DG Rijkswaterstaat/RIKZ

WL | delft hydraulics

Sediment budget analysis and testinghypotheses for the Dutch coastalsystem

November, 2006

Z4100

Report

DG Rijkswaterstaat/RIKZ

Prepared for:

VOP II-1.2 Long term coastal management and

DC05.20 North Sea and Coast

Prepared for:

RijkswaterstaatRIKZ

Sediment budget analysis and testinghypotheses for the Dutch coastalsystem

E. Elias, M. van Koningsveld, P.K. TonnonZ.B. Wang

Report

November, 2006

VOP II-1.2 Long term coastal management

and

DC05.20 North Sea and Coast

VOP II-1.2 Long term coastal management Z4100.00 November, 2006Detailed sediment budget analysis of the Dutch Wadden Sea

WL | Delft Hydraulics 1 — 1

Contents

1 Introduction .................................................................................................... 1—1

1.1 Context ................................................................................................ 1—1

1.2 Objective of the study .......................................................................... 1—1

1.3 Research approach ............................................................................... 1—2

1.4 Setting of the study area ....................................................................... 1—2

1.4.1 Wadden Sea............................................................................. 1—2

1.4.2 Holland Coast.......................................................................... 1—3

1.4.3 Zeeuwse Delta ......................................................................... 1—5

2 The analysis approach .................................................................................... 2—1

2.1 The marine and coastal analysis toolbox............................................... 2—1

2.2 Sediment budget analysis ..................................................................... 2—3

2.3 Validation of the UCIT approach.......................................................... 2—7

3 The Wadden Sea ............................................................................................. 3—1

3.1 Present knowledge of the Wadden Sea.................................................. 3—1

3.1.1 Interaction between the Wadden Sea and the coast ................... 3—1

3.1.2 Large scale sand balance of the Wadden Sea (Walburg)............ 3—1

3.1.3 Sand balance for the individual inlets....................................... 3—2

3.2 Analysis of the western Wadden Sea .................................................... 3—5

3.3 Analysis of all inlets............................................................................. 3—7

3.3.1 Marsdiep Inlet ......................................................................... 3—7

3.3.2 Vlie inlet ............................................................................... 3—14

3.3.3 Eierlandse Gat Inlet ............................................................... 3—19

November, 2006 Z4100.00 VOP II-1.2 Long term coastal managementDetailed sediment budget analysis of the Dutch Wadden Sea

1 — 2 WL | Delft Hydraulics

3.3.4 Ameland Inlet ....................................................................... 3—19

3.3.5 Friesche Zeegat ..................................................................... 3—19

3.4 Reflection and formulation of hypotheses .......................................... 3—31

4 The Holland coast........................................................................................... 4—1

4.1 Present knowledge of the Holland Coast .............................................. 4—1

4.2 Analysis of the Holland Coast.............................................................. 4—4

4.2.1 Fixed-map grid data: Vaklodingen data.................................... 4—5

4.2.2 Fixed-map grid data: JARKUS data......................................... 4—9

4.3 Analysis of sub-areas of the Holland coast ......................................... 4—10


5 The Delta area ................................................................................................ 5—1

5.1 Present knowledge of the Delta area..................................................... 5—1

5.2 Analysis of the Delta area .................................................................... 5—2

5.3 Analysis of sub elements of the Delta area ........................................... 5—4

5.3.1 Haringvliet .............................................................................. 5—4

5.3.2 Grevelingen............................................................................. 5—7

5.3.3 Eastern Scheldt........................................................................ 5—9

5.3.4 Western Scheldt..................................................................... 5—12


6 Hypothesis testing by data analysis................................................................ 6—1

6.1 Introduction ......................................................................................... 6—1

6.2 Basic data ............................................................................................ 6—1

6.3 Empirical relations for morphological equilibrium ............................... 6—3

6.4 Characteristics related to tidal asymmetry ............................................ 6—9

6.5 Conclusions ....................................................................................... 6—12

7 Hypothesis testing by process modelling........................................................ 7—1



7.1 Introduction ......................................................................................... 7—1

7.1.1 Research Questions ................................................................. 7—2

7.2 Method and Model ............................................................................... 7—2

7.3 The effect of basin geometry and bathymetry on flow........................... 7—7

7.3.1 Flow and sediment transports...................................................7—7

7.3.2 Tidal asymmetry and tidal propagation characteristics..............7—7

7.4 Conclusions ......................................................................................... 7—9

8 Preliminary synthesis ..................................................................................... 8—1

8.1 Mega scale: Behaviour of the Dutch coastal system.............................. 8—1

8.2 Macro and Meso-scale: Interaction between elements of the DutchCoastal System.....................................................................................8—2

8.3 Future work ......................................................................................... 8—4

9 Literature........................................................................................................ 9—1

A Figures Wadden Sea ........................................................................................ A–1

B Basic data derived from the hypsometry data Wadden Sea ........................... B–1

B.1 Marsdiep............................................................................................... B–1

B.2 Eierlandsegat ........................................................................................ B–4

B.3 Vlie....................................................................................................... B–7

B.4 Amlanderzeegat .................................................................................. B–10

B.5 Friesche Zeegat ................................................................................... B–13

C Model results.................................................................................................... C–1



1 Introduction

1.1 Context

Since 1990 Dutch coastal management adopted a policy of ‘dynamic preservation’ of thecoastline. Since then coastal managers applied regular beach and shoreface nourishments tomaintain the coastline, based on annual profile assessments, at a position not landward of anagreed upon threshold value – the so called Basal Coastline (BCL). As a result of this policyan average annual volume of 6 Mm3 of sand was nourished in the nearshore zone.

Based on analyses of the Dutch coastal foundation as a whole (defined as the area betweenthe NAP -20 m contour and most landward extent of the dune barrier), Mulder (2000)suggested that an average annual volume of 12 Mm3 is needed to compensate the loss and tocompensate for the effect of sea-level rise. Nederbragt (2006) introduced a conceptualmodel for the working of the Dutch coastal system as a whole which thus also includes thetidal basins in the Wadden Sea and the Western Scheldt. In spite of the apparent uncertaintyin the estimated overall sand loss the main conclusion, viz. that the system as a whole isloosing sand on a structural basis, has remained over the years.

In a context of sustainability this structural sand loss was considered unacceptable. As aresult, a large scale coastal management objective was officially added to the nationalcoastal management policy. Since 2004 sand losses in the coastal system (following theabove definition) need to be compensated. Mulder (2000) and Van Koningsveld and Mulder(2004) describe potential outlines of a management strategy that deals with this addedobjective.

Concerning the above large scale approach to coastal management a number of crucialquestions remains:

How does the Dutch coastal system as a whole (in this case including the Wadden Seaand Zeeuwsche delta) actually work?How do the different morphologic elements of this system exchange sediment?How is this new large scale coastal management strategy likely to affect the system?

1.2 Objective of the study

This report describes research on the above described topics conducted in the framework ofthe Generic Coastal Research programme as part of the work package Long term coastalmanagement.

The objective of the study is to obtain more insight into the long-term morphologicaldevelopment of the Dutch coastal system, which is needed for supporting the coastalmanagement strategy.



1.3 Research approach

Despite a long history of monitoring and measurements our knowledge of themorphodynamic behaviour of e.g. the Wadden Sea as a whole and the individual inletsystem is still limited. Previous studies agreed on the net sediment influx into the basin sincebegin last century, but estimates of the present import rates and future evolution vary. Withthe ongoing measuring and recent development of tools for analysis (UCIT) the study oflonger time series may provide further understanding. Therefore, as a first step in this study,the sand balance of the Dutch coastal system is analysed again. Based on this a number ofhypotheses will be formulated of how the system is supposed to work and how the differentmorphologic elements are thought to exchange sediment.

