0307 011 9871

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An Integrated Technical Research of Beach Nourishment Design

for the Flemish East Coast

Marc Huygens' & Marc Willems'

•Hydraulics Laboratory, TW15V, Faculty of Applied Sciences, Ghent University,Sint-Pietersnieuwstraat 41, B-9000 Gent, Belgium Marc.Huygens@rug.ac.be

• Flanders Hydraulics, Administration Waterways and Coast UN Flemish Community, Berchemlei115, B-2140 Borgerhout, Belgium, MarcL.Willems@lin.vlaanderen.be

)

I. INTRODUCTION

Due to the high soda-economical, cultural, ecological and

recreational value. the persistent regression of the coastline

along the Flemish East Coast forms an acute threat forcing

the coastal management to a comineous maintenance of

the local beach area. Effective intervention is needed to

ensure a sufficient safety level of both shore and

hinterland protection. Under incident wave attack, the

natural shore profile transports its beach sand offshore.

from where the longshare flow gradient removes tbe

foreshore breaker berm. inducing a gradually developed

structural erosion problem of the Knokke-Zoute beaches.

"Appelzak" even more landwards, just in front of the

groynes before the coast of Knokke; while the refracted

and diffracted waves nearly perpendicularly approach the

beach. As a result, a more intensified beach degradation

appears in the area together with a dramatic silting up of

the entrance gully to the highly valuable "Zwin" nature

reserve, downstream the coastal zone.

The seaward extension of the harbour of Zeebrugge

Figure I. Siwarion plan Flemish East Coast

)

(1977-1986) consolidates this

reserve-'Betgian - DUlCh._- border:,

morphological

Figllre 2. Hydromorphological processesDifferential evolution map Flemish East Coast

A rehabilitation of the natural sea· land environment, new

technical potentialities and political accents have made

that since the seventies preference is given to "soft", eco­

friendly measures, i.e. beach nourishment. taking into

account the natural dynamics of the shore profile [1].

Starting from the sand supplement as the initial working

method and within the framework of the coastal policy, an

evaluation is set up to provide a fundamental knowledge

of the complex coastal processes to sustain future

disequilibrium. The resulting easterly longshore tidal flow

re-orients itself and shifts the local tidal trench called

management decisions.

Inaugural International Conference on Port and Maritime R & D and TechnologySingapore, 29 - 31 October 2001

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506

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Figure 3. Slorm erosion at Knokke-Zoule beach

Figure 4. Si/ling up oJthe ZWin~e;irance

A first important beach replenishment (7 000 000 m' sand

over a 8 km reach) was executed in 1977 in the frame of

the Zeebrugge-extension and not as a result of it. Due to

the deteriorated structural erosion in the coastal area of

Knokke-ZoUle, a second nourishment ( 730 000 m) sand

overa reach of2.9 km) was already necessary in 1986. [SJ

Once more, intense local erosion at the beach of Knokke­

Zoute transponed the total sand volume out of the region

in a period of on Iy 5 years.

Therefore, in order to identify the drastic instability and

the morphological impact on the area, an extended

research program is set up to explore a basic

understanding of the local beach morphology. As a

starting point for the research, a detailed analysis of all

existing information is compiled. The high imponance

(touristical - ecological - economical - social) of the

beach region under consideration explains the wealth of

available data [6J. A profound analysis gives a long-term

development picture of the coastal area. Structural erosion

is clearly noticed along the projecting beach area in

Knokke-20ute (2]. An exponential variation in the annual

retreat volumes is noticed: initially (1986~ 1987) an erosion

rate of 100 m'/m/year was noticed, while a mean value of

__ Pot'''' ..·.. _.(.l.JcZ < ".f)

__ ' __ I"·HZ<".J)

"',~, "~I"""""""''::''

" .........~~.- ~.--:-,-,:-:,:-:,--:.--:-,-,:-:.:-:.-'..:-:,,_,~,~,~,~..:-:,,"-':,,==,,=,=.=,=.~.:-:",--l"

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40 m3/m/year over the last 13 years (1986-1999) is

calculated.

Figure 5. Sand balanceJar the Flemish East Coasl

(1986-1998 data Eurosense·Belforop ~

The important sand volume eroded from the beach zone,

both in vertical (deepening) and horizontal (onshore

retreat) directions, does not settle down at the unaltered

foreshore and tidal gully; but just disappears out of the

cross·shore unit. Even the foreshore starts eroding after 5

years, due to the sand deficit at the beach. It is clearly

identified that, due to the complex: interaction of wave­

induced on- and offshore transport, longshore tidal drift

and the impact of the breakwater obstruction by the

harbour extension of Zeebrugge, a traditional beach

nourishment will not provide a complete and durable

solution for the coastal defence ofthe Flemish East coast.

