1. Consolidation and stratification within a Muddy, Partially Mixed Estuary: A Comparison between Idealized and Realistic Models for Sediment Transport in the York River Estuary, Virginia
Danielle R.N. Tarpley, Courtney K. Harris, Carl T. Friedrichs
Cohesive Processes
Objective & Questions
Surface charge on clay particles leads to:
Flocculation which impacts settling velocity.
Consolidation on the seabed that reduces erodibility.
Sediment-induced stratification can limit sediment entrainment.
Sediment transport models often neglect these processes, which are especially important in fine-grained estuaries.
Figure 1: Representative images of flocculated particles using Environmental Scanning Electron Microscopy (ESEM) techniques (Garcia-Aragon et al., 2011).
Objective:
Explore the impacts of sediment-induced stratification using an idealized and realistic numerical model.
Research Questions:
What are the relative roles of sediment-induced stratification, and bed consolidation and swelling in determining limits to resuspension in an idealized estuary?
How do the roles of consolidation and stratification compare to the impacts of bathymetric changes and lateral advection within the York River estuary?
Figure 2: Postma diagrams of thresholds for erosion and deposition according to average particle size (Grabowski et al., 2011)
Model Designs
Longitude
Latitude
Figure 5: The three-dimensional York River estuary model grid, each square represents 5 model grid cells (Rinehimer, 2008; Fall et al. 2014).
REALISTIC YORK RIVER MODEL (Figure 5)
Forcings: river discharge, wind, salinity & tidal height from observations
Processes:
Sediment-induced stratification
Bed consolidation & swelling
Grid Resolution
170 m resolution along
110 m resolution across
20 vertical layers
10 bed layers
IDEALIZED MODEL (Figure 4)
Scaled similar to York River Estuary, VA.
120 m3 s-1 river discharge
0 – 26 psu salinity range
Tidally forced 12-hour period
Grid Resolution
500 m along estuary
40 vertical layers
10 bed layers.
ETM
Distance Along-Estuary (km)
Figure 4:
Top: Grid for the idealized quasi 2-dimensional estuary. Blue dot represents the location of the model data used to calculate ETM estimates.
Bottom: Salinity structure for idealized two-dimensional estuary with the location of the estuarine turbidity maximum (ETM) marked.
CONSOLIDATION: Erosion Formula
Critical shear stress varies with depth in the bed, and time (Figure 3), following Sanford (2008).
Figure 3: Conceptual diagram of bed consolidation and swelling (Rinehimer, 2008). The Tc and Ts represent the consolidation and swelling time scales, respectively and τceq represents the equilibrium bed critical shear stress for erosion profile.
Results
York River Estuary Model:
Effects of sediment-induced stratification:
Bed thickness (Fig. 10A):
Stratification slightly reduces bed erosion, i.e. decreasing the bed thickness.
Erodible mass (kg m-2) WP GP CB
(A) Neglects stratification
Figure 9: Modeled daily averaged erodible bed mass (kg m-2) at 0.4 Pa (i.e. Dickhudt et al., 2009). Effects of sediment – induced stratification: (A) Neglected; (B) Included.
Animation
Idealized Estuary
Standard (Std.) run is reference:
Bed thickness (Fig. 6A):
Stratification decreases deposit (89%);
Consolidation increases deposit (49%);
Combination decreases deposit (97%).
Applied bed stress (Fig. 6B):
Stratification reduced significantly.
Suspended mass (Fig. 6C):
Stratification - decrease 72%
Consolidation - increase 88%
Combination - decrease 36%
Erodibility:
Stratification reduced calculated erodibility (Fig.7).
ETM is most erodible (Fig. 6)
ANIMATION: Suspended sediment concentration (color), along – channel velocities (arrows), and salinity (black contours) along the estuaries. Top: idealized estuary; Bottom: York River model. A
(B) Includes stratification
Table 1: Description of the parameters changed between each simulation compared in this stud.
Stratification
Critical shear stress
Std. No
0.1 Pa
Run 1
Yes
Run 2
Depth varying
Run 3
Applied bed stress (Fig. 10B):
No significant change between runs.
Suspended mass (Fig. 10C):
Stratification decreased suspended concentra-tion at the ETM (i.e. near West Point). B
Bed thickness (m)
Applied bed stress (Pa)
Depth integrated, suspended sediment mass (kg m-2)
u velocity (m s-1)
Distance along estuary (km) B C D A
Figure 10: Along channel transect of York River model : (A) bed thickness (m) at end time; (B) daily average bed stress (Pa); (C) daily averaged, depth-integrated total suspended mass (kg m-2); (D) daily averaged, along estuary near-bed velocity (m s-1).
