Conclusions

References

Acknowledgments

Sensitivity Tests

Cohesive Sediment ModelModeling System

Future Work

Including Cohesive Sediment Processes in the Community Sediment Transport Modeling System (CSTMS) for the York River estuary, Virginia

Danielle Tarpley, Kelsey Fall, Courtney K. Harris, Carl T. FriedrichsAbstract

To further understand estuarine sediment dynamics in the York River estuary, Virginia, the Community Sediment Transport Modeling System (CSTMS) was implemented in a three-dimensional domain using the Regional Ocean Modeling System (ROMS). In addition to accounting for suspended sediment transport, erosion, and deposition, this version of the CSTMS includes cohesive processes via consolidation and swelling of the sediment bed, which change the critical shear stress of the seafloor in response to sedimentation. The model includes wind forcing based on observed timeseries of wind speed and direction, as has recently been modified through adjustments to the open boundary conditions to improve representation of salinity. Model estimates for the summer 2007 has shown good agreement with observed sediment concentration, bed stress, current velocity, salinity and tidal amplitude. We analyzed the sensitivity of calculations of the total eroded mass to the bed consolidation time scale and the critical shear stress for erosion. Further analysis showed model sensitivities to the swelling time scale and the user defined initial and equilibrium critical shear stress profiles.

• Model Grid – 92 cells across channel (cell width 110 m)– 334 cells along channel (cell width 170 m)– 20 vertical water column layers, higher resolution

near the water and sediment surfaces.– 20 layer sediment bed.

York River

• Critical Shear Stress

• Deposition:• Erosion:• Sediments

– Two sediment classes:• Settling velocities: 0.8 mm/s (Flocs) & 2.4 mm/s (Pellets)

Figure 1: York River estuary model grid and ROMS model vertical structure.

Figure 2: Average (dashed lines) and equilibrium (solid lines) critical stress profiles for April and September, 2007. Equilibrium profiles obtained by a power-law fit to the observed values (Rinehimer, 2008). Symbols show observed data from Dickhudt et al. (2008).

• Partially mixed estuary with tidal range from 0.7 m to 0.85 m from the mouth to the head.

• Total length of ~50 km• Seasonal secondary turbidity maximum (STM)

mid-estuary• Location of the Multidisciplinary Benthic

Exchange Dynamics (MUDBED) project, since 2006.– Observations show a physically dominated system

near the head of the estuary and a biologically dominated system near the mouth of the estuary.

– The STM occurs in the transition zone between a physically and biologically dominated system.

Tidal May – Aug July – Aug

USCG 0.22 0.20

CB 0.25 0.23

TC 1.55 1.51

Table 1: RMSE comparing modeled and measured water level at United States Coast Guard (USCG) pier in Yorktown, VA, Claybank (CB) and Taskinas Creek (TC) on the York River.

Figure 3: Model estimates of seabed properties after two months spin-up. (a) Erodibility of the seabed, calculated as the thickness of the layer having a critical shear stress exceeded by 0.2 Pa. (b) Fraction of the surficial sediment in the “faster settling” size class. (c) Average settling velocity of surficial sediment (Sherwood et al., In Progress).

Current Speed (cm/s)

Concentration (mg/L) Bed Stress (Pa)

Figure 5: ADV (upper) and model (lower) estimated (a) current speed, (b) concentrations, (c) bed stresses for the top 20% of tidal cycles with strongest bed stresses. Error bars denote ±1 standard error (Fall et al. 2014).

Water Colum

nSeabed

Taskinas Creek

Dickhudt, P., Friedrichs, C., and Sanford, L. (2008). Mud matrix solids fraction and bed erodibility in the York River, USA and other muddy environments. In Proceedings of the 9th International Conference of Nearshore and Estuarine Cohesive Sediment Transport Processes, 2007. submitted.

Fall, K.A., Harris, C.K., Friedrichs, C.T., Rinehimer, J.P., and Sherwood, C.R. (2014). Model Behavior and Sensitivity in an Application of the Cohesive Bed Component of the Community Sediment Transport Modeling System for the York River Estuary, VA, USA. Journal of Marine Science and Engineering 2, 413-436.

Rinehimer, J.P. (2008) Sediment Transport and Erodibility in the York River Estuary: A Model Study. Master’s Thesis, School of Marine Science, College of William and Mary, Gloucester Point, VA, USA.

Sanford, L.P. (2008). Modeling a dynamically varying mixed sediment bed with erosion, deposition, bioturbation, consolidation, and armoring. Computers and Geosciences 34, 1263-1283.

