Controls on particle settling velocity and bed erodibilty in the presence of muddy flocs and biologically packaged pellets: Modeling study utilizing the York 3-D Hydrodynamic Cohesive Bed Model *Kelsey A. Fall 1, Courtney K. Harris 1, Carl T. Friedrichs 1, and J. Paul Rinehimer 2 1 Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 2 Department of Civil and Environmental Engineering, University of Washington, Seattle, WA

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: ETM --Lower York Biological site: No ETM --Mid York Intermediate site : Seasonal STM

Study Site: York River Estuary,VA (MUDBED Long-term Observing System)  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. ADV observed Settling Velocity (W sBULK ) and Bed Erodibility (ε) Fugate and Friedrichs,2002; Friedrichs et al., 2009; Cartwright, et al and Dickhudt et al., 2010

Study Site: York River Estuary,VA (MUDBED Long-term Observing System) Cartwright et al.,2009 ADV observed Settling Velocity (W sBULK ) and Bed Erodibility (ε) Strong Observed Transition between Regime 1 and Regime 2: June-August 2007

(a) Tidal Current Speed (cm/s) Tidal Velocity Phase (θ/π) Increasing U Decreasing U (b) Bed Stress (Pa) (c) Concentration (mg/L) Regime 1 Regime 2 Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), and Concentration(c)) Regime 1: Flocs -High C at relatively low τ b (trapping of fines) -Lower τ b despite higher similar current speeds Regime 2: Pellets+Flocs -Lower C at high τ b (dispersal of fines, pellets suspended) Tidal Velocity Phase (θ/π) Increasing U Decreasing U Tidal Phase Average Analysis (Fall, 2012): Average ADV data (current speed, concentration, bed stress and settling velocity) over the tidal phases with the strongest bed stresses for each regime to obtain representative values of each parameter throughout a tidal phase.

A. ADV estimated W sBULK ADV observations suggest different particles in are suspended during Regime 1 than Regime 2. Increasing U and τ b Tidal Velocity Phase (  ) (Note that Bulk Settling Velocity, w sBULK = /c set is considered reliable for mud only during accelerating half of tidal cycle.) ADV Observations: Velocity Phase Averaged Analysis (W sBULK ) W sBULK = (c/(c-c wash ))*W s ) Regime 1: Flocs+Fines -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 (~1.5 mm/s) -Influence of pellets on W sBULK

Intermediate SiteBiological SitePhysical Site Regime 2: (High W sBULK and Low ε) -Little or no stratification -Dispersal of fines and/or flocs - No STM -Bed stress no longer suppressed Regime 1: (Low W sBULK and High ε) -Stratified -Trapping of fines and/or flocs (STM) -Suppressed bed stresses York River Conceptual Model (Dickhudt et al., 2009) Observations suggest seasonal variability in W sBULK and ε at the Intermediate Site attributed to presence of STM.

Overarching Goal: Use 3-D hydrodynamic cohesive bed model to explore the fundamental controls on W sBULK and ε in muddy estuary. A Preliminary Study: Application of bed consolidation and swelling model (Sanford, 2008) in realistic 3-D domain. 1.Implement a three-dimensional, numerical model that includes bed consolidation and swelling in the York River Estuary (Rinehimer,2008). 2.Evaluate “standard” model behavior compared to ADV tidal phase observations during a transition from Regime 1 to Regime 2 at the Intermediate Site (June-August 2007). 1.Investigate sensitivities of the bed consolidation and swelling model a.Cohesive bed swelling time (T s ) b.τ c equilibrium profile (τ ceq ) c.Initial bed sediment bed τ c (τ cinit )

Overarching Goal: Use 3-D hydrodynamic cohesive bed model to explore the fundamental controls on W sBULK and ε in muddy estuary. A Preliminary Study: Application of bed consolidation and swelling model (Sanford, 2008) in realistic 3-D domain. 1.Implement a three-dimensional, numerical model that includes bed consolidation and swelling in the York River Estuary (Rinehimer,2008). 2.Evaluate “standard” model behavior compared to ADV tidal phase observations during a transition from Regime 1 to Regime 2 at the Intermediate Site (June-August 2007). 1.Investigate sensitivities of the bed consolidation and swelling model a.Cohesive bed swelling time (T s ) b.τ c equilibrium profile (τ ceq ) c.Initial bed sediment bed τ c (τ cinit )

Community Sediment Transport Modeling System (CSTMS): York River 3-D Hydrodynamic Model (Rinehimer, 2008) 3-D ROMS model grid showing every 5 th grid cell. Figure by C. Harris Water Column Seabed CSTMS description see Warner et al 2008

Consolidation Model (Sanford,2008) (Cohesive Sediment Bed - τ cr vary with depth) Implementation in ROMS – CSTMS: see Rinehimer et al Depositional Beds Consolidate: become less erodible with time. τ ceq τcτc τcτc τ min τ ceq τcτc τcτc τ ceq = Equilibrium critical stress profile; is function of depth (z). τ c = Modeled critical stress profile; is function of depth (z), location (x,y) and time (t). T c, T s = timescales for consolidation (1 day) and swelling (10 days). Erosional Beds Swell: become more erodible with time.