Figure 7: Estimated erodible bed mass (kg m-2) at 0.4 Pa (i.e. Dickhudt et al., 2009) for Run 2 and 3 at the mouth, ETM, and estuary head.
Erodible mass (kg m-2)
Location in estuary
Figure 6: Longitudinal section of idealized estuary: (A) bed thickness (m) day 365; (B) daily average bed stress (Pa); (C) daily averaged, depth-integrated total suspended mass (kg m-2); (D) daily averaged, along estuary near-bed velocity (m s-1).
Distance along estuary (km)
Bed thickness (m)
Applied bed stress (Pa)
Depth integrated, suspended sediment mass (kg m-2)
u velocity (m s-1) B C D A
Erodibility:
Stratification reduced the calculated erodibility at the ETM (Fig.8) and on the shoals (Fig. 9).
Otherwise did not significantly change erodibility elsewhere
Figure 8: Estimated erodible bed mass (kg m-2) at 0.4 Pa (i.e. Dickhudt et al., 2009) for York River model with and without sediment induced stratification (green and black) at Gloucester Point (GP), Claybank (CB) and West Point (WP; the ETM). Note: At WP, the non-stratified erodibility greatly exceeds the y-axis limit.
ETM Sediment Trapping:
Model trapped sediment and produced an ETM (Estuarine Turbidity Maximum) at West Point (Fig. 10).
ETM evolves to be most erodible part of the estuary (Fig. 8)
Stratification limited sediment entrainment at the ETM (Fig. 8 & 10).
ETM Sediment Trapping:
Stratification limits sediment entrainment at the ETM (Fig. 6 & 7).
Combined consolidation and stratification limited suspended concentrations (Fig. 6).
Conclusions
References
Future Work
SEDIMENT – INDUCED STRATIFICATION:
Decreased the bed thickness and suspended sediment in both the idealized and realistic estuarine model.
Other impacts of sediment-induced stratification were not apparent in the York River model.
The regions most significantly effected by sediment-induced stratification is the estuarine turbidity maximum (ETM) and shallow shoals.
CONSOLIDATION:
Limits erosion downstream of the ETM, but high erosion upstream remained, in the idealized estuary.
COMBINATION:
Stratification governs the vertical suspension of the differing size classes and consolidation confines sediment to the bed. The combination produces a reasonable ETM location and magnitude
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Dickhudt, P.J., Friedrichs, C.T., Schaffner, L C., Sanford, L.P., Spatial and temporal variation in cohesive sediment erodibility in the York River estuary, eastern USA: A biologically influenced equilibrium modified by seasonal deposition. Marine Geology, 267(3),
Fall, K.A., Harris, C.K., Friedrichs, C.T., Rinehimer, J.P., Sherwood, C.R., 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(2),
Garcia-Aragon, J., Droppo, I.G., Krishnappan, B.G., Trapp, B., Jaskot, C., Erosion characteristics and floc strength of Athabasca River cohesive sediments: towards managing sediment-related issues. Journal of Soils and Sediments, 11(4),
Rinehimer, J.P., Harris, C.K., Sherwood, C.R., Sanford, L.P., Estimating cohesive sediment erosion and consolidation in a muddy, tidally-dominated environment: Model behavior and sensitivity. Estuarine and Coastal Modeling, Proceedings of the Tenth Conference, 5-7.
Sanford, L.P., Modeling a dynamically varying mixed sediment bed with erosion, deposition, bioturbation, consolidation, and armoring. Computers & Geosciences, 34(10),
Traykovski, P., Geyer, R., Sommerfield, C., Rapid sediment deposition and fine‐scale strata formation in the Hudson estuary. Journal of Geophysical Research: Earth Surface, 109(F2).
Woodruff, J.D., Geyer, W.R., Sommerfield, C.K., Driscoll, N.W., Seasonal variation of sediment deposition in the Hudson River estuary. Marine Geology, 179(1),
Investigate variation in bed erodibility over varying time scales: Flood-ebb, spring-neap, seasonal, etc.
Examine influence of bathymetric changes and advection on suspended sediment concentrations and bed erodibility
Include aggregation and breakup of flocculated particles (Fig. 11); FLOCMOD: population size class model.
Figure 11: Cycle of deposition and resuspension of cohesive sediment involved in particle aggregation and breakup (Maggi, 2005).
Community Surface Dynamics Modeling System; Boulder, CO; May2016.