Sherwood, C.R., Aretxabaleta, A., Harris, C.K., Rinehimer, J.P., Ferré, B., Verney, R. (In Progress). Cohesive and mixed sediment model: Extension of the Community Sediment Transport Modeling System. In Prep. for Ocean Dynamics.

Warner, J.C., Geyer, W.R., Lerczak, J.C. (2005). Numerical modeling of an estuary: A comprehensive skill assessment. Journal of Geophysical Research 110, C5.

Figure 9: Modeled salinity transect of a 2-D estuary ROMS test case

• Update the York River forcings to represent a more recent year.

• Implement cohesive sediment in a 2-D estuary test case (Figure 9)

Model development benefited from efforts by J. Paul Rinehimer done as part of his Master’s Thesis, and work completed by a Christopher Newport University Governor’s School summer intern, Jessica Sydnor. Funding from NSF – OCE-1061781 and OCE-0536572.

a

USCGS0 = 25S0 = 32S0 = 29

GP

S0 = 25

S0 = 32

S0 = 29

b

Figure 8 (Right): Daily instantaneous model calculated τcr profiles (colored lines) shown with user defined equilibrium (τceq) and initial bed profiles (τcrinit) for model runs with swelling times defined as (a) 2 days, (b) 25 days, and (c) 50 days. The black arrows show the direction that the instantaneous τc will nudge toward (Fall et al., 2014).

Figure 6: Maximum total suspended sediment mass over a tidal cycle for varying values of consolidation time (Tc). Assuming consolidation rates ranging from Tc = 1 to 48 hours. Values represent the maximum estimated for each tidal cycle (Rinehimer, 2008).

Figure 7: Daily instantaneous model calculated τc profiles (colored lines) shown with the profile used to parameterize these models (black lins). The equilibrium profile (τceq) and initial bed profile (τcrinit) were defined based on (a) September and (b) April profiles (Fall et al. 2014).

• Salinity and water level compared to observations– Setting S0 =29 improved the modeled salinity (Fig.

4) versus S0 = 25 in Fall et al. (2014). – Largest water level error from tidal propagation up

river (Table 1).

Figure 4: Modeled salinity compared to measured salinity at (a) United States Coast Guard (USCG) pier in Yorktown, VA and (b) Gloucester Point (GP), VA. The lowest root mean square error (RMSE) from July to August for both sites was found when S0 = 29, 1.66 and 1.12 for USCG and GP, respectively.

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)

a b c

• A 3-D numerical model for the York River estuary was developed– The addition of a depth varying critical shear stress was

implemented • The model was most sensitive to user defined

parameters such as the critical shear stress equilibrium profile and initial critical shear stress profile– Observational data needed to constrain these parameters

• Model output compared relatively well with both hydrological parameters and sediment transport parameters

September (less erodible) April (more erodible)

Ts=25 Days Ts=25 Days

Note: τceq = τcrinit a b

Ts=50 DaysTs=25 DaysTs=2 DaysMin. 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

a b c

• Sensitivity to both the τcrinit and the τceq profiles.– Positive feedback between erodibilty and concentration when τcrinit profile more

erodible bed (as in April).– Instantaneous τc profiles shift toward zero (Fig. 7).

– Introducing Beryllium-7 as a tracer.

– Incorporating flocculation population dynamics.

– Including effects of sediment induced stratification.

• Incorporate these advances into the 3-D York River estuary model.

• Salinity– Along channel:

• Model estimates– Bed stress, bed mass, average settling velocity and

suspended sediment concentration,– Erodibility based on consolidation time (Tc), swelling time

(Ts) and a depth varying critical shear stress (Fig. 3).

• Model compared better to with measured current speed and suspended sediment concentrations than with bed stress (Fig. 5).

• Sensitivity to consolidation time (Tc) and swelling time (Ts).

– Longer Tc produced more asymmetry between spring and neap tide (Fig. 6)

– Longer Tc also produced a more erodible bed (Fig. 6).

– Longer Ts allowed minimal adjustment of the τc (Fig. 8)

Less Erodible

Septτceq=1.0m0.62

Aprilτceq=0.4m0.55

More Erodible

• Regional Ocean Modeling System (ROMS)– Solves the hydrostatic Reynolds-averaged Navier-

Stokes equations on a curvilinear orthogonal grid with vertical stretched terrain-following coordinates.

• Community Sediment Transport Modeling System (CSTMS) with a cohesive bed sub-model– Sub model based on Sanford (2008) including– Swelling, consolidation, and an increase in τc with

depth in the sediment