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

14

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: