1. Comparison of modeled and observed bed erodibility in the York River estuary, Virginia, over varying time scales
Danielle Tarpley, Courtney K. Harris, Carl T. Friedrichs
Virginia Institute of Marine Science

2. ETM
STM
Rinehimer, 2008

3. Motivation: Sediment availability impacts sediment distribution
ETM
STM
Motivation: Sediment availability impacts sediment distribution
Rinehimer, 2008
Scully and Friedrichs, 2007

4. Motivation: Impacts of bed erodibility on the near bed suspended sediment concentration
Bed Stress
Near-Bed C
Red =
0.1 Pa
1 kg/m2
at 0.2 Pa
150 mg/L
7 Jul 2007
10 km
Dominate flux during on spring
Dominate flux on the shoal
Indicates the bed erodibility may be variable and needed to better understand the sediment fluxes in the estuary.
Fall et al., 2014
In an estuary, bed stresses are tidally driven and therefore relatively uniform spatially
Patterns in SSC align with patterns in bed erodibility

5. Observations indicate cycles in sediment availability, impact sediment fluxes. While modeling efforts indicate bed erodibility impacts sediment availability.
Objective:
Examine the change in bed erodibility in the York River estuary over varying time scales:
Observational and modeled seasonal variation
Observational and modeled spring/neap variation

6. Observational measurements of erodibility
GUST erosion microcosm
Applies a known stress to the sediment surface
Measures suspended sediment mass
Turbidity meters
Filtered water sample
U-GEMS Manual Version 1.0, Green Eyes LLC

7. Numerical Methods: Hydrodynamic and sediment transport model
Longitude
Latitude
Regional Ocean Modeling System (ROMS)
Resolution (~150m)
Forcings
River discharge, wind, salinity & tidal height from observations
Sediment-induced stratification
Bed Consolidation and Swelling
Resolution 170 m along-channel
Open boundary is set to match observed salinities and open boundary at the river is fresh (salinity)
USCG
Every grid cell shown represents 5 grid cells

8. Sediment Bed Model
Bed consolidation model adjustment to equilibrium
Recent
Erosion
Deposition
More
Erodible
Less
Erodibility defined as amount of sediment mobilized at a shear stress threshold
Erosion formula:
Critical shear stress varies with depth in the bed, and time, following Sanford (2008):
Define critical shear stress: the shear force applied to the bed in which erosion occurs

9. Model comparison to tidally averaged ADV observations near Clay Bank, York River estuary
Fall et al., 2014
Comparable accelerating and decelerating current speeds
Matching peak SSC but over estimating over the rest of the tidal phase
Slight over estimate of the applied bed stress

10. Seasonal Erodibility Winter: December – February Spring: March – May
Summer: June – August
Fall: September – November
Seasonal Erodibility
Dickhudt et al., 2009

11. GUST erosion measurements at Clay Bank, York River
Years *
Shoal
GUST erosion measurements at Clay Bank, York River

12. Contrast Spring Season Summer Season High river discharge (top)
More erodible.
Summer Season
Low river discharge (bottom)
More consolidated bed.

13. Seasonal patterns
Higher erodibility corresponding with elevated river discharge
Locally at Clay Bank
Throughout the York
Potential drivers
More recent deposition of fine sediment
Shift in salinity stratification as observed in the Hudson River (Ralston et al., 2011).

