How to harm on ize a ccessib ility , s a fe ty n e ss and n a tu ra ln ess?
January 2001
Improving navigation conditions
in the Westerschelde and
managing its estuarine environment
20753
How to harmonize
accessibility safetyness and naturalness?
,
J. J. Peters
R. H. Meade
W. R. Parker
M. A. Stevens
January 2001
Foreword
In 1999, th e Dutch an d Flem ish g o v e rn m e n ts d e cid e d to co n d u ct a c o m p reh en siv e,
multidisciplinary research project th a t w ould result in a long-term vision (LTV) ab o u t th e
further evolution of th e W e ste rsc h e ld e . T h e m ain is s u e s c o n sid e re d in this project a re
th e im provem ent of th e navigation c h a n n e ls (accessibility), th e risk for flooding
(s a fe ty n e ss) a n d th e p reserv atio n of th e e stu a rin e ecology (n atu raln ess). A “cluster
morphology” w as s e t up for investigating th e morphological a sp e c ts, com m on to all th re e
m ain LTV issu e s.
By th e e n d of 1999, th e Port of A ntw erp called on a te a m of international e x p e rts to
provide advice on th e feasibility of a further d e ep e n in g of th e navigation ro u te in th e
W e s te rs c h e ld e b e tw ee n th e s e a an d A ntwerp. At th e re q u e st of th e LTV clu ster
morphology, regular con tacts w ere estab lish ed and information w as e x c h a n g e d b etw een
th e team s.
T h e expert team appointed by th e Port of Antwerp issu ed a first report in July 2000. This
“b a se lin e report” p la c e s th e p re s e n t W e s te rs c h e ld e in th e co n tex t of its historical
evolution and gives the b a sis for a discussion about th e future c h an g e s, both natural an d
anthropogenic.
T h e final report p re s e n te d h e re co n tain s s o m e original view s an d id e a s ab o u t th e w ay
to m a n ag e th e W este rsc h e ld e . O n e of th e s e id e as is an alternative dredging an d
dum ping strateg y , com bined with p o ssib le m odifications of th e river training w orks.
T h e s e a s p e c ts a re d e v elo p e d in m ore detail in a technical a d d en d u m .
T h e expert team w ants to thank th e Port of Antwerp and th e m em b ers of th e LTV c lu ster
m orphology for th e constructive an d collaborative spirit in which th ey could work. W e
hope this work h a s contributed to th e further m a n a g e m e n t of this s o v alu ab le e stu a rin e
environm ent.
J e a n J a c q u e s P e te rs
T e a m L eader
CONTENTS
EXECUTIVE SUMMARY
1
INTRODUCTION ...............................................................................................................
1.1
Initial T erm s of R eferen c e a n d O b jectiv es ............................................. .
1.2
Linking to LTV ........................................................................................................
1.3
Final R eport O bjectives .....................................................................................
2
SUMMARY O F KEY PO IN TS O F TH E BASELINE R E P O R T ............................... 3
2.1
N atural M orphological E v o lu tio n .......................................................................... 3
2.2
A nthropogenic M orphological C h a n g e s ..............................................................5
2.3
C h a n g e s in Tidal R e g i m e ....................................................................................... 6
2.4
Salinity an d Mixing P r o c e s s e s .................................................................................7
2.5
M orphodynam ics of th e W e s te rs c h e ld e ........................................................... 7
2.5.1 M ovem ent of C h a n n e ls ........................................................................... 8
2.5.2 G eological Control of E stu arin e M orphodynam ics ....................... 10
2 .6 R elationship with th e V lakte v an d e R a a n .......................................................10
2.7 Dynamic E q uilibrium ............................................................................................ 11
2.8 S u m m a r y ...............................................
14
3
DREDGING AND ITS INFLUENCE ON TH E ESTUARIAL DYNAMICS . . . . 15
3.1
G en eral C om m ents ...................................................................
15
3.2 History of dredging in th e W e s t e r s c h e l d e ....................................................... 16
3.3 Tidal p a ra m e te rs sin c e 1938 ........................................................................... 17
3.4
S u m m a r y .............................................................................................................. 19
4
FUTURE D E V E L O P M E N T S .............................................................................................20
4.1
O bjectives .............................................................................................................. 20
4.2 M an ag em en t M ethods an d T o o l s ...................................................................... 20
4.4 Role of D epoldering ................................................................................................22
4.5 A lternative D redging S tra te g ie s .........................................................................23
4.6 R e v e rs ib ility ............................................................................................................ 24
4.7 Possibility for Further D eep en in g ...................................................................... 24
5
R EC O M M EN D A TIO N S....................................................................................................... 26
5.1 D ata R e v ie w ........................................................................
26
5.2 D ata C o lle c tio n ..........................................................................................................27
5.3 Monitoring .............................................................................................................. 27
5.5 Trial of A lternative S t r a t e g i e s .............................................................................. 29
5.6 Institutional A s p e c t s ................................................................................................ 29
6
CONCLUSION ................................................................................................................... 30
R E FE R E N C E S
1
1
1
1
31
EXECUTIVE SUMMARY
T h e natural evolution of th e morphology of th e W este rsch e ld e h a s continued for sev e ra l
th o u san d years, following a pattern of grow ing an d c o ale scin g s h o a ls an d b a n k s linked
with a simplifying an d d e ep e n in g ch an n el netw ork. This h a s led naturally to in c re a se d
tidal penetration into th e estuary. This natural d ev elo p m en t path h a s b e e n perturbed,
possibly a c c e le ra te d or e n h a n c e d , by poldering an d o th e r reclam ation a n d stabilising
activities which sta rte d in pre m ediaeval tim es.
T h e se factors have resulted in the p re se n t situation in w hich th e continuing evolution of
tidal m orphology an d dynam ics le a d s to higher tidal levels p en etratin g further up the
e stu a ry with a tte n d a n t risks of flooding an d c h a n g e s in m orphology, an d leading to
reduced morphological diversity and c o n se q u e n t reductions in ecological diversity. Tidal
energy is th e principal driving force for morphological ch an g e. Examination of tim e-series
of tidal p a ra m e te rs an d dredging q u an tities d o e s not rev eal an y c lea r an d unequivocal
effects of th e major dredging and deepening cam paigns on tidal propagation p aram eters.
Any effects a re indistinguishable from c h a n g e s in tidal p a ra m e te rs which h a v e tak en
place befo re sta rte d a significant dredging in th e W e ste rsc h e ld e , d o w n stream of Bath.
E stuarial m orphological m a n a g e m e n t stra te g ie s n e e d to h a v e a s their ta rg e t th e
satisfaction of th e req u irem en ts for safety, navigation an d ecology through th e
co n serv atio n or th e redev elo p m en t of m orphological diversity. M an ag em en t stra te g ie s
sh o u ld re st upon a thorough u n d erstan d in g of th e natural physical p ro c e s s e s in th e
estuary b a se d principally on field o b serv atio n an d e x p erien c e su p p o rte d by a n a ly s e s of
historical a n d geological d a ta an d th e u s e of ap p ro p riate physical an d num erical
modelling tech n iq u es.
T h e a n a ly sis of existing d a ta sh o w s th at a “do nothing” or “le av e everything a s it is”
m a n ag e m e n t stra teg y is neither a practical nor a su sta in a b le option in th e co n tex t of
either safety, navigation, or environm ental quality.
T h e p ro p o se d a p p ro a ch of m orphology in th e L.T.V. project, which is b a se d 'la rg e ly on
m odel stu d ie s an d sed im en t b u d g e ts which h a v e g re a t u n certainties. C om parison of
results of th e LTV stu d y m ethod with specific d o c u m en ted c a s e s of dredging, d isp o sal
a n d m orphological dev elo p m en t d o e s not su p p o rt th e validity of th e LTV a p p ro a ch to
defining perm issible in tra-estu arin e d isp o sal quantities. T h e LTV proposal, which
involves th e rem oval of sed im en t from th e estu ary , is re g a rd e d by th e ex p ert te a m a s
having g re a t uncertainty a n d d o e s not provide a s e c u re b a sis for future m a n ag e m e n t
d e c isio n s or m eth o d s. In particular th e rem oval of sed im en t from th e W e ste rsc h e ld e
sy stem is not within th e generally a c c e p te d m eaning of “su sta in a b le ” or “rev ersib le”.
T h e stra te g y a d v o c a te d by th e ex p ert gro u p re c o g n ise s th e n e e d to m e et ta rg e ts of
safety, transport and ecology using an holistic approach to estu arin e m an ag em en t b a s e d
on s e c u re field d a ta an d ap p ro p riate interpretation of field o b serv atio n s a s well a s th e
u s e of appropriate physical an d num erical m odels. T h e p ro p o se d u s e of pilot stu d ie s of
m orphological m a n a g e m e n t satisfies th e crucial o b s e rv a n c e of req u irem en ts for d u e
p ru d e n c e in achieving sustainability an d reversibility in m a n a g e m e n t stra te g ie s.
Introduction
1
INTRODUCTION
1.1
Initial T erm s of R e fe re n c e an d O b jectiv es
T h e initial T erm s of R eferen ce aim ed at bringing together a team of international experts,
who would review th e existing docum entation on th e W estersch eld e and would form ulate
an independent advice on the technical feasibility of a further d e ep e n in g of th e m aritim e
a c c e s s to th e Port of A ntwerp. T h ree rep o rts would b e issu ed . T h e first o n e , called th e
“baseline report”, issu e d in July 2000, p la c e s th e p re s e n t W e s te rs c h e ld e in th e context
of the long-term trend of th e e stu a ry ’s behaviour. T h e s e c o n d report w ould a n a ly se th e
dredging w orks an d th e relationship with th e m orphological evolution. T h e third report
would focus on the feasibility of a further d e e p e n in g of th e m aritim e a c c e s s to th e Port
of Antwerp (PA) and form ulate recom m endations about th e future dredging strateg y . For
re a so n s explained hereafter, it w as decided to m erg e rep o rts 2 an d 3 an d to is su e them
a s a single final report. An A ddendum of o th er technical is s u e s h a s b e e n p rep ared .
1.2
Linking to LTV
Tw o m o n th s after starting its activities, th e ex p ert te a m w a s a s k e d to liaise with a
working group on “morphology”, acting in th e Long-Term -Vision (LTV) project lau n ch ed
by th e Dutch and Flemish G overnm ents in th e binational Technical S ch eld e Com mission.
Although this collaboration h a s slow ed dow n th e work rhythm of th e PA ex p ert team , it
h a s very m uch en rich ed its output. M em bers of th e te a m p articip ated in LTV m eetings
in Belgium an d in T h e N etherlands. Advice w a s given ab o u t th e w ork in th e LTV C luster
M orphology. T h e e x p e rt’s first report - th e b a se lin e report - w a s included in an audit
perform ed on LTV in O c to b er 2000.
1.3
Final R eport O bjectives
T he objectives of th e final report deviate som ehow from th e contractual o n es. This report
m erg es th e p ro p o se d rep o rts 2 an d 3. It m a k es an overview of th e resu lts o b tain ed by
th e expert team , taking into a cco u n t th o s e resu lts of th e th re e c o m p o n e n ts of th e LTV
working g ro u p s re la ted to th e dredging w orks in th e W e s te rs c h e ld e an d their likely
im pact on a s p e c ts su c h a s accessibility, n a tu ra ln e ss an d safety. B asic q u e stio n s
a d d re sse d in this final report w ere indicated already in th e b a se lin e report, which should
also b e c o n sid ered a s an annex.
T h e q u estio n s p re s e n te d in th e b a se lin e report w ere:
What could be the future evolution of the estuary under the
influence of natural factors and human impact, and what are the
possibilities to further improve the navigation conditions without
damaging the estuarine environment?
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 1 o f 32
Introduction
On the longer term, what kind of management is needed to have
the estuary evolving towards an even more healthy morphology
than the one that existed some fifty or sixty years ago?
T h e following m ay ad d ed :
On the shorter term, what are the dredging strategies that couid
serve both purposes: to further improve the navigation conditions
and to steer the morphological changes in a direction favourable for
the naturalness and safeness of the Westerschelde?
What kinds of works, other than dredging, may contribute to the
recovery of the Westerschelde’s morphology?
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 2 o f 32
Summary o f key points / conclusions o f baseline report
2
SUMMARY O F KEY PO IN TS O F TH E BASELINE R E PO R T
2.1
N atural M orphological Evolution
C oastal a re a s and estu aries w e re s h a p e d during th e H olocene period. S cien tists a g re e
a b o u t th e m agnitude of th e e u sta tic s e a level rise during th e transition from th e
P le is to c e n e geoiogicai period - th e last ice a g e , ab o u t 21 kyr B P 1 - to th e H olocene 10 kyr BP - (Fig. 2.1-01). During this period s e a level ro s e from ab o u t -125 m to - 30 m
relative to th e p resen t level, which w as alm ost re a ch e d so m e 3 kyr ago. T he stabilisation
of th e o c e a n levels in re c e n t tim es d o e s not m e an th at th e re a re no fluctuations
anym ore. T h e p resen t rate of m ean s e a level rise am ounts to ab o u t a q u arter of a m etre
per century. It is influenced by clim ate c h a n g e s , a m ajor c o n ce rn to environm entalists.
This continuing long term trend m ust b e re c o g n ise d a n d a cc o u n te d for w hen a s s e s s in g
flood risk. Land su b sid e n c e m ay also co ntribute to th e d a n g e r for cata stro p h ic flooding
of c o asta l regions, like in s o m e a re a s bordering th e North S e a .
Before th e Atlantic O c e a n invaded th e a re a b e tw ee n E urope an d England, th e m ajor
E uropean rivers Rhine, M euse an d S ch eld e d ischarged so m ew h ere in the middle of w hat
is now th e North S e a (Fig. 2.1-02). In a first p h a s e , th e s e a intruded th e river c o u rse s,
creating estuaries. W hen o cean levels stabilized, th e further m orphological evolution of
the North S e a c o a sts d ep en d e d on th e relative m ag n itu d e of th e hydraulic m e ch an ism s
and forces originating from s e a and river. S o m e c o a s ts w ere eroded by tides an d w a v es,
w hile o th e rs w e re rep len ish ed by se d im e n ts eith er originating from th e rivers or from
c o a s ta l ero sio n . W h en th e ch an n el b etw ee n E ngland an d F ra n c e w a s invaded by th e
Atlantic O cean , s h o re erosion yielded larg e a m o u n ts of detrital m aterial. T h o se e ro d ed
from the cliffs along th e French c o ast w ere tran sp o rted by th e currents in a n o rth -eastern
direction, along w hat b e ca m e th e Belgian, Dutch an d part of th e G erm an coastline. T h e
p ath of th e se d im e n t is determ ined by th e to p ography, by th e direction of th e residual
tidal c u rren ts, an d by th e w inds com ing predom inantly from th e so u th w est. All th e s e
factors induce th e n et d isp lacem en t of w ater a n d sed im en t in n o rth -ea stern direction.
S o m e 5 kyr BP, th e sed im en t transit h ad c re a te d a ridge of sed im en t, with high sh o als.
T h e s e form ed th e border of a large lagoon into which Rhine, M eu se an d S c h e ld e
d ischarged their fresh w ate r an d continental se d im e n t load. T h e S c h e ld e river supplied
a rather limited quantity, co m p ared to th e two first o n e s. Van V een (1950) p re s e n te d a
s k e tc h of w hat it m ust h a v e looked like in R o m an tim es, 2 kyr BP (Fig. 2.1-03). S in ce
th e n , th e larger q uantities of s a n d an d gravel tra n sp o rte d by R hine and M eu se rivers
partially filled the northern an d n o rth e a ste rn p art of th e lagoon, while m arine se d im e n ts
w ere m oved into it by the s e a currents, through b re a c h e s created during sto rm s. A round
1 kyr B P, th e lagoon h ad partly silted up in m any a re a s , its so u th ern part mainly with
m arine se d im e n ts. O n a d o cu m en t from RIKZ, an e x ten d e d a re a of F lan d ers an d
Z eelan d is c o m p o se d of sch o rre an d p e a t (Fig. 2.1-04).
’O ne thousand years is 1 kyr; BP is ‘Before Present’
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 3 o f 32
Summary o f key points / conclusions o f baseline report
Archeological evidence indicates hum an se ttle m e n ts from th e s to n e - a n d b ro n ze a g e in
th e a re a mainly d o w n stre am of H answ eert, an d from R om an tim es c lo se to V lissingen.
S om e research ers co n sid er that in R om an iim es, the mainly p e a t a re a of th e “d e lta ” w as
invaded by th e s e a through th e river m ouths b e c a u s e of th e s e a level rise (Eck, v an , B.,
1999). T he expert te am favours the hypothesis - explained a b o v e a n d a s put forw ard by
Van V een - th a t th e lagoon w a s first c re a te d a s a c o n s e q u e n c e of th e e m e rg e n c e of a
s a n d barrier form ed by a lito ra l” m arine sed im en t tran sp o rt. This p ro c e ss b e c a m e
possible w hen th e s e a level rise slow ed dow n, letting c u rre n ts a n d wind w ork to g e th e r
to build up th e islands and dunes. T h e lagoon m ust hav e functioned a s an alm ost c lo se d
s e a , isolated from th e newly c re a te d North S e a . T h e river m o u th s w ere a t th e land
bo rd ers of th e lagoon, to th e E ast, not at th e s e a b o rd er to th e W est.
Until aro u n d a th o u s a n d y e a rs ag o an th ro p o g en ic c h a n g e s in th e W e s te rs c h e ld e ’s
morphology w ere negligible while th e m orphological c h a n g e s resulting from th e eu static
s e a level rise w ere strong. T h e s e m a d e th e so u th ern part of th e lagoon - in th e “Lower
L ands” (T he N etherlands) - evolve to w ard s a n intricate sy ste m of islands, s h o a ls and
ch an n els. A ccording to C o en (1988), tid es re a c h e d A ntw erp for th e first tim e b etw een
1.4 and 1 kyr BP (Fig. 2 .1 -0 5 & 06). T h e tidal w av e could easily d issip a te in th e lagoon
an d did not p e n e tra te very far into th e river re a c h e s. S u b seq u en tly , th e m ore inland
prop ag atio n of th e tide m ust h a v e b e e n th e result of a n in c re a s e d flow co n v ey a n c e
capacity of th e m ain c h a n n e ls in th e lagoon, e n h a n c e d by th e accretio n of th e shallow
a r e a s by siltation, a n d th e scouring of th e c h a n n e ls by th e tidal flow. At th at time,
anthropogenic c h a n g e s w e re insignificant. T h e lagoon in th e so -called “d e lta ” of Rhine,
M euse an d S c h e ld e w a s progressively evolving tow ards an a re a with g ro u p s of sh o als
s e p a ra te d by m ajor c h a n n e ls through which th e tide could p ro p a g a te further inland.
In th e a b s e n c e of any hum an im pact, th e sy ste m of s h o a ls an d active tidal c h an n e ls
would have developed further, with a p ro g ressiv e accretion of higher g ro u n d s to slikke,
schorre and finally islands, while th e seco n d ary channels would co n tin u e to decline. T h e
main channels, of which th e H onte w a s o n e of th e d e e p e s t, w e re in fact c re a te d by th e
scouring by the tidal currents through b re a c h e s in th e littoral s a n d barrier, during heavy
storm s. T he fate of th e H onte s e a b ran ch w ould h a v e b e e n to further silt up, w e re it not
draining th e S c h e ld e river basin. Indeed, th e river sy ste m in this c atc h m e n t form s th e
“lung” th ro u g h which th e e stu ary m ay respire, a s th e tidal flows p ro p a g a te into it. T he
Zwin, another such s e a branch created by th e breaching of th e c o a sta l s a n d barrier, did
not drain a large river c atc h m e n t an d it silted up naturally, th ough th e p ro c e s s w as
a c c e le ra te d by hum an intervention.
T h e m outh a re a of th e H onte - th e p re s e n t W e s te rs c h e ld e - underw ent, an d still
undergoes, significant natural c h an g es. T h e Flem ish B anks a re a se rie s of large m arine
sa n d ridges, deposited by th e w ater an d sedim ent circulation in th e S o u th ern Bay of th e
North S e a . Until ab o u t 200 y BP (1800), th e ridges p e n e tra te d well into th e funnels h a p e d mouth a re a of th e Honte. T h e rapid m orphological evolution of th e m outh a re a ,
o v e r a t le a s t th e last two centu ries, is likely to b e linked to th e c h a n g e s in th e
W esterschelde. T h e recent works along th e Belgian coast, including th e exten sio n of th e
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 4 o f 32
Summary o f key points / conclusions o f baseline report
Z e e b ru g g e harbour, will inevitably h a v e h a d an effect but o n e which is at p re se n t
unquantified.
2 .2
A nthropogenic M orphological C h a n g e s
S e a level re a c h e d is p re s e n t g e n eral level ab o u t 3 kyr BP after which it h a s oscillated
around its present level, though co n tem p o rary know ledge s u g g e s ts an underlying tren d
of a rise at ab o u t on q u a rte r of a m etre p e r century. O n c e th e s e a level h a d stab ilised
th e fine s u s p e n d e d load carried by th e tidal flow d e p o sited m arine clay on th e sh o als
which rose high enough to allow new hum an settlem ents. T h e settlers progressively built
le v e e s to p rotect th e m s e lv e s from th e h ig h e st tide levels. In so m e c a s e s , le v e e s w ere
e re c te d by consolidating th e natural le v e e s th a t a re form ed during inundation by th e
deposition of s a n d ridges along th e m o st activ e tidal ch an n els.
T h e flow of th e S c h e ld e river d isc h a rg e d into w hat is now th e O o ste rsc h e ld e (E astern
Scheldt). T h e H onte - th e p re s e n t W e s te rs c h e ld e d o w n stream of Bath - w a s in fact
form ed by a se rie s of s e a b ra n c h e s s c o u re d in th e lagoon bottom by th e tidal c u rren ts
during storm events. T h e s e s e a b ra n c h es surrounded sh o als on which m an -m ad e lev ees
reduced progressively th e ex ten t of th e inundation a re a . Confining th e flow in c h a n n e ls
by reducing th e inundation a re a e n h a n c e s th e penetration of th e tidal w ave. T his results
in higher p e ak levels an d in c re a se d flood risk so th a t settle rs build s u c c e ssiv e ly higher
lev ees.
W ith th e higher le v ee s, polders b e c a m e m ore v ulnerable to catastro p h ic inundations,
during which new , d e e p c h a n n e ls w e re sco u red by tidal c u rren ts through th e new
b re a ch e s. T h e settlers, reclaiming th e inundated land, rebuilt lev ees not a c ro ss th e newly
form ed channels, but rather following their banks. As a result, the plan form of th e le v ee s
m a k e s so m e sh a rp c h a n g e s in direction. T h e re p e a te d p ro c e ss of land reclam ation,
lev ee breaching, inundation, levee building and again land reclam ation sta rte d in th e 1 1 th
century. In th e 15th an d 16th c e n tu rie s, h eav y floods re s h a p e d significantly th e
morphology of the W esterschelde. In th e 17thcentury th e p re sen t s h a p e a p p e a rs, though
still with m any secondary c h an n e ls a n d m any flooded sh o al a re a s . M orphologically, th e
W estersch eld e is not a typical m ean d erin g c o a sta l plain estu ary . This explains why it is
still evolving, despite th e m any civil engineering w orks undertaken to control th e estuary.
Figure 2.2-01 show s th e result of su c c e ssiv e flooding and land reclam ation o v e r th e p a s t
8 0 0 years. Interpretation is obviously m a d e difficult by th e fact th a t s o m e a r e a s h a v e
b e e n reclaim ed a n d flooded s e v e ra l tim es. S o m e inundations w e re m an -m ad e, often
s tra te g ic during w ar tim e. This is illustrated by th e co m p ariso n of th e z o n e s poldered
b e fo re 0.7 kyr BP a n d n e v e r flooded a g ain (in Fig. 2.2-01) with th e m ap of 0 .7 kyr BP
(Fig. 2.6-01).
W ith reliable c h arts becom ing av ailab le sin ce th e beginning of th e 19th century, it is
possible to a s s e s s quantitatively th e im pact of h um an interventions (Fig. 2.2-02). From
th e s e c h a rts an d from th e older, m o re qualitative m ap s, it m ay b e c o n clu d ed th at
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 5 o f 32
Summary o f key points / conclusions o f baseline report
poldering an d cutting off s e c o n d a ry c h a n n e ls a c c e le ra te d an d m odified significantly th e
natural tre n d of th e m orphological c h a n g e s . T h e analysis of this im pact is not yet
sufficient to m ake a g o o d e stim ate of its m ag n itu d e and further stu d y work is n e e d e d .
2 .3
C h a n g e s in Tidal R egim e
T h e long term m orphological tren d an aly sis p re s e n te d in th e b a se lin e report sh o w s
e v id e n c e of th e form ation of a limited n u m b er of m ajor s e a b ra n c h e s. T h e tidal-w ater
s to ra g e a r e a in th e H onte - th e s e a b ran ch in th e R h in e-M eu se-S ch eld e lagoon th a t
cap tu red th e S c h e ld e river flow - w a s p ro g ressiv ely re d u c ed by siltation on s h o a ls an d
scouring of th e main s e a b ra n c h e s. B efore any significant im pact of hum an activities,
th e s e n atural p ro c e s s e s favoured th e inland p en etratio n of th e tidal w ave. S in ce th e
Middle A ges, poldering an d cutting-off m any se c o n d a ry c h a n n e ls h a v e e n h a n c e d
significantly th e inland intrusion of th e tid e s long b efo re any dredging o p eratio n s, which
sta rte d at th e en d of th e nineteen th century.
A ppendix 2 of th e b a se lin e report p re s e n ts th e resu lts of an evaluation of tidal
observations within the S c h e ld e for th e period 1880 to 2000, m ore th a n 6 nodal cycles.
T h e s e d a ta w ere co m p ared with o b serv atio n s o u tsid e th e e stu a ry (O o sten d e) a n d at
s ite s in th e so u th ern North S e a . T his an aly sis co n clu d ed th a t c h a n g e s in tidal
p a ra m ete rs within th e e stu a ry (high w a ter height, tidal ran g e, high w ater interval) w e re
far g re a te r th an c h a n g e s in equivalent tidal ch aracteristics in th e so u th ern North S e a .
T he c h an g e s a re not cyclical but indicate a shift in o n e direction o v e r th e period of 100
or so y e a rs d a ta. T h e s e d a ta a re d is c u s s e d further in sectio n 3.3. of this docum ent.
C om parative consideration of th e tem poral c h a n g e s in tidal ch arac te ristic s a n d intertidal
a r e a is informative. B etw een 1800 a n d 1980, s o m e 58% of th e initial (1800) intertidal
a r e a h a s b e e n lost to poldering. It is co n clu d ed th a t this m ay b e th e principal
a n th ro p o g e n ic factor c a u sin g th e c h a n g e s in tidal pro p ag atio n during th e period of
o b serv atio n s. It is likely th a t dredging sin c e 1970 h a s contributed to th e e a s ie r
p e n e tra tio n of th e tidal w a v e in th e e stu ary . In a re c en t p ap er, A ren d s an d L angerak
(2000) c a m e to a similar conclusion. H ow ever, they do not co rro b o rate with facts their
statem en t that “channel enlarging by dredging d o m in ated (since 1970) th e d ev elo p m en t
of the estuary.” T he g raphs they p re se n t (Fig. 2.3-01 & 2.3-02) a re in line with th e tren d
over th e p a s t millennium a n d c o rre sp o n d to th e c h a n g e s d u e to natural factors. T his is
not to s a y th a t dredging h a s no effect but rath er th a t it is o n e factor am o n g st se v e ra l
others an d that the relative significance of the different factors is still unknown. Major an d
continuing c h a n g e s in c h an n e l g eo m etry h a v e b e e n o b se rv e d in th e p a st d e c a d e s . A
typical exam ple is th e c h a n g e of th e M iddelgat, b etw een T e rn e u z e n and H answ eert,
from a predominantly flood channel in th e 1960's to a predom inantly eb b c h an n el by th e
1970's, th e G at van O s s e n is s e taking o v er a s th e flood c h an n el (s e e Fig. 2.2-02).
D ata with which to e v a lu a te o th er m orphologically relev an t c h a n g e s h a v e not b e e n
readily a c c e ssib le to th e Port of A ntw erp ex p ert te am
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 6 o f 32
Summary o f key points / conclusions o f baseline report
2.4
Salinity a n d Mixing P ro c e s s e s .
T h e mixing of th e fresh w ate r river flow with th e salt s e a w a ter d e term in e s th e specific
w a te r a n d salt circulation in e stu a rie s. Salinity d a ta a re av ailab le only for th e p a s t fifty
years, or so. B esides ex cep tio n s, salinity d a ta w ere not collected in th e W e ste rsc h e ld e
in view of analysing in a sy ste m a tic w ay th e im pact th e m orphological c h a n g e s could
h a v e on th e mixing p ro c e s s e s . This is unfortunate, b e c a u s e salinity is an excellent
p a ra m e te r to a s s e s s th e variation in flow c o n v ey a n c e of th e estu ary .
S everal authors h av e rep o rted resu lts of m e a s u re m e n ts of th e longitudinal, tra n sv e rsa l
and vertical stru ctu re of salinity in th e W e s te rs c h e ld e (Fig. 2.4-01 )(D e P au w & P e te rs,
1973; P eters, 1975; P e te rs & Sterling, 1976; P e te rs & W ollast, 1976; P e te rs & W ollast,
1980). T h e s e stu d ie s sh o w e d th at th e S c h e ld e E stuary m ay b e subdivided in 3 z o n e s,
w h e re w ater an d salt circulation h a v e different p attern s. T h ey rev eal th e high mixing
potential in th e W estersch eld e dow nstream of H answ eert, w h ere th e distinctive flood and
eb b c h a n n e ls recirculate th e w ater aro u n d an d over th e large sh o al a re a s . Mixing is
e n h a n c e d by th e p re s e n c e of a re a s w h e re w ater c an b e retain ed during s o m e tim e a r e a s th a t c a n b e called tem porary d e a d z o n e s - while th e re st of th e flow c o n tin u es
m oving. T h e mixing pow er of th e flood an d eb b c h an n e l p a tte rn s dim inishes
progressively u p stre am of H answ eert, a s th e m orphology c h a n g e s steadily from th e
distinct m ultiple-channel to w ard s a sin g le ch an n el p attern .
T h e c h a n g e s in th e mixing pow er a n d its distribution along th e W e ste rsc h e ld e
d e te rm in e s th e vertical stratification of salinity an d velocity, h e n c e th e s u s p e n d e d
sedim ent circulation. It explains th e p re s e n c e of th e so -called “turbidity m axim um ”, also
th e mud deposits o b serv ed upstream of th e Dutch-Belgian border. S u s p e n d e d sed im en t
budgets w ere established on th e b asis of system atic m e a su re m e n ts perform ed b etw een
1967 a n d 1976 (P e te rs & Sterling, 1976; P e te rs & W ollast, 1976). Although they
fo cu ssed on th e e x iste n c e of th e m ud accum ulation in th e a re a of th e Port of A ntwerp,
thus on th e tran sp o rt of s u s p e n d e d m aterials, th e d a ta sh o w a significant n et tran sp o rt
of marine sedim ents b e c a u s e of the stratified flow. Bed m aterial transported a s pure b e d ­
load w as not m easured, but th e re is e n o u g h ev id en ce to s u g g e s t th a t th e m o v em en t of
se d im e n t a s pu re b ed-load m ust b e varying all over th e W e ste rsc h e ld e , with in so m e
a r e a s net inland o riented m ovem ent of b e d m aterial, m o re in te n se th an w ithout th e
existence of the stratification. How m uch this affects th e natural m orphological c h a n g e s
is not known, but one m ay su p p o se that the further simplification of th e W e s te rs c h e ld e ’s
m orphology - natural a n d a n th ro p o g en ic - m ay modify th e s e p a tte rn s of b e d sed im en t
m ovem ent, a s will b e d is c u s s e d further.
2 .5
M orphodynam ics of th e W e s te rs c h e ld e
T h e m orphodynam ic beh av io u r of c o a sta l plain e stu a rie s is in g en eral still poorly
understood. B ecau se of their complexity, investigators te n d w rongly to favour modelling
m ore than field observations and physical interpretation of th e field d ata. Very interesting
study w ork w a s m ad e in th e p a st a b o u t th e m orphology of th e W e s te rs c h e ld e an d th e
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 7 o f 32
Summary o f key points / conclusions o f baseline report
available information is unfortunately not well u s e d in investigations.
T h e physical p ro c e ss e s governing th e c h a n g e s in morphology of an estuary a re basically
th e sa m e a s th o se in rivers. T h e changing p re ssu re gradients and reversing flows of tidal
m ovem ent obviously com plicates th e interpretation. Large efforts h a v e yet to b e m ad e
in u nderstanding th e physical p ro c e s s e s governing th e m ovem ent of c h a n n e ls, sh o a ls
an d bars, crossings etc. T h e s h a p e of a s'noai or a b ar is d eterm in ed by th e flow pattern
a n d th e sedim ent transport, mainly th e tra n sp o rt of b ed m aterial. M ethods w e re already
d e v e lo p e d for interpreting th e m orphological c h a n g e s in large alluvial rivers (P e te rs &
Sterling, 1975; C oen, P e te rs & R o o v ers, 1977; P e te rs & W en s, 1991). T h ey rely on
u n d e rstan d in g th e m orphological p ro c e s s e s , using field d a ta to g e th e r with modelling.
M odelling, both physical a n d m athem atical, is u s e d a s a tool to a s s is t in th e
understanding of th e p ro c e s s e s , not to m a k e predictions. Indeed, in m o st applications,
m odels for predicting m orphological c h a n g e s a re producing poor resu lts b e c a u s e th e
m o d els can not re p ro d u c e correctly s o m e of th e b a sic physical p ro c e s s e s . T herefore,
“geographical” analysis of p a st m orphodynam ic evolutions is e sse n tial in addition to th e
traditional flow -sedim ent m odelling. T h e re is no re a so n to p re te n d th a t th e b a sic
p ro c e sse s governing th e m orphological c h a n g e s in an estu arial s a n d b e d environm ent
would differ from th o s e in an alluvial river.
T h e b aselin e report p re s e n ts a prelim inary an aly sis of th e W e s te rsc h e ld e
m orphodynam ics. C h arts of th e W e s te rs c h e ld e in 1636, 1800, 1865, 1938 a n d 1972
s e rv e d to illustrate th e mobility of th e m orphological fe a tu re s: th e shifting and
deform ation of b a r a re a s , th e m o v em en t a n d deform ation of c h an n e ls, c h a n g e s in th e
location a n d elevation of c ro ssin g s. T h e b a se lin e report c o n clu d e s th at th e long-term
trend in morphological change, a s a c o n se q u e n c e of th e p o st-P le isto c en e s e a level rise,
is still going on. This is now confirm ed by a c lo ser an aly sis of tide d a ta, a s p re s e n te d in
this final report. M oreover, th e b a se lin e report d is c u s s e s th e a n th ro p o g en ic c h a n g e
b e fo re major dredging sta rte d in th e W este rsc h e ld e . It co n clu d e s th a t poldering and
cutting off se c o n d a ry c h an n e ls, which sta rte d in th e Middle A ges, h a v e e n h a n c e d and
acc e le ra te d th e natural trend.
T he series of m ore precise charts betw een 1800 and 1972, p re sen te d in Figure 10 of th e
baseline report, h a s b een com pleted with th e additions of ch arts for th e 17 C entury and
for 1997 (Fig. 2.2-02). T h e inform ation p re s e n te d in this figure provides no indication of
any obvious or dram atic c h a n g e s in th e p attern of s h o a ls an d c h a n n e ls after th e o n se t
of m ajor dredging p ro g ram m e s in 1970. T h e s h a p e s of b a rs and c h a n n e ls h av e
continued to evolve a s they h a d d o n é prior to dredging an d dum ping in th e
W este rsch e ld e .
2.5.1
M ovem ent of C h an n e ls
T h e m orphodynam ic behaviour of th e W estersch eld e u p stream Vlissingen is d eterm in ed
by the sedim ent transport capacity of th e ch an n els a n d their d e g re e of freed o m to m ove
laterally. This is b e st illustrated in Figure 2.5-01. W h en o b serv in g th e s e c h a rts, it should
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 8 o f 32
Summary o f key points / conclusions o f baseline report
b e re m e m b ered th a t th e ap p aren tly m e an d e rin g s h a p e of th e W e s te rsc h e ld e is
accid en tal, th e result of repetitive flooding a n d land reclam ation. T h e e a s te rn ch an n el
(G at van O ss e n iss e and O verloop van H answ eert) n a s b e en evolving trem endously over
th e p a s t 2 0 0 y e ars, a very sh o rt period for su c h a c h a n g e . It b e c a m e a continuous
channel after th e forties an d took o v er th e role of flood ch an n el from M iddelgat only in
1969. T h e morphology of th e bar-com plex located betw een th e two main c h a n n e ls is still
evolving, not only a s a resuit of sed im en t tra n sp o rt on an d along th e b a rs, but a lso with
th e shift of th e G a t v an O s s e n is s e c h a n n e l to th e e a s t (Fig. 2.5-01). T h e P laten v an
Hulst, a slikke a n d sch o rre a re a rem n an t of th e e x te n d e d m arsh land (s e e Fig. 2.5-02
(17th Century)) - is still e ro d e d by flood an d e b b flows. T h e b an k d e fe n c e s - m an -m ad e
h ard p o ints - north an d so u th of th e a re a m a k e th e ch an n el b e n d becom ing m ore
p ronounced. T h e middle b ar sh o w s a discontinuity in its s h a p e ju st a c ro s s th e eroding
part of th e P laten v an Hulst.
In all th e d o cu m en ts co n su lted by th e e x p ert team , n o w h ere is this kind of evolution
a n aly se d morphologically, i.e. taking into a cc o u n t th e sed im en t tra n sp o rt to g e th e r with
th e erosion potential of th e ch an n el b a n k s. In th e b a se lin e report is m entioned th a t th e
s h a p e of the e a s te rn ch an n el (G at v a n O s s e n is s e - O verloop van H answ eert) b e c a m e
tortuous, a morphologically “unhealthy” situation. From th e visual an aly sis of th e ch arts,
it s e e m s th a t this evolution is “n atu ral”, influenced by poldering an d ch an n el cut-off’s.
T h e im pact of dredging and dum ping is unknow n, but is likely to b e limited. However, this
a n d similar is s u e s merit m ore re s e a rc h w ork, not fo re se e n in o u r assig n m en t.
B ank e ro sio n an d th e accretion of a b a r a c r o s s th e b an k a re linked, but th e link is
complex. T h e flow pattern and th e sedim ent m ovem ents a re continuously chan g in g with
th e tide, and they d ep en d on th e m agnitude of th e tidal amplitude. Also sto rm s affect th e
sedim entation m echanism s. D espite th e com plexity of th e s e p ro c e s s e s , th e m o v em en t
of th e c h a n n e ls a re quite p ro g ressiv e an d slow. T h ey m ove continuously a s a resu lt of
e ro sio n a n d deposition p ro c e s s e s a t a tim e s c a le of m onths an d y e ars. T h e sed im en t
tra n sp o rt is rarely in equilibrium with th e se d im e n t tran sp o rt cap acity of th e flow. T h e
actual tra n sp o rt d e p e n d s on th e av ailab le s o u rc e s of sed im en t.
In the overall sedim ent budget of th e W estersch eld e, th e a v erag e input of sed im en t m ay
b e considered a s negligible, co m p ared to th e actu al m ovem ent of sed im en ts inside th e
system . T hough th e re s e e m s to b e a c o n s e n s u s ab o u t a n internal circulation of
sed im en t, th e re a re different opinions a b o u t th e w ay it h a p p e n s. In th e cell co n cep t,
d e v e lo p e d by Delft Hydraulics, th e circulation of sed im en t is estim ate d from sed im en t
transport form ulas. T h e s e form ulas w e re e sta b lish e d for statio n ary flow conditions a n d
provide th e solid d ischarges in th e h y p o th esis th a t th e tran sp o rt cap acity is tran sp o rtin g
fully available sedim ent. T h e m odels do not co n sid e r a sorting of th e sed im en t p articles
by size, although river bed d a ta show a h etero g en e o u s spatial distribution of th e m edian
se d im e n t size an d silt co n ten t all o v er th e W e ste rsc h e ld e (Fig. 2.5-03 & 04).
Sedim entation in quiet flow z o n es m ay c re a te consolidated sed im en t, le ss easily e ro d e d
than th e m edium size sed im en t tra n sp o rte d an d d e p o site d in activ e flow z o n es.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 9 o f 32
Summary o f key points / conclusions o f baseline report
A free lateral - and thus also vertical - c h a n g e s in tidal c h an n e l layout a re h a m p ered by
th e p re sen c e of m an-m ade hard points, like d y k es, g ro y n es, le v e e s, etc. D ifferences in
ero sio n re s ista n c e of th e bottom se d im e n ts b e c a u s e of th e s iz e distribution variation
influence th e s e c h an n el m ovem en ts. H ow ever, th e c h an n el bottom an d b a n k s m ay b e
c o m p o se d of geologically older m aterials th a t m ay h a v e a still stro n g er re s ista n c e to
erosion; th e s e a re called th e geological controls.
2.5.2
G eological Control of E stu arin e M orphodynam ics
From m a p s show ing th e positions of b a n k s an d c h a n n e ls during th e period 1800 to
p resen t it a p p e a rs th a t th e re a re a re a s through which th e c h a n n e ls h a v e not m igrated.
T h e q uestion a ris e s a s to w h eth er th e s e a r e a s h a v e not b e e n “cycled” b e c a u s e of th e
morphological a n d positional en v elo p e of c h an n e l form or b e c a u s e they a re c o m p o se d
of older m aterials which a re resistan t to erosion, th e re b y limiting or inhibiting ch an n el
migration. P re se n c e of clay layers in th e bottom of the W este rsch e ld e h a s b e e n rep o rted
(Fig. 2.5-05)
It w ould not b e unu su al or u n e x p ec te d for so m e significant topography to h av e
developed during low or high s ta n d s of s e a level. It is know n (p erso n al com m unication
from d re d g e rs) th at in c h an n el re a c h e s su c h a s B o rssele, th e s te e p ch an n el w alls a re
form ed in c e m e n te d m aterials an d th a t v ario u s d e p o sits having significant erosion
re s is ta n c e occur at levels higher th an th e ta rg e t c h an n e l d e p th of - 1 4 m. G eological
investigations m ight give a clue ab o u t th e a g e of this bottom m aterial, possibly older
d e p o sits from a n earlier s e a tra n sg re ssio n an d their su rfa c e m orphology d ev elo p ed
during o th er s e a levels. In th e s e c irc u m sta n c e s th e m orphological evolution m ay be
d e te rm in e d by th e “to p o g rap h y ” an d p ro p erties of any ero sio n re sista n t sed im en t a s
much as, or in so m e c a s e s possibly m ore th an , hydrodynam ic p ro c e s s e s an d sed im en t
transport.
2 .6
R elationship with th e Vlakte v a n d e R aan
T h e V lakte v an d e R aa n w a s for long c o n sid e re d a s lying o u tsid e th e W este rsc h e ld e ;
V lissingen - B re sk e n s w a s th e s e a b o u n d ary of th e estu ary . In a study report for th e
model North S e a - S chelde E stuary (P e te rs & Sterling, 1976), a first attem pt w a s m ad e
to a n a ly se m orphologically th e interaction b etw een th e V lakte v a n d e R aa n an d th e
W este rsch e ld e . N ow adays, this interaction b etw een th e tw o a re a s is c o n sid e re d a s
im portant. H ow ever, m a n a g e m e n t of th e two a re a s is not y et well integrated.
Until the early nineteenth century, th e a re a s e a w a rd s of V lissingen co ntinued to h a v e a
strong m arine character, with th e Flem ish B an k s extending from th e French b o rd er into
th e mouth of th e Honte. S everal ch an n els conveyed th e tidal flow in and out th e estu ary .
T his w a s th e e n d p h a s e of a long term m orphological evolution, during which the
S c h e ld e river flow c h a n g e d its d isc h a rg e point, first into th e M eu se in R om an tim es (2
kyr BP), th e n in th e O o ste rsc h e ld e (1.4 kyr BP) an d finally in th e H onte (1.2 kyr BP)
(Coen, 1988). M aps from 0 .7 kyr BP (Fig. 2.6-01) rev eal th e com plexity of th e ch an n el
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 10 o f 32
Summary o f key points / conclusions o f baseline report
sy ste m - s e a b ra n c h e s - in w hat at th a t tim e rem ain ed of th e lagoon. Until s o m e 1000
y e a rs ag o , th e m orphology of th e H onte w a s com plex, shallow a n d p re s e n te d a high
resistance to th e tidal flow. T h e c u rre n ts entering an d leaving th e e stu a ry in V lissingen
rem ained limited, a s com pared to w hat o ccurs today. H ow ever, a s th e tidal ra n g e at th e
m outh of th e H onte w a s larger th a n th e o n e at th e m outh of th e O o ste rsc h e ld e , th e
scouring enlarged progressively th e H onte s e a branch. T h e tidal prism in th e H onte w as
initially sm all, but in c re a s e s p rogressiv ely w hen tid es intruded further in th e w idening
channels. W hen th e H onte b e c a m e sufficiently en larg ed , it took o v er th e S c h e ld e river
flow an d tid e s intruded still m ore easily. This e n h a n c e d th e e x c h a n g e of sed im en t
betw een th e W estersch eld e and th e Vlakte van d e R aan, that b e c a m e th e o u ter estuary.
T h e charts of the W estersch eld e (Fig. 2.2-02) show that th e e n tra n c e of th e e stu a ry h a s
evolved significantly. T h e lay out with th e s e q u e n c e of Flem ish B an k s pen etratin g
to w a rd s th e bottleneck V lissingen - B re sk e n s h a s b e e n p ro g ressiv ely re p la ce d by a
com plex of b a n k s which now m ak e up th e Vlakte v an d e R aan.
2.7
Dynamic Equilibrium
A central a s p e c t of contem porary m orphological stu d y is th e c o n c e p t of “dynam ic
equilibrium”. In this c o n c e p t a property, condition or v ariab le c a n flu ctu ate random ly, or
periodically, about a notional m ean condition or value su ch th a t th e a v e ra g e v alu e of th e
property, condition or variable, ta k e n o v er an ap p ro p riate tim escale, d o e s not c h an g e.
For th e S c h e ld e E stu ary th e minimum ap p ro p riate tim escale m ay b e th e 18.6 y e ars of
th e nodal cycle or se v e ra l su c h cy cles (e.g. 37 .2 or 55 .8 y ears).
In respect of th e morphological developm ent of th e S c h e ld e E stu ary a n u m b er of is su e s
n eed to be a d d re sse d in this context and th e se can b e exam ined a s 2 s e ts of q u estio n s.
Q uestion A.
If th e m orphological equilibrium is th e result of hydrodynam ic fo rces an d
th e resulting sed im en t tran sp o rt:
A 1.
Is th e sy ste m co n strain e d or free to c h a n g e ?
A2.
A re th e vario u s forcing functions in a s ta te of dynam ic equilibrium
or statio n ary o v er a “long period”?
A3.
Is th e re sp o n se of th e morphology in equilibrium with th e forcing or
d o e s its re s p o n s e lag, an d d o e s this lag vary with th e ra te of
c h a n g e of th e function?
Q uestion B.
If th e m orphological equilibrium involves th e m o v em en t of sed im en t an d
its redistribution within th e sy ste m leading to m orphological ch an g e:
B1.
Is th e total quantity of sed im en t in th e sy ste m c o n sta n t?
B2.
Is th e redistribution of sed im en t within th e sy ste m c o n sta n t?
B3.
W h a t is th e tim escale of th e redistribution a n d th u s th e
m orphological re s p o n s e ? (S e e A3).
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 11 o f 32
Summary o f key points / conclusions o f baseline report
A.1 : T h e c h an n el/b an k s y ste m s of th e S c h e ld e a p p e a r to h a v e p e rm a n en t
co m p o n en ts, a re a s of b a n k through w hich th e c h a n n e ls h a v e not m igrated. Are
th e s e h ard sp o ts which e x ercise a determ inistic an d limiting influence on
morphological c h an g e ? S o m e ch an n els a re now d e e p e r th an they h av e ev er b e en
a n d h a v e s h a p e s an d sid e s lo p e s not c o n siste n t with their being form ed in
erodible sedim ent. Anecdota! e v id e n c e in d icates th at s o m e c h a n n e ls a re form ed
in c e m e n te d sed im en ts.
T h ere is thus a clear possibility th at p a rts of th e c h a n n e ls a n d b a n k s a re not free
to b e c h a n g e d by hydraulic a n d hydrodynam ic p ro c e s s e s an d a s su c h do not
constitute an “equilibrium” sy stem but are, in part at least, a fixed sy stem . In o th e r
p la c e s historical re c o rd s indicate th a t th e geology an d b an k protection
unquestionably form fixed controls.
A.2 : T h e main forcing function is th e tide. Tidal c h arac te ristic s vary on 18.6 y e a r an d
shorter cycles. However, th e s e a level is rising in a non-equilibrium m an n er. M ore
than this how ever, observatio n s sh o w g ro s s c h a n g e s in im portant tidal v ariab les
(range, asym m etry, p h a se , etc) in a non-cyclical w ay over tim e-scales longer th an
th e nodal cycle. On this b a sis w e s u g g e s t th at th e b a sic dynam ic forcing within
th e system is not in a s ta te of equilibrium and a s su ch w e a re c o n c e rn e d th a t any
m orphological m a n a g e m e n t stra te g y b a s e d on an a ssu m p tio n of dynam ic
equilibrium m ay b e unsoun d .
A.3 : T h e re sp o n se of any material, or by analogy a sy stem , to a forcing or s tr e s s m ay
b e c h a ra c te rise d by th e ratio (De) of th e relaxation tim e of th e sy ste m to th e
period of th e s tre ss .
For exam ple, in the c a s e of sim ple m aterials, w here De « 1 th e m aterial is a fluid,
w h e re De » 1 th e m aterial is a solid. In th e c a s e of estu arial sed im en ts, th e
individual sed im en t particles re sp o n d in stan tan eo u sly to th e co n tem p o rary fluid
forcing. However, th e residual m o v e m e n ts of th e sed im en t leading to ero sio n or
sedim entation respond to th e longer sc a le cycles. T h e erosion of so m e sed im en ts
m ay resp o n d only to e x tre m e e v e n ts. U n-erodible sed im en t d o e s not resp o n d .
T here is thus a variable h y steresis in th e re s p o n s e of th e sed im en t population to
th e hydrodynam ic forcing a n d th e re is, a s c an b e e x p ec te d , a n unknow n a n d
u n predictable lag b e tw ee n an y c h a n g e s to th e hydrodynam ic forcing a n d th e
re s p o n s e of th e m orphology resulting from sed im en t erosion, tra n sp o rt or
deposition. However, this d o e s not invalidate th e c o n c e p t of dynam ic equilibrium
if th e forcing is in dynam ic equilibrium . It just m e a n s th at th e prediction of th e
equilibrium m orphology is very difficult. N ev erth eless, if, a s is sh o w n by
observation, th e forcing function is not in dynamic equilibrium, th e n n either will b e
th e morphology. For th e s e re a so n s w e do not consider that th e m orphology of th e
W estersch eld e should b e a ss u m e d to b e in a s ta te of dynam ic equilibrium on any
practically relevant tim escale.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page ¡2 o f 32
Summary o f key points / conclusions o f baseline report
Turning to th e q u estion of sed im en t b u d g e ts p o s e d in Q u estio n B:
B.1 an d B.2.
T he determ ination of sedim ent budgets, which m ay b e reg ard ed a s th e n et import
or export, gain or loss, of sedim ent in a n a re a is notoriously difficult, especially in
circum stances of reversing flow an d lateral differentiation of longitudinal tran sp o rt.
This is e x a c e rb a te d in regions w h e re th e main tra n sp o rt v e cto rs ro tate with
channel m eandering, w here lateral transport occurs (as is th e c a s e in m o st th reedim ensional flow sy ste m s) a n d w h e re th e n et tra n sp o rt is e stim ate d a s th e
difference b etw een 2 large q u an tities d eterm in ed with only poor precision.
At p resen t w e have b e e n u n ab le to identify any o b serv atio n s or co m p u tatio n s of
net transport of sed im en t a c c o m p a n ied by e stim a te s of uncertainty. As su c h w e
consider that th e condition of th e sy stem with resp ect to th e in c re a se o r d e c r e a s e
of its sed im en t population is unknow n.
B.3.
T h e re is yet no system atic, q u an titativ e a n d au d ited a s s e s s m e n t available of
either natural or anthro p o g en ic redistribution of sed im en t within th e sy ste m an d
its a ss o c ia te d m orphological c h a n g e . S u rv ey s a re infrequent an d related to
specific locations. S y stem w ide c h a rts rely on d a ta collected over sev e ra l y e a rs
rather than being reasonably synoptic (e.g. period of 1-3 m onths). No sy stem atic
s e a s o n a l co m p ariso n s, e v en of sen sitiv e or dynam ic a re a s , a re available.
In considering th e s e 2 g ro u p s of q u e stio n s w e co n clu d e that:
1)
T h e a ssu m p tio n of dynam ic m orphological equilibrium is not justifiable on th e
b a s is p ro p o se d h ere, though it m ay provide a b a sis for exploring th e p o ssib le
application of o n e type of m a n a g e m e n t tool.
2)
T h e assu m p tio n of dynam ic equilibrium c a n n o t b e e sta b lish e d for th e sed im en t
budget which is not adequately docum ented, either on a sy ste m w ide b a sis or for
a re a s within th e sy stem . This is a crucial g a p in information.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 13 o f 32
Summary o f key points / conclusions o f baseline report
2 .8
Sum m ary
A s a resu lt of th e continuing, fluctuating, rising s e a ievei ana' th e geological,
geom orphological and sedim ent transport p ro c e sse s related to it, the a re a of w hat is now
th e W estersch eld e Estuary h a s experienced, and continues to ex p erien ce, a continually
changing p attern of tidal hydrodynam ics an d related m orphological developm ent. T his
naturally changing d ev elo p m en t is still ongoing a n a in certain fa c e ts, su c h a s tidal
ch aracteristics, c h a n g e s a re m ore rapid th an o u tsid e th e e stu a ry sy stem . It is evident
that th e p resen t morphology of th e estu ary is not just a product of sed im en t tra n sp o rt but
is influenced by geology an d hum an actio n s. Although th e relative im p o rtan ce of th e s e
influences is quantitatively undefined, from first principles th e effects of poldering w ould
b e e x p ec te d to p ro d u ce th e o b se rv e d c h a n g e s in th e tidal regim e. It is c o n sid ered th a t
th e m orphological evolution of th e W e s te rsc h e ld e is also linked to th e evolution of
c o astal a re a s su c h a s th e V lakte v a n d e R aan. W h ate v e r m orphological m a n a g e m e n t
s tra te g ie s a re d e v elo p e d will h a v e to ta k e into acco u n t th e continuing dynam ic n a tu re
of th e system , its links to o th e r c o a sta l s y ste m s a n d its links to th e re st of th e S c h e ld e
c atch m en t. As a c o n s e q u e n c e of th e continuing c h a n g e s in m orphology and tidal
dynam ics, it is evident th a t in term s of safety from flooding, th e co n serv atio n or
im provem ent of navigation an d th e su sta in a b le m a n ag e m e n t of th e environm ent, a “do
nothing” or “k eep things a s they a re ” m a n a g e m e n t stra teg y for m orphology an d ecology
is neither a viable nor a su sta in a b le option.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 14 o f 32
Dredging and its influence on the estuarial dynamics
3
DREDGING AND ITS INFLUENCE ON TH E ESTUARIAL DYNAMICS
3.1
G en eral C om m ents
Inform ation ab o u t dredging w orks in th e W e ste rsc h e ld e is available in sev e ra l
d o c u m e n ts. T h e expert te a m ack n o w le d g e s th e u n certain ties in th e d a ta found in
different reports. T h e se uncertainties h a v e little influence on th e a s s e s s m e n t to b e m ad e
of the impact the works h av e on th e estuarial m orphology an d ecology. H ow ever, in view
of a th e fu tu re m a n a g e m e n t of th e e stu a ry , th e te a m a d v ise s th e creation of a unified
Belgian-Dutch d a ta b a se , with thoro u g h d a ta quality control. N ev erth eless, it is realised
that there will alw ays rem ain u n certain ties, linked to th e n a tu re of th e dredging activity,
either for d e ep e n in g an d w idening of th e c h a n n e ls, or for s a n d mining.
T h e te a m re a lise s how difficult it is to g e t a clear picture of th e relationship b etw een
dredging effort an d navigation depth . M any facto rs in terv en e in th e p ro c e ss, especially
th e self-scouring capacity of th e flow a s related to th e morphology of th e W e ste rsc h e ld e .
A section on dredging efficiency in relation to m orphology is included in th e A ddendum .
O n e of th e key q u e stio n s w hich h a s to b e a d d re s s e d is :
“Is it feasible, with th e available inform ation, to draw s o m e co n clu sio n s
a b o u t th e im pact th a t dredging a n d dum ping h av e on th e m orphological
c h a n g e s of th e W e s te rs c h e ld e ? ”
A nother question which th e te a m co n sid e r relev an t is:
“C an dredging and dum ping b e u s e d a s a tool to influence m orphological
evolution with th e objective of m aking th e m orphology “healthier”, i.e.
preserving or enhancing the ecological v alu e by in creasin g m orphological
diversity, reducing th e risk of inundation an d optim ising th e dredging, so
th a t larg er dep th cari b e o b tain ed with th e sm allest p o ssib le dredging
v o lu m e s? ”
O ne poorly understood elem ent is th e relationship betw een m orphology (mainly th e planform), the dredging an d the elevation of c ro ssin g s. During a W orld B ank fu n d ed project
on the maritime reach of a large alluvial river, th e team leader w as a sk e d to esta b lish th e
requirem ents of dredging equipm en t for a given targ et navigation d ep th . T h e study
re v e ale d that, for ab o u t 90 y e a rs of o b serv atio n s, th e relationship b e tw ee n th e
navigation depth achieved for a given dredging effort and th e volum e of d red g ed material
to b e rem oved w as mainly d eterm in ed by th e self scouring (auto-dredging) capacity of
th e river. T h e plan-form s h a p e of th e c h a n n e ls w a s a key elem ent. H ow ever, sed im en t
over- or under-loading of s o m e re a c h e s played an im portant role. This kind of an aly sis
w as not in th e assignm ent of th e expert team , though it would h av e b e en very useful an d
should certainly b e a part of an y future m a n a g e m e n t plan dev elo p m en t.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 15 o f 32
Dredging and its influence on the estuarial dynamics
3.2
History of dredging in th e W e s te rs c h e ld e
T h e dredging activities for improving th e navigation conditions in th e W e s te rsc h e ld e
sta rte d at th e en d of th e 19th century (Fig. 3.2-01). Initially, m a in te n an c e dredging w a s
mainly n eed ed in Belgium. After 1927, d re d g e sites w ere also found in T h e N eth erlan d s,
u p s tre a m of H answ eert. S in ce W orld W ar II, th e volum e of sed im en t d re d g e d in th e
W estersch eld e (capital an d m aintenance) in c re a s e d steadily, from a b o u t 2 million cubic
m e tre s in 1945 to a b o u t 8 million a t th e e n d of th e sixties. T h e dredging locations
extended progressively sea w ard s, reach in g th e b o ttlen eck a re a of th e e stu ary b etw een
V lissingen an d B re sk e n s in 1973, with th e d re d g e site of B orssele.
T he navigation d epths increased progressively with th e d eep en in g of th e c ro ssin g s. T h e
p e rcen tag e figures pre 1920 are b a se d on very sm all volum es, often zero, an d so sm all
c h a n g e s produce large p ercen tag e fluctuations. In 1970, at th e start of th e first intensive
d e ep e n in g program m e, th e Port of A ntw erp w a s a c c e ssib le for sh ip s with a d rau g h t of
about 13 m. At that time, the volum e of m aterial d red g ed annually in th e W e ste rsc h e ld e
re a c h e d a m axim um of m ore th an 16 million cubic m etres, ab o u t half of it in Belgium.
During th e 1971-1976 d e ep e n in g p ro g ram m e, th e s h a re in Belgium of this volum e
d e c re a se d to about 20 % (Fig. 3.2-02). C onsequently, the volum es d red g ed b e tw ee n th e
D utch-Belgian b order an d th e s e a h a v e sin c e b e c o m e by far th e largest. In th e period
1971-1976, th e c ro ssin g s d o w n stream of th e Zandvlietsluis w ere d e e p e n e d by 1.5 to
2 m. It is interesting to note that th e stea d y trend after th e d e ep e n in g p rogram m e sh o w s
an in c re a se of drau g h t an d a d e c r e a s e of vo lu m es d red g ed . T h e dredging efficiency
obviously plays a n im portant role in this evolution, but it is worth analysing th e role
played by th e continuing alteration of th e W e s te rs c h e ld e ’s m orphology, exem plified by
th e significant c h a n g e in th e ch an n el configuration b e tw ee n T e rn e u z en en H an sw eert
which occurred at th e en d of th e sixties.
T he deepening program m e perform ed at th e en d of th e nineties implies a larger dredging
effort. It will b e interesting to o b serv e th e further trend, either a reduction in th e dredging
effort or an in crease of th e volum es to b e d red g ed for maintaining th e new ta rg e t d e p th s.
Until 1924 th e sed im en t rem oved from c ro ssin g s w a s initially u s e d a s land fill m aterial.
After that, the p ercen tag e of sedim ent d u m p ed b ack in th e river in c re a se s, mainly in th e
D utch part. T h e stra teg y a d o p ted until recently w a s to d isp o se a s m uch a s p o ssib le of
th e d re d g e d soil in se c o n d a ry c h a n n e ls, mainly in flood c h an n e ls. T h e m orphological
im pact of this stra teg y is stro n g e r in th e w ide m ulti-channel sy stem in th e Dutch p art th e s e a branch - th an in th e m uch sim pler Belgian part.
T h e modelling perform ed in LTV with a “cell” m odel le a d s to th e conclusion th at th e
“overloading” of th e flood channels will m a k e th em die. T h e expert te a m w orked out th e
d a ta about volum es dredged in the various cells. Details a re given in the A ddendum . T h e
variations of th e se volum es b etw een 1961 an d 1997 show n that b efo re 1970, dum ping
w as negligible do w n stream of H an sw eert (cells 3 an d 4). T h e significant m orphological
c h a n g e s w ere natural. Large volum es w ere dum ped in cell 5 from th e en d of th e sixties,
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 16 o f 32
Dredging and its influence on the estuarial dynamics
in either flood or eb b channels. This in creased with th e first d eep en in g program m e 1971 1976 a n a w a s said to h a v e resu lted in o v er siltation of th e flood c h an n e ls. T h e expert
te a m b e lie v es th a t th e m orphological c h a n g e s w e re mainly d u e to natural factors, a s
explained in th e A ddendum .
M odels w ere u se d in LTV for determ ining th e m axim um quantities th a t m ay b e d u m p ed
in a given “cell”. A s is d o cu m en ted in th e A ddendum , th e s e allow ed q uantities w ere
e x c e e d e d o v e r se v e ra l y e a rs in m ore p la c e s, w ithout resulting in a d eg rad atio n by
siltation. In this co n tex t th e ex p ert te a m c o n sid e rs th a t th e m odels do not provide a
sec u re basis for th e developm ent of m a n a g e m e n t stra te g ie s. R eco m m en d atio n s b a s e d
upon th e s e m odel c o n c e p ts should not b e re g a rd e d a s d e p en d a b le .
T he team p ro p o se s that a com prehensive study of th e m orphodynam ics of this a re a and
th e application of alternative dredging s tra te g ie s for m orphological m a n a g e m e n t in this
a re a should b e m a d e a s explained in S ection 4.5.
3.3
Tidal p a ra m e te rs sin c e 1938
Previous an aly ses of tidal d a ta h av e exam ined g ro ss, long term c h a n g e s o v er th e period
1880 -1 9 9 0 . It is of interest to exam ine similar tidal p a ra m e te rs in m o re detail during th e
period from 1938 to th e p re se n t, w hen m ore significant dredging w a s perform ed for
improving th e a c c e s s to th e Port of A ntw erp, an d for s a n d mining. At this tim e only tidal
d a ta a v e ra g e d over 10 y e ar intervals is u se d . T h e s e d a ta a re not sy n ch ro n ised with
particular p a rts of th e nodal cycle a n d so s o m e p ro c e s s related a s p e c ts m ay b e
o b scu red . Further w ork on this is n e e d e d .
D redging activity co m p rises:
a) Capital and M aintenance dredging with relocation of material within th e system .
b) C apital an d M aintenance dredging with rem oval of m aterial from th e system .
c) S a n d mining with rem oval of m aterial from th e sy stem .
T h e rates of ch an g e of six tidal p aram eters, M ean High W ater, M ean Low W ater, M ean
Tidal R a n g e , M ean Duration of Flood, M ean D uration of E bb a n d High W ater Interval
from Vlissingen h av e b e en exam ined using d a ta su p p lied by M inisterie v an d e V laam se
G e m e e n s c h a p , A ntw erpse Z e e h a v e n d ie n st. For V lissingen, T e rn e u z en , H answ eert,
B ath, H edw igpolder-P rosperpolder, Lillo-Liefkenshoek an d A ntw erpen, for e a c h tidal
p a ra m e te r, th e g ro ss m ean an n u al ra te of c h a n g e w a s estim ate d by calculating th e
difference betw een 10 year d a ta blocks, assu m in g this ch an g e took place over 10 y e ars.
N egative v a lu e s indicate a rev ersal of tren d . T h e resu lts a re show n in Figs 3.3-01. to
3.3-06. Also show n a re th e actual c u rv e s of c h a n g e . T h e s e d a ta sh o u ld b e co m p ared
with th e c u rv e s for dredging, (Figs 3.2-01 to 03 in S ectio n 3.2). T h e o n s e t of th e m ajor
dredging c a m p a ig n s in 1970 is show n by th e vertical w hite line on th e g rap h s.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 17 o f 32
Dredging and its influence on the estuarial dynamics
3.3.1 M ean High W a te r (Fig. 3.3-01)
in th e baseline report it w a s noted that th e heights of high w a te r h a v e risen all alo n g the
estuary during th e period u n d er con sid eratio n , it is also evident th a t th e position of th e
m axim um in w ater level h a s m oved approxim ately 33 Km into th e e stu a ry from n e a r
Antwerp to near S t A m ands, an a v e ra g e of th e o rd e r of 300 m p e r y ear. T h e increasing
asym m etry of th e m e an high w a ter profile is also evident in Fig B.1. of th e B aseline
R eport. H ow ever, although th e c h a n g e s a re c o n siste n t a s n o ted , a n d for all locations,
th e h eight of high w a ter in c re a s e s steadily, th o u g h with sm all fluctuations, th e ra te of
ch an g e is relatively stea d y throughout th e period co n sid ered . S o m e locations a p p e a r to
show fluctuations aro u n d 1970, o th e rs sh o w fluctuations th ro u g h o u t th e period.
3.3.2 M ean Low W a te r (Fig. 3.3-01)
T h e heights of low w a te r w e re not e v alu a te d in th e B aseline R eport for th e re a s o n s of
variability. T rends a re both up or down depending on location th ough th e re s e e m s to b e
a s u d d e n but apparently tem porary drop in level after 1970, visible on th e d a ta for th e
p e rio d s 1970 to 1980 an d 1980 to 1990. For m ost locations th e ra te of c h a n g e is
variable, but so m e inflection occurs betw een th e 1960-1970 an d 1970-1980 d a ta blocks.
T h e s e a re th e effects of single d a ta points an d th e re a re s o m e points which m ay b e in
error. T h e 1980-1990 d a ta blocks a p p e a r to fall b ack within th e e n v elo p e of p re 1960
d ata. T h e c h a n g e in 1970 c a n h a v e m ore th an o n e explanation. O n th e o n e h an d a
natural morphological c h a n g e in th e flood and e b b ch an n el sy ste m “M iddelgat - G a t van
O ss e n iss e ”, which resulted in m uch w ider c h an n els and lower cro ssin g s, so th a t th e ebb
flow w a s e a s e d . O n th e o th e r hand, th e d e ep e n in g by dred g in g of th e navigation
channel, that m ade th e c ro ssin g s d e e p e r so th a t th e main c h an n e l could e a s ie r convey
th e w a ter at th e low est tide levels. Both m o d e s of c h a n g e - th e natural an d th e
anthropogenic - h a v e h a p p e n e d in th e period 1969-1975.
3.3.3 M ean Tidal R a n g e (Fig. 3.3-02)
T h e stu d ie s u n d e rta k en for th e B aselin e R eport re v e ale d th a t th e tidal ra n g e h a s
increased throughout th e e stu a ry an d th a t th e locus of m axim um ra n g e h a s m oved upestuary from th e vicinity of Prosperpolder/Lillo L iefenshoek in 1888-1895 to th e vicinity
of T e m s e /S t A m ands by 1981/1990, a d ista n c e of th e o rd e r of 4 4 Km, a m e an rate of
m ore th an 400 m p er year. T h e stu d ie s s u g g e s t th a t th e c h a n g e s a re m ore v ariab le in
th e sh o rt term an d th a t further investigations of th e c h a n g e s in th e sectio n from
H answ eert to A ntw erp for th e period 1888 to 1950 n e e d to b e related to m orphological
c h a n g e s in this section of th e estuary. From Prosperpolder th e m ean tidal ra n g e a p p e a rs
to h a v e in c re a se d m o st strongly sin ce th e 1950 to 1960 period. In th e resu lts of the
p re s e n t study, th e c u rv e s for e a c h location a re different. T e rn e u z en , Bath, LilloL iefkenshoek an d A ntw erpen, show a p e a k in th e 1970-1980 d a ta block but th e 19801990 d a ta point retu rn s to th e p re 1970 en v elo p e. V lissingen is quite different show ing
a p e a k in th e 1950-1960 d a ta block, a trough in 1960-1970 d a ta block an d a return to
th e p re 1950 e n v elo p e in 1970-1980 a n d 1980-1990. H an sw eert sh o w s a drop in th e
1980-1990 d a ta block com pared to th e pre 1980 d a ta. Of th e long term d a ta s e ts , Bath,
□Ilo an d A ntw erpen sh o w m arked in c re a s e s in tidal ra n g e after 1950.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 18 o f 32
Dredging and its influence on the estuarial dynamics
3 .3 .4 M ean Duration of Flood an d E bb T ide (Fig. 3.3-03)
This p a ra m ete r w a s not ev alu a te d in th e B aselin e R eport b e c a u s e of th e m any facto rs
which can influence it. T h e various effects which may influence th e time of low w a te r an d
high w ater lead to g re a te r variability in th e s e d a ta. In th e an aly sis of th e ra te of c h a n g e
in Flood T ide duration no e stu a ry w ide tre n d is clear. S o m e locations, V lissingen,
T e rn e u z en , an d H answ eert, show s te a d y or slightly declining tren d s, o th e rs sh o w
m arked reductions to a minimum in 1960-1870. T h e ra te s of c h a n g e a re very variable.
3.3.5 High W ate r T im e Interval (Fig. 3.3-04)
In th e b a se lin e study it w a s show n th a t th e tim e b e tw ee n high w ater at V lissingen an d
o th e r locations h a s d e c re a s e d during th e period of th e d a ta . T h e tide p ro p a g a te s a t a
g re a ter s p e e d . H ow ever th e an n u al ra te of c h a n g e is variable th ro u g h o u t th e period of
d ata. Small c h a n g e s in o b se rv e d interval h a v e a dram atic effect on th e g ra p h s. T h e re
is so m e indication that th e rate of d e c re a s e h a s slow ed but the d a ta n e e d s further quality
control.
. 3.3.6 D iscussion.
T h e o n set of m ajor dredging c a m p a ig n s in 1970 is m arked by th e w hite vertical line on
th e illustrations. In s o m e g ra p h s th e re is a coincident excursion in th e d a ta points. In
others there is an excursion prior to th e o n s e t of th e dredging cam p aig n . In o th e rs th e re
is no evident effect. For s o m e p a ra m e te rs th e excursion s e e m s to b e tem p o rary with
recovery by 1980. C om parison of th e s e d a ta with reco rd s of d re d g e d v o lu m es (Fig 3.201 ) d o e s not sh o w any clear correlation b e tw ee n th e e stu a ry w ide tre n d s or ra te s of
dredging and the p atterns of c h a n g e s in tidal pro p ag atio n p a ra m e te rs relev an t either to
safety or navigation. Further, m ore detailed, an aly sis is n e ed e d .
3 .4
Sum m ary
C om parison of available dredging d a ta for th e period 1895 to 1997 with v ario u s tidal
p ro p ag atio n p a ra m e te rs for th e period 1880 to 1990 d o e s not rev eal any significant
c h a n g e s in th e trend, or ra te of c h a n g e , in tidal p a ra m e te rs which c a n re a so n a b ly b e
attrib u ted to th e dredging, especially to th e dredging c am p a ig n s of 1970 an d 1995. In
that th e tidal propagation param eters within th e W estersch eld e a re believed to b e linked
mainly to morphological c h an g es, in a feed-back type of system , it would a p p e a r th a t th e
s c a le of dredging h a s b e e n su c h th a t it h a s, so far, not p ertu rb ed th e overall
m orphological c h a n g e s , an d th ere-b y c h a n g e d th e overall ch arac te ristic s of tidal
propagation, beyond w hat h a s occurred “naturally” during th e previous 100 y e a rs or so.
P o ssib le ex ceptions to this a re ap p aren tly tem p o rary effects on th e ra te s of c h a n g e of
Low W ater levels and apparently tem porary effects on th e ra te of c h a n g e of High W ater
time interval although th e s e se e m to h av e returned to p re 1970 levels by 1980/90. M ore
d etailed investigation of th e s e is s u e s is n e e d e d .
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 19 o f 32
Future developments
4
FUTURE DEVELOPM ENTS
4.1
O bjectives
T h e principle objectives of future d e v elo p m en t should b e to:
4 .2
•
E n su re th e safety of th e a r e a s surrounding th e S c h e ld e E stu ary an d th e
safety of u s e rs of th e estu ary .
•
E n su re th e continued s u sta in a b le u s e of th e e stu a ry for navigation an d
com m ercial benefit
•
E n su re th e su sta in a b le m a n a g e m e n t of th e environm ent of th e estu ary .
M an ag em en t M ethods an d T ools
T h e p re sen t system of m anagem en t is fo c u sse d on navigation. It relies on p ro c e s s e s of
relocation of sedim ent within the system by dredging cro ssin g s (“d rem p el”) a n d dum ping
in a variety of locations. T h e re is no integration with oth er dredging activities an d no
coordination of dredging with ecological m a n a g e m e n t goals.
T h e re is a widely held c o n s e n s u s th a t in th e a b s e n c e of any dredging activity th e
W este rsch e ld e would evolve to a single c h an n e l sy stem . T h e principal objective of th e
LTV stu d y is to identify stra te g ie s a n d m e th o d s which will allow m a n a g e m e n t of th e
s y ste m to satisfy th e requirem en ts for N avigation, S afety an d Environm ent. T h e
c o n se n s u s m ongst technical specialists is th a t this is b est m et by a m ultichannel sy stem .
It is, therefore, n e c e s s a ry to h a v e a c c e s s to m a n ag e m e n t tools which will allow th e
prediction of th e effects of particular actio n s in ord er th a t m a n a g e m e n t s tra te g ie s m ay
b e d e s ig n e d an d specified to a ch ie v e s u sta in a b le m an ag em en t. It is our view th a t no
single tool can provide all the n e ce ssa ry a n sw e rs and that th e tools u s e d m ust b e b a s e d
upon a reliable u n d erstan d in g of th e sy ste m , th e w ay it w orks an d th e facto rs which
govern its m orphological evolution sin c e m orphological diversity is b a sic to ecological
diversity.
Two m ain g roups of m a n ag e m e n t a p p ro a c h e s m ay b e identified:
1. Predictive Sim ulations
T h e se tools include num erical sim ulation of flows an d sed im en t tran sp o rt, sc h e m a tise d
num erical sim ulations of th e m orphological evolution of spatial units an d physical
sim ulations of ch an n el or bank se c to rs . T hey a re b a s e d on d a ta an aly sis and
observation, assum ption, w ide ranging sim plifications, o th er m odels an d e x p erien ce. In
this a p p ro a ch , th e e sse n tia l m a n a g e m e n t tool is th e m orphological m odel. H ow ever,
there a re several natural a sp e c ts of th e sy stem which w e believe m ak e com plete or e v en
su b stan tial reliance on su ch tools a s inadvisable.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 20 o f 32
Future developments
T h e s e m ay b e su m m a rised a s follows:M orphology an d m orphological d ev elo p m en t d o e s not d e p e n d solely on
hydrodynamic an d sedim ent transport p ro c e ss e s but is influenced, possibly in key
p la ce s, by th e geology of th e a re a .
T he morphological boundaries of th e W estersch eld e system , th e dykes, s e a w alls
an d training stru ctu res, m ean th a t th e re a re non-natural limits an d non-natural
c o m p o n en ts in th e m orphology determ ining m ech an ism s.
M anagem ent tools b a se d on assu m p tio n s of dynam ic equilibrium do not ta k e into
a cc o u n t th e non-stationarity of hydrodynam ic forcing or th e long term effects of
such p ro c e ss e s a s poldering or land creation on th e tidal d ynam ics of th e sy stem .
T his is further com plicated by th e e x p ec ta b le h y ste re sis in th e m orphological
re s p o n s e of th e sy ste m to c h a n g e s in tidal d ynam ics which m a k e s m odel
calibration a n d validation extrem ely difficult.
T he various com ponents of th e simulation sy stem s h av e unknown uncertainty d u e
to u ncertainties in sed im en t tra n sp o rt calculations or o b serv atio n s. In particular
it is not know n if th e sy ste m a s a w hole, a n d especially th e W e s te rs c h e ld e
(B reskens/V lissingen to, say , th e D utch-B elgian border) is gaining or losing
sedim ent. This is a key a sp e c t of th e overall system d ev elo p m en t a n d in th e view
of th e expert te a m c a s ts very se rio u s d o u b ts on th e LTV p ro p o sal th a t sed im en t
from crossings should b e relocated se a w a rd of a B reskens-V lissingen line rath er
than within th e W e s te rsc h e ld e e stu a rin e system .
2. Field O b serv atio n an d E xperim entation.
M any w orkers ex p erien c e d in field o b serv atio n of fluvial an d estu arial p h e n o m e n a a re
also u se rs of numerical simulations and so recognise th e difficulties fa c e d by sim ulations
d u e to th e com plexity of th e natural sy stem . T hey re c o g n ise th e limitations on th e
reliability of field o b serv atio n s of sed im en t tra n sp o rt a s m uch a s th e u n certain ties in
sim ulations. N e v e rth e le ss field o b serv atio n s, which include bathym etric d a ta, flow an d
sedim ent transport observations, observation of th e p attern s of b ed-form s or th e internal
s tru c tu re of s a n d b a n k s, c h an n el w alls, c ro ssin g s, etc, provide crucial information on
w hat actually occurs a s o p p o sed to w hat would h appen according to theory. T h e s e ty p es
of field o b se rv a tio n s form th e b a sic platform for any u n d erstan d in g of th e m echanistic
fram ew ork within which m orphological d ev elo p m en t o ccu rs. Direct o b serv atio n is now
possible with a d e g re e of resolution, sophistication, convenience, econom y and versatility
not im agined 20 y e a rs ago. It offers a greatly im proved m ethod of u n d erstan d in g
m orphological p ro c e s s e s a n d check in g th e validity of sim ulations.
This su rg e in o b servational capability brings into realistic reckoning th e u s e of carefully
d e sig n e d experim ents to te s t e sse n tia l h y p o th e se s regarding s u c h fa c to rs a s tim escales, spatial-scales and intensity of sed im en t tran sp o rt, d isp ersa l of m aterial d red g ed ,
c h a n g e s in b e d m orphology on m odel tim e-step s c a le s an d th e m onitoring of th e local
m orphological im pact of particular relocation m e th o d s an d p ro g ram m es. T h e g re a t
a d v a n ta g e of su c h o b serv atio n s an d ex p erim en ts is th a t all p ro c e s s e s an d p ro c e ss
s c a le s a re included. No v ariab les a re om itted or simplified.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 21 o f 32
Future developments
For th e s e re a s o n s w e believe th a t m a n a g e m e n t m eth o d s n e e d to b e ev o lv ed using a
combination of field observation an d field an d laboratory experim entation in com bination
with num erical or physical sim ulation an d m orphological an aly sis an d interpretation.
T h e s e ap p ro ach es n e ed to b e placed in th e context of e x p erien ce in th e stu d y of natural
sy ste m s. Indeed, th e difficulties fa c e d by modelling req u ires th e b e s t p o ssib le
understanding of the morphological c h an g e s. Therefore, experts from different disciplines
m ust collaborate. As an ex am p le th e e x p ert te a m e m p h a s is e s th a t very little h a s b e e n
d o n e in th e p a st with th e available historical information to clarify th e detail of th e
evolution of th e W e s te rs c h e ld e m orphology during historical a n d arch aeo lo g ical tim es.
4.3
M an ag em en t of th e C ontem p o rary Situation
T h e deepening to 14 m com pleted in 1997 re p re se n ts the latest in a series of non-natural
m orphological d ev elo p m en ts. T h e re is a c o n s e n s u s technical view th a t th e sy ste m will
evolve tow ards a single channel sy stem , which, it is believed, will h a v e serio u s a d v e rs e
c o n se q u e n c e s for safety from flooding an d ecology and so th e re is a n e e d to institute an
in te g ra te d sedim entation m a n a g e m e n t stra te g y d e sig n e d to m aintain a m ultichannel
system . This requirem ent exists independently of any p ro p o sa ls to d e e p e n th e c h an n e ls
in th e W esterschelde. T h e proposal for adoption of a “do nothing” or “k e ep things a s they
a r e ” m a n a g e m e n t stra te g y is u n su sta in a b le in th e context of th e o b s e rv e d tre n d s in
estuarial evolution. T he stu d y a n d o th e r re co m m en d atio n s p re s e n te d below (S ection 5)
a re n e c e s sa ry w h a tev e r th e decision reg ard in g d eep en in g .
4 .4
R ole of D epoldering
Poldering h a s b een p ro g ressiv e. N atural lë v e e s w ere elev a te d an d co n so lid ated by th e
local people. H ow ever, th e history of poldering c o n sists in a p ro c e s s of building le v ee s
which could b e b re a c h e d during flood or sto rm s. T h e p ro c e ss of losing land, reclaim ing
it again and building other le v ee s w as re p e ate d many tim es. T ogether with th e cutting-off
of s e c o n d a ry c h an n e ls, it p ro d u ced in th e W e ste rsc h e ld e a m orphology which w a s a
com plex series of interconnected s e a b ran ch es. This evolved to w ard s a n e stu a ry w h o se
geom etry conveys th e tidal flow better than before. However, th e natural tren d m a d e th e
shoals an d bars silt up and m ain c h a n n e ls d e e p e n . In th e a b s e n c e of a further in c re a se
in tidal am plitude with time, th e tidal prism would h a v e d e c re a s e d . T his would h a v e
re d u c ed th e tidal “pum ping”, leading th e W e s te rsc h e ld e to evolve to w ard s a single
channel system . Fortunately, th e steadily d e e p e r penetration of tides into th e e stu a ry an d
the in crease in tidal am plitude in th e W estersch eld e, co m p en sated for th e loss of s to ra g e
by siltation. Land reclam ation by poldering w a s reducing th e sto ra g e a re a - not th e
volum e - sin ce it e n h a n c e d th e tidal intrusion.
