Faculty of Geosciences
Research group River and delta morphodynamics
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Modelling shoal margin collapses and their morphodynamic effect on channels and shoals in a sandy estuary
Wout M. van Dijk
1,*Dick R. Mastbergen
2, Jebbe J. van der Werf
2, Jasper R.F.W. Leuven
1and Maarten G. Kleinhans
1(
*presenting author W.M.vanDijk@uu.nl, www.woutvandijk.com)
1
Fac. of Geosciences, Dept. of Physical Geography, Universiteit Utrecht, Utrecht, The Netherlands,
2Unit Marine and Coastal Systems, Deltares, Delft, The Netherlands
Background and Model Setup
Conclusions
Sensitivity Analysis
Channel bank failure and collapses of shoal margins have been recorded system- atically in Dutch estuaries for the past 200 years (Wilderom, 1961-1979). In many locations collapses reoccur at intervals of several years to decades. The effects of these collapses on the morphodynamics of estuaries are unknown.
Objective:
1. Develop universal parameterisations for shoal margin collapses.
2. Analyse the near-field morphodynamics and far-field effects on flow pattern and channel-bar morphology.
Wout van Dijk
Initial versus Yearly Collapses
References & acknowledgements
Channel Network Analysis (Kleinhans et al., 2017)
Fig. 3) The space-time diagram for three cross-locations (Fig. 1c) shows sediment spreading from artifical sill.
shoal margin collapses
-60 10
bed elevation (m)
0 10
±
distance (km) 5
A
B
C
D
E F G
H
I J
A,B,C,.. locations as in figure 4 & 6
1960 2015
±
shoal margin collapses bank protection
0 10
distance (km) 5
Vlissingen
Borssele
Hansweert
Terneuzen
Ossenisse Walsoorden
shoal margin boundary
-30 5
bed elevation of 2015 (m)
-5 5
DoD of 2015-2014 (m)
A B C
A
cross-sections Fig. 3 collapse
Fig. 1) Shoal margin collapse of July 2014 that eroded 800,000 m3 of the tidal flat of Walsoorden (a). Yearly bathymetry measurement of 2015 (b). DEM of Difference of illustrates the location of the 2014 collapse (c).
Fig. 2) Shoal margin migration map shows that shoal margin collapses occur at shoal margins that do not migrate laterally (Van Dijk et al. Subm).
What are shoal margin collapses?
• Shoal margin collapses occur on the inner side of a bend.
• Flow slides, i.e., liquefaction or breaching.
• ~1 Mm
3eroded sediment → perturbation.
How big, and can we predict shoal margin collapses?
• Shoal margin collapses in the Western Scheldt have a median size of 34,000 m
2, and a median volume of 100,000 m
3, and both are log-normal distributed.
• The geometric shape of a collapse follows 1/3 ellipsoid.
• Location of collapses can be predicted by the relative shoal margin height (H
r) as well as slope (α
r), according to the following equation:
where F is the frequency of shoal margin collapses (SC), ψ is relative density and L
smis the total length of shoal margin in the estuary. Western Scheldt; SC
avg= 5 per year and L
sm= 300 km.
The Western Scheldt model setup
• Multi-channel systems (ebb- and flood-dominated).
• Spring-neap cycles, tidal range 3.5-5 m.
• Relatively fine sediment, 200 μm.
• Delft3D model schematisation.
• 2 years hydrodynamic simulation; MorFac of 20.
• Tested 2 scenarios: 1. Initial 10 collapses of 1 Mm
32. Yearly collapses
Fig. 4) Along estuary time-space diagram showing the spreading of collapsed sediment and mean width-averaged bed elevation difference between run with and without the large collapses.
Fig. 5) Modelled shoal margin collapse locations for the scenario with initial 10 collapses.
Fig. 6) a, DEM of Difference shows the difference between run with and without collapses after 40 yrs, and contours indicate distribution of the tracer sediment. Along estuary time-space diagram of the mean bed
elevation difference (b) and collapsed sediment spreading (c), with indicated disturbance direction.
• Shoal margin collapses excite the channel network of sandy estuaries.
• Volume of the collapse and grain-size determine magnitude and direction.
• Short-term: the morphodynamics are affected by changing bed elevation in longitidunal direction but not in transverse direction. Residual currents were not affected by the collapses.
• Long-term: the disturbance stimulate morphological changes, especially when the disturbance reaches a channel junction affecting the bed elevation in trans- verse direction as well.
• Shoal margin collapses reduces the depth of the channels and increases the number of smaller channels on the tidal marshes according, which has nega- tive effects for the fairway but positive effects for ecological habitats.
Wilderom (1979), Resultaten van het vooroeverondervoor langs de Zeeuwse stromen (in Dutch). Technical report, Rijkswaterstaat, 1979.
Kleinhans et al. (2017), Computing Representative Networks for Braided Rivers. 33rd Intern. SoCG 2017, pp. 48:1-48:15.
Van Dijk et al. (Subm.), Probability and causes of shoal margin collapses in sandy estuary. to Earth Surface Processes and Landforms.
pre-print available through doi:10.17605/OSF.IO/MYRKW (EarthArXiv)
Vici grant to MGK by the Netherlands Organisation for Scientific Research (NWO). We gratefully acknowledge Marco Schrijver (Rijkswater- staat), and Marcel Taal (Deltares) for insightful discussions. We thank Deltares for providing the schematization of the Western Scheldt, and Rijkswaterstaat for providing the SCALWEST model.
Sensitivity of the grain-size
• Finer material spreads faster and deposits at the channel flanks.
• Finer material is predominantly seaward → the residual current.
• Coarser material is predominantly landward → tidal asymetry.
Sensitivity of the collapsed size and location
• Sediment from the larger collapses are spread over longer distances.
• Larger collapses show tens of centimeters variation of the mean bed elevation, compared to centimeters for smaller collapses.
• Sediment is transported in ebb and flood direction.
• Dominant sediment transport direction follows residual current.
Initial Collapses
• Spreading of the collapsed material is not uniform throughout the estuary.
• Migration of disturbance is dominant in one direction, depending on - Flood-dominated channel (secondary channel), or
- Ebb-dominated channel (main channel).
• Less spreading in the secondary channel (e.g. locations E and F).
• Mean bed level difference excites over time.
equation 1
F SC ] g = 11 b H
rl
2.5b cot a 9.5
rl
5b l 10 1
-10 0.05- }^ hL
smSC
avgtime (yrs)time (yrs)
distance from Western Scheldt mouth (km)
5
0 10 15 20 25 30 35 40 45 50
collapsed tracer sediment (L) 102 104.5
-1.5 1.5
mean bed level diffence (m)
A B C D G E F H I J
0 40
20 30
10 0 40
20 30
10
-10 10
DoD (m)
0 10
±
distance (km) 5
tracer sediment (L) 1000100
10
a
b
c
locations see Fig. 5
width-averaged on curvilinear grid
ex cit e
migration
Fig. 7) Yearly collapses against run without collapses shows migration, excitation and damping of disturbance.
-60 10
bed elevation (m)
0 10
±
distance (km) 5
0 50 100 150 200
0 20 40 60 80
-60 -50 -40 -30 -20 -10 00 10 20
40 60 80
-60 -50 -40 -30 -20 -10 0 10 0
20 40 60 80
count (-) count (-)count (-)
count (-)
elevation (m) elevation (m)
main channel
secondary channel chutes
marshes
b, initial topography c, no collapses
d, initial collapses e, yearly collapses
a
Fig. 8) Various network scales (a) and its depth distribution initially (b) and after 40 yrs morphodynamic modelling (c-e).
• Various network scales are determined: main, secondary, chute and marsh.
• Initial there is a deep main channel due to dredging activities.
• Morphodynamic modelling decreases the channel depth main channel.
• Model with initial collapses develops towards model without disturbances.
• Yearly collapses reduces the depth even more: main equals secondary channel.
Location map of the initial 10 collapses of 1 Mm
30 100 200 morphological time (days)
0 100 200
morphological time (days)
0 100 200
morphological time (days)
30 32 34 36 38 40 42 44 46 48 50
30 32 34 36 38 40 42 44 46 48 50
30 32 34 36 38 40 42 44 46 48 50
tracer sediment (L) 100
104
100 104
mean bed elevation difference (m) -0.1
0.1
distance from Western Scheldt mouth (km)
100,000 m3 H (see Fig. 5) I J
1,000,000 m3
tracer sediment (L) time (yrs)
distance from Western Scheldt mouth (km)
5
0 10 15 20 25 30 35 40 45 50
-1.5 1.5
mean bed level diffence (m)
0 40
20 30
10
width-averaged on curvilinear grid
shoal margin collapse
size
e = excite d = dampen m = migrate
e+m e+m
m m
d
