Post on 23-Mar-2021
Abdel Nnafie
Tomas Van Oyen
Bart De Maerschalck
Dunes and Estuaries 2015
conference
September 16-18, 2015
Bruges, Belgium
On the use of idealized morphodynamic models to address complex coastal management problems
Existing modeling approaches
Morphodynamic models can be classified into 3 groups (Hommes et al. 2007, Briere et al. 2010):
Conceptual models: focus on general behavior without use of fundamental equations. Associated model concepts derived from observation;
Complex process-based models: resolve all physical processes in a realistic setting. Reproduce a natural system as completely and as accurately as possible;
Idealized process-based models: resolve only essential physical processes in a idealized setting. Focus on fundamental understanding.
Coastal problems
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Coastal problems difficult to address:
involve broad range of time
and space scales
(seconds to hundred of years).
Source: Stive et al., 1995
Coastal problems
Generally, it is believed that complex models would perform best
on long time scales as they most closely resemble the natural
system.
However, in case of complex models:
Quantitative match with observations does not
guarantee that model will do well at longer time scales.
Problem of ‘getting right results for the wrong reasons’.
Computationally very expensive.
Several studies demonstrated that Idealized+conceptual
models are powerful tools to address coastal problems on long-
time scales.
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Idealized+conceptual models
Idealized models:
Used to gain insight into essential physical mechanism.
Low computationally costs, can be applied on long time
scales.
Drawback: quantitative results should be treated with
caution.
Conceptual models:
Computationally very fast.
Drawback: model based on empirical relations rather
than fundamental physical equations.
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Objective of this workshop
Show three case studies where idealized and
conceptual models were used to address complex
coastal problems:
Case study 1: Response of continental shelf
sand ridges to sand extraction.
Case study 2: Effects of sea level rise on
continental shelf sand ridges.
Case study 3: Effects of tidal asymmetry on
the sand exchange between Scheldt estuary
and the mouth area.
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Continental sand ridges
Atlantic Ocean
New York Long Island
Fire Island
(m)
4004 ¢7 N
4004 ¢2 N
4003 ¢6 N
4003 ¢0 N
4002 ¢3 N
7304 ¢7 W 7303 ¢5 W 7302 ¢4 W 7301 ¢2 W 7300 ¢0 WBathymetric map (NOAA, 2013)
Examples of continental
shelf ridges
Case study 1:
Sand ridges important for coastal
stability
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Sand ridges contribute to stability of the coast:
they absorb energy of incoming waves during storms
they act as a natural sand source for nearshore zone
(Schwab et al. 2000, 2013)
Case study 1:
Increasing offshore sand extraction
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Extracted volumes of offshore sand extractions (also from sand ridges) is
increasing.
Atlantic
Ocean
New York Long Island
Fire Island
(m)
4004 ¢7 N
4004 ¢2 N
4003 ¢6 N
4003 ¢0 N
4002 ¢3 N
7304 ¢7 W 7303 ¢5 W 7302 ¢4 W 7301 ¢2 W 7300 ¢0 W
Bathymetric map (NOAA, 2013)
Case study 1:
Aims of the idealized model
• General aim:
Increase fundamental understanding of effects
of sand extractions on dynamics of sand
ridges.
• Specific coastal questions:
Will the ridge recover after extraction? If yes,
how long does it take?
What are the implications for beach stability?
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Case study 1:
Idealized model
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Idealized process-based model:
bottom evolution (~ 101-102 years)
Currents (~ days)
Waves ( ~ seconds)
Sediment transport (~ days)
Case study 1:
Idealized geometry
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nearshore zone not modeled
bo om
innershelf(~6km)
~ 20 m
storm-driven flow v
q
waveray
x = L
z y
x
outershelf
beach
nearshorezone(~2km)
hzb
x
sea surface z= 0
x = Lx(x, y, z) = 0
~ 14 m
Case study 1:
Methodology
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A. Spin up the model until shoreface-connected sand ridges
obtained.
C. Beach stability:
Analyze sediment flux between the nearshore zone (beach)
and the continental shelf.
Case study 1:
A: Spin up of the model
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x (
km
)
y (km)
heig
ht
(m)
t/Tm
t/Tm
Bed level (m)
Spin up time t/Tm~ 1 (Tm~4500 years)
Case study 1:
B: Extract sand ~1 million m3
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close
center
far
Case study 1:
Response of ridge to extraction
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y (km)
y (km)
x (
km
) B
ed-le
ve
l z
b (
m)
Yes, ridge recovers on time scales of decades to centuries.
However: its does not entirely resemble its natural pattern.
Sand source needed for recovery: local redistribution of sand.
Case study 1:
Extracting close and far
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Extracting far from nearshore zone: strong restore of
ridge.
Sand sources: local redistribution of sand + import
from outer shelf.
Extraction close to nearshore zone: weak restore of
ridge.
Sand sources: local redistribution of sand + local
import from nearshore zone.
Erosional hotspots at the beach might occur.
Case study 1:
Geometry of the sand pit
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Wide-shallow Elongated1 Elongated2
Wide-shallow sand pit: strong recovery of ridge.
Elongated sand pits: weak recovery of the ridge
Case study 1:
Summary case study 1
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After sand extraction, ridge recovers on time
scales of decades to centuries.
In general: sand needed for refill of pit comes
from local redistribution of sand.
Strong (weak) recovery of sand when extracting
far (close) from (to) nearshore zone.
In case of extracting close: sand import from
nearshore zoneerosional hotspots Erosional hotspots
My beautiful house, WHYYYYYYYYYYY…..!
Case study 1:
For more information…
Please refer to Nnafie, A. and de Swart, H. E. and Calvete, D. and Garnier, R.
Modeling the response of shoreface-connected sand ridges to sand
extraction on an inner shelf. Ocean Dynamics, 1–18, 2014.
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Case study 1:
Atlantic Ocean
New York Long Island
Fire Island
(m)
4004 ¢7 N
4004 ¢2 N
4003 ¢6 N
4003 ¢0 N
4002 ¢3 N
7304 ¢7 W 7303 ¢5 W 7302 ¢4 W 7301 ¢2 W 7300 ¢0 WBathymetric map (NOAA, 2013)
Case study 2: Effects of sea level
rise (SLR) on shelf sand ridges
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Shoreface-connected
sand ridges
Shoreface-detached
sand ridges
Past Sea Level Rise (SLR) has had a profound impact on formation and
long-term evolution of sand ridges.
Case study 2:
Aims of the idealized model
• General aim:
Increase fundamental understanding of effects
of SLR on dynamics of sand ridges.
• Specific coastal problems:
What are effects of future SLR on formation
and long-term evolution of ridges?
Will the future SLR drown the ridges, or would
these ridges be able to keep up with SLR?
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Case study 2:
Combining idealized and
conceptual models
Besides resolving short-time scale processes (~ second-days),
long-term modeling of these ridges must also resolve long-time
scale processes (~ decades-centuriesSLR, shoreface retreat,
subsidence, changes in shelf geometry…)
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Idealized process-based model:
bottom evolution (~ 101-102 years)
Currents (~ days)
Waves ( ~ seconds)
Sediment transport (~ days)
SLR, retreat… ( ~ 101-102 years)
Conceptual model for
relation shelf evolution and
SLR+retreat…,
Case study 2:
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Sea Surface
Side View continental shelf
Sea bottom
Conceptual model shelf evolution
old coast
Case study 2:
Results from case study 2
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waves
Storm-drivencurrent
Shoreface
retreat
New ridges form and grow in time. Old ridge continue
to grow, but eventually, they will drown.
Orientation difference is observed between new and
old ridges in agreement with observations.
Case study 2:
Future SLR and shelf ridges
• Based on models results, it can be stated that in case
of future SLR (together with coastal retreat):
New ridges will be created in the new area.
Old ridges, that were already form, will
continue to grow, but eventually, they will
drown
• In case of only SLR (without coastal retreat), results
(not shown) demonstrate that
Ridges can keep up with SLR if the rate of
the latter is not too high, otherwise, they will
stop growing and will drown.
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Case study 2:
For more information…
Please refer to Nnafie, A., H. E. de Swart, R. Garnier, and D. Calvete, Effects of sea
level rise on the formation and drowning of shoreface-connected sand
ridges, a model study, Continental Shelf Research, 80, 32–48,
doi:10.1016/j.csr.2014.02.017, 2014.
A. Nnafie, H.E. de Swart, D. Calvete, R. Garnier, Dynamics of
shoreface-connected and inactive sand ridges on a shelf, Part 2: The
role of sea level rise and associated changes in shelf geometry,
Continental Shelf Research, Volume 104, Issue null, Pages 63-75.
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Case study 2:
Case study 3:
Scheldt mouth area
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Morphology: complex pattern with shoals and channelslooks like
an ebb-tidal delta. However, tidal bars also present.
Source: Damen et al. 2014
Case study 3:
What makes morphodynamic
modeling of this area so difficult?