A second step is to use process models to investigate these hypotheses and identify whichprocesses are responsible for the observed and hypothesised behaviour.

The results of both steps together should result in a recommendation on how to approach themodelling of the long term coastal behaviour of the Dutch coastal system.

This report deals mainly with the first step. In this stage of the project the research isprimarily focussed on the data analysis. Test of the hypotheses is only started for theWadden Sea. For the Holland Coast and Zeeuwsche Delta only data analysis is carried out.

The report starts with a description of the method that is used in the data analysis (Chapter2). To establish some confidence in this method some of the findings reported in literatureare reproduced. In the following three chapters (3, 4 and 5) the detailed data analysis of thethree sub-systems, Wadden basins and ebb deltas, Holland Coast and the Delta coast ispresented. Each of these three chapters finishes with a number of hypotheses on the systembehaviour derived from the analysis. In Chapters 6 and 7 a start of the test of thesehypotheses for the Wadden Sea has been made by further data analysis and by process-modelling. The report ends with a synthesis how the whole Dutch coast behaves in Chapter8.

1.4 Setting of the study area

For the analysis the whole Dutch coast system is divided into three sub-systems, viz. theWadden Sea area, the Holland Coast and the Zeeuwse Delta.

1.4.1 Wadden Sea

Figure 1.1 provides an impression of the Wadden Sea area, with (from left to right) the inletsystems of Texel, Eierlandse Gat, Vlie, Ameland and the Frisian inlet. Due to the absence ofsuitable field data the Eems-Dollard is not investigated in this study. The black lines definethe approximate location of the tidal watersheds and are used as separation of the individualinlets.



Figure 1.1: Overview of the tidal inlet systems in the Dutch Wadden Sea.

Rijkswaterstaat frequently performs bed level measurements in the Wadden Sea. Since 1987the ebb-tidal deltas are measured in 3-year intervals and the basins every 6 years. For eachinlet this data is stored in 20x20m resolution in the so-called Vaklodingen database.

Before 1987 bathymetric data is available with larger time intervals, and probably of loweraccuracy as smaller scale measurements of varying origin and accuracy were used toconstruct these maps. The data is stored on 250x250 m interval grids. Extensive descriptionsof the measurements and conversion to complete maps of the Western Wadden Sea aredocumented in e.g. Rakhorst (1986), Glim et al. (1998) and de Boer et al (1991). Maps ofthe Frisian Inlet were constructed in the Kustgenese project (sub-project: ISOS*2), see Oost(1995) for details.

In a recent study by RIKZ (Walburg, personal communication) these data were used toobtain insight in the in the sediment balance of the Dutch part of the Wadden Sea. Wereanalyse the data aiming to identify unresolved questions requiring further research.

1.4.2 Holland Coast

Figure 1.2 provides an impression of the Holland coast area. For the analysis of this centralpart of the Dutch coast a number of data sources are available:

Since 1843 beach lines have been recorded almost yearly at approximately 1 km intervalfor the entire Dutch coastal system bordering the North Sea



Since 1926 (the closure of the Zuiderzee) the Dutch coastal system is monitored withso-called Vaklodingen (see Wadden description) currently stored in 20x20 m resolutionand ordered in a so-called fixed map system (Kaartbladen).Since 1963 a transect system (JARKUS) was defined. From then on the near shore partof the Dutch North Sea coast was monitored yearly, along these JAKUS transects whichare spaced 200 to 250 m apart.In recent years the JARKUS transect measurement have also been interpolated to a20x20 m grid. We have reorganised that data into the same fixed map system(Kaartbladen) that was used to structure the Vaklodingen data.Surrounding the nourishments, which occurred on a regular basis especially since 1990,a number of additional soundings were performed to monitor the nourishment evolution.

Figure 1.2: Overview of Holland Coast System.

To analyse the sediment budget of the Holland coast the above mentioned data sources willbe used. Emphasis will lie on the Vaklodingen data and the gridded JARKUS data.



1.4.3 Zeeuwse Delta

Figure 1.3 provides an impression of the Zeeuwse Delta showing the location of the analysisareas Voordelta, Grevelingen, Oosterschelde and Westerschelde. Analysing these areas thesame data was used as in the analysis of the Wadden area, viz. 20x20m Vaklodingen datastored in the fixed map format defined by Rijkswaterstaat.

Figure 1.3: Overview of Delta Coast System.



2 The analysis approach

2.1 The marine and coastal analysis toolbox

Starting 2003, WL|Delft Hydraulics initiated the development of marine and coastal analysistoolbox. This initiative was triggered by the practical observation that in studies of coastalsystems there was (and is) a tendency to reinvent the wheel. Especially when dealing withthe same coastal system great benefits may be expected from an analysis toolbox that servesas a collective memory for researchers as well as end users.

Figure 2.1. Conceptual structure of UCIT (pronounced as Use it!) combining data, models,coastal state indicators (CSI’s) and a potential link to a geographical information system(GIS) (Source: Van Koningsveld et al., 2004).

Van Koningsveld et al. (2004) describe the first conceptualization of a toolbox that actuallycan work as a collective memory for both researcher and end users (see Figure 2.1). It worksmainly by facilitating easy access to and integration of various types of measurement data,morphological models and coastal state indicators (i.e. specific parameters on whichdecisions are based). A primary benefit of this approach is an increased efficiency in dealingwith the ‘traditional’ coastal problems for which long standing approaches are used. Asecondary but by no means lesser benefit is the creation of an environment where innovativetechnologies can be employed to supplement the traditionally derived information or even togenerate new, previously unavailable, information in support of coastal management.

The toolbox as suggested by Van Koningsveld et al. (2004) has been constructed andimplemented in the WL|Delft Hydraulics research environment with a first focus on theDutch coastal system. More recent versions have been set up in a more generic sensefacilitating application to other coastal areas as well.

Figure 2.2. Schematic representation of the marine and coastal analysis toolbox.

MySQLdatabase

Generalengines

Applications Output

Marine and coastalanalysis toolbox

Otherdatabases Data I/O

routines

databasecom layer



Figure 2.2 schematically represents the abstract structure of the toolbox currently used. Atthe far left the foundation of the entire toolbox structure is depicted, viz. a MySQL-databasecontaining the most important coastal bathymetry data sets (NB: other databases may beused as well). For the Dutch case the database currently contains:

the profile measurements (in Dutch: JARKUS raaien) as contained in the JARKUSdatabase,The special soundings surrounding various shoreface nourishment projects (eitherinterpolated to the JARKUS transects (Terheijde, Katwijk en Noordwijk) or in grid form(Bergen and Egmond).the fixed map grid Vaklodingen (in Dutch: Kaartblad data vaklodingen),as stored in theWADI database,the fixed map grid gridded JARKUS data (in Dutch: Kaartblad data JARKUS) asprovided by Rijkswaterstaatthe beach line data (in Dutch: Strand lijnen data) since 1843 as provided byRijkswaterstaat23 years worth of information concerning off shore boundary conditions (tide, setup,wind, waves etc.)a record of all dredging and dumping activities that have taken place in the Dutchcoastal zone.

In the context of the developments surrounding the ARGUS system, databasecommunication routines have been developed that enable the mathematical analysissoftware package Matlab to exchange information with the pre-mentioned MySQL-database. As a result it is now quite straightforward to extract any selection of bathymetricinformation from the database and store it in a variable that can then be processed byMatlab. The fact that Matlab is (becoming) the scientific language in the field of coastalmorphology is obviously an important pro in its acceptance.