S o m e m e asu re s w ere p ro p o sed to alleviate th e increasing flood risk a ss o c ia te d with th e
stro n g e r penetration of th e tidal w av e. D epoldering is o n e of th e s e . R ecently,
reconnecting the O o stersch eld e to th e W e ste rsc h e ld e w a s a lso co n sid e re d , but only for
th e hig h est flood levels, effective mainly during ex trem e sto rm s.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 22 o f 32
Future developments
S o m e a re a s se le c te d for depoldering w e re se c o n d a ry c h a n n e ls which h a d b e e n cut-off
(Fig. 4.4-01). This might s e e m logical, bu t m eanw hile, th e active ch an n el lay-out h a s
ch an g ed and there is no special re a so n for not depoldering a t o th er p laces. D epoldering
sh o u ld b e co n sid e re d not only to in c re a s e th e flood-w ater s to ra g e a re a , but also for
m odifying th e lay-out of th e le v ee s. Doing s o would influence th e flow a n d sed im en t
p a tte rn , correcting in so m e p la c e s th e plan-form of th e W e ste rsc h e ld e , w h e re it h a s a
neg ativ e im pact on th e d ev elo p m en t of th e ch an n els. This depoldering is to b e
c o n sid ered w here th e W e s te rs c h e ld e h a s its typical multiple ch an n el sy ste m , in Dutch
territory. D epoldering in th e Belgian territory would h a v e a s its objective fluvial flood
w ater storage, rath er th an changin g th e w ay le v e e s guide th e flood flow. T his m e a s u re
m u st b e studied in a m orphological p e rsp e c tiv e which is m ore th an ju st flow an d
sed im en t transport.
4 .5
Alternative D redging S tra te g ie s
It is th e view of th e expert te a m th a t hitherto dredging in th e W e s te rs c h e ld e w a s
o rg a n is e d with no or limited in te re st for steerin g th e overall e stu a rin e m orphology.
D um ping s ite s w ere se le c te d on th e b a s is of th e c h a n g e s in th e local m orphology,
favouring th e disposal of d re d g e m aterial in sec o n d a ry c h an n e ls, mainly th e flood
ch an n els.
For th e reach b etw een H answ eert a n d th e international D utch-B elgian border, it h a s
b e e n written in th e LTV reports th a t th e larg er volum es d u m p ed in th e flood c h a n n e ls
after 1970 resulted in th e dying out of th e s e flood ch an n els. In a m eeting b e tw ee n LTV
C luster M orphology an d th e Port of A ntw erp th e expert te a m , it w a s a g re e d to d is c u s s
th e evolution of th a t a re a. T h e c o m m e n ts p re s e n te d h e re after a re d e tailed in th e
A ddendum .
W ithout challenging th e idea th a t dum ping large am o u n ts of sed im en t in flood ch an n el
may have this effect, th e te a m c a m e to th e conclusion th a t o n e of th e likely re a s o n s for
th e s e flood channels to becom e le ss active w as the m orphological c h a n g e s in th e re a ch
betw een T erneuzen an d H answ eert. Indeed, by th e en d of th e sixties, th e O verloop v a n
H answ eert o p en ed completely, m aking th e G at v an O ss e n iss e th e flood c h an n e l in stead
of th e M iddelgat. W ith th e fixed points controlling th e m orphological c h a n g e s in th at
region - am ong which a re th e protruding jetties of th e harb o u r of H an sw eert - this
evolution oriented the flood flows m ore tow ards th e Plaat v an W alsoorden. T h e se a w a rd
tip of this b a r h a s re g re s s e d p rogressiv ely , so that, after p a ssin g H answ eert, th e flood
flow enters into a reach w here th e flow lo ses its erosive pow er b e c a u s e of a n ex p an sio n
in th e c ro ss section (Fig. 4.5-01).
T h e alternative relocation stra te g y w ould aim at resh ap in g specific a r e a s in which
m orphological c h a n g e s a re evolving to w a rd s a le ss fav o u rab le lay-out su c h a s , for
exam ple, a re d u c ed self scourin g of th e c ro ssin g s o r th e dying out of im portant
secondary channels. In th e c a s e of th e re d u c e d activity of th e S c h a a r v a n W a a rd e an d
S c h a a r van Valkenisse, the P laat v an W alsoorden n e e d s to b e developed so that it could
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 23 o f 32
Future developments
m ake a better bifurcation of the flood flow, guiding en o u g h flood-w ater to w ard s th e flood
c h a n n e ls. This strateg y requires a p ro g ram m e of careful investigation a n d monitoring,
m ore so p h isticated than th e p re s e n t d a ta acquisition program m e.
D re d g ed soils a re presently d isp o se d by h o p p e r d re d g e s in d e e p c h an n e ls. In th e
alte rn ativ e strategy, th e se d im e n ts w ould b e partly d isp o se d in shallow w ater, so th a t
other equipm ent would be n eeded. T h e sed im en t would b e rem o v ed from th e h o p p er by
m e a n s of a' floating line and brought to th e d isp o sal site. T h e tech n iq u e is perfectly
fe a sib le technically, an d would n e e d to b e a s s e s s e d econom ically. H ow ever, th e
ecological (morphological) benefit n e e d s to b e co n sid e re d w h en establishing th e
c ost/benefit ratio.
4 .6
Reversibility
T h e new, alternative strateg y m ust b e tried out a n d an obvious q u estio n c o n c e rn s th e
reversibility of th e p ro c e ss, in c a s e th e m orphological c h a n g e s would a p p e a r to b e
negative. This issue merits careful consideration. It m ust b e clearly p la ce d in th e co n tex t
of leading th e W e ste rsc h e ld e to w ard s a m ore fav o u rab le m orphological situation, in
te rm s of s a f e n e s s and n a tu ra ln ess. T h e a d v a n ta g e of recu rren t m e a s u re s (soft
engineering) v e rsu s training w orks (hard engineering) is th at th e recu rren t m e a s u re s a re
a d ju sta b le an d reversible. Hitherto, m orphological m a n ag e m e n t d ecisio n s regarding
coastal plain estuaries like the W este rsch e ld e h av e favoured h ard en g in eerin g solutions
like training w orks w h o se long-term im pact is uncertain. S o m e g ro y n e s or sp u r d y k es
w ere d esigned on the b asis of m odel te s ts with th e m orphological situation of th a t tim e.
D ue to m orphological c h a n g e s they a c t now in a very different situation. From th e
analysis m ad e by the expert team , it a p p e a rs that so m e of th e se training stru c tu re s h a v e
to d ay a n eg ativ e im pact on th e m orphological d ev elo p m en t but ch an g in g or rem oving
th e m d o e s not a p p e a r to b e c o n sid ered . T his a s p e c t should b e included in future
strateg ic considerations.
A key fa c et of future m a n a g e m e n t is th e initiation of a co m p re h en siv e monitoring
program m e. C om prehensive d o e s not m e an ex p en siv e, an d th e d a ta acquisition m ust
b e well thought out. T h e ex p ert te a m b e lie v es th a t this kind of monitoring is not only
n e e d e d but is both econom ically an d technically feasible, c o st effective an d beneficial.
4 .7
Possibility for Further D eep en in g
D ue to natural p ro c e s s e s , th e navigation d e p th s in th e W e s te rsc h e ld e h a v e in c re a se d
over the p a st millennia, a s th e tidal w av e p e n etrate d further an d m ore easily inland. T his
tre n d w a s e n h a n c e d by reducing th e lateral mobility of th e c h a n n e ls in th e e stu a ry by
poldering, bank protection an d cutting off seco n d ary channels. D ue to th e lack of reliable
historical data, it is not feasib le to m ak e an a c c u ra te e stim ate of th e evolution with tim e
of th e natural depth on th e c ro ssin g s. It w ould n o n e -th e -le ss b e w orth m aking a s e m iquantitative analysis of available b athym etric d a ta.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 24 o f 32
Future developments
T he evolution of th e first significant d eepening in th e early se v e n tie s is d o cu m en ted . T h e
available information should b e a n aly se d m orphologically, i.e. a s s e s s in g th e result of
d e e p e n in g by dredging with th e lay-out of th e c h a n n e ls a n d b ars. S u ch a n an aly sis is
n e e d e d to get m ore insight in th e sed im en t tra n sp o rt m e ch a n ism s an d either confirm or
n e g ate th e opinion s h a re d by m any sp ec ia lists of th e W e s te rs c h e ld e th at d e ep e n in g of
th e c ro ssin g s a s su ch would not influence negatively th e ongoing trend, th e o n e th a t
would occur anyhow, ev en without any dredging (the so -called “au to n o m o u s” evolution).
Scientific opinion is divided ab o u t th e im pact of dum ping th e in creasin g am o u n ts of
d re d g e m aterial, a s would b e required with a further d e ep e n in g .
T he studies conducted with m odels in Delft Hydraulics, d is c u s s e d in th e b a se lin e report,
lead to th e conclusion that there are limitations on th e volum e th a t c a n b e d u m p ed in th e
seco n d ary channels of th e W e ste rsc h e ld e . T h e expert te a m e x p re s s e d its re serv a tio n s
about the u s e of th e s e m odels. N onetheless, o n e idea put forward in LTV is to export th e
sedim ent to th e North S e a . This implies a enlargem ent of th e W e s te rs c h e ld e ’s geo m etry
in th e h y p o th e sis th a t th e e stu a ry h a s very little input of se d im e n t from o utside. In th e
a b s e n c e of reliable an d quality a s s u re d e v id en c e to th e contrary, th e export of d re d g e
material s e e m s d an g ero u s and possibly harm ing th e environm ent by d eg rad atio n of th e
sh o als, b ars, slikke an d schorre.
If export of sedim ent could lead to u n fo reseeab le morphological evolutions, redistribution
of th e sed im en t within th e e stu a ry m u st b e continued. E x p erim en ts on a new dum ping
stra te g y , su c h a s th e o n e p ro p o sed for th e a re a b e tw ee n H an sw eert an d th e DutchBelgian border, should b e te ste d . If p ro v ed to b e beneficial for halting or slow ing dow n
th e ecological an d morphological d eg rad atio n of th e W e ste rsc h e ld e , this stra teg y could
th e n b e te s te d on a larger sca le .
A further deepening is feasible, if d e sig n e d a n d co n d u cted in th e fram e of a rem odelling
of th e e stu a rin e m orphology. It should b e d e sig n e d with th e objective of co n serv in g
n a tu ra ln ess, s a f e n e s s an d accessibility.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 25 o f 32
Recommendations
5
RECOMMENDATIONS
5.1
D ata Review
T h e b a se lin e stu d y so u g h t to identify th e g en eral fram ew ork within which th e
p ro p o sals for d e e p e n in g should b e exam ined. This stu d y identified a nu m ber of
primary issu e s which a re fundam ental to m orphological m a n ag e m e n t. T his study
a lso identified further a re a s w h e re m ore e x ten siv e an d m ore d etailed d a ta
a ssem bly, review an d analy sis is n e e d e d . T h e s e include:
T h e geology of th e S c h e ld e E stuary (bottom an d sub-bottom ) a s it m ay
affect th e ability of ch an n els to m igrate u n d er purely hydrodynam ic forces.
T his is b a sic to th e viability of v ario u s a p p ro a c h e s to predicting future
m orphology.
A detailed analysis of th e migration p a tte rn s of th e c h a n n e ls to identify
th o se a re a s which m ay not hav e b een c ro s s e d by c h an n e l m igration. This
links with a s s e s s m e n t of th e geology of th e e stu a ry an d o th e r m an -m ad e
h ard points.
An a s s e s s m e n t of tidal propagation for the period 1948 to 1958 in an effort
to d e te c t any signal from th e c h a n g e s in intertidal v o lu m es an d
distributions c o n se q u e n t upon th e flooding of previously p o ld ered lan d s in
1953. T his e x ercise will identify th e sensitivity of th e sy ste m to lo s se s in
intertidal a re a an d indicate the viability, or o th erw ise, of p o ssib le
depoldering projects.
T h e topographic evolution of th e s e a a re a s in front of th e e stu a ry narrow s
a t V lissingen/B resken s should b e an aly sed . It is now generally a c c e p te d
th a t th e S c h e ld e E stu ary e x te n d s well w estw ard from th e previously
c o n sid e re d limits at V lissingen. T h e evolving m orphology of this a re a is
re le v a n t to tidal prop ag atio n an d to th e overall se d im e n t supply into th e
“middle” estuary b etw een Vlissingen and Bath. No sy ste m a tic a n d detailed
analy sis of th e available d a ta h a s b e e n co m pleted
S e d im e n t T ype distributions. T h e m orphological re s p o n s e will b e
influenced by th e nature of sedim ent types, particularly lightly c e m e n te d or
o v e r c o n so lid ated sed im en ts having re s ista n c e to co n tem p o rary
hydrodynam ic conditions.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 26 o f 32
Recommendations
5.2
D ata Collection
A num ber of a s p e c ts of th e sy ste m which require u n d erstan d in g suffer from th e
lack of b asic d a ta . T h e s e include:
H ydrodynam ics
T he d e g re e to which tra n s v e rs e w a te r su rfa ce g rad ien ts influence en erg y
dissipation an d m ay influence sed im en t tra n sp o rt is unknow n sin c e th e
p atterns of w ater s u rfa c e elevation is only know n at th e e stu a ry m argins.
Modern survey techniques allow precise an d very rapid evaluation of su c h
factors.
S edim ent B udgets
S o m e a tte m p t m ust b e m a d e to estab lish w h e th er or not th e “m iddle”
estu ary is gaining or losing sed im en t, or is in b a lan c e . A careful
program m e of analysis an d m e a su re m e n t n e e d s to b e u n d e rta k en to look
a t this is s u e which is fu n d am en tal to th e m a n a g e m e n t philosophy which
m ay b e d ev elo p ed .
P a tte rn s of S ed im en t Circulation
T here is little doubt that sed im en t m o v es a ro u n d th e e stu a ry in a com plex
m anner. T he pathw ays followed by the sed im en t h a v e b e e n e v a lu a te d by
a n u m b er of qualitative a n d highly theoretical m eth o d s. H ow ever, this
factor is crucial to th e d ev elo p m en t of m a n a g e m e n t tools an d th e d esig n
of m anagem ent strateg ies. This study will require a ra n g e of te c h n iq u e s to
b e d e p lo y ed in o rd er to first d ev elo p a qualitative u n d erstan d in g of th e
sed im en t tra n sp o rt p attern an d su b se q u e n tly a q u antitative e stim a te of
fluxes along key se c to rs. S u c h a study would underpin th e w hole of any
m orphological m a n a g e m e n t strateg y .
5.3
Monitoring
R eg u lar monitoring of both dredging a re a s an d d isp o sal site s is a b a sic
input to all a s p e c ts of study.
Annual e stu a ry w ide bathym etric su rv e y s should b e u n d e rta k en an d
com pleted within a period of no m ore than 2 w e ek s. T his is p o ssib le using
m odern Lidar a n d S w a th e B athym etry tech n iq u es.
Tidal o b serv atio n a n d an aly sis should include M id-estuary d a ta points.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 27 o f 32
Recommendations
5.4
S tu d ies
A c o m p re h en siv e study p ro g ram m e is esse n tial to a c h ie v e th e b e st p o ssib le
u n d e rstan d in g of how th e W e s te rs c h e ld e ’s m orphology d ev elo p s. Four
a p p ro a c h e s a re n e ed e d :
•
detailed analysis of historical re c o rd s an d d a ta
•
field investigations an d field experim entation
•
s c a le or physical modelling,
•
m athem atical or num erical m odelling
A nalysis of historical d a ta sh o u ld re c eiv e a high priority, starting with settin g up
an international d a ta b a s e , with a G eo g rap h ic Information S ystem .
T he field studies a re m uch m ore th a n th e d a ta collection for feeding th e m odels,
or than the monitoring n e e d e d to collect d a ta for a s s e s s in g th e im pact of w orks.
T h ey include field investigations for co m p ariso n with stu d ie s on historical d ata,
acquisition of new d a ta an d specific ex p erim en ts to te s t u n d erstan d in g of
p ro c e sse s a s well a s c h e c k m odel c o n c e p ts or resu lts. T h e stu d y w ork n e e d s to
b e a ss ig n e d to a n international (T h e N eth erlan d s & F landers), multi-disciplinary
team .