._,•.-..­._,..,._-·G....~-_~~~....... ',..• o 1M ~ * _ * ~ ~ _

Crll5l-shon1 d11tance trom M..ll x IrnJ

Inaugural IllIcmalional Conference 011 Pan and Marilime R & D and TechnologySingapore, 29 - 31 October 2001

507

(synthetic sequence of tidal combinations) and possible

coastal defence measures (e.g. proposed sand supplement

schemes).

In order to generate appropriate model conditions the

determining local hydraulic and morphological

characteristics near Knokke-Zoute are collected and

analyzed. During a field measuring campaign in the tidal

gully "Appelzak" detailed flow measurements reveal the

temporal and spatial distribution of the longshore flow

patterns. Together with the available wave data and tidal

characteristics in the area, driving hydrodynamic forces

for the transport mechanisms are gathered up into

representative synthetic tidal sequences. Detailed sediment

investigations identify the mobility ofthe local sea bottom.

Figure 6. Field records of the cross-shore profile

(/986-/998 data Eurosense-Belfolop @)

2. RESEARCH PROJECT

To ensure a proper coastal defence design, a thorough

knowlegde of the local beach morphology is needed. As

an initial link in the integrated "coastal zone

management"-chain, an integrated hydromorphological

study is explored. By using physical model tests together

with computer simulations and in-situ measurements, a

complete synergy between all components leads to a fully

integrated description. An interactive collaboration with

all partners (administration, management, consultancy and

laboratory partners) develops an integral technical

approach. Within this framework, an absolute safety-level

definition strictly induces both hydrodynamic impacts

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r-~~;:=::.?,_O::_--------------------'::'::'~"::-::..j't

-Reference Supp/etlon 1986

-&- PronJe Supplet10n 2 (dSO '" 300 I'm)

__ PronJe SuppleUon 3 Slope 1fJO

->- Perched Beach with gravel toe (proff/e 4)

LWS_

,g JN

">• I~

E0

" ·10

"..u .J0~

·5

.,0 50 100 1~0 200 2~0 300 350 400 450 500

Cross-shore Distance from seawall X (m)

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Figure 7. Beach supplement profiles

The initial physical scale model tests, performed in a 1D­

wave flume (linear scale 1I25) at the Flanders Hydraulics

Laboratory Borgerhout, generate an overall qualification

ofthe beach nourishment stability as a cross shore unit. In

the geometrically undistoned physical scale model,

hydrodynamics are scaled down following the Froude

approach. Since breaking wave turbulence is identified as

the main sediment transport driving force, the sand of the

moveable bed is modelled by the so-called Dean Number

HlwT (fall speed parameter criteria). These model laws

preserve similarity in wave form, sediment fall path,

wave· induced velocities, break point, wave breaker type

and wave

decay in the surf zone [Hughes).

Initially, the morphological evolution of five sand

supplement schemes under identical hydrodynamic impact

(perpendicular wave attack and tidal water elevations) is

comparatively validated. As an intermediate result, a

specific beach supplement profile (form 2 with horizontal

terraces) is indicated as the most stable cross-shore form,

Inaugural International Conference on Port and Maritime R & D and TechnologySingapore, 29 . 31 October 2001

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1!oO JOO 150 300 350 ~ 450

Figure 8. ID-physical scale model Resulting VolumeBalance Development

Therealistic sediment transport development.

Comparisons with detailed field measurements of the

local bathymetry for the reference 1986-supplement show

the complex wave-Iongshore tidal current interaction not

to be represented in the ID-physical scale modelling.

Therefore, an extended physical model (linear scale 1/60)

is explored in a computer-controlled 20 wave tank

installation [4]. The local complex hydrodynamics, as a

combination of perpendicular random waves, longshore

ebb-flood currents and vertical tidal variation, generate a

Figure 9. Numerical SBEACH-simulations vsRepresenralivejield records

These calibrated computer results show a fairly good

agreement with the field records. Finally, based on the

reference runs for the I 986-supplement, a numerical

comparison between all proposed supplement cross-shore

forms suggests once more a "stable" beach nourishment

scheme. Again, the supplement profile 2 with the

horizontal platforms and the perched beach with gravel

foot protection (profile 4) are validated as valuable coastal

defence systems for the study area. In this stage of the

research program, the traditional sand supplement with the

adapted cross-shore profile 2 is pointed out as the most

stable and economical solution.

simultaneously combination of all three hydrodynamic

components in a physical scale model is quite unique. The

infrastructure at the Flanders Hydraulics Laboratory

(Borgerhout) provides not only a fully computer­

controlled input (spatially and temporally variation) of

incident waves, longshore currents and tidal water

elevations; but also a stand-alone laserscansystem to

record the resulting bathymetries in the model. Breaking

(storm) waves transport the beach material offshore to the

seaward limit of the foreshore, into the tidal gully

"Appelzak" from where it is carried away by the

predominantly northeasterly-flowing tidal current. A good

agreement between physical test results and the in situ data

----.:...-

- ~···SOWI ...... 'U__c_,-.SJ._ .••_"... _FWt_· ••_ ..

_e-,.-n....... ll.-"~ L!=-~-~~.....~;·~'~,·~-~'~..~=L --=::::,..J.. '"

together with a perched beach (supplement form 4). A

"stable" beach supplement profile is, at this stage of the

research, described as a cross-shore unit that keeps the

sand as high as possible and close to the landward sea wall

in the profile. The basic idea behind this approach is to

prevent the cross-shore wave-induced transport to remove

the supplement sand offshore from the beach to the

foreshore; because there the locallongshore tidal flow will

interactively rupture the formation of a breaker berm.

Simultaneous numerical simuIations with SBEACH [7]

and LITPACK [5J reveal some interesting agreements and

operational (sensitive) features ofboth software tools. The

dynamic LITPACK-simulation shows a more explicit

morphological response; while the SBEACH-software

offers a more "equilibrium-oriented" smooth profile

development. The numerical simulation with SBEACH is

artificially adapted to the contineous interaction of the

normal wave-induced cross-shore transport with the

longshore ebblflood current: at intermediate stages the

foreshore breaker berm is artificially removed from the

resulting cross-shore profile before the next time step in

the calculation is performed.

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Inaugural Intcrnalional Conference on Port and Maritime R & D and TechnologySingapore. 29 - 31 October 2001

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for the 1986-reference confirms these morphological

processes in the area as the main cause of the local

structural erosion problem. The excessive impact of the

synthetic hydrodynamic cycle (with two consecutive 100

year-storms in a sequence of 27 tides) induces an

overestimation of the sediment transport development in

the model. The resulting bathymetry corresponds (based

on the eroded sand volumes) with a field picture after 2-4

years. Again. based on these reference-results a qualitative

comparison between the proposed profile supplement

schemes is worked out.

r

i-·~J-'A---

I -I j-,-.-.-.-.~-.-_---.M-----j "..._ ............ 111_"1

'''''CI'loHItM"",,4-'gr_I.. ,

U

L_.ld::::=:-::::=:::::·==::::::--:::::.===7I .....-

Consec..u.. h,.,.-,,.....k s..n ......

Figure 10. Cumulative Sand Balances in 2D-scale model.

It is c1e:lrly identified that the preliminary "stable"

profile supplement 2 (with the horizontal terraces) doesn't

remain stable under the combined wave-current impact.

The longshore flow partly removes the underwater plateau

(Z = -1.00 m) where the breaker berm was kept in the ID­

model. As a result, sand is no longer hold high on the

beach profile. Due to the more concave initial cross-shore

profile (horizontal terraces) the sand is even rapidly

removed offshore under wave impact, leading to an

identical long-tenn equilibrium. As a consequence, the

structural erosion in the coastal area of the Flemish East

Coast will not be simply resolved by a traditional beach

nourishment scheme with adapted cross-shore profile.

An alternative solution, a perched beach protected by

a seaward gravel foot, is identified as a more suitable

coastal protection system for the area. The gravel toe (dso

= 14 mm) not only supports the beachward sand massive:

but also catches the offshore transported sand in an

effective coarse gravel frame to build a protective breaker

berm on the seaward end ofthe gravel massive on one side

and acts on the other side as a dike protection against

longshore tidal flow for the landward beach supplement.

Since a concentrated erosion in' the tidal gully is noticed,

special attention should be paid to the stability of the

gravel foot under the combined impact of (breaking)

waves and longshore current. By that, this perched beach

with gravel foot protection can be seen as an intermediate

stage (both from technical as from ecological point of

view) to the fully (underwater) breakwater-option. The

resulting, cumulative volume variation (over a cross-shore

distance of 600 m from the sea wall) clearly shows a less

erosive sand balance for this allemative: the gravel toe

protection of the more concave supplement sand massive

works well under the combined wave-tidal flow impact.