Morphological evolution mouth area is affected by:
• Changes in hydrodynamic conditions (tide, waves, wind…), sediment
composition (sand, mud…)…
• Past and present human interventions in this area (construction of port of
Zeebrugge, dredging/dumping…
• Past and present human intervention in the estuary (closing connection
eastern and western Scheldt, closing of tidal basin near Terneuzen,
dredging/dumping) changes in sand exchange between estuary/mouth area 29
Case study 1:
Sand balance in the Scheldt estuary
• Since 1955 alternating periods with sand import and
export occurred.
• Estuary shows sand exporting trend over the last two
decades.
• Possible cause: change in tidal asymmetry due
bathymetric changes induced by human
interventions (dredging, dumping…)..Sea level
rise?
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Case study 3:
Observed changes in M4
characteristics in the Scheldt
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Adopted from Kuijper & Lescinski, Deltares, 2012
Case study 3:
Objectives of the idealized model
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• General aim:
Increase fundamental understanding of the
mechanisms controlling morphodynamic
development of the mouth.
• Specific coastal problems:
Quantify Morphodynamic effects of:
A changing M4 component;
A width narrowing at Vlissingen;
Using different grain sizes;
Closing tidal basin near Terneuzen;
Construction of port of Zeebrugge;
…..
Case study 3:
Scheldt estuary+mouth area
+open sea
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𝑻𝒉𝒆 𝑵𝒆𝒕𝒉𝒆𝒓𝒍𝒂𝒏𝒅𝒔
𝑩𝒆𝒍𝒈𝒊𝒖𝒎
𝑵𝒐𝒓𝒕𝒉 𝑺𝒆𝒂
𝑬𝒔𝒕𝒖𝒂𝒓𝒚
𝑺𝒆𝒂 𝒃𝒐𝒖𝒏𝒅𝒂𝒓𝒊𝒆𝒔
𝑴𝒐𝒖𝒕𝒉 𝒂𝒓𝒆𝒂
𝑵
Case study 3:
Idealized geometry
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𝑾(𝒙) 𝑥
𝑦
𝑴𝒐𝒖𝒕𝒉 𝑵𝒐𝒓𝒕𝒉 𝑺𝒆𝒂 𝑬𝒔𝒕𝒖𝒂𝒓𝒚
(Ghent)
H(𝑥)
𝑧 = 0
𝑥 = 0 (𝑊𝑒𝑠𝑡 − 𝑘𝑎𝑝𝑒𝑙𝑙𝑒)
(b) Side view
𝑥 = 15 km (𝑽𝒍𝒊𝒔𝒔𝒊𝒏𝒈𝒆𝒏)
𝑩𝒐𝒕𝒕𝒐𝒎
(a) Top view
𝑥
• 𝐅𝐨𝐫𝐜𝐢𝐧𝐠: 𝑴𝟐 + 𝑴𝟒
• Open searectangular basin
• Estuaryexponentially converging width
• Bathymetry:
Linear decreasing depth in x-direction
Constant depth in y-direction
Case study 3:
Effects of changes in M4
amplitude/phase
𝑵𝒐 𝒆𝒙𝒕𝒆𝒓𝒏𝒂𝒍
𝑴𝟒
𝑺𝒕𝒓𝒐𝒏𝒈 𝒆𝒙𝒕𝒆𝒓𝒏𝒂𝒍
𝑴𝟒
Case study 3:
Preliminary results from
case study 3
With increasing amplitude M4:
estuary exports more sand, which is
deposited in the mouth area.
A phase difference between M4 and M2
can change an estuary from a sand importing
system to a sand exporting system.
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Case study 3:
Summary/Conclusions
Long-term evolution of coastal systems involves a broad
range of time and space scales
difficult to model
Idealized and conceptual morphodynamic models are
designed to increase fundamental understanding of the
underlying physical mechanisms of coastal problems.
Results from case studies demonstrate that idealized and
conceptual models can be used to (qualitatively) address
complex coastal problems.
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Starting the debate….
1. Complex models cannot be used to address
coastal problems on long time scales (decades to
centuries).
2. Idealized and conceptual models are powerful
tools to (qualitatively) address coastal problems.
3. Long-term coastal management problems should
be addressed using different kinds of models
(parallel modeling approach).
38 Completely disagree rather disagree neutral rather agree completely agree
Thank you
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Hommes et al. (2007): “by using different types of
models together (parallel or complementary
modeling approach), it is possible to answer coastal
management questions effectively and with a higher
degree of predictive power”
Hydrodynamic calibration
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Case study 3: