1) The document presents numerical modeling results studying net transport of fine sediments in an idealized estuary resembling the Sea Scheldt using Delft3D. 2) The initial results show that estuarine circulation alone can produce an estuarine turbidity maximum, but sediment-induced buoyancy destruction is needed to concentrate sediments near the bed. 3) Tidal asymmetry on its own does not induce net sediment transport; water-bed exchange is also required.

1. IAHR, June 29, 2015
net transport of fine sediments in a narrow,
converging estuary – the Sea Scheldt
Han Winterwerp, Miguel de Lucas Pardo, Bas van Maren,
Julia Vroom & Zheng Bing Wang
Deltares and Delft University of Technology

2. the hyper-turid Ems River
Emden
ebb
Qriv < 30 m3/s
flood
salinity
suspended
sediment
salinity
suspended
sediment
Talke et al., 2009 - data Aug 2, 2006
no relation between SPM & salinity
tidal
excursion
Herbrum (weir)

3. historical evolution of the tidal range
summary tidal evolution
0
2
4
6
1875 1900 1925 1950 1975 2000 2025
year
tidalrange[m]
Antwerp
Bremen
Hamburg
Nantes
Papenburg
tidal amplification in Loire (Nantes) and Ems (Papenburg)
can be explained for 50% by deepening and narrowing and
for 50% by sediment-induced reduction in hydraulic drag

4. our research question – the stability diagram
0
0 0.2 0.4 0.6 0.8 1
volumetric concentration 
fluxRichardsonnumberRif
Ricr
super-saturated
sub-saturated
U2
U1
U2 < U1
present Scheldt
conditions
present Ems &
Loire hyper-turbid
conditions
potentialenergy/kineticenergy
how do we get to hyper-turbid state?

5. our research question
reduced river flushing
too much deepening
tidal
amplification
reduction in
hydraulic drag
increase tidal
asymmetry
pumping of mud
this takes time (decades?)
too big ships
what triggers the increase
in the net up-estuary
transport of mud, e.g.
the “pumping of mud”

6. net sediment transport processes
net sediment balance SPM over
water column
water-bed
exchange
∆SPM/∆t = import by estuarine circulation 2DV or 3D not relevant
import (or export) by tidal asymmetry see next slide necessary
export by river flushing depth-averaged not relevant
source by bed/bank erosion depth-averaged -
sink by sedimentation depth-averaged -
transport by tidal advection = gross effect
2DV or 3D: vertical SPM gradients required for net transport
vertical gradients are affected by sediment-induced stratification

7. net transport by tidal asymmetry
we start here, thus we look for balance between:
• estuarine circulation
• river-induced flushing
• tidal asymmetry in vertical mixing and slack water
net transport by tidal asymmetry SPM over
water column
water-bed
exchange
alluvial bed
(hyper-turbid estuary)
peak flow asymmetry depth-averaged necessary
ebb/flood asymmetry
(settling lag)
depth-averaged necessary
starved bed
(“normal” estuary)
internal asymmetry
(vertical mixing)
2DV or 3D necessary
slack water asymmetry
(scour lag)
depth-averaged necessary
4
T U∝
( )d d T
u t
2
z Uε ∝
( ) 0
d d u
u t =

8. asymmetry for starved bed conditions
for starved-bed systems: slack-water asymmetry &
asymmetry in mixing
-2
-1
0
1
2
0 5 10 15 20
verticalmixingεz[m2/s]
time t [hrs]
mixing ~U^2
22 -φ4 = ...o
flood
ebb
22 -φ4 = ...o
flood = 1.26×ebb
-2
-1
0
1
2
0 5 10 15 20
velocityU[m/s]
time t [hrs]
U4 = 0.2 U2
22 -φ4 = ...o
ucrit
flood
ebb
HWSLWS

9. our approach
• idealized, converging estuary with constant depth
• dimensions, tidal forcing and river flow from Sea Scheldt river
• no intertidal area, hence flood-dominant
• implemented in Delft3D
• switch on/off various processes one by one (or combinations)
• vary boundary conditions (Qriv and external M4 amplitude & phase)
• vary internal processes (equation of state, erosion/deposition)
ETM
Bath
Ghent
idealized model of Sea Scheldt (+ small part Western Scheldt)
tideb.c.
Qriv
3km
104 km

10. first results – development of asymmetry
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0 5 10 15 20 25 30 35 40 45
velocityU[m/s]
time t [hrs]
flow velocities in idealized estuary
sea boundary (x = 0)
max sal. intrusion (x = ~18 km)
x = 40 km
LWS
HWS
mouth x = 0 maximum salinity intrusion x = 40 km
LWS HWS LWS HWS LWS HWS
45 min 24 min 33 min 24 min 24 min 21 min
• profound development of asymmetry in peak velocity, hence vertical mixing:
induces up-estuary transport
• tLWS < tHWS, hence down-estuary transport, but reducing in up-estuary direction

11. first results – effect estuarine circulation
simulating 3 months sediment input from sea-side:
• no water-bed exchange
• no sediment-induced buoyancy destruction
salinity sediment (SPM)

12. first results – effect buoyancy destruction
with buoyancy destructionwithout buoyancy destruction
sediment-induced buoyancy destruction =
effect sediment-induced stratification on vertical turbulent mixing
maximal up-estuary ETM after 6 months simulation time

13. cumulative sediment flux through kmr 5
without buoyancy destruction
with buoyancy destruction

14. discussion and conclusions
• we used Delft3D for studying net transport of fine sediments in
schematized estuary, resembling Sea Scheldt,
• this approach seems promising, but many more analyses are
required,
• the first results indicate:
• ETM can be produced by estuarine circulation alone,
• sediment-induced buoyancy destruction keeps fines closer to
the bed, reducing horizontal transports,
• the realistic effect of sediment-induced buoyancy destruction is
large, hence cannot be omitted in sediment transport studies,
• tidal asymmetry without water-bed exchange does not induce
net transport of fine sediment,
• these numerical results are in line with theory, but have never
been quantified.