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0307 011 9871
111111111111111111111111111111111111111111111 505
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 [email protected]
• Flanders Hydraulics, Administration Waterways and Coast UN Flemish Community, Berchemlei115, B-2140 Borgerhout, Belgium, [email protected]
)
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
- '--~~ -...,.- -'- ....... .... ~ ~,'" ~ --~....".-,~. -... -..,.."---~
506
)
)
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"
n_~__~ol~l_ri__
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
.=-------------------------,
)
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)
)
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
• .r"",,;,,;r_ __ ~ ~ ..- ...~~~~ .... ""....... -•.".....- ~":" ~ ,-- ~ .."...---~~~f~
<
508
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.
• ...----------ji!: - - - - - -: I:•- .
L •i .~ ,
L:] ~
)
)
Inaugural Intcrnalional Conference on Port and Maritime R & D and TechnologySingapore. 29 - 31 October 2001
)
)
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
51l