Transcript ppt

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. Friedrichs
Abstract
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.
Modeling System
• Deposition: D = w sC
• Erosion: E = M(t b - t cr (z))
• Sediments
• Regional Ocean Modeling System (ROMS)
– Solves the hydrostatic Reynolds-averaged NavierStokes equations on a curvilinear orthogonal grid
with vertical stretched terrain-following
coordinates.
ì 1
• Critical
ï T (t ceq (m) - t cr (m))
Shear ¶t (m) ï c
cr
0
Stress ¶t = í
ï 1
ï - T (t ceq (m) - t cr (m))
î s
York River
t cr (m) < t ceq (m)
t cr (m) = t ceq (m)
t cr (m) > t ceq (m)
Current Speed (cm/s)
Concentration (mg/L)
b
a
Bed Stress (Pa)
c
Tidal Velocity Phase
(θ/π)
Decreasing U
Increasing U
Tidal Velocity Phase
(θ/π)
Decreasing U
Increasing U
Tidal Velocity Phase
(θ/π)
Decreasing U
Increasing U
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 Column
Seabed
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).
a
Note: τceq = τcrinit
b
– Observational data needed to constrain these
parameters
More
Erodible
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).
a
April (more erodible)
Ts=25 Days
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).
b
GP
S0 = 25
S0 = 32
– 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).
S0 = 29
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.
• 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
Table 1: RMSE comparing modeled and
measured water level at United States Coast
time (Ts).
Guard (USCG) pier in Yorktown, VA,
– Longer Tc produced more asymmetry between spring and York River.
Tidal May – Aug
neap tide (Fig. 6)
USCG
0.22
– Longer Tc also produced a more erodible bed (Fig. 6).
CB
0.25
– Longer Ts allowed minimal adjustment of the τc (Fig. 8)
Claybank (CB) and Taskinas Creek (TC) on the
• Sensitivity to both the τcrinit and the τceq profiles.
TC
July – Aug
1.55
0.20
0.23
1.51
– Positive feedback between erodibilty and concentration when τcrinit profile more
erodible bed (as in April).
– Instantaneous τc profiles shift toward zero (Fig. 7).
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).
τcrinit
τceq
Note: τceq ≠ τcrinit
Ts=2 Days
Rapid adjustment from τcrinit to τceq.
b
τcrinit
τceq
Ts=25 Days
Some adjustment from τcrinit to τceq.
c
τcrinit
τceq
Ts=50 Days
Min. adjustment from τcrinit to τceq.
• Model output compared relatively well with
both hydrological parameters and sediment
transport parameters
Future Work
• Update the York River forcings to represent
a more recent year.
• Implement cohesive sediment in a 2-D
estuary test case (Figure 9)
USCG
S0 = 25
S0 = 32
S0 = 29
• Salinity and water level compared to
observations
a
September (less erodible)
Ts=25 Days
• The model was most sensitive to user defined
parameters such as the critical shear stress
equilibrium profile and initial critical shear stress
profile
Less
Erodible
– 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 Grid
Figure 1: York River
estuary model grid and
ROMS model vertical
structure.
April
τceq=0.4m0.55
• Model estimates
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).
– The addition of a depth varying critical shear stress
was implemented
Sept
τceq=1.0m0.62
Sensitivity Tests
• 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.
– 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.
– Along channel:
æ
S0 é
X öù
S(x) =
ê1+ tanhç2 - ÷ú
2 ë
b øû
è
• Settling velocities: 0.8 mm/s (Flocs) & 2.4 mm/s (Pellets)
– Sub model based on Sanford (2008) including
– Swelling, consolidation, and an increase in τc with
depth in the sediment
– 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.
• A 3-D numerical model for the York River estuary
was developed
• Salinity
– Two sediment classes:
• Community Sediment Transport Modeling
System (CSTMS) with a cohesive bed sub-model
Taskinas Creek
Conclusions
Cohesive Sediment Model
– 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.
Figure 9: Modeled salinity
transect of a 2-D estuary
ROMS test case
References
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.
Acknowledgments
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.