Field investigations should b e s ta rte d on a re a s of direct interest, su c h a s around
Vlissingen (for the new C ontainer Term inal project) o r W also o rd en - W a a rd e (for
th e selection of dum ping sites). M odern field d a ta acquisition tools n e e d to b e
applied, a m o n g st o th ers, to m e a s u re d a ta th a t a re n o w ad ay s m ore often
p ro d u ced only by m odels:
ADCP or O SCAR for current m easu rem en ts (quantitative) an d s u s p e n d e d
particulate m atter (qualitative)
R em ote se n sin g (satellite im ag es, also ra d a r im ages)
S pecial float te c h n iq u e s for m easu rin g s u rfa c e flow p attern (D G PS
positioning)
D G P S to g e th e r with tide g a u g e s for longitudinal an d tra n s v e rs e w ater
s u rfa c e slo p e m e a s u re m e n ts
S e d im e n t sam pling, both s u s p e n d e d load, b e d load a n d n e a r b e d
transport, an d sed im en t siz e analysis
S o n a r an d s e a b e d classification sy ste m s for description of sed im en t
distributions
sed im en t tra n sp o rt z o n e s a n d relative sed im en t tra n sp o rt intensities.
T his list is not e x h au stiv e a n d n e e d s to b e d is c u s s e d with th e co m p eten t
authorities.
S cale an d num erical m odels sh o u ld b e u s e d to v isu alise an d a n a ly se th e typical
three-dim ensional flow m ech an ism s an d th e w ay they affect sed im en t m ovem ent.
To sta rt with, this c an b e d o n e on a fixed-bed m odel, a s th e o n e existing in
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 28 o f 32
Recommendations
F la n d e rs H ydraulics. S pecial te c h n iq u e s m ay help analy sin g th e distribution of
s h e a r s tre s s in relation to th e flow p attern . T h e sc a le effects induced by th e
distortion of th e m odels h a v e to b e a s s e s s e d carefully, but distortion a s su c h m ay
b e a c c e p ta b le for sem i-quantitative investigations.
N um erical m odels n e e d to b e e v a lu a te d thoroughly. It might b e useful to h a v e
m ore th a n o n e m odel u s e d to c o m p a re results. S e v e ra l tw o- a n d th reedim ensional w e re d e v elo p e d in v ario u s E u ro p ean countries.
5.5
Trial of A lternative S tra te g ie s
Using appropriate d a ta small sca le te s ts of alternative m a n a g e m e n t s tra te g ie s for
particular c ro ssin g s sh o u ld b e p la n n ed an d ex ecu ted . T his w ould involve both
monitoring of th e dredging an d disposal, th e related m ovem ents of se d im e n ts and
c h a n g e s in m orphology.
5.6
Institutional A sp e c ts
It is c lea r th a t m orphological m a n a g e m e n t h a s to b e a p p ro a c h e d on an e stu ary
wide b a sis an d n e e d s to in teg rate an appreciation of all th e relev an t activities in
the system . T his will n e e d th e full collaboration of all authorities having in terests
in th e S c h e ld e E stuary. An international collaboration b e tw ee n th e N eth erlan d s
a n d Belgium m ust b e furthered. T h e p lan n ed reo rg an isatio n in th e Flem ish
adm inistration o p e n s new possibilities for improving th e internal collaboration,
setting up integ rated d a ta b a s e s an d conducting relev an t stu d ie s.
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 29 o f 32
Conclusions
6
CONCLUSION
D eepening th e channel to A ntw erp is feasib le. To s e c u re th e aim s of th e LTV project it
will require an integrated program m e of stu d y an d monitoring. T h ere are no technological
barriers to monitoring relev an t p ro c e s s e s b u t th e re is a n e e d to p la ce existing or future
morphological m anagem ent strateg ies on a s e c u re technical and factual b a s e . T his b a s e
would involve an understanding of th e se d im e n t tran sp o rt in th e W e s te rs c h e ld e sy ste m
a s a w hole, which includes th e V lakte v a n d e R aan , th e integration of all sed im en t
removal and relocation strateg ies into th e morphological and environm ental m a n ag e m e n t
strateg y a n d th e monitoring of existing a n d future p ro c e d u res an d effects.
D evelopm ent of the m an ag em en t stra teg y will require th e definition of ta rg e ts for safety
by th e reduction in tidal levels, for environm ent by creation or redistribution of habitat an d
for navigation by optimisation of channel m ain ten an ce and all o th er dredging p ro c e d u re s
an d their integration into th e m orphological an d ecological m a n a g e m e n t p ro c e s s e s .
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 30 o f 32
References
R E FE R E N C E S
AMINAL - RIK2 - ZMF (1995). D e S c h e id e , e e n stroom n atu u rtalen t (in Dutch)
ARENDS, A., & A. LANGERAK (2000). C a u s e s an d effects of high w a ter rise in
th e W este rn S cheldt e stu ary , MAFF (UK) co n fe ren c e for c o asta l an d river
e n g in e e rs (in English)
COEN, I., J.J. PE T E R S & P. R O O V ER S (1977). M ateb a 14 - M ethod to m a n a g e
d red g in g o p eratio n s on th e b a sis of an aly sis of m orphologic evolutions applications, report Hydraulic L aboratory B orgerhout (in French)
C O EN , I. (1988). O n tsta a n e n ontwikkeling van d e W e ste rsc h e ld e , in “D e
S ch eld e, T o e g a n g tot A ntw erpen”. Tijdschrift W ATER, Nr. 43/1 (in Dutch)
M INISTERIE VAN DE VLAAMSE G EM EEN SCH A P. O verzicht v an d e
tijw aarnem ingen in het Z e e sc h e ld e b e k k e n (in Dutch)
ECK, van, B. (1999). D e S ch eld eA tlas, e e n b eeld v a n e e n estu ariu m (in Dutch)
HEIDEMIJ - RIKZ - RA (1996). H erstel N atuur W e ste rsc h e ld e - A lternatieven (in
Dutch)
MOL, G. (1995). De W e s te rs c h e ld e : e e n re su ltaa t van m enselijke ingrepen.
R ap p o rt RIKZ-95.030. R ijk sw aterstaat, Rijksinstituut voor Kust en Zee/RIKZ,
M iddelburg (in Dutch)
PAUW , De, N. & J .J. P E T E R S (1973). C ontribution to th e stu d y of th e salinity
distribution an d circulation in th e W e ste rn S ch eld t E stuary, C om m ission
Interm inistérielle d e la Politique Scientifique. P ro g ram m e N ational B eige R e c h e rc h e s et D éveloppem en t s u r l'E nvironnem ent P h y siq u e et Biologique.
Projet Mer : 1-58 (in French)
PE T E R S , J .J . (1975) . M éc a n ism e s d e m élan g e d e s eau x d a n s l’e stu a ire d e
l'Escaut, (W ater mixing m ech an ism s in th e Scheldt estuary); A nnales d e s Travaux
Publics d e B elgique : 2-1975 :1 0 1 -1 2 0 (in French)
P E T E R S , J .J . & A. STERLING (1976). H ydrodynam ique e t tra n sp o rts d e
sédim ents d e I'estuaire d e I'Escaut, (Hydrodynamics and sedim ent transport in th e
S c h e ld t estuary); C om m ission Interm inistérielle d e la Politique Scientifique.
Program m e National B eige - R e c h e rc h e et D éveloppem ent su r l’E nvironnem ent
P h ysique et Biologique. Projet M er : 1 0 : 1-70 (in French)
P E T E R S , J .J . & R. W OLLAST (1976). R ole of th e sed im en tatio n in th e self­
purification of th e S ch eld t e stu ary , Third F ed eral Inter-Agency S ed im en tatio n
Conf. - D enver (USA), M arch 2 2 -2 5 , 1976 : 3 : 77-86 (in English)
PETER S, J.J. & R. W OLLAST (1980). T ran sfe r of m aterials in e stu a rin e z o n e s.
Sym posium on T ran sp o rt P r o c e s s e s in E stu arin e an d N ear-S h o re Z o n e s,
International Council for th e Exploration of th e S e a (ICES), 68 thStatutory M eetingC o p en h a g e n (in English)
P E T E R S , J .J . & F. W E N S (1991). M ain ten an ce dredging in th e navigation
c h a n n e ls in th e Z aire inner delta, C O P E D E C III C o n feren ce, M om basa, 16 20/09/1991 (in English)
PE TE R S, J.J. & A. STERLING (1975). M ateb a 12 - M ethod to m a n a g e dredging
o p eratio n s on th e b a sis of an aly sis of m orphologic evolutions (in French)
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 31 o f 32
References
PIETERS, T., C. STORM, T. WALHOUT, & T. YSEBAERT (1991). H et S ch eld eesiuarium , m é ér d a n e e n v aarw eg . N ota G W W S -91.081. R ijksw aterstaat,
Rijksinstituut voor K ust en Zee/R IK Z e n Directie Z e e lan d , M iddelburg (in Dutch)
SCHELDE INFORMATIE CENTRUM (1999). De ScheldeA tlas, e e n beeld van e en
estu ariu m (in Dutch)
SCHNEIDER R.R., E. BARD an d A.C. MIX (2000). Last lee Age global o c e a n and
land su rfa ce te m p e ratu res: T h e EPILOG initiative, in IGBP N ew sletter 43 (in
English)
SHENNAN I. (1987). H olocene sea-level c h a n g e s in th e North S e a , in “S ea-lev el
C h a n g e s ”, ed. TOOLEY M.J. & SHENNAN I. (in English)
VEEN, van, J . (1950). Eb- e n v lo e d sc h a a r s y ste m e n in d e N e d erlan d se
getijdew ateren, Koninklijk N ed erlan d s A ardrijkskundig G e n o o tsc h a p (in Dutch)
WARTEL, S., F. DE MEUTER & A. RINGELE (1983). N ote concerning th e origin
of th e S cheldt e stu a ry bottom sed im en ts, Bull. K. Belg. Inst. Nat. W et. 55-1 (in
English)
Improving navigation conditions in the Westerschelde and managing its estuarine environment
Page 32 o f 32
Illustrations
0-01
The southern part of the Thine, Meuse and Schelde delta
PUTTEN
WAARD
1HOLEN
\
f :
TOELICH TING
VLAANDEREN
I
Improving navigation conditions in the W esterschelde a n d managing its estuarine environm ent
page FI/23
Illustrations
0-02
Topo-bathymetry of the Westerschelde, situation 1992
WALCHEREN
VEERSE MEER
THOLEN
Middelburg
Kanaal d oor W alcheren
ZU D - BEVELAND
Kanaal d o o r
Zuid - B evelaridx
Vlissinge
olenglaapO,
Platen van
Ossemsse
\/s/ieVtnge0
S ch eld e Rijn Kanaal
H ooge Platen
a a t v a r r = ^ 3 /*K
Walsoorden
Springergeul
& N o o rd
Platen v a n \ / ^ ^
Valkenissèi
^
ge Spnnger
M iddelm aat
schor
T ern eu zen
V erdronken Land van S a eftin g e
ZEEUWS-VLAANDEREN
Kanaal van G en t
naar Terneuzen
NEDERLAND
Dieptegegevens t.o.v. NAP (m ) 1 9 9 2
BELGIE
10 km
from figure IA in De ScheldeAtlas, een beeld van een estuarium , 1999, Schelde Inform atie C entrum
Improving navigation conditions in the W esterschelde a n d managing its estuarine environm ent
page F2/23
Illustrations
2.1-01
Eustatic sea level rise since 21 kyr BP
o
o
o
o
o
o
O '
00
o
o
o
c-
o
o
o
vO
o
o
o
m
o
o
o
O
O
o
m
o
o
o
CM
O
O
O
pH
5
0
S id e r e a l A g e
-5
-10
-1 5
-20
-2 5
-5 0
-7 5
-1 0 0
m oving average
A lti tu d e (m )
-1 2 5
Data between -21 kyr and -9 kyr from SC H N EID ER R.R., E. BA RD and A.C. M IX, 2000,
Last lee Age global ocean and land surface tem peratures: T he EPILO G initiative, in 1GBP Newsletter 43
Data between -8.5 kyr and -1 kyr from SH EN N A N L, 1987, Holocene sea-level changes in the North Sea,
in “Sea-level Changes”, ed. TOOLEY M.J. & SH EN NA N 1.
Improving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page F3/23
Illustrations
2.1-02 The North Sea at the end of the Pleistocene
Noordzee
D o g g e rsb a n k
S c h e ld e
M aas
Ref.: De Schelde, een stroom natuurtalent, 1995, published by A M IN A L - R IK Z - ZM F
Im proving navigation conditions in the Westerschelde a n d m anaging its estuarine environment
page F4/23
Illustrations
2.1-03 The North Sea and Rhine-Meuse-Schelde “delta” in Roman times
W insum
Koudum
ifn n g en
Vollenhove
f
'
hlleg ersberg
S
• Donken
M
,
I
after VAN VEEN, 1950. Eb- en vloedschaar systemen in de N ederlandse getijdwateren,
Koninklijk Nederlands A ardrijkskundig Genootschap
Im proving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page F5/23
Illustrations
2.1-04 The Schelde area around 1 kyr BP
Duinen
0
1 2 .5
2 5 km
S ch orren
H oge zandgronden
H oogveen
from figure 7 in De ScheldeAtlas, een beeld van een estuarium , 1999, Schelde Inform atie Centrum
Im proving navigation conditions in the W esterschelde and m anaging its estuarine environm ent
page F6/23
Illustrations
2.1-05 Evolution of tidal amplitude along the Schelde from 1550 till 1970
tid a l a m p lit u d e (m )
5
4
3
2
1
0
\
o
CO
d i s t a n c e f r o m m o u th (k m )
100
50
150
from COEN 1., 1988, O ntstaan en ontw ikkeling van de W esterschelde, Tijdschrift WATER 43/1
2.1-06 Water levels in Antwerp since -2 kyr BP
j
s to rm tid e s
w a t e r e le v a ti o n s (m )
¡HP
lí
/
i-
lí
'/
' ••
„.e e\e
Yd
s
..
lo w w a t e r
fluvial re g im e
tra n s itio n
tid e -in flu e n c e d
-l
500
1000
1500
p re se n t
from Fig. 10 in COEN 1., 1988, O ntstaan en ontw ikkeling van de W esterschelde, Tijdschrift W ATER 43/1
Improving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page F7/23
Illustrations
2.2-01 Land reclaimed in the Westerschelde at different times since the Middle Ages
W ALC H E R EN
Z U ID -B E V E L A N D
»
r
□
□
□
voor de 1 3 e eeuw
1 3 e eeuw
1 4 e eeuw
■
■
■
1 5 e eeuw
I k e eeuw
1 7 e eeuw
■
■
■
10 km
18e eeuw
1 9 e eeuw
2 0 e eeuw
from: De ScheldeAtlas, een beeld van een estuarium , 1999, Schelde Inform atie Centrum
Zones reclaim ed before and during the indicated century; the figure does not represent the situation o f polders
at that moment, because some reclaim ed areas were lost later on during storms
Improving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page F8/23
Illustrations
2.2-02 The morphological changes of the Westerschelde from the 17th Century to 1997
Cm A
1636
1800
1865
original figure from PETERS J.J. and A. STERLING, 1976, Hydrodynamique et transports de sédiments de l’estuaire de l’Escaut.
ICWB-C1PS Projet Mer, Vol. 10. T his figure from 1800 to 1972 was completed with the figures from the 17th C entury (see 2.5-02)
5 m e te r
8 m e te r
and the situation of 1997, courtesy Flanders Hydraulics
Im proving navigation conditions in the W esterschelde and m anaging its estuarine environment
GLLWS
page F9/23
Illustrations
2.3-01 Increase of mean high water at Vlissingen relative to the mean sea level
io
y e a rl y in c r e a s e (cm )
8
6
4
2
0
2
■4
1900
1910
1920
1930
1950
1960
1970
1980
1990
2000
1990
2000
2.3-02 Increase of mean high water at Bath, relative to the mean high water at Vlissingen
25
y e a rly in c r e a s e (cm )
20
15
10
5
0
1900
1910
1920
1930
1940
1950
1960
1970
1980
from A REN DS, A., & A. LA N G ER A K , 2000. C auses and effects o f high water rise in the W estern Scheldt estuary;
paper presented in 1999 on the M A FF (U K) conference for coastal and river engineers
Improving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page F10/23
Illustrations
2.4-01 Longitudinal salinity profile and its shift with tide and river discharge
ci %
15
w ith tid e = ^ = 5 ^
s la c k h ig h w a t e r
s la c k lo w w a t e r
w ith riv e rd iso b a rg e
d is ta n c e f r o m m o u t h (k m )
0
50
100
150
from Fig. 7, in PETERS J.J. and A. STER LIN G , Hydrodynamique et transports de sédim ents de l’estuaire de l’Escaut.
