2009 International Sediment Transport and Sedimentation Symposium
Understanding Sediment Source To
Sink – On Modeling Several Critical
Processes in the Nearshore
Tian-Jian Hsu (Tom)
Assistant Professor
Center for Applied Coastal Research
Civil and Environmental Engineering
University of Delaware
Fate of Terrestrial Sediment in Coastal Ocean (Sediment source to sink)
- Why is it important?


Coastal Ecosystem
1) Heath of coral reef
2) Contaminant Transport
In “Mud, Marine Snow and Coral Reefs” by Eric Wolanski, Robert Richmond,
Laurence McCook and Hugh Sweatman, American Scientist, 91 (1), p. 44.
Carbon Sequestration
Yes, sediment transport is
related to GLOBAL WARMING!
Particulate Organic Carbon and
nutrients are carried by river
borne sediments.


Seabed Properties
 They determine surface wave
propagation and acoustic wave
transmission.
Adopted from MARGINS Science Plans 2004. National Science Foundation.
Waterway Navigation
 dredging
A soft mud bed at a tidal flat
in Willapa Bay, WA
Where are the big signals?
Large rivers in passive margin – Amazon, Mississippi
Small mountainous river in active margin – e.g., Kaoping (Taiwan), Jhoushui (Taiwan),
Waipaoa (New Zealand), Santa Clara and Eel (USA). See Milliman & Syvitski (1992) J.
Geology.
Adopted from MARGINS Science Plans 2004.
National Science Foundation.
Physical Settings of Small Mountainous River
• Episodic high sediment yield triggered by large rainfall due to typhoon or
tropic cyclone (sediment concentration often exceeding several g/l).
• Highly salt-stratified frontal zone.
• Energetic wave condition.
Left: SAR image of Jhoushui river mouth courtesy of Dr. Chang at Center for Space &
Remote Sensing Research, National Central Univ., Taiwan. Right: Figures of
Landslides and river flooding after typhoon Mindulle (July 2005) are provided by Dr.
S.-J. Kao, Academia Sinica, Taiwan.
ROMS model results for highly salstratified frontal zone adopted from
Hetland & MacDonald (2008)
Spreading in the near-field
Merrimack River plume, Ocean
Modeling, 21, 12-21.
Motivation
Large-scale coastal models, e.g. Delft3D, MIKE21, NOPP-CSTMS (ROMS), ECOMSED (POMS)
and many others, require accurate parameterization on small-scale processes.
NOPP-CSTMS (Community Sediment Transport Model System):
CSTMS model results provided by Chris Sherwood (USGS) and Rocky Geyer (WHOI). http://www.cstms.org
~10m
~20cm
Wave boundary
layer process and
near-bed
sediment
transport require
grid resolution ~ 1
mm
Frontal dynamics requires
grid resolution ~ cm
Fluid Mud Modeling
Develop a numerical modeling framework for fine sediment transport based on
Fast Equilibrium Eulerian Approximation (Ferry & Balachandar 2001) to the Eulerian
Two-phase formulation – Mixture Approach

Mass and momentum equations:



Turbulent & viscous stresses; rheological stresses.
Sediment-driven gravity flow.
Turbulence closure: damping of turbulence due
to sediment density stratification and viscous
drag in turbulent fluctuation.
z
dilute
turbulent
suspension
lutocline
 < o

Rheological stress: e.g., Bingham-plastic.
Important to hydrodynamic dissipation.

Floc properties and floc dynamics assuming
fractal structure. Winterwerp (1998), Khelifa &
Hill (2006), Son & Hsu (2008).
mobile fluid mud

Erodibility (Type I erosion): critical bottom stress
depends on cumulative eroded mass.
Consolidating bed

