1 university of utrecht modelling lateral entrapment of sediments in well-mixed estuaries photo:...
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1 University of Utrecht
Modelling lateral entrapment of sediments in well-mixed estuaries
Photo: mudbanks in the Ems
Huib de Swart
Karin Huijts, Henk Schuttelaars, Arnoldo Valle-Levinson
Institute for Marine and Atmospheric Research Utrecht
2 University of Utrecht
Lateral entrapment of suspended sediment: James River estuary Introduction
Washington
Chesapeake Bay
100 km
Sediment trappedat the left bank
From ADCP data A. Valle-Levinson(view into estuary)
3 University of Utrecht
Introduction
Here: simple model to study physical processesthat cause lateral trapping of sediment
Specific focus:1. Coriolis deflection of tidal currents and
of along-estuary density-driven flow
2. Lateral density gradients
Main question:
why is sediment in James River estuarytrapped at the left bank?
4 University of Utrecht
• Geometry• Local (L~10 km)
• Along-estuary uniform
• Here: lateral bathymetry of James estuary
• 3D Shallow water equations • Tidal estuary
• Density gradients (prescribed)
• Coriolis
• M2 + M0
• Sediment mass balance equations• Advection diffusion equation for suspended sediment
• No mean lateral sediment transport
• Non-cohesive sediment, uniform size
• M2 + M0
Domain and equations of motion Model
sea
river
5 University of Utrecht
Analysis Model
where primes~tides and bars~time mean
• Scaling and perturbation analyses → reduce eq's to essential physics
• Analytical solutions:
• and 'ccc
Tidal flow equations Residual flow equations2
2
2
2
0
z
z
u ufv g A
t x z
v vfu g A
t y z
v w
y z
.0
,
,
2
2
0
2
2
0
z
w
y
v
z
vA
ygz
y
guf
z
uA
xgz
x
gvf
z
z
yx uuuu '
6 University of Utrecht
Effects of tides and density gradients on the transport is investigated
separately by substituting and :
1. Tides:
2. Along-estuary density gradient:
3. Lateral density gradient:
Mean lateral sediment transport
Mean lateral transport due to
0
dzcv-D
0
dzcv x-D
0
dzcv y-D
Model
+
'ccc
By definition: T = 0
dzvc-D
yxvvvv '
7 University of Utrecht
Results
Mean sediment concentration (mg l-1)
Case 1: Tides and along-estuary density gradient
0
dzcv-D
0
dzcv x-D
xv'v 3 cm/s
0.5 cm/s
Highest concentrations near the bed → Lateral near-bed flow crucial for lateral transport
trapping at right bank
Tra
nspo
rt
Mean lateral sediment transport induced by…
xv v
8 University of Utrecht
Mean density-driven flow
ResultsCase 1: Tides and along-estuary density gradient
In deep channel: inflow
Coriolis deflection =>
Sediment is transported to the right bank
9 University of UtrechtTrapping at left bank
Case 2: Tides and both horizontal density gradients Results
Mean flow component due to lateral density gradient (cm/s)
← max. 6 cm/s
Mean sediment concentration Mean sediment transport due to…
…along-estuary density gradient
…tides…lateral density gradient
0
dzcv y-D
xv'v 3 cm/s
0.5 cm/s
6 cm/syv
Tra
nspo
rt
salter
fresher
10 University of Utrecht
Tidal amplitude Comparison to observations
Courtesy observations:
A. Valle-Levinson
12 University of Utrecht
Mean concentration Comparison to observations
Lateral density gradient mechanism dominates in James River!
13 University of Utrecht
Conclusions
Sediments are trapped at the left bank of the James-transect, as
• Lateral near-bed flow induced by lateral density gradient
induces mean sediment transport towards that side
• Tides erode bed sediments
The effects of tides and density gradients on lateral sediment trapping can be studied separately, providing insight in underlying mechanisms.