Model includes bed consolidation BUT neglects aggregation and disaggregation of particles. Sediment Bed Model Standard Set Up One-meter thick sediment bed. Twenty layers. Upper layers ~ 1 – 2mm thick. Thick (~1m) layer at bottom. Two sediment types Initially: uniform distribution. Settling velocities: Flocs:0.8 mm/s Pellets: 2.4 mm/s. Model Simulation Time One month spin-up to develop: Spatial variability in grain size distribution. June-August 2007

Bed Consolidation Model (Sanford, 2008) (Cohesive Sediment Bed - τ cr vary with depth) τ ceq Profiles Obtained by Power Law Fit to Observations More Erodible Less Erodible Sept τ ceq =1.0m 0.62 April τ ceq =0.4m 0.55 (Rinehimer, 2008) User Defined τ ceq Profile: τ ceq =0.4m 0.55 (April) Initial Sediment Bed τ c Profiles= September τ c Profile

Overarching Goal: Use 3-D hydrodynamic cohesive bed model to explore the fundamental controls on W sBULK and ε in muddy estuary. A Preliminary Study: Application of bed consolidation and swelling model (Sanford, 2008) in realistic 3-D domain. 1.Implement a three-dimensional, numerical model that includes bed consolidation and swelling in the York River Estuary (Rinehimer,2008). 2.Evaluate “standard” model behavior compared to ADV tidal phase observations during a transition from Regime 1 to Regime 2 at the Intermediate Site (June-August 2007). 1.Investigate sensitivities of the bed consolidation and swelling model a.Cohesive bed swelling time (T s ) b.τ c equilibrium profile (τ ceq ) c.Initial bed sediment bed τ c (τ cinit )

Bed Stress (color) Depth Int. Current (aroows) Near Bed SSC 0.2 Pa Near Bed Settling Velocity Timeline ( river discharge) 15 Date Preliminary Standard Model Run June-August 2007

Movie goes here Bed Stress (color) Depth Int. Current (aroows) Near Bed SSC 0.2 Pa Near Bed Settling Velocity Preliminary Standard Model Run June-August 2007

Model vs. ADV : Velocity Phase Averaged Analysis Current Speed (cm/s)Concentration (mg/L)Bed Stress (Pa) Regime 1(blue) vs. Regime 2 (green) ADV Observations Model  Resolves similar current speeds between regimes.  Resolves difference in concentration between regimes  Does not resolve difference in bed stress between regimes. Increasing U Decreasing U Tidal Velocity Phase (θ/π) Increasing U Decreasing U Tidal Velocity Phase (θ/π) Increasing U Decreasing U Tidal Velocity Phase (θ/π)

Model vs. ADV : W sBULK Velocity Phase Averaged Analysis Regime 2 Regime 1 W sDEP (mm/s) A. ADV estimated W sBULK Removing C WASH and solving for settling velocity of the depositing component (W sDEP = (c/(c-c wash ))*W sBULK ) Increasing U and τ b Tidal Velocity Phase (  ) Increasing U and τ b B. Model estimated W sBULK

Overarching Goal: Use 3-D hydrodynamic cohesive bed model to explore the fundamental controls on W sBULK and ε in muddy estuary. A Preliminary Study: Application of bed consolidation and swelling model (Sanford, 2008) in realistic 3-D domain. 1.Implement a three-dimensional, numerical model that includes bed consolidation and swelling in the York River Estuary (Rinehimer,2008). 2.Evaluate “standard” model behavior compared to ADV tidal phase observations during a transition from Regime 1 to Regime 2 at the Intermediate Site (June-August 2007). 1.Investigate sensitivities of the bed consolidation and swelling model a.Cohesive bed swelling time (T s ) b.τ c equilibrium profile (τ ceq ) c.Initial bed sediment bed τ c (τ cinit )

A.Calculated τ c Profiles Cluster Around user defined equilibrium profile (τ ceq ) and displaying initial bed profile (τ crinit ) Sensitivity to Cohesive bed swelling time (T s ) T s =50 DaysT s =25 DaysT s =2 Days Short Swelling Time Bed quickly becomes more erodible. Long Swelling Time Bed is more consolidated (less erodible). Swelling Time= 25 days Bed adjusts. Min. adjustment from τ crinit to τ ceq. Rapid adjustment from τ crinit to τ ceq. Some adjustment from τ crinit to τ ceq. τ crinit τ ceq Note: τ ceq ≠ τ crinit τ crinit τ ceq τ crinit τ ceq