A first layer in the toolbox is a so called General engine layer. Engines are Matlab routinesthat process the data extracted from the database to come to basic information that can beused in the analysis process. Examples of engines are routines that convert data to othercoordinate systems or concatenate a number of smaller datasets to one larger dataset and soforth. The engine routines are very generic and once programmed should not change muchanymore.

A second layer in the toolbox is formed by the Applications. Applications are routines thatcombine all pre-mentioned elements to generate information that is relevant to analysts.They have often a batch-like character and/or include a graphical user interface for easyaccess. For example by performing a batch of dune erosion calculations the analyst cangenerate a number of erosion points that together form a dune erosion line (In Dutch:afslaglijnen). formed by Tools. Tools perform standard analyses on the information that thatis produced by the engines. Another application example consists for example of routinesthat are used to determine a Momentary Coast Line.

By storing, maintaining and disseminating the data, data I/O routines, general engines andapplications at a central location, slowly but surely a toolbox emerges to which analysts andend users naturally gravitate for their basic information need. This will promote



collaboration and the exchange of ideas which is beneficial for the coastal morphologycommunity.

In the next section the elements that were added (as a new application) to the toolbox toenable sediment budget analyses are described. Much of the development work on this newapplication was performed in the context of the matching Delft Cluster project North Seaand Coast (DC-05.20).

2.2 Sediment budget analysis

For the sediment budget analysis reported in this document the fixed-map gridVaklodingendata (in Dutch: kaartblad data) from the WADI database and gridded JARKUSdata made available separately by Rijkswaterstaat were used primarily.

In the WADI database all digitally available Vaklodingen, from 1926 until now, are stored.Most recent data that was available but not yet in WADI was processed into the properformat by WL|Delft Hydraulics. The data have been subdivided in fixed maps of 10 kmwide and 12.5 km high, within which the data is interpolated (by RWS RIKZ) to a 20 x 20m grid. Figure 2.3 shows how a number of these fixed maps together cover the entire Dutchcoastal system.

Figure 2.3. Overview of the fixed map locations for Vaklodingen.

In the JARKUS database all digitally available transect measurements, from 1963 until now,are stored (see Figure 2.4a). The transect data was interpolated by Rijkswaterstaat to a20x20m grid. This interpolated data was processed by WL|Delft Hydraulics to the same



fixed map grids as were used for the Vaklodingen data (see Figure 2.4b). The data have beensubdivided in fixed maps of 10 km wide and 12.5 km high, within which the data isinterpolated (by RWS RIKZ) to a 20 x 20 m grid. Figure 2.3 shows how a number of thesefixed maps together cover the entire Dutch coastal system.

(a) (b)Figure 2.4. (a) Overview of JARKUS transect locations, (b) Overview of the fixed maplocations for the gridded JARKUS data (NB: the gridded JARKUS grid maps clearly covera narrower area that the Vaklodingen grid maps).

For each of the fixed maps bathymetric data that have been collected in the past areavailable. Figure 2.5 shows an example of Vaklodingen data that were labelled to be from1981. Data that were measured roughly before the mid 1980’s have been interpolated byRWS RIKZ to generate good coverage for the separate Wadden basins and ebb-tidal deltas.Data from later years were not pre-combined and hence give a more scattered coverage.



Figure 2.5. Example of data available from the fixed maps (this case 1981)

As part of the marine and coastal analysis toolbox, a number of Matlab routines areavailable to facilitate sediment budget analysis on coastal systems with data stored in a fixedmap format as described above (see Figure 2.6).

A first step in the sediment budget analysis is to generate for an arbitrary polygon a gridwith maximum data coverage for a predetermined ‘Year 1’. To do this a routine wasdeveloped to find all available fixed-map grids within an arbitrary selection polygon. Nextone grid is generated onto which data from each selected fixed map can be projected. It is ofcourse possible that for the indicated ‘Year 1’ not all fixed maps have data. To come to aproper sediment budget analysis, however, it is highly preferred to have maximum datacoverage inside the polygon defined by the user. There are several approaches conceivableto fill in the blank spots. For this specific case a routine was developed that searches thedatabase backward in time (and if needs be forward) to fill in the blanks and generate a datacoverage that is as close to optimal as possible.

A second step is to do the same for a ‘Year 2’.

In Step 3 the data from ‘Year 1’ is subtracted from ‘Year 2’ to generate a map of changes. Afourth and final step determines, from the map of changes, the overall volume change forthe given period.



Figure 2.6. Illustration of the steps in determining sediment budgets. (Upper left panel:bathymetry ‘Year 1’, Upper right panel: bathymetry ‘Year 2’, Lower panel: difference map).

In the Wadden analyses for this study the following approach was selected. For years before1985 an optimal data coverage could be obtained by searching the data base backward intime only. The fact that RWS RIKZ combined and interpolated the data for this period inorder to get optimal data coverage for the basins as well as the ebb-tidal deltas made thisapproach viable in the case of the Waddensea. From 1985 onward interpolated data fieldswhere constructed every five years until 2000. This means that for the years 1985, 1990,1995 and 2000 maximum coverage was established with the nearest older as well as thenearest younger data (in both cases with a 15 year search window). Data availability around2005 was limited to none. For the value for the targetyear was found by linear interpolationin time between these nearest older and younger values for each separate data point. In caseswhere either the older, younger or both values are not available a no-data value (NaN in caseof Matlab) results from this procedure. Similarly for the maximum data coverage maps, the‘difference map’-procedure also returns a no-data value when one or both of the datapointsfor ‘Year 1’ and ‘Year 2’ are not available. Data points with a no-data value do notcontribute to the overall volume difference.

Because the sediment budget analyses are run for approximately 8 – 12 different years aswell as on a great number of polygons and sub-polygons the whole procedure is to beautomated as much as possible. However, occasionally data points in the volumedevelopment plots are less reliable. In most cases this is caused by one or both of the targetbathymetries having (very) limited data coverage. To make the quality of the data coverageimmediately apparent in the volume evolution plots the following procedure was applied toeach of the sediment budget analyses.



For each polygon a number of volume change calculations are made (8 – 12). Because thedata in the data base is based on a fixed map format it is known beforehand how many gridpoints actually lie within each polygon. By counting how many of these data points containa no-data value we can establish the coverage percentage for each volume estimation. Nextfor each volume estimation we determine the amount of under-coverage by subtracting thecoverage percentage for a given volume estimation from the maximum coverage percentageof all volume estimations (NB: this maximum is not necessarily 100% as it depends on theshape of the polygon with respect to the available data).

To get an idea of the potential effect of this under-coverage percentage it is multiplied withthe maximum volume difference present in all volume calculations for that polygon. Theresulting number is plotted around each volume estimation using Matlabs error bar function.However, it is very important to realise that these values are not actually error bars! Theyare intended as quick visual clues on the quality of the data coverage only. Even though insome cases the errorbars may indicate a large uncertainty due to poor data coverage, it maystill be possible that a resulting volume estimate is not so poor when the data that isavailable covers the most dynamic parts of the bathymetry. For now the error bars should beinterpreted by the analyst as a clue to treat that particular result with caution. Alternativedisplay methods may need to be developed if it turns out that the use of error bars createstoo much confusion.

It is good to note that in most cases the volume estimations based on Vaklodingen data from2000 will have large error bars around it. Main cause of this is that only limited data isavailable from the period after 2000. Furthermore the reader should realise that the volumeevolution graphs have not yet been corrected for human interventions (dredging anddumping). Main reason for this is the limited information on the exact locations of theseinterventions. This presents a problem especially for the sub-polygons. For the Hollandcoast a first attempt to correct the observations has been made. For the Wadden and Deltaarea this issue remains to be resolved.