Future work: Directly resolve bed consolidation and
fluidization (e.g., Gibson et al. 1967). Evolution of floc
aggregate structure.
o <  < gel
 > gel
~10g/l
~100g/l
Winterwerp & van Kesteren (2004)
Numerical Implementation
•2DV turbulence-averaged fine sediment modeling framework is solved
numerically by revising a 2DV time/depth resolving wave model (COBRAS,
Lin and Liu 1998, JFM). A non-hydrostatic 2D RANS solver with complex
bathymetry.
•COBRAS can calculate surface wave propagation using volume of fluid (VOF)
scheme and is a powerful tool for nearshore where wave effects are important.
•When sediment concentration and salinity  0, numerical model reduces to
original COBRAS for clear fluid RANS model.
Model results on surf zone wave provided by Alec Torres-Freyermuth, CACR, U. Delaware.
App #1: Sediment-laden River Plume & Initial Deposition
Positively Buoyant Plume (~<40g/l):
Negatively Buoyant Plume: (>40g/l)
 Frontal trapping, lift-off, etc (e.g., Armi &
Farmer 1986; Geyer & Kineke 1995; Wu et al.
2006; Lee et al. 2009).
 Hyperpycnal flow (Wright et al. 1988, Nature,
vol. 332, p. 629)
 Sediment settling (Hill et al. 2000):
1) Primary particle (↓ 0.1 mm/s)
2) Flocculation process (↓ 1 mm/s)
3) Convective sedimentation (↓ 1 cm/s)
(e.g., Green 1987;
Parson, Bush & Syvitski 2001;
McCool & Parson 2004)
 Buoyancy reversal (due to rapid sediment
deposition, Hurzeler et al. 1996; Hogg et al.
1999)
Wright & Nittrouer [1995], Estuaries.
Hyperpycnal plume – silt
River discharge, U0=20 cm/s, cin=47 g/l, Salinity of seawater 35 ppt, ccric=42 g/l
Silt: d=20 µm, s=2650 kg/m3, Ws=0.36 mm/s
Salinity
(ppt)
Sed.
conc.
(g/l)
Hypopycnal plume (convective sed.)– silt, clay (primary particle)
River discharge, U0=20 cm/s, cin=26.5 g/l, Salinity of seawater 35 ppt, ccric=42 g/l
Silt: d=20 µm, s=2650 kg/m3, Ws=0.36 mm/s
Salinity
(ppt)
Sed.
conc.
(g/l)
Convective Sedimentation/Instability
• Earlier studies explain this phenomenon
based on double-diffusion analogy for
salt-finger: e.g., Green (1987), Sedimentology;
Chen, (1997) Deep-Sea Research; Hoyal, Bursik,
and Atkinson (1999) Marine Geology.
• McCool & Parsons (2004) carry out first
experiment for convective instability with
ambient shear flow. They observe
convective instability with sediment
concentration as low as 380mg/l.
Parsons, Bush, Syvitski,, 2001, Sedimentology, 48, 465-478.
Science Issue #1:
Is double-diffusion
the only explanation
of convective
instability? How
about gravitational
settling itself?
McCool & Parsons, 2004, Cont. Shelf Res., 24, 1129-1142.
Sicence Issue #2: Can convective instability occur in the field?
Recent field study by Warrick et al. (2008, Cont. Shelf Res.) at Santa Clara River, CA
observe rapid sediment settling from bottom tripod, which was indirectly related to
convective instability (not direct field evidence).
Our numerical model results suggest
when inlet height is larger (~meters),
strong interfacial mixing (K-H waves,
Holmboe waves) can suppress
convective instability:
1) It is likely that convective instability
can occur in the field but with much
larger sediment concentration, i.e.,
10~20 g/l.
 Important for small mountainous
river.
2) This problem is strongly related to
plume dynamics (e.g., hydraulic
control, Armi and Farmer 1986), and
interfacial mixing.
3) What is the effect of surface gravity
waves?
Resuspension



Currents (tidal-, wind-driven, etc)
and waves can suspend high
concentration cohesive sediment –
Fluid mud
Wave dissipation over mud.
Wave-supported gravity-driven
mudflows (e.g., Ogston et al. 2000;
Traykovski et al 2000; Wright et al.
2001).
Adopted from MARGINS Science Plans
2004. National Science Foundation.
Traykovski et al. [2000], Cont. Shelf Res.
App #2: Detailed modeling of wave-supported gravity-driven mudflow and its
parameterization (Hsu et al. 2008, JGR - Ocean, accepted).
Wright et al. 2001; Marine Geology
- Parameterization for coastal modeling
1DV fluid mud model results of wave supported
gravity-driven mudflow at Po prodelta.
EUROSTRATAFORM data obtained in collaboration
with Peter Traykovski (WHOI).
App #3: Wave-mud Interaction – 2DV Modeling
The presence of mud can significantly attenuates wave energy (e.g.,
Gade 1958; Dalrymple & Liu 1978; Sheremet & Stone 2003)
Rheological stress closure:


0
 s   s 0 1 

 kb 0  u z 
ka
u
   s
z
s
xz
f
Stiffness:
kb =>Bingham-type
kb =>Newtonian-type
Cnoidal Wave Train Propagating over Muddy Seabed
*Numerical modeling of wave-mud interaction is carried out by postdoc Alec Torres-Freyermuth
Wave characteristics:
H=0.72 m
T=6 s
L=28 m
Mesh characteristics:
Min (y)=2 mm
Min (x)=2.5 cm
Fluid-mud characteristics:
D=22 m (floc)
f=1340 Kg/m3
nf=2.1
Ws~0.1 mm
Wave Amplitude Dissipation
The wave amplitudes present a monotonic decrease in amplitude at various
frequency, suggesting both direct interaction (long wave and mud) and wave-wave
interaction cause wave attenuation.
kh=1
@ x=185 m
Clear fluid
Clear fluid with
increased
Ks=400mm
Nonlinear wave effects
- Ongoing work
Fluid mud
Mud-laden Wave Boundary Layer Dynamics
Clear fluid
Fluid mud
(a)
(b)
Ongoing and Future Works
- related to sediment Disposal off Small Mountainous River
1) Improved modeling on sediment-laden plume dynamics for high
sediment yield condition.
- Small-scale modeling (2DV~1m X 100m);
- Coastal Modeling, e.g., Community Sediment Transport Modeling System (3D
~10m X 5km X 5km)
Schematic plot of sediment-laden river plume produced in collaboration with Dr. Rocky Geyer, WHOI
2) Effects of waves on plume dynamics and sediment deposition (and vice versa)
a) NearCoM Simulation (REF/DIF1+SHORECIRC)
Model results provided by Drs. Fengyan Shi and Jim Kirby
University of Delaware. See
http://chinacat.coastal.udel.edu/~kirby/programs/nearcom/
b) 2DV Buoyant plume simulation with
free-surface (use VOF scheme):