Sensitivity to Cohesive bed swelling time (T s ) Regime 1Regime 2 B. Phase Averaged Concentration  Model estimated suspended sediment concentration is sensitive to T s.  A T s = 25 days may be a more reasonable estimate for T s in this system than previously used 50. A. Calculated τ c Profiles Cluster Around user defined equilibrium profile (τ ceq ) and displaying initial bed profile (τ crinit ) T s =50 DaysT s =25 Days Min. adjustment from τ crinit to τ ceq. Rapid adjustment from τ crinit to τ ceq. Some adjustment from τ crinit to τ ceq. τ crinit τ ceq τ crinit τ ceq Increasing U Decreasing U Tidal Velocity Phase (θ/π) Increasing U Decreasing U Tidal Velocity Phase (θ/π) τ crinit τ ceq Note: τ ceq ≠ τ crinit

Sensitivity to Initial Bed Profile (τ crinit ) B. Calculated τ c Profiles Cluster Around τ cinit (based on data) A. τ ceq profiles obtained by power law fit to Gust (Rinehimer,2008). September (less erodible)April (more erodible) T s =25 Days Sept April The current version of the model has a more difficult time nudging the bed τ c profiles to the τ ceq profile when a more erodible τ crinit was used. Note: τ ceq = τ crinit

A. τ ceq profiles obtained by power law fit to Gust (Rinehimer,2008). September (less erodible)April (more erodible) T s =25 Days Sept April Sensitivity to Initial Bed Profile (τ crinit ) B. Calculated τ c Profiles Cluster Around τ crinit (based on data) C. Phase Averaged Concentration  Model estimated suspended sediment concentration is sensitive to initial bed τ cr profile because the model run time is short when compared to T s and T c.  For this particular version of the model the estimated suspended sediment concentration is more sensitive to initial bed τ cr profile than T s. Increasing U Decreasing U Tidal Velocity Phase (θ/π) Increasing U Decreasing U Tidal Velocity Phase (θ/π) Note: τ ceq = τ crinit

Conclusions and Future Work  This study showed the application of the 3-D Hydrodynamic York River Model (Rinehimer, 2008), a three-dimensional numerical model that included bed consolidation and swelling, in the York River Estuary, Virginia.  A standard model simulation showed that the York River 3-D model could be a useful tool in investigating the fundamental controls on bed erodibility and settling velocity in a muddy estuary.  Simulated observed current speeds and concentrations over a tidal phase.  Resolved the difference in concentration and settling velocity between regimes over a tidal phase.  Simulate observed bed stresses during Regime 2. Future work will involve turning on sediment induced stratification in the model with aim to simulate realistic stresses for Regime 1.  The bed consolidation model (Sanford, 2008) was found to be sensitive to bed swelling time, τ cr equilibrium profile and τ cr initial profile.

10/10 Acknowledgements Justin BirchlerFunding: Adam Miller Julia Moriarty

Study Site: York River Estuary,VA (MUDBED Long-term Observing System) 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 benthic ADV tripods (1) and monthly bed sampling cruises (2) provide long-term observations within a strong physical-biological gradient. Schaffner et al., 2001 (1) MUDBED Benthic Tripod ADV (2) MUDBED Sampling Cruises

(a) Tidal Current Speed (cm/s) Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI (b) Bed Stress (Pa) (d) Concentration (mg/L) (c) Drag Coefficient C WASH Regime 1: Fines+Flocs -High freshwater discharge -High C at relatively low τ b (trapping of fines) -Lower τ b despite higher similar current speeds….Why?? Regime 1 Regime 2: Pellets+Flocs -Decreased freshwater discharge -Lower C at high τ b (dispersal of fines, pellets suspended) Regime 2 ADV Observations: Velocity Phase Averaged Analysis Tidal Velocity Phase (θ/π) Increasing IuI Decreasing IuI

Bed Consolidation Model Initial Sediment Bed τ c Profiles (red-x) and τ ceq Profiles for Sept. and April User Defined τ ceq Profile: τ ceq =0.4m 0.55 (more erodible April) April τ ceq =0.4m 0.55 Sept τ ceq =1.0m 0.62 Initial Sediment Bed τ c Profiles= September τ c Profile

“Standard” Model Simulation Study Period: June-August 2007  Strongest Observed Transition  Continuous ADV data available (MUDBED)