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

1. 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

2. Motivation: Determine fundamental controls on sediment settling velocity and bed erodibility in muddy estuaries (X-rays courtesy of L. Schaffner) Study site: York River Estuary, VA 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 MUDBEDproject ADV tripods provide long-term observations within a strong physical-biological gradient. 1/11 Schaffner et al., 2001

3. Observations provided by an Acoustic Doppler Velocimeter Sensing volume ~ 35 cmab ADV after retrieval ADV at deployment (Photos by C. Cartwright) • -- 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 2/11 Fugate and Friedrichs ,2002; Friedrichs et al., 2009; Cartwright, et al. 2009 and Dickhudt et al., 2010

4. Seasonal Variability in bulk settling velocity (WsBULK) and bed erodibility (ε) is observed at the Intermediate Site. 3- day Mean WsBULK from fits to <w’c'> = WsBULK<C> using ADVs 2 1.5 Biological site WsBULK~1 mm/s WsBULK (mm/s) 1.0 Intermediate site WsBULKvaries from ~ 0.5 mm/s (Regime 1) to ~ 1 mm/s (Regime 2) 0.5 0 3-day mean of ε from fits to M = ε τbusing ADVs 6 5 4 Intermediate site ε varies from ~ 3 kg/m2/Pa (Regime 1) to ~ 1 kg/m2/Pa (Regime 2) 3 ε (kg/m2/Pa) 2 Biological site Generally < 1 kg/m2/Pa 1 3/11 Cartwright et al., 2009

5. What is happening at Intermediate Site when Regime 1  Regime 2? 3- day Mean WsBULK from fits to <w’c'> = WsBULK<C> using ADVs 2 1.5 Biological site WsBULK~1 mm/s WsBULK (mm/s) 1.0 Intermediate site WsBULKvaries from ~ 0.5 mm/s (Regime 1) to ~ 1 mm/s (Regime 2) 0.5 0 3-day mean of ε from fits to M = ε τbusing ADVs 6 5 4 Intermediate site ε varies from ~ 3 kg/m2/Pa (Regime 1) to ~ 1 kg/m2/Pa (Regime 2) 3 ε (kg/m2/Pa) 2 Biological site Generally < 1 kg/m2/Pa 1 Cartwright et al., 2009 3/11

6. Phase-Averaged Settling Velocity for Two Regimes (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 7/11

7. Tidal Analysis highlights differences in Regime 1 and Regime 2. (a) Tidal Current Speed (cm/s) (b) Bed Stress (Pa) 45 0.25 0.2 30 0.15 0.1 15 0.05 (c) Drag Coefficient (d) Concentration (mg/L) 0.0016 200 0.0012 150 0.00008 100 CWASH 0.00004 50 CWASH 1 0 0.5 0.5 1 0 Tidal Velocity Phase (θ/π) Tidal Velocity Phase (θ/π) Decreasing IuI Increasing IuI Decreasing IuI Increasing IuI 5/11

8. Tidal Analysis highlights differences in Regime 1 and Regime 2. (a) Tidal Current Speed (cm/s) (b) Bed Stress (Pa) 45 0.25 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 0.2 30 0.15 0.1 15 0.05 Regime 1 (c) Drag Coefficient (d) Concentration (mg/L) 0.0016 200 0.0012 150 Regime 1 0.00008 100 CWASH 0.00004 50 CWASH Regime 1 1 0 0.5 0.5 1 0 Tidal Velocity Phase (θ/π) Tidal Velocity Phase (θ/π) Decreasing IuI Increasing IuI Decreasing IuI Increasing IuI 5/11

9. Tidal Analysis highlights differences in Regime 1 and Regime 2. (a) Tidal Current Speed (cm/s) (b) Bed Stress (Pa) 45 0.25 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 0.2 30 Regime 2 0.15 0.1 15 0.05 (c) Drag Coefficient (d) Concentration (mg/L) 0.0016 200 Regime 2: Pellets+Flocs -Lower C at high τb -Increase in Cd (Water column less stratified) Regime 2 0.0012 150 0.00008 100 CWASH 0.00004 50 CWASH Regime 2 1 0 0.5 0.5 1 0 Tidal Velocity Phase (θ/π) Tidal Velocity Phase (θ/π) Decreasing IuI Increasing IuI Decreasing IuI Increasing IuI 5/11

10. Erosion -- Once tbincreases past a critical stress for initiation (tcINIT), C continually increases for both Regime 1 and for Regime 2 -- 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 τcDEP flocs = ~ 0.08 Pa (a) (b) Regime 2 Regime 1 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. 6/11

11. Phase-Averaged Settling Velocity 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 7/11

12. Phase-Averaged Settling Velocity 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) (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 Regime 2 WsBULK = <w’c’>/<c> (mm/s) WsDEP = (c/(c-cwash))*WsBULK (mm/s) Regime 1 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 8/11

13. Phase-Averaged Settling Velocity 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) (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 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 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 8/11

14. Phase-Averaged Settling Velocity 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 = ~ 1.5 mm/s at peak tb Assume: fF = 5/8, fP = 3/8 This gives: WsPELLETS= ~ 2 mm/s (b) (a) Sediment Bulk Settling Velocity, WsBULK Regime 2 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 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 8/11

15. Influence of Stress History on Bed Erodibility for Two Regimes 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 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 Erodibility, (kg/m2/Pa) Regime 2: Erodibility (ε) decreases with greater stress, possibly associated with the effects of bed armoring by the pellet component. Regime 2 25 or 120 Hour Averaged Bed Stress (Pa) 9/11

16. 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 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 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. 10/11

17. Questions? Acknowledgements MarjyFriedrichs Tim Gass Wayne Reisner Funding: Julia Moriarity Carissa Wilkerson 11/11