How to harm onize accessibility, safetyness and naturalness?
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How to harm on ize a ccessib ility , s a fe ty n e ss and n a tu ra ln ess?
January 2001Improving 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 2001Foreword 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
1.1 Initial T erm s of R eferen c e a n d O b jectiv es ............................................. . 1
1.2 Linking to LTV ........................................................................................................ 1
1.3 Final R eport O bjectives ..................................................................................... 1
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 31EXECUTIVE 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 32Introduction
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 32Summary 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 32Summary 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 funnel- s 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).
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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 32Summary 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 ree-
dim 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 32Summary 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 plan-
form), 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 32Dredging 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.
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