*Kelsey A. Fall 1 , Courtney K. Harris 1 , Carl T. Friedrichs 1 , and J. Paul Rinehimer 2

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. Fall1, Courtney K. Harris1, Carl T. Friedrichs1 , and J. Paul Rinehimer2

1Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 2Department 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


Spatial variability in WsBULK and bed ε between 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.

ADV observed Settling Velocity (WsBULK) and Bed

Erodibility (ε)

Fugate and Friedrichs ,2002; Friedrichs et al., 2009; Cartwright, et al. 2009 and Dickhudt et al., 2010


Cartwright et al.,2009

ADV observed Settling Velocity (WsBULK) and Bed

Erodibility (ε)

Strong Observed Transition between Regime 1 and Regime 2: June-August 2007

(a) Tidal Current Speed (cm/s)

15

30

45

Tidal Velocity Phase(θ/π)Increasing U Decreasing U

(b) Bed Stress (Pa)(c) Concentration (mg/L)

0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25

Regime 1

Regime 1

Regime 1

Regime 2

Regime 2

Regime 2

Velocity Tidal Phase Averaged Analysis (Current Speed (a), Bed Stress (b), and Concentration(c))

Regime 1: Flocs

-High C at relatively low τb (trapping of fines)

-Lower τb despite higher similar current speeds

Regime 2: Pellets+Flocs

-Lower C at high τb (dispersal of fines, pellets suspended)

Tidal Velocity Phase(θ/π)Increasing U Decreasing U

0 0.5 1

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 WsBULK ADV observations suggest different particles in are suspended during Regime 1 than Regime 2.

Increasing U and τb

Tidal Velocity Phase (q/p)0.1 0.2 0.3 0.4 0.5

(Note that Bulk Settling Velocity, wsBULK = <w’c’>/cset is considered reliable for mud only during accelerating half of tidal cycle.)

ADV Observations: Velocity Phase Averaged Analysis (WsBULK ) W

sBU

LK =

(c/(

c-c w

ash))

*Ws )

Regime 1: Flocs+Fines-Lower observed WsBULK at peak |u| and τb (~0.8 mm/s)

Regime 2: Pellets+Flocs-Higher observed WsBULK at peak |u| and τb (~1.5 mm/s)-Influence of pellets on WsBULK

Intermediate SiteBiological Site Physical Site

Regime 2: (High WsBULK and Low ε)

-Little or no stratification-Dispersal of fines and/or flocs- No STM-Bed stress no longer suppressed

Regime 1: (Low WsBULK 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 WsBULK 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 WsBULK 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).

3. Investigate sensitivities of the bed consolidation and swelling modela. Cohesive bed swelling time (Ts)b. τc equilibrium profile (τceq )c. Initial bed sediment bed τc (τcinit )






Community Sediment Transport Modeling System (CSTMS): York River 3-D Hydrodynamic Model (Rinehimer, 2008)

3-D ROMS model grid showing every 5th grid cell.

Figure by C. Harris

Water Colum

nSeabed

CSTMS description see Warner et al 2008

Consolidation Model (Sanford,2008)(Cohesive Sediment Bed - τcr vary with depth)

Implementation in ROMS – CSTMS: see Rinehimer et al. 2008

Depositional Beds Consolidate:become less erodible with time.

τceq

τc

τc

τmin

τceqτc

τcτceq = Equilibrium critical stress profile; is function of depth (z).

τc = Modeled critical stress profile; is function of depth (z), location (x,y) and time (t).

Tc, Ts = timescales for consolidation (1 day) and swelling (10 days).

Erosional Beds Swell: become more erodible with time.

( )b cE M

( ) ( ) /cceq c c

z Tt

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 - τcr vary with depth)

τceq Profiles Obtained by Power Law Fit to Observations

More Erodible

Less Erodible

Septτceq=1.0m0.62

Aprilτceq=0.4m0.55

(Rinehimer, 2008)

User Defined τceq Profile: τceq=0.4m0.55 (April)

Initial Sediment Bed τc Profiles= September τc Profile






Bed Stress (color)Depth Int. Current (aroows)

Near Bed SSC Erodibility @ 0.2 Pa Near Bed Settling Velocity

Timeline ( river discharge)

15

Date

Preliminary Standard Model RunJune-August 2007

• Movie goes here

Bed Stress (color)Depth Int. Current (aroows)

Near Bed SSC Erodibility @ 0.2 Pa Near Bed Settling Velocity

Preliminary Standard Model RunJune-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)