IC W B-CIPS Projet Mer, Vol. 10
Im proving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page FI 1/23
Illustrations
2.5-01 Evolution of the Westerschelde between Terneuzen and Walsoorden from the 17th Century to 1997
1865
i
\
1938
1 9 9 5 -9 7
1972
GLLWS
5 m e te r 8 m e te r ■
excerpt from Fig. 2.2-01
Improving navigation conditions in the W esterschelde a n d m anaging its estuarine environment
page F12/23
Illustrations
2.5-02 Map of the Westerschelde in the 17th Century
ü ..» * * *
s
z
¿>
ih
f
l
M
i
e
£
%
M
î
s:
\
s
V
T p s
Ref.: MERCATOR-HONDS1US-JANSSONIUS Atlas or a Géographie Description of the World, 1636, A m sterdam
Improving navigation conditions in the W esterschelde a n d managing its estuarine environm ent
page F I3/23
Illustrations
2.5-03 Bottom sediment size
V lissin g e i
T e rn e u z e n
175
—
N A P -2 m
200
( in d ic a tie in te rg e t ijd e g e b ie d e n )
250
300
>300
□
g e e n m e tin g e n
v e rric h t
from Fig. 32 in De ScheldeAtlas, een beeld van een estuarium , 1999, Schelde Inform atie Centrum
2.5-04 Silt content in bottom sediments
V lis sin g e i
385
380
375
G egevens 1 9 9 4
T e rn e u z e n
S lib g e h a lte ( % )
—
N A P -2 m
50
100
( in d ic a tie in te rg e tijd e g e b ie d e n )
I
I
1 0 km
g e e n m e tin g e n
v e rric h t
from Fig. 33 in De ScheldeAtlas, een beeid van een estuarium , 1999, Schelde Inform atie Centrum
Improving navigation conditions in the Westerschelde and m anaging its estuarine environment
page F14/23
Illustrations
2.5-05 Clay as possible morphological control points
F L U S H IN G
b à -a l - h o e k
.
OOSTERWEEL
^AN TW ER P
BURCHT
h e m i k s e m
PLIOCENE
LiMo F o r m a t ion
MIOCENE
B erc h e m Form ation
:
OL I G O C E N E
B o o m Clay
Top of B o o m Cl ay
T o p o f S a n d s o f M e r k s e m l m e t e r s b e l o w N.A.P )
( m e t e r s b e l o w N.A.P
Ref.: W ARTEL, S., F. DE M EU TER and A. R IN G ELE, 1983. Note concerning the origin o f the Scheldt estuary bottom sediments,
Bull. K. Belg. Inst. Nat. Wet. 55-1
Im proving navigation conditions in the W esterschelde a n d m anaging its estuarine environm ent
page F15/23
Illustrations
2.6-01 Zeeland 0.7 kyr BP
______
^
a
a
ZEEDYK z Ltuii'L iv --- •—
_____
.
t
i SCGUDES
A
’ •2 A
_
NORTBEVELANT
V O IR T Z F .E
---------
'A. A
•Vv
_ _
S trien e
oS teen b ergh en
A
/V
r^ rÀ ^ 'A L G H E R E N
^ ^ A M id d e lb u rch
Berghen
S .A
ZUITBE V E L A N T
IV iciinghe :
WULPEN
K aed san t
y. ^
¿ ^ 'C O E S A N T
^
Î oL
V a lk enesse
_________
Home
n tv liet
. f Oostbur
* " Kn
i Cr \
E im a n
Hulst
Vpv
A xel
Stuvesant
Brugghe
A n tw e r p e n
from Fig. 5 in COEN I., 1988, O ntstaan en ontw ikkeling van de W esterschelde, Tijdschrift W ATER 43/1
Im proving navigation conditions in the W esterschelde a n d m anaging its estuariae environm ent
page F16/23
Illustrations
3.2-01
Time series of annual dredged volumes from 1895 to 1998
v o lu m es in m 3 x IO6
io
to ta ls
N e th e rla n d s
B elgium
h-1
CO
CO
sO
t—1
I—*
vO
I—i
sO
nO
I—*
«O
sO
sO
The lines with 3-year moving average data indicate the broad trends
3.2-02 Percentage of volumes dredged in Belgium and in The Netherlands
0%
100%
75%
25%
50%
50%
25%
75%
100%
0%
M
CO
-O
CD
I—i
o
vO
>0
sO
O'
sO
CD
-o
vO
ro
o
from data AWS
Im proving navigation conditions in the Westerschelde a n d m anaging its estuariae environm ent
page F17/23
Illustrations
3.3-01 High water and low water at selected stations between Vlissingen and Antwerpen - Changes and rates of change
5.25
Vlissingen
5.25
M ean high w ate r/M ean low w a te r
height (m)
5 .0 0
Terneuzen
5.25
M ean high w ate r/M ean low w a te r
h eig h t (m)
5 .0 0
H an sw eert
5,25
M ean high w a te r/M e a n low w a te r
h eig h t (m)
5 .0 0
B ath
M ean high w a te r/M e a n low w a te r
h eig h t (m)
5 ,0 0
4,75
4,75
4,75
4,75
4,50
4,50
4,50
4,50
4.25
4.25
4.25
4,25
4 .0 0
4 .0 0
4 .0 0
0,75
0,75
0,75
0,75
0,50
0,50
0,50
0,50
0,25
0,25
0,25
0,25
0,00
0,00
0,00
0,00
co
^ ^
H edw igpolder-P rosperpolder
5,25
M ean high w ate r/M ean low w ate r
height (m)
5 ,0 0
Li Ilo-Liefkenshoek
5,25
M ean high w a te r/M e a n low w a te r
h eig h t (m)
5 ,0 0
S u m m a ry
A ntw erpen
M ean high w a te r/M e a n low w a te r
heig h t (m)
5 ,0 0
r\j
o
co
Antwerpen
, Lillo-Liefkenshoek
M ean high w a te r/M e a n low w a te r
h eig h t (m)
5 ,0 0
Bath
4,75
4,75
4,75
4,50
4,50
4,50
4,25
4,25
4,25
4,25
0,50
0,50
0,50
Vlissingen
4 ,0 0
0,75
0,25
0,25
0,00
CO
-o
V lissingen
* M ean High W a ter
» M ean Low W ater
0,00
I—
'
sO
CO
CO
I—
'
sO
i—
1
nO
M
sO
—
M ean High W ater
CO
— M ean Low W ater
M
CD
I—
'
* M ean High W ater
* M ean Low W ater
H ed w ig p o ld er-P ro sp erp o ld er
►M ean Low W ater
I—
'
CO
I—
'
sO
ro
I—'
o
sO
B ath
0,05
* M ean High W a ter
ra te of ch an g e in heig h t
(m /year)
0,025
■M ean High W ater
CO
sO
H an sw eert
0,05
ra te of ch an g e in heig h t
(m /year)
Lillo-Liefkenshoek
0,00
h-1
sO
T erneuzen
0,05
ra te of chang e in height
(m /year)
Bath
0,25
0,00
CO
Vlissingen
Terneuzen
Hansweert
» M ean Low W a ter
0,05
ra te o f ch a n g e in h eig h t
(m /year)
0,025
0,025
-0,025
-0,025
-0,025
-0,025
-0,05
-0,05
-0,05
-0,05
Lillo-L iefkenshoek
0,05
ra te of chan g e in height
(m /year)
• M ean High W ater
* M ean Low W a ter
A ntw erpen
0,05
■M ean High W ater
ra te of change in heig h t
(m /year)
■M ean Low W ater
* M ean High W a te r
►M ean Low W a ter
0,025
0,025
0,025
-0,025
-0,025
-0,025
-0,05
-0,05
-0,05
from data AWS
Improving navigation conditions in the Westerschelde a n d managing its estuariae environment
page F18/23
S u m m a ry
0,05
rate of ch an g e in heig h t
(m /year)
0,05
ra te o f ch an g e in h eig h t
(m /year)
0,025
W
-0,025
-0,05
Illustrations
3.3-02 Mean tidal range at selected stations between Vlissingen and Antwerpen - Changes and rates of change
Vlissingen
(m)
Terneuzen
M ean tid a l ran g e
(m )
Bath
H an sw eert
(m)
M ean tid a l ran g e
M ean tid a l ran g e
(m)
5 ,0 0
5 ,0 0
5 ,0 0
5 ,0 0
4,50
4,50
4,50
4,50
4 ,0 0
4 ,0 0
4 ,0 0
4 ,0 0
3,50
3,50
3,50
3,50
H edw igpolder-P rosperpolder
(m)
L illo-L iefkenshoek
M ean tid a l ran g e
(m)
5 ,0 0
A n tw erpen
M ean tid a l ra n g e
S u m m a ry
Antwerpen
M ean tid a l ran g e
(m )
5 ,0 0
(m)
M ean tid al ran g e
5 ,0 0
M ean tid a l ra n g e
Lillo-Liefkenshoek
5 ,0 0
Prospel
Bath
4,50
4,50
4,50
4,50
Hansweert
Terneuzen
4 ,0 0
4 ,0 0
4 ,0 0
4 ,0 0
Vlissingen
3,50
3,50
3,50
3,50
CO
V lissingen
Terneuzen
0,1
R ate of ch a n g e in m ean tid al ran g e
(m /year)
R ate o f ch a n g e in m ean tid al ran g e
co
>o
n
O
sO
H an sw e ert
0,1
(m /year)
R a te of ch a n g e in m ean tid al ran g e
co
CO
■£>
B ath
0,1
(m /year)
R ate o f ch a n g e in m ean tid a l ran g e
0,1
(m /year)
0,05
0,05
0,05
0,05
-0,05
-0,05
-0,05
-0,05
- 0,1
- 0,1
-0 , 1
- 0,1
sO
00
o
H ed w igpolder-P ro sp erp o ld er
L illo -L iefk en sh o ek
0,1
R ate of ch a n g e in m ean tid al ran g e
ro
o
(m /year)
o
o
O
o
o
A ntw erpen
0,1
R ate o f ch a n g e in m ean tid al ran g e
V0
S u m m a ry
0, 1
R ate of ch a n g e in m ean tid a l ran g e
R a te o f ch a n g e in m ean tid a l ra n g e
0,1
(m /y ear)
0,05
0,05
0,05
0,05
-0,05
-0,05
-0,05
-0,05
- 0, 1
- 0,1
-0,1
- 0 ,1
from data AWS
Improving navigation conditions in the W esterschelde and managing its estuarine environment
p age F19/23
Illustrations
3.3-03 Duration of flood and ebb tide at selected stations between Vlissingen and Antwerpen - Changes and rates of change
7:30
Vlissingen
7:30
Duration of tide phase
(hrs)
7:30
ebb
6:30
6:00
flood i
6:00
5 :0 0
>0
I—
'
sO
xO
7 :0 0
6:30
6:30
ebb
flood
6:00
6:00
flood :
00
H-1
sO
x£>
sO
ISJ
I—
I
sO
I—1
O
sO
00
Lillo-Liefkenshoek
Duration of tide phase
7:30
CO
M
M
>o
xO
xO
CO
7:30
CO
I—
I
sO
I—
I
>o
xO
i—
i
SÛ
Summary
Duration of tide phase
(hrs)
7 :0 0
M
o
Antwerpen
Duration of tide phase
(hrs)
7 :0 0
5 :0 0
5 :0 0
i—'
sO
7:30
5:30
5:30
I—<
Hedwigpolder-Prosperpolder
(hrs)
Duration of tide phase
(hrs)
ebb
ebb
5 :0 0
•£>
Duration of tide phase
7 :0 0
5:30
CO
Bath
flood
5:30
CO
7:30
(hrs)
7 :0 0
6:30
Hansweert
Duration of tide phase
(hrs)
7 :0 0
7
Terneuzen
Duration of tide phase
(hrs)
ebb
7 :0 0
ebb
ebb
Antwerpen
7 :0 0
6:30
6:30
6:30
6:00
6:00
6:00
Lillo-Liefkenshoek
Bath
Terneuzen
6:30
Vlissingen
Hansweert
flood
5:30
flood
5:30
5:30
flood
5 :0 0
5 :0 0
M
CO
N>
O
-D
sO
CO
CO
I—1
xO
sO
xO
ro\3
xO
xO
Vlissingen
Rate of change in tide duration
6:00
5 :0 0
i—1
CO
CD
00
xO
xO
h-1
xO
I—I
xO
0,5
CD
o
M
CO
xO
I—I
ro
o
xO
Bath
Hansweert
T erneuzen
(mln/year)
Rate of change in tide duration
(m ln /y ear)
ro
xO
xO
(mln/year)
Rate of change in tide duration
(min/year)
Rate of change in tide duration
0,5
0,5
0,5
ebb «
ebb
0
»
/
0
\
ebb
i-
¡t
0
n
SSS!Sili\ . lii
O
O
C
D
x
O
x
O
x
O
x
O
x
O
v
O
c o x o o i - ' r o v j j i u i
o
o
o
o
o
o
o
o
xO
-J
o
-0,5
-0,5
-0,5
-1
-1
-1
XÛ
00
o
Hedwigpolder-Prosperpolder
Rate of change in tide duration
•0,5
Antwerpen
Lillo-Liefkenshoek
Rate of change in tide duration
(m ln /y ear)
Rate of change in tide duration
(m in/year)
Summary
Rate of change in tide duration
(m in/year)
(m in/year)
ebb
ebb
0,5
0,5
0,5
0,5
•0,5
•0,5
-0,5
-0,5
ebb
from data AWS
Improving navigation conditions in the W esterschelde and m anaging its estuarine environment
page F20/23
Illustrations
3.3-04 High water time interval at selected stations between Vlissingen and Antwerpen - Changes and rates of change
2:30
V lissingen - Terneuzen
2:30
M ean high w a te r tim e in terv al
(hrs)
2:30
V lissingen - H edw igpolder-P rosperpolder
(hrs)
V lissingen - H an sw e ert
2:30
M ean high w a te r tim e in terv al
(hrs)
2:00
2:00
1:30
1:30
1:30
1:00
1:00
1:00
0:30
0:30
0:30
0:00
0:00
0:00
V lissingen - L illo -L iefk en sh o ek
M ean high w a te r tim e in terv al
2:30
M ean high w a te r tim e in terv al
(hrs)
V lissingen - A n tw erpen
2:30
(h rs)« = = —
M ean high w a te r tim e in terv al
(h rs)
2:00
2:30
V lissin g en - B ath
S u m m a ry
M ean high w a te r tim e in terv al
M ean high w a te r tim e in terv al
(hrs)*®*»
2:00
2:00
2:00
2:00
1:30
1:30
1:30
1:30
1:00
1:00
1:00
1:00
0:30
0:30
0:30
0:30
Antwerpen
Lillo-Liefkenshoek
Hansweert
Terneuzen
0:00
0:00
0:00
0:00
I—•
I—'
CO
sO
>£>
I—'
sO
V lissingen - T erneuzen
R ate of ch a n g e in high w a te r tim e in terv al
(m in/year)
(m in/year)
O
sO
sO
from data AWS
Improving navigation conditions in the Westerschelde a n d m anaging its estuarine environm ent
page F 2I/23
SÛ
V lissingen - B ath
(m in /y ea r)
R ate of ch a n g e in high w a te r tim e interval
-0,5
•0,5
S u m m a ry
V lissingen - A ntw erpen
(m in/year)
: R ate of ch a n g e in high w a te r tim e in terv al
(m in/year)
R a te o f ch a n g e in high w a te r tim e in terv al
-0,5
-0,5
CO
sO
>o
-0,5
-0,5
I—I
N)
O
t—»
sO
0,5
-0,5
I—'
sO
0,5
0,5
CO
M
0,5
0,5
CO
co
sO
(m in/year)
R ate of c h a n g e in high w a te r tim e in terv al
(m in/year)
R ate o f ch a n g e in high w a te r tim e in terv al
sO
V lissingen - H an sw e ert
V lissingen - L illo -L iefk en sh o ek
V lissingen - H ed w igpolder-P ro sp erp o ld er
R ate of ch a n g e in high w a te r tim e in terv al
M
>o
CO
I—<
I—'
nO
sO
i—'
sO
M
CO
CO
i—»
sO
sO
o
Illustrations
4.4-01 Areas proposed for depoldering
H e r s t e l Notuur W e s t e r s c h e l d e
LEGENDA:
D
□
bebouwing ( k e r n e n en i n d u s t r i e g e b i e d e n )
tater
tra
schorren sl'kksfî sn plets
A..
B ..
r
buitendijkse maatregelen
ontpoldering
l o k a t i e s p r o j e c t e n , blad 1
biensedij nDs MiSctrsçsIs
School:
« e g en
0
10
i
w w aw w c=3w w
km.
Noord
h e id e m i j a d v ie s
Opdrachtgever:
S jonuori 1996
B8
Ref.: Herstel Natuur Westerschelde - Alternatieven, 1996, H EIDEM IJ - R IK Z - RA
Improving navigation conditions in the Westerschelde a n d managing its estuarine environment
page F22/23
Rijkswaterstaat, directie Zeeland
Project: 95010 (UWS)
Illustrations
4.5-01 Test area for alternative dredging strategy - Proposal PA Expert Team
Dieptegegevens (m) 1992
C hart data from R IK Z, situation 1992; original data in NAP, approxim ately transform ed in GLLWS by lowering with 2 m
Improving navigation conditions in the W esterschelde a n d m anaging its estuariae environm ent
page F23/23