In the next section a brief validation of the approach, against results of an extensive studyperformed by Rijkswaterstaat RIKZ, is presented. Results of the more detailed sedimentbudget studies are presented in Chapter 4.

2.3 Validation of the UCIT approach

A comparison of the Walburg data sets (Figure 2.7 top) and the UCIT representation (Figure2.7 bottom) of the sediment volume changes in the ebb-tidal deltas (left) and basins (right) isshown in Figure 2.7. In general a good correspondence between Walburg and UCIT isobserved.



Zandvolumes van de binnendelta's van de Waddenzee

-100

-50

0

50

100

150

200

250

300

1920 1930 1940 1950 1960 1970 1980 1990 2000jaar

zand

volu

me[

106 m

3 ]

Friesche ZeegatAmelander ZeegatHet VlieEierlandse GatMarsdiep

Figure 2.7: Sediment volume changes of ebb deltas (left) and basin (right) based on Walburg(top) and UCIT (bottom). NB: the UCIT results have not (yet) been corrected for humaninterventions.

In Marsdiep the decreasing ebb-tidal delta volume and the increase in the basin isreproduced. Magnitudes differ slightly as UCIT (for the moment) is based on thebathymetric maps only and no additional corrections have been applied. In the smallEierlandse Gat inlet (where additional corrections are minor) a good representation ofsedimentation and erosion magnitudes is seen. The basin development of Vlie inlet is verywell reproduced in magnitude and characteristics. On the ebb-tidal delta the decrease inerosion rates during the recent 1985-2000 period is not reproduced by UCIT that isdominated by near linear trends, although magnitudes remain in similar ranges. Thisdiscrepancy possibly relates to the UCIT interpolation between the various datasets. Basindevelopments in the Amelander zeegat show similar developments. On the ebb-tidal deltathe erosion rates are significantly lower according to the UCIT program. As, for example, asignificant number of nourishments have been executed within this polygon, in particular onthe coast of Terschelling, this may well have to do with the fact that the volume changescalculated by UCIT have not been corrected for human interventions yet. In Friesche Zeegatwe observe discrepancies in the basin and ebb-tidal delta changes. In the basin sedimentchange is overpredicted by 50 Mm3, whereas in the ebb-tidal delta a similar underprediction occurs. Possibly this is due to slightly different grid schematisation.

Nevertheless the correspondence, qualitatively as well as quantitatively, between the UCITresults and the Walburg results provides confidence in the further application of the UCIT



method for detailed analysis of the sediment budgets. Ongoing analysis to improve results,e.g. by including correction for human interventions, is being performed.



3 The Wadden Sea

3.1 Present knowledge of the Wadden Sea

3.1.1 Interaction between the Wadden Sea and the coast

Stive and Eysink (1989) were among the first to note the sand demand of the Wadden Seabasin as a main factor in structural large sand losses from the adjacent barrier, ebb-tidaldeltas and the North-Holland coast. These losses are partly related to relative sea-level rise;one of the characteristic features of the Wadden Sea is its continuous sedimentation in thetidal basins in order to keep pace with the increase in accommodation space due to therelative sea level rise (Louters and Gerritsen, 1994). Additionally, a vast amount of sedimentimport resulted in the infilling of closed channels after the closure of the Zuiderzee in 1932(Elias et al., 2003; Elias et al., 2005), and closure of the Lauwerzee in 1969 (Oost, 1995).This infilling dominated much of the morphodynamic changes over the last decades, andespecially in case of Marsdiep is expected to continue to dominate the evolution of the inletfor many years to come.

3.1.2 Large scale sand balance of the Wadden Sea (Walburg)

Figure 3.1 (top): Volume change of all ebb-tidal delta’s and basins relative to the 1997volume (increase means sedimentation and decrease means erosion), and (bottom) yearly



changes (positive means sedimentation and negative means erosion) in volumes (red =basin, and dashed = ebb-tidal delta).

Many sediment budget studies of (parts of) the Wadden Sea have been documented inliterature. These studies use similar underlying field data. In recent studies by RIKZ(Nederbragt, and Walburg, personal communications) these data were checked and madeconsistent for obtaining insight in the sediment balance of the Dutch Wadden Sea. As firststep the ‘Walburg’ data have been reviewed to identify unresolved questions requiringfurther research.

The overall trend of sediment transport from the coast into the basin is illustrated in Figure3.1. On the gross scale a reasonable correspondence exists between sediment loss from thecoast and sedimentation in the basin. Looking at the period 1930 - 1994 the sedimentation inthe basin of 486 Mm3 is near-equal to the erosion of the ebb-tidal delta 480 Mm3. In 1997the near-balance is somewhat distorted as both ebb-tidal delta and basin increase in volume.By plotting a range of trendlines through the observations we can estimate the sedimentimport to range between 6.5 and 8 Mm3/year, with a mean value of 7.2 Mm3/year. Thisamount is approximately double of what would be needed to compensate for 20 cm/centurysea-level rise (3.1 Mm3/year), and plausibly relates to the response of Texel and FrieseZeegat basin to damming.

Remarkably, we cannot observe a clear decreasing trend in sediment import rate (see theyearly averaged changes). What we expected is that immediately after the closure of part ofthe basins the sediment import rates significantly increase, and as the morphology adaptssediment import reduces.

3.1.3 Sand balance for the individual inlets

An underlying assumption in previous sediment budget studies of the Wadden Sea was thatthe individual inlets form 'more-or-less' unconnected sub-systems separated by the tidalwatersheds. The location of these tidal watersheds corresponds roughly with theVaklodingen areas. Analysis of the individual inlet systems was therefore mostly based onthe corresponding Vaklodingen dataset.



Figure 3.2: Sand Balances Western Wadden Sea (based on Vaklodingen data by Walburg,Note that the Friesche Zeegat only includes the area which is still open to sea after theclosure of Lauwers Sea in 1969)

Figure 3.2 illustrates the evolution of ebb-tidal delta and basin for the individual inletsystems. With the exception of Eierlandse Gat and Amelanderzeegat the inlets show adecrease of ebb-tidal delta volume and an increase in basin volumes. As basin and ebb-tidaldelta form a sand-sharing-system (Oost, 1995) a corresponding rate of sedimentation anderosion is expected, which is not observed in the sand balances of the individual inletsystems. For example in Texel basin a large sediment import was observed following theClosure of the Zuiderzee (completed in 1932), while at present the sediment volume changefluctuates around zero. Based on this mis-thought decrease in sediment influx, it wasconcluded that the basin had completed much of the adaptation to the closure. Comparingthe sediment volume change of basin and ebb-tidal delta we can observe an imbalancebetween ebb-tidal delta and basin; the ebb-tidal delta looses nearly 100 Mm3 of sediments,while the basin volume remains constant. A similar but opposite discrepancy is observed inVlie inlet.



Figure 3.3: Detail of the recent changes in sediment volume of the Western Wadden Seainlets.

Figure 3.3 illustrates the imbalance in detail by addressing the recent changes (1986 –present) for the Western Wadden Sea inlets. In total the ebb-tidal deltas loose -95 Mm3 overthe period 1986-1997, while the basin gains a near-equal amount of sediment (+ 105 Mm3).The corresponding sediment volume changes indicate a net sediment import into the westernWadden Sea in the order of 9.5 Mm3/year, of which most is supplied by the adjacent barrierislands and ebb-tidal deltas (8.5 Mm3/year); the 1 Mm3/year discrepancy betweensedimentation and erosion is an indication of the net supply of sediment due to littoral drift.

