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

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-rays courtesy of L. Schaffner) Schaffner et al., /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 (τ b ): ρ* Bulk Settling Velocity (w sBULK ): /c set Erodibility (ε) given by ε = τ b /M, where M is depth-integrated C 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., /11

Biological site Generally < 1 kg/m 2 /Pa Intermediate site ε varies from ~ 3 kg/m 2 /Pa (Regime 1) to ~ 1 kg/m 2 /Pa (Regime 2) ε (kg/m 2 /Pa) Seasonal Variability in bulk settling velocity (W sBULK ) and bed erodibility (ε) is observed at the Intermediate Site. 3-day mean of ε from fits to M = ε τ b using ADVs Biological site W sBULK ~1 mm/s Intermediate site W sBULK varies from ~ 0.5 mm/s (Regime 1) to ~ 1 mm/s (Regime 2) 3- day Mean W sBULK from fits to = W sBULK using ADVs W sBULK (mm/s) Cartwright et al., /11

Biological site Generally < 1 kg/m 2 /Pa Intermediate site ε varies from ~ 3 kg/m 2 /Pa (Regime 1) to ~ 1 kg/m 2 /Pa (Regime 2) ε (kg/m 2 /Pa) 3-day mean of ε from fits to M = ε τ b using ADVs Biological site W sBULK ~1 mm/s Intermediate site W sBULK varies from ~ 0.5 mm/s (Regime 1) to ~ 1 mm/s (Regime 2) 3- day Mean W sBULK from fits to = W sBULK using ADVs W sBULK (mm/s) Cartwright et al., 2009 What is happening at Intermediate Site when Regime 1  Regime 2? 3/11

W sBULK = / (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK Phase-Averaged Settling Velocity 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 -Higher observed W sBULK at peak |u| and τ b (~1.2 mm/s) -Influence of pellets on W sBULK 7/11 (Note that Bulk Settling Velocity, w sBULK = /c set 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) (c) Drag Coefficient C WASH (a) Tidal Current Speed (cm/s) Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI 5/11

(a) Tidal Current Speed (cm/s) 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) (c) Drag Coefficient C WASH Regime 1: Flocs -High C at relatively low τ b -Lower ADV derived Cd (more stratified water column) -Lower τ b despite higher similar current speeds Regime 1 Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI 5/11

(a) Tidal Current Speed (cm/s) 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) (c) Drag Coefficient C WASH Regime 1: Flocs -High C at relatively low τ b -Lower ADV derived C d (more stratified water column) -Lower τ b despite higher similar current speeds Regime 2: Pellets+Flocs -Lower C at high τ b -Increase in C d (Water column less stratified) Regime 2 Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI 5/11

Concentration (mg/L) (a)(b) 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. τ cDEP flocs = ~ 0.08 Pa Washload (~20%) Flocs (~80%) Washload (~20%) Flocs (~50%) Pellets (~30%) Bed Stress (Pa) Concentration (mg/L) τ cDEP flocs = ~ 0.08 Pa τ cINT = ~ 0.05 Pa τ cINT = ~ 0.02 Pa Regime 1 Regime 2 -- Once  b increases past a critical stress for initiation (  cINIT ), C continually increases for both Regime 1 and for Regime 2 Erosion -- 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. Deposition 6/11

W sBULK = / (mm/s) (a) Sediment Bulk Settling Velocity, W sBULK Phase-Averaged Settling Velocity 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 7/11 (Note that Bulk Settling Velocity, w sBULK = /c set is considered reliable for mud only during accelerating half of tidal cycle.)

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) Phase-Averaged Settling Velocity for Two Regimes Remove c wash Regime 1 Regime 2 Tidal Velocity Phase (  ) Regime 1 Regime (b) Depositing component of Settling Velocity, W sDEP Increasing |u| and τ b 8/11 Recall: peak τ b ~ 0.15 Pa for Regime 1, and peak τ b ~ 0.22 Pa for Regime 2

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) Phase-Averaged Settling Velocity for Two Regimes Remove c wash Regime 1 Regime 2 Tidal Velocity Phase (  ) Regime 1 Regime (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 8/11 Recall: peak τ b ~ 0.15 Pa for Regime 1, and peak τ b ~ 0.22 Pa for Regime 2

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) Phase-Averaged Settling Velocity for Two Regimes Remove c wash Regime 1 Regime 2 Tidal Velocity Phase (  ) Regime 1 Regime (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 = ~ 1.5 mm/s at peak  b Assume: f F = 5/8, f P = 3/8 This gives: W sPELLETS = ~ 2 mm/s 8/11 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

25 or 120 Hour Averaged Bed Stress (Pa) 25 Hour Averaged Erodibility, (kg/m 2 /Pa) Daily-averaged erodibility is correlated either to 5-Day-averaged  b (Regime 1) or to daily-averaged  b (Regime 2), revealing two distinct relationships between ε and  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. 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 (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 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 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. 10/11

Acknowledgements Marjy Friedrichs Tim Gass Wayne Reisner Funding: Julia Moriarity Carissa Wilkerson Questions? 11/11