimes: (i) Regime 1: a period dominated by muddy flocs [lower W s, higher ε]. (ii) Regime 2: a period characterized by pellets mixed with flocs [higher W s, 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 (C WASH ) is always present during both Regimes. Once stress (τ b ) exceeds an initial critical value (τ cINIT ) of ~ 0.02 to 0.05 Pa, sediment concentration (C) continually increases with τ b for both Regimes. As τ b decreases, cohesion of settling flocs to the surface of the seabed is inhibited for τ b larger than ~ 0.08 Pa for both Regimes. Subtraction of C WASH from W SBULK for Regime 1 results in a stable floc settling velocity of W sFLOC ≈ 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 W sFLOC and C WASH from W SBULK for Regime 2 finally yields W SPELLET ≈ 2.4 mm/s. During Regime 1, ε increases with  b averaged over the previous 5 days, consistent with cohesive bed evolution; while for Regime 2, ε decreases with daily  b, 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 Hour Averaged Erodibility, (kg/m 2 /Pa) 120 Hour Averaged Bed Stress (Pa) 25 Hour Averaged Erodibility, (kg/m 2 /Pa) Reveals two distinct relationships between ε and  b. 9/10 b. Daily-averaged ε vs. 5-day-averaged  b a. Daily-averaged ε vs. daily averaged  b

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 Hour Averaged Erodibility, (kg/m 2 /Pa) 120 Hour Averaged Bed Stress (Pa) 25 Hour Averaged Erodibility, (kg/m 2 /Pa) R= R= Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2 Reveals two distinct relationships between ε and  b. 9/10 b. Daily-averaged ε vs. 5-day-averaged  b a. Daily-averaged ε vs. daily averaged  b

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 Hour Averaged Erodibility, (kg/m 2 /Pa) 120 Hour Averaged Bed Stress (Pa) 25 Hour Averaged Erodibility, (kg/m 2 /Pa) R= R= R= R= Influence of Stress History on Bed Erodibility for Regime 1 and Regime 2 Reveals two distinct relationships between ε and  b. 9/10 b. Daily-averaged ε vs. 5-day-averaged  b a. Daily-averaged ε vs. daily averaged  b