Looking at the sediment budget of the individual inlets we can observe the largest changesin Marsdiep and Vlie. The sediment volume of the Marsdiep ebb-tidal delta decreased over75 Mm3 since 1980. In the basin the sediment volume remained near constant (althoughsome fluctuations occur). A similar (but opposite) discrepancy between ebb-tidal delta andbasin evolution is observed in Vlie inlet. The Vlie ebb-tidal delta initially erodes (nearly 50Mm3), obtaining an equilibrium state around 1990. Meanwhile the sediment volume of thebasin increased over 100 Mm3.

The large differences between ebb-tidal delta and basin behaviour of Vlie and Texel inlet isremarkable. Empirical inlet knowledge teaches that ebb-tidal delta and basin form a coupledmorphological system. In principal sediment volume increase of the basin is due to adecrease of the ebb-tidal delta and visa versa. The corresponding rates of erosion andsedimentation of the entire system and the imbalance in the individual subsystems are aclear indication that the individual inlets exchange sediments. Moreover no equilibrium stateis reached in Marsdiep.

It is our hypothesis that the sediment import into Marsdiep is comparable to the period justafter the closure. The decreasing sediment trend in the basin relates to the sedimentredistribution from Marsdiep to Texel Inlet. This hypothesis is investigated by detailedanalysis of the Western Wadden Sea area. Sediment budgets for this detailed analysis arederived from approximately 80 years of measurement data supplied by RijkswaterstaatRIKZ, using the Marine and Coastal Analysis Toolbox developed by WL|Delft Hydraulics.



Apparently sediment budgets on the scale of the whole basins are not sufficient tounderstand what exactly is happening. Analysis on more detailed scales is required tounderstand the behaviour of the system.

3.2 Analysis of the western Wadden Sea

The large changes in sediment volumes of ebb-tidal deltas and basin are clearly reflected bythe changes in Wadden Sea topography (Figure 3.4). The sedimentation-erosion patterns inthe lower plot of Figure 3.4 summarize the morphologic evolution. Since closure of theZuiderzee the western part of the Wadden Sea has undergone drastic changes. Largesedimentation areas are observed in the terminal parts of channels (see, e.g. the Vlieter,Vliestroom) where tidal currents reduced to almost zero and the loss of discharge caused thechannels to accrete rapidly (Berger et al., 1987; Oost and de Boer, 1994). Sedimentation wasalso observed on the major shoal areas such as Lutjeswaard, Balgzand and Vlakte vanOosterbierum. An opposite response is present in the area between Waarden andKornerderzand where erosion prevails. This erosion is related to the scouring of the channelTexelschaar and continues up to present.

The basic layout of channels and shoals has remained remarkably similar over the lastdecades (century) despite the large sedimentation in the basin. The main channelTexelstroom-Doove Balg has maintained its eastward direction with the upper part(Texelstroom) stretching along the Texel coastline and the lower part extending along theAfsluitdijk. The large sedimentation and erosion values observed at the channel location arean indication of lateral channel migration (Oost and de Boer, 1994). ‘Uitbochten’ ofTexelstroom induces an eastward extension at the expense of the Waarden shoals, which isvisible in erosion of the eastern channel embankments. In general context largesedimentation values (> 2 m) occur in the channels, and either indicate an infilling of aclosed off channel or lateral channel migration. The Doove Balg channel increases in depthdue to the eastward deflection of the flow at the closure dam. Large sedimentation occurs onthe spill-over lobe (Kornwerderzand) facing Doove Balg, this sedimentation area extendseastward along the Frisian coast.

Vlie inlet is dominated by large sedimentation along the margins of the basin. A major partof this sedimentation is related to the infilling of the former channels to the Zuiderzee (at thelocation of Imschot). Significant sedimentation is also observed along the Frisian coastlinewhere the shoal areas between Grienderwaard and the Vlakte van Oosterbierum accretedsignificantly. The notably smaller erosion values of the Vlie ebb-tidal delta and coastlinecompared to the sedimentation in the basin render it likely that much of the sediments aresupplied by the Marsdiep basin. The Texelschaar scouring must have contributedsignificantly to the infilling near Imschot, while in addition the net eastward directed windand wave driven currents transport sediments from e.g. the Doove Balg spill-over lobeeastward.

The changes in the Eierlandse Gat inlet are moderate compared to the large changes in Vlieand Texel basin. In general sedimentation prevails on the shoal areas, while channels seemto be dominated by erosion.



Figure 3.4: (top to bottom) Bathymetries of the Wadden Sea for 1926, 1990 andsedimentation-erosion patterns.



Conjugate to the large changes in the basins large alterations in ebb-tidal delta morphologyhave occurred. The ebb deltas are dominated by erosion (see Figure 3.4) and sedimentredistribution. A general trend, which is observed in all inlets, is a landward redistribution ofsediment; sediments erode from the seaward margins of the delta and are transportedlandward, thereby increasing shoal heights and the deltas extend alongshore. An interestingphenomenon is that with the exception of Friese Zeegat, all ebb-tidal deltas show apreferential erosion of the updrift side and sedimentation on the downdrift side of the delta(updrift and downdrift in respect to the direction of tidal wave propagation that is fromsouth to north near Texel and west to east along the Wadden Sea).

In the case of Texel inlet the updrift erosion is shown to relate to effects of Closure of theZuiderzee. Elias et al. (2003, 2004) recognized that the southward developing channels are aforced response to closure of the ZuiderZee. The closure dam increased the tidal prims inthe remaining part of the basin and the increased outflow on to the ebb-tidal delta was moresouthward directed due to the enlarged importance of Texelstroom. This triggered southwardchannel development. The recent erosion of the western margin of the ebb-tidal delta relatesto the landward redistribution of the abandoned ebb-shield Noorderhaaks.

Although a major part of the former Zuiderzee connected to the Vlie inlet, the changes in theebb-tidal delta were less drastic. Nevertheless, the main ebb delta channel increased in depthand is nowadays more updrift directed. Especially the downdrift shoal height has increasedconsiderably at the expense of the seaward extension of the shoal. Similar to the basin thechanges in the Eierlandse Gat inlet are small compared to the large changes of Vlie andTexel inlet, and need to be addressed on a local (inlet) level.

3.3 Analysis of all inlets

3.3.1 Marsdiep Inlet

The sediment volume changes of the Marsdiep ebb-tidal delta show a trend of erosion(Figure 3.5). Over 270 Mm3 is eroded from the ebb-tidal delta and adjacent coasts in theperiod 1926-2000. A major part of the eroded deposits (140 Mm3) is related to the landwarddisplacement of the ebb-tidal delta margin [sub-areas 6, 7, 10 & 11], and the scouring oflarge tidal channels in a southward direction (70 Mm3 [2]). The initial response of accretion[2, 1950] might be related to inaccuracies in the measurements, or an alternative explanationis that the ebb-tidal delta initially expanded as the tidal prisms enlarged after closure,pushing sediments from the proximal part of the delta seaward. Landward sedimentredistribution increases the size of Noorderhaak [3] since 1970. This increase is partlyvisible in the formation and northward extension of the Noorderlijke Uitlopers of theNoorderhaaks spit and the increased area of the supra-tidal Noorderhaaks. .

Sedimentation in areas 5 and 9 is plausibly related to the southward expansion ofZuiderhaaks, increasing the sediment volume.