Model vs. ADV : WsBULK Velocity Phase Averaged Analysis

Regime 2

Regime 1

WsD

EP (m

m/s

)

A. ADV estimated WsBULK

Removing CWASH and solving for settling velocity of the

depositing component

(WsDEP = (c/(c-cwash))*WsBULK )Increasing U and τb

Tidal Velocity Phase (q/p)

Increasing U and τb

B. Model estimated WsBULK

0.1 0.2 0.3 0.4 0.5






A. Calculated τc Profiles Cluster Around user defined equilibrium profile (τceq) and displaying initial bed profile (τcrinit )

Sensitivity to Cohesive bed swelling time (Ts)

Ts=50 DaysTs=25 DaysTs=2 Days

Short Swelling TimeBed quickly becomes more erodible.

Long Swelling TimeBed is more consolidated (less erodible).

Swelling Time= 25 daysBed adjusts.

Min. adjustment from τcrinit to τceq.Rapid adjustment from τcrinit to τceq. Some adjustment from τcrinit to τceq.

τcrinit

τceq

Note: τceq ≠ τcrinit

τcrinit

τceq

τcrinit

τceq

Sensitivity to Cohesive bed swelling time (Ts)

Regime 1 Regime 2

B. Phase Averaged Concentration

Model estimated suspended sediment concentration is sensitive to Ts.

A Ts = 25 days may be a more reasonable estimate for Ts in this system than previously used 50.

A. Calculated τc Profiles Cluster Around user defined equilibrium profile (τceq) and displaying initial bed profile (τcrinit )

Ts=50 DaysTs=25 DaysMin. adjustment from τcrinit to τceq.Rapid adjustment from τcrinit to τceq. Some adjustment from τcrinit to τceq.

τcrinit

τceq

τcrinit

τceq





τcrinit

τceq

Note: τceq ≠ τcrinit

Sensitivity to Initial Bed Profile (τcrinit ) B. Calculated τc Profiles Cluster Around τcinit (based on data)

A. τceq profiles obtained by power law fit to Gust (Rinehimer,2008).

September (less erodible) April (more erodible)Ts=25 Days Ts=25 Days

Sept

April

The current version of the model has a more difficult time nudging the bed τc profiles to the τceq profile when a more erodible τcrinit was used.

Note: τceq = τcrinit

A. τceq profiles obtained by power law fit to Gust (Rinehimer,2008).

September (less erodible) April (more erodible)Ts=25 Days Ts=25 Days

Sept

April

Sensitivity to Initial Bed Profile (τcrinit ) B. Calculated τc Profiles Cluster Around τcrinit (based on data)

C. Phase Averaged Concentration

Model estimated suspended sediment concentration is sensitive to initial bed τcr profile because the model run time is short when compared to Ts and Tc.

For this particular version of the model the estimated suspended sediment concentration is more sensitive to initial bed τcr profile than Ts.





Note: τceq = τcrinit

Conclusions and Future WorkThis 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, τcr equilibrium profile and τcr initial profile.

10/10

AcknowledgementsJustin Birchler

Funding:Adam MillerJulia Moriarty


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)

15

30

45

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

(b) Bed Stress (Pa)

(d) Concentration (mg/L)

0 0.5 1

50

100

150

200

0.05

0.1

0.15

0.2

0.25

(c) Drag Coefficient

0 0.5 1

0.00004

0.00008

0.0012

0.0016

CWASH

CWASH

Regime 1: Fines+Flocs-High freshwater discharge

-High C at relatively low τb (trapping of fines)

-Lower τb despite higher similar current speeds….Why??

Regime 1

Regime 1

Regime 1

Regime 1

Regime 2: Pellets+Flocs

-Decreased freshwater discharge

-Lower C at high τb (dispersal of fines, pellets suspended)

Regime 2Regime 2

Regime 2

Regime 2

ADV Observations: Velocity Phase Averaged Analysis

Tidal Velocity Phase(θ/π)Increasing IuI Decreasing IuI

Bed Consolidation Model

Initial Sediment Bed τc Profiles (red-x) and τceq Profiles for Sept. and April

User Defined τceq Profile: τceq=0.4m0.55 (more erodible April)

Aprilτceq=0.4m0.55

Septτceq=1.0m0.62

Initial Sediment Bed τc Profiles= September τc Profile

“Standard” Model Simulation

Study Period: June-August 2007

Strongest Observed Transition

Continuous ADV data available (MUDBED)

Documents

*Kelsey A. Fall 1 , Courtney K. Harris 1 , Carl T. Friedrichs 1 , and J. Paul Rinehimer 2