While the resulting sand volumes for both "traditional"

sand supplements (the 1986-reference and the new design

with horizontal terraces) have a quite similar magnitude.

the beach nourishment scheme with a gravel toe at the

foreshore reduces the erosive sand volume with 40% ! The

general bathymetric differential map for the supplement

profile with gravel massive in figure 12 confirms the

above mentioned morphological trends: very little sand

removal on the beach area. an explicit sedimentation

above the gravel bed and a concentrated erosion in the

tidal gully.

509

IlIjugur:J..l IJlI~malional Conference on Port and Maritime R & D and TechnologySingapore. 29 - 31 OclObcr 2001

510

Figure I J. Profile development underfully 2D­

hydrodynamic impact

Longshore distance Y Cm) ,.....,....,

350

300

250

;0--~. r.",~

"

Perched BeachGravel foot

6 x Spring - InitialR07 -ROO

Accretion

.>1.20m

.O.60m_t.20mr::::J 0.00 m _ 0.60 m

Erosion

D -0.60 m _ 0.00 m

-1.20 m Q -0.60 m

• -1.80 m _ -1.20 m

• -2.40 m ((» -1.80 m

D <·2.40 m

IBottom Variation !1 Z (m) Ie .1" ._, . t ... 1.11 ... ~ .....

-:?Se-2e3·150·"ee ·52 e 50 1ee 150 2ee 250Figure J2. Differential bottom map for the perched beach with gravel joot.

technical features to the stakeholders. This integrated

3. CONCLUSIONS

As a result. by deploying a complete synergy offield data,

computer results and physical scale modelling. a better

inSight into the morphological and hydrodynamic

behaviour of ani ficial beach nourishment for the Flemish

coaSl forms a major step to a more reliable and

scientifically based design oflocal coastal defence systems

for sandy beaches. An optimum beach protection for the

studied coastal area should surpass the traditional sand

supplement if one is looking for a long·term stable beach

policy. The feeding of the beach with external sand

provides a temporal remedy; but doesn't solve the

structural erosion development in the area. A perched

beach with gravel foot protection at the foreshore can be

an alternative solution to the actual, regular maintenance

supplement in this case.

But what is even more important is the reflection of these

Inaugural International Conference on POri and Maritime R & D and TechnologySingapore, 29 - 31 October 2001

research should help both to create a social platform and to

carry conviction for the future sustainable coastal

management in the area.

4. REFERENCES

[1] CUR RijkswaterSlaat and Delft Hydraulics (1987),

Report 130 Manual on Artificial Beach Nourishment,

CUR Gouda ISBN 90 212 6078 6,

[2) De Wolf P., Fransaer D. and Houthuys R. (1999),

Morphological trends of the Belgian Coast shown by 20

years of remote·sensing based surveying, Proceedings

Coastal Zone '99.

[3] Fredsoe 1. and Deigaard R. (1992), Mechanics of

coastal sediment rransport, Advanced Series on Ocean

Engineering Volume 3, World Scientific Publishing Co.

Singapore.

[4] Hughes S.A. (1993), Physical Models and Laboratory

Techniques in Coastal Engineering, Advanced Series on

Ocean Engineering Volume 7, World Scientific Publishing

Co. Singapore.

[5] Hedegaard LB., Roelvink lA., Southgate H" Pechon

P., Nicholson 1. and Hamm L. (1992), Intcrcomparison of

coastal profile models, Proceedings IeeE 23, ASCE.

[6J Kerckaert P., Macs E., Fransaer D. and Van

Rensbergen J. (1990), Monitoring shore and dune

morphology using bathymetric soundings and remote

sensing techniques, Proceedings SII 27th; lot. PIANC

Congress, Osaka.

[7] Larsan M. and Krau, N.C. (1989), SBEACH:

numerical tool for simulating storm-induced beach

change, Technical Report CERe 89-9, US Army

) Engineering Waterways Experiment Station, Coastal

Engineering Research Center, Vicksburg MS.

[8J Roovers P.P.L., Kerckaert P., Burgers A., Noordam A.

and De Candt P. (1981), Beach Protection as part of the

harbour extension at Zeebrugge, Proceedings 25th Int.

PIANC Congress, Edingburgh.

[9] Warnier F., Raes E., Malherbe B. And Lahousse B.

(1994), Execution aspects and monitoring ofa new beach

nourishment concept at De Haa: combined feed benn and

profile nourishment, Proceedings ICeE 24, ASCE.

)

Inaugural IIl\~malional Conference on Port and Maritime R & D and TechnologySingapore. 29 - 31 Oclo~r 2001

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