The recent measurements (1986 to present) show an ongoing erosion of the ebb-tidal deltamargin. The decreasing erosion rates of the nearshore area [2] are expected as the main



channels developed until around 1990 and remained stable ever since. The areas outside theebb-tidal delta perimeter [5,9,12,8] are less frequently measured and data shows largescatter. Interesting is the increasing erosion rate of the Texel foreshore [4,8] and the stability(increase?) of the North-Holland coast volume. Analysis of the recent foreshoredevelopments would greatly benefit from detailed analysis of the MKL volume changes.

In the basin large sedimentation occurs. Initially the erosion volume of the ebb-tidal deltafollows the sedimentation volume of the basin, but since 1986 both ebb-tidal delta and basinare loosing sediment, this deficit indicates that analysis of basin changes on level of thewhole basin is not sufficient to understand the observed alterations. Based on the observedsedimentation and erosion patterns (Figure 3.4) a subdivision of the Marsdiep basin is madein 5 selected areas (Figure 3.6), which are divided in a northern [1,2,3] and southern [4,5]part separated by the relative stable (in position) Texelstroom/Doove Balg channel.

The area definition in detail is as follows:(1). spill-over lobe of Doove Balg(2). Waarden / Scheurrak(3). Waarden / Texelstroom(4). Balgzand(5). LutjesWaard

Analysis of these selected areas might provide more insight in the evolution of the basinsince completion of the Afsluitdijk in 1932.

On the gross scale the basin can be subdivided in (1) an eroding part [2,3], the Waardenshoals north of Texelstroom, and (2) a sedimentation area south of Texelstroom [4,5] and (3)the spill-over lobe formation facing Doove Balg [1] near Kornwerderzand. At this latterlocation large sedimentation was observed after closure due to infilling of the closed offchannels, and eastward reorientation of e.g. Doove Balg along Afsluitdijk. The observedstability during the last decades could be a temporary fluctuation, or an indication of a stateof (dynamic) equilibrium wherein sediment supply by Doove Balg is balanced by eastwardtransport along the Frisian coast under influence of the prevailing wind and waveconditions. This would explain the bulky accretion along the Frisian coast.

North of Texelstroom erosion prevails on the Waarden shoal area [domain 2 and 3]. Thiserosion is a continues process with erosion rates in the order of 0.75 – 1 Mm3/year. Thereare no indications of a decreasing trend. The erosion in the northern part of Waarden [2] isfor a major part related to the scouring of Texelstroom, while in the central part [3] thedevelopment of Texelschaar, and eastward expansion of Texelstroom dominates.Texelschaar formed after closure of the Zuiderzee north of the existing Scheurrak channel.

On the shoal areas Balgzand and Lutjeswaard, south of Texelstroom and Doove Balgsedimentation prevails. This sedimentation was initially related to the rapid infilling ofclosed off channels such as Vlieter, but also the shoals areas increase in height. Thesedimentation in and south of Lutjeswaard is a continuous process with sedimentation ratesof 1 – 1.5 Mm3/year. The origin of the large fluctuation in volume during the period 1965-1985 is unclear.



Large sedimentation was also observed in the Balgzand area. Sedimentation rates of 2Mm3/year occurred during an 50-year period preceding the closure. During the last decadesa decreasing trend is observed, which is a possible indication of a near-equilibrium state inthis part of the basin.

Summarizing; since closure of the Zuiderzee continuous sediment transport from the ebb-tidal delta into the basin has occurred. The decreasing trend in basin sedimentation is notan indication for equilibrium. The recent net sediment loss of the basin, despite the largeinflux of sediment, show that much of the sediments are transferred to the Vlie Vaklodingenarea.

In the basin we observe an eroding part (north of Texelstroom) and an accreting part (southof Texelstroom). Erosion occurs due to the scouring of tidal channels in the Waarden shoalarea. The decreasing sedimentation rates near Balgzand indicate that this area is reachingequilibrium. Interestingly, the sedimentation areas (with exception of Lutjeswaard) havedecreasing sedimentation rates, while in the erosion areas rates seem to increase.

The large fluctuation in basin volume around 1980 is related to fluctuations in thesedimentation areas [1, 4, 5]. No fluctuation in the eroding areas is observed. Is this anindication of some change in forcing conditions resulting in more sediment loss to the Vlieinlet? Note the corresponding increase in sediment volume in the central part of Vlie inletsince 1980.


3 — 1 0 WL | Delft Hydraulics


WL | Delft Hydraulics 3 — 1 1



Figure 3.5: Sediment budget of the Marsdiep ebb-tidal delta and adjacent coast. The verticalaxis indicates cumulative sedimentation in Million m3.



Figure 3.6: Sediment budget of Marsdiep Basin. The vertical axis indicates cumulativesedimentation in Million m3.



3.3.2 Vlie inlet

The ebb-tidal delta and adjacent coastlines of Vlie inlet show a general trend of erosion(Figure 3.7 and 3.8). Since closure of the Zuiderzee over 150 Mm3 of sediments have beeneroded, mainly from the southern and western margin of the ebb-tidal delta [1,5]. TheWalburg observations indicated that since 1990 the erosion rates decrease and a more-or-lessstable ebb-tidal delta volume is observed. This stabilization is not clearly visible in theUCIT results that are based on interpolation in time between the nearest available data, andneeds further investigation.

In the basin erosion is only observed near the inlet gorge [2], where channel relocationinduces a loss of sediment until around 1985, ever since a stability is reached. The stabilityof proximal part of the basin and (possible) stability in ebb-tidal delta volume are anindication of an equilibrium state. In the remainder of the basin sedimentation prevails, thatpeaks along the Frisian coastline [6,7,8] where over 135 Mm3 accretes, and in the closed offchannels south of Imschot [6] 127 Mm3.

The sudden increase in sediment volume since 1980 reflects the switch from an eroding toaccreting mode in the central part of the basin [3,4,5]. This is probably due to the transportof large amounts of sediment from Marsdiep to Vlie basin as in Marsdiep erosion increasesaround this period.

The infilling of the central part of the basin and the decreasing trend of sedimentation nearthe watershed of Terschelling [9] and along the Frisian coast [6,7] are indications of thegradual infilling of the basin.

The larger sedimentation in the basin compared to the erosion of the ebb-tidal delta showsthat a net supply of sediment is delivered from the adjacent Marsdiep. Texelschaar andScheurrak are plausible sediment contributors for the sedimentation in area 6. Wave andwind-driven transports, over the Doove Balg spill-over lobe and through Boontjes,contribute to the infilling of the shoal areas along the Frisian coast.

Summarizing; Mardiep and Vlie inlet form a single morphological system. The initialadaptation of the basin was infilling of closed of channels. The basin fills in with sedimentto retain equilibrium at the expense of the adjacent ebb-tidal deltas and coasts.Sedimentation areas near the margins (Balgzand, Terschelling water shed) are the first toregain equilibrium. Nowadays, the central part of the basin accretes. The Vlie ebb-tidaldelta and upper-part of the basin might have reached an equilibrium state, and now Texelinlet is the principal source of sediment for the entire system.







Figure 3.7: Sediment budget of Vlie ebb-tidal delta. The vertical axis indicates cumulativesedimentation in Million m3.



Figure 3.8: Sediment budget of Vlie basin. The vertical axis indicates cumulativesedimentation in Million m3.



3.3.3 Eierlandse Gat Inlet

Eierlandse Gat inlet is governed by erosion of ebb-tidal delta and basin (Figure 3.9 and3.10). Erosion of the basin concentrates in the Robbengat and Keteldiep channels [1], nearthe inlet. On the shoals, near the tidal watersheds sedimentation prevails.

Similar to the basin the ebb-tidal delta indicates an eroding trend. Although real quantitativeconclusions cannot be made due to the large scatter that dominates the sediment budgets,and the 1926 starting value that is an outlier in the data range.