14. Spring/neap cycle in bed erodibility
Days starting from 1 April 2010
Gust eroded mass
(kg/m2) at 0.2 Pa
Tidal range
(m)
r =
r =
(a)
(b)
Kraatz et al, 2013
cores from spring of 2007 exhibited significantly more eroded mass (p < ) than those from 2010
Erodibility depended on the where the system was in the spring/neap cycle with the erodibility being highest during spring tides.
Correlation with time alone is poor
Time and tidal stage explain 94% of the variance
Observations correlate best with a 5 day lag

15. On average higher erodibility during spring tide
MODEL summer average for the entire timeseries during spring and neap tides
On average higher erodibility during spring tide
Channel no difference
Values lower than 2010 observations
Shoal
Channel

16. MODEL combined daily average bed erodibility for spring and neap tides
Shoal
Erodibility (kg m-2)
Channel
Erodibility (kg m-2)
Days past the onset of the tidal cycle

17. MODEL spatial variation in spring/neap daily averaged bed erodibility
West Point
Gloucester Point
Clay Bank
Longitude
Log10 kg m-2
Latitude
Fall et al., 2014
Neap
Spring
Higher erodibility during spring
Higher erodibility in the upper and middle river

18. Conclusions
Bed erodibility patterns from a three-dimensional model of sediment processes in the York River that includes sediment-induced stratification, and bed consolidation and swelling compare well with observational patterns.
Both the observations and model results indicate that bed erodibility and therefore sediment availability vary over a range of timescales.
At the STM erodibility was higher:
In the spring season
During spring tides
The model also indicates that spatial differences in bed erodibility evolve between the channel and shoal.
The sediment bed at the STM develops to be more erodible in the shoal.

19. Future Work
Examine spatial variation in bed erodibility over longer time scales
Consider variation of bed erodibility on additional time scales
Flood/ebb tidal cycle
Interannual
Storms
Since storms can increase sediment transport flux by a factor of 100 (Gong and Shen, 2009)
Consider the addition of other cohesive processes
Flocculation

20. Thank You!
Courtesy of Pat Dickhudt

21. Additional slides

22. 1. Introduction – What is bed erodibility. How do we measure it
1. Introduction – What is bed erodibility? How do we measure it? Why do we care?
2/16
-- Along most “drowned river” coastal plain estuaries, tidal velocity (and thus tidal bed stress) is relatively uniform with distance along the estuary.
-- So changes in suspended sediment concentration (C) along estuaries must be due at lowest order mainly to changes in bed erodibility.
-- So if you care about sediment concentration (C) in estuaries, you need to care about bed erodibility, which in turn is characterized by M = eroded mass in kg/m2 at a given stress.
Bed Stress
Bed Erodibility
Near-Bed C
7 Jul 2007
7 Jul 2007
7 Jul 2007
10 km
10 km
10 km
00:40 – 01:30
Red =
1 kg/m2
at 0.2 Pa
Red =
150 mg/L
Red =
0.1 Pa
Daily averaged magnitudes from 3-D ROMS model of York River Estuary (Fall et al., 2014) utilizing depth-limited, temporally-evolving cohesive erodibility model of Sanford (2008).

23. Fall et al., 2014

24. Highest sediment not correlated with highest stress but high erodibility
Fall et al., 2014

25. Motivation: Sediment availability impacts sediment distribution
Indication of possible changes in bed erodibility

26. Seasonal changes in bed erodibility

27. Kraatz, 2013

28. Observations show seasonal and spatial variation in bed erodibility
Seasonal difference
During some seasons the erodibility was higher than the shoal
- - Can cut if needed
Dickhudt et al., 2009

29. Spin up
Neap
Spring
This time period should be after the secondary turbidity maximum has vacated the area
Change in water level over a 6 hour period (big=spring tide)

30. Average higher erodibility during spring tide
Shoal
Channel
Average for the entire timeseries during spring and neap tides
Average higher erodibility during spring tide

31. Erodibility during spring tide remains higher ~6x higher
Combined daily average bed erodibility for spring and neap tides
Shoal
Erodibility (kg m-2)
Days past the onset of the tidal cycle
Erodibility during spring tide remains higher
~6x higher
Observations correlate best with a 5 day lag after the onset of the tide
Model shows ~8 day lag for peak erodibility

32. On average higher erodibility during spring tide
MODEL summer average for the entire timeseries during spring and neap tides
On average higher erodibility during spring tide
Channel: less of a variation
Values lower than observations
Shoal
Channel