3.3.4 Ameland Inlet

Changes in the Amelander zeegat on the scale of the inlet are minor compared to the largechanges in the adjacent Vlie and Marsdiep inlets. The sedimentation-erosion patterns ofFigures 3.11 and 3.12 are governed by a channel migration rather then large scour. In totalwe observe an increase in both the basin and the ebb-tidal delta volume. However, relativelarge uncertainty exists in the ebb-tidal delta measurements.

In the basin sedimentation prevails along the Frisian coast [3,4]. Erosion prevails along theFriesche zeegat tidal watershed. Plausibly this erosion is related to a southward expansion ofthe main basin channel that relocates its associated spill-over lobe eastward.

On the ebb-tidal delta a clear redistribution of sediments is present from the updrift to thedowndrift part of the ebb-tidal delta. The channel develops in the updrift direction. The ebb-tidal delta evolution might be governed by cyclic migration of the channels and shoals(Israel, 1998).

3.3.5 Friesche Zeegat

Developments in Friesche Zeegat (Figure 3.13 and 3.14) are dominated by the closure of theLauwerzee in 1969 that reduced the tidal prisms from 305 Mm3 to 100 Mm3. Thesignificantly reduced tidal prisms induced major morphological changes. The ebb-tidal deltavolume was too large in relation to the tidal prism and erosion of the ebb-tidal delta wasexpected and has occurred. Roughly 100 Mm3 was eroded (but large scatter occurs in theaccuracy of the measurements).

Since closure large sedimentation increased the shoal heights in the western part of the basin(Pinkegat 1) and east of Zoutkamperlaag [3]. Eastward extension of Zoutkamperlaag causederosion in areas 3 and 4.

It looks like that the largest part of the response to the closure of Lauwers Sea has tekenplace although the system is not in full equilibrium yet.







Figure 3.9: Sediment budget of Eierlandse Gat ebb-tidal delta. The vertical axis indicatescumulative sedimentation in Million m3.



Figure 3.10: Sediment budget of Eierlandse Gat basin. The vertical axis indicatescumulative sedimentation in Million m3.







Figure 3.11: Sediment budget of Ameland ebb-tidal delta. The vertical axis indicatescumulative sedimentation in Million m3.



Figure 3.12: Sediment budget of Ameland basin. The vertical axis indicates cumulativesedimentation in Million m3.







Figure 3.13: Sediment budget of Friesche Zeegat ebb-tidal delta. The vertical axis indicatescumulative sedimentation in Million m3.



Figure 3.14: Sediment budget of Friesche Zeegat basin. The vertical axis indicatescumulative sedimentation in Million m3.

3.4 Reflection and formulation of hypotheses

The sediment demand of the Western Wadden Sea is a major factor in the Dutch sedimentbudget which is illustrated by the near 500 Mm3 of sediment import since 1930. Not only isthere a direct exchange of sediment between the Wadden Sea and the adjacent barrier coasts,but also the Dutch coastline is influenced. Analysis of the large-scale sand balance of theWadden Sea area shows that much of the sediment exchange is resolved by local interactionbetween ebb-tidal delta and basin. These exchanges are much larger then the sedimentinflux from the Holland coast. Detailed analysis of the individual inlet systems shows thatthe ebb-tidal deltas are rapidly eroding; sediment is transported landward from the ebb-tidaldelta margin towards the inlet throat and into the basins. The sediment import into the basinhas been significantly larger then the amount of sand needed to compensate for sea-levelrise. This gives rise to the following hypothesis:

Hypothesis (1): At the present Sea-Level Rise rate the total basin volume of the DutchWadden Sea tends to decrease as the sedimentation rate averaged over all the basins ishigher then needed to compensate for Sea-Level Rise.

During the last decades (since 1990) trend changes can be observed in the developments ofthe ebb-tidal deltas. Such trend changes might be related to maintaining the coastline at a



fixed position by nourishments. If we maintain the coastlines this effectively means that wesupply the system with the sediments, so that limited availability of sediment cannot occur.Hence,

Hypothesis (2) Maintenance of the coastlines along the islands will stimulate thesedimentation of the Wadden Sea in the long term.

or

Hypothesis (3) The sand deficit of the Wadden Sea due to sea-level rise can be solved byextra nourishment along the coasts of the islands and at the ebb-tidal deltas.

The major changes in the Wadden Sea have occurred after human intervention. Especiallythe role of Closure of the Zuiderzee has been very important. Closure of the Zuiderzeeincreased tidal prisms and separated the relative shallow shoal areas of the Zuiderzee fromthe deep Wadden Sea. The remaining basin is too deep in relation to the tidal prism andmorphological adaptation of the inlet resulted in large sediment import; the Marsdiep andVlie channel systems need to find new equilibrium configurations adapted to the presence ofthe closure dam. From the budget analysis it appears that prior to closure Vlie and Marsdiepwere connected both feeding the Zuiderzee in harmony, but now they are competing for thelimited left-over space. It is clear that the Vlie and Texel inlet cannot be separated into twoindividual inlet systems. The first signs of this competition indicate that the Marsdiep hasextended its drainage area eastward at the expense of the Vlie channel system. Largesedimentation was observed along the Frisian coast and in the central part of the basin. Howthis two-basin system further develops can have significant influence on the sedimentbudget of the coastal system although it is not yet clear what the influence exactly is. For thecoastal management it can mean that the nourishment demand alone the North Hollandcoast increases while that on the Wadden Sea Islands decreases.

Hypothesis (4): Multiple inlet systems cannot be stable as Marsdiep is the dominant inletVlie inlet will further decrease its volume in the long term.

Analysis of the sediment budgets of ebb-tidal deltas and basins of the individual inletsystems shows that basin sedimentation and ebb-tidal delta erosion are closely related. Thisimplies that much of the sediments needed by the basin are supplied by the ebb-tidal deltas.In case of Texel inlet as much as 5 to 6 Mm3/year erodes from the ebb-tidal deltas. Lookingat the evolution of the basin, which needs a lot of sediment to regain equilibrium (hundredsof millions) and the only limited sediment volume in the ebb delta. We hypothesize on twoopposite developments:

Hypothesis (5): Limited sediment availability in the ebb-tidal deltas and adjacent coastseventually reduces the sediment influx and leads to deepening of the Wadden Sea.

or

Hypothesis (6): Limited sediment availability in the ebb-tidal deltas and adjacent coastsbut continued large sediment influx into the Wadden Sea leads to significantly enlargederosion of the adjacent coastlines (including Holland coast).



From the analysis of the Marsdiep and Vlie changes it is clear that these inlets cannot belooked at separately. The basins form coupled systems that exchange sediment (fromMarsdiep to Vlie inlet). Looking at the individual sand budget of the Marsdiep Vaklodingenarea one could conclude that the effects of Closure have minimized. Reanalysis of thetopographic changes, taking Vlie inlet into account, clearly shows that the effect of theZuiderzee is still far from damped out.

Hypothesis (7): The effect of Closure of the Zuiderzee is still far from damped out,sediment influx related to regaining equilibrium will dominate sediment budget for manydecades to come.

More insight in the role of the Wadden Sea would be obtained by continuing the analysis inthe remaining parts of the Dutch Coast. Nevertheless, based on this first analysis the crucialrole of the sediment exchange between basin and adjacent coasts has been shown. Long-term model approaches systematically checking the influence of sediment influx into theWadden Sea due to different scenarios of development could develop more understanding inthe sediment budget, and help to predict erosion rates.



4 The Holland coast

This chapter discusses the sand balance analysis of the Holland Coast. The Holland Coast isthe central part of the Dutch coast between Den Helder to Hoek van Holland that behaves asa beach-dune system in strong interaction with the barrier island coast in the north and thedelta coast in the south. The northern (north of Egmond) and southern (south ofScheveningen) sections of the Holland coast suffer from structural erosion because of e.g.the sediment importing capacity of the neighbouring tidal inlets. In Section 4.2 availableliterature and data used for the analysis is discussed and in Section 4.3 two methods used toderive sand balances for the Dutch coast are discussed. The results of the analysis arepresented in Section 4.4.

4.1 Present knowledge of the Holland Coast

The ‘closed’ Holland coast has a length of approximately 118 km (ranging from Den Helderin the North (km 0) to Hoek van Holland in the South (km 118,5)) and consists of a concavesandy coastline. This closed coastal section is only interrupted significantly by the harbourmoles of Scheveningen and IJmuiden and the bastion formed by the Hondsbosche enPettemer sea defence.

In the past two centuries the coast of the so-called ‘Kop van Noord-Holland’ (from RSP1 km2 to km 31) and of Delfland (from RSP km 98 to km 118) has been strengthened bij groinswith a lengths ranging from app. 80-100 m. Furthermore some form of sea defence has beenpresent between Camperduin and Petten (RSP km 20 to 26) since the 15th century.Continuing erosion on the north and south of this sea defence has led to the current bastionformation.

The harbour moles of IJmuiden, built at the time the North Sea Canal from Amsterdam toIJmuiden was constructed (in the period 1865-1879) and extended between 1962 and 1967),and of Hoek van Holland (first constructed fin the period 1864-1874, extended in the period1968-1972) are of significant impact from a morphological perspective. They form a barrierfor all along shore sediment transport and are for that reason dominating the morphologicaldevelopments in their vicinity.

Since 1963 the Dutch coast is measured yearly along a fixed system of transects that arespaced approximately 250 m apart, the so-called JARKUS measurements. Based on thesemeasurements Wijnberg (1995) identifies a number of five coastal sections along theHolland coast that exibit distinct morphodynamic behaviour:

1 The Rijks-StrandPalen (RSP), or RSP monuments are spaced roughly 250 m apart along the entireDutch North Sea coast. Along the closed Holland coast this system of monuments represents thedistance in kilometers to Den Helder, the so-called RSP (Rijks-StrandPaal) position.



Tabel 4.1: Characteristic morphodynamics in five coastal sections along the Holland Coast(according to Wijnberg (1995).Section Characteristics

Section 1, RSP km 3-8 Coastline recession, steepening of the coastal profile, small sand bars.Direct influence by ebb-delta Marsdiep. Morfological changes in thearea tide dominated and driven by the landward channel migration ofthe Nieuwe Schulpengat.

Section 2, RSP km 8-23 Coastline recession. Local progradation resulting from extensivenourishment. Indirect effects from the Marsdiep ebb-delta. Transitionfrom tide to wave domination. Tendency for profile steepeningresulting from chaning bathymetry outside the surfzone. Variation inshoreface morphology giving rise to variable wave attack on coastaldefences.

Section 3, RSP km 23-55 Coastline recession and a stable coastline respectively. Morphologicalchanges wave dominated. Variations in profile steepness. Periodicbehaviour of the multiple barsystem (timescale 15 years)

Section 4, RSP km 56-98 Stable coastline and profile steepnes with small fluctuations from yearto year. Periodic behaviour of the multiple barsystem (timescale 4-5years)

Section 5, RSP km 98-118 Generally small coastline fluctuations (beach stabilized with groins)with exception of the sedimentation around the harbour moles ofScheveningen and Hoek van Holland and following sand nourishment.Profile steepness variations due to human intervention in the system.No permanent presence of sandbars

Van de Rest (2004) provides an overview of studies regarding the historic coastlinedevelopment of the Holland coast. Most studies show coastal erosion north of Egmond andsouth of Scheveningen and accretion in the area in between. According to Van Vessem(1990) the average erosion south of Scheveningen is approximately 0.35 m/year in theperiod 1850-1990 whereas the average erosion north of Egmond is 0.9 m/year (mainlyattributed to the influence of the Marsdiep). Van Vessem (1990) estimates an accretion in thearea in between of about 0,25 m/year with locally higher values around the harbour moles ofIJmuiden.

A more detailed analysis was performed by Bouwmeester et al. (1994), see Table 4.2. Basedon an analysis of JARKUS measurements over the period of 1963 – 1992, Bouwmeester etal. (1994) present the natural trends for subareas along the Dutch coast. They distinguish theshoreface(NAP -6m to NAP -1 m), the beach (NAP -1m to NAP +3m) and the first dunerow(above NAP +3m). The resulting volumes where corrected for human interventions, in thiscase mainly nourishments. Based on their analysis Bouwmeester et al (1994) conclude thatthe development of the Holland coast is characterized by short erosion areas alternated withshort sedimentation areas. This fluctuating pattern is thought to be correlated with themigration of sandbars. Further more they find that the development of the shoreface, thebeach and the migration of sandbars is highly coupled



Tabel 4.2: Trends of erosion and sedimentation, corrected for nourishments (according toBouwmeester et al., 1994).Section RSP-line trendShoreface (zone between NAP -6m and NAP -1m):Den Helder- de Kaap 0.90-7.89 significant erosive trend, stronger in the last 10 yearsde Kaap- Pettemer zeewering 7.89-17.00 mostly mild accretionPettemer- and Hondsb. zeewering 20.27-26.15 erosive trendHondsb. zeewering- Castricum 27.00-47.00 fluctuating coastline (surfzone ridges)Castricum- just before IJmuiden 47.00-49.25 lee side erosiondirectly north of IJmuiden 49.25-50.25 significant accretiondirectly south of IJmuiden 56.25-59.50 significant accretionBloemendaal-Zandvoort 60.50-68.00 significant erosionZandvoort-Noordwijk 68.00-92.00 from a positive to a negative trendDuinoord-Wassenaar-zuid 92.00-97.00 significant erosionScheveningen-km 113 102-113 statistical but not significant accretion

113-117 significant accretiondirectly north of HvH 117-118 erosionBeach (zone between NAP-1m and NAP +3m):Den Helder-Egmond aan zee 0-39.00 near complete retreat, except for km 9Egmond-Castricum 39.00-47.50 alternating erosion and sedimentationCastricum- just before IJmuiden 47.50-49.25 erosiondirectly north of IJmuiden 49.25-50.25 significant accretionRijnland 56.25-101.4 same trend as the shoreface, however, still

significantDelfland 102-117 significant accretiondirectly north of HvH 117-118 erosionDunes (zone above NAP +3m):Den Helder-Castricum 0-47.50 no clear pattern, alternating erosion/sedimentationCastricum- even voor IJmuiden 47.50-49.25 erpsopmdirect ten noorden van IJmuiden 49.25-50.25 strong accretionRijnland 56.25-101.4 same trend as the shoreface and beach, however,

prevailing accretion only turns to erosion at km 94.Delfland 102-117.5 significant accretiondirectly north of HvH 117.5-118 erosion

Based on the various budget studies Van de Rest concludes the following concerning thecoastal evolution (exluding nourishments) in the nearshore zone NAP+3/-8m:

From Hoek van Holland to Den Helder an erosive trend exists of 150.000-300.000m3/year;From Egmond (RSP km 40) to Den Helder a structural erosive trend may be observedwith small accreting areas;Between RSP km 40 and 47 the coast is relatively stable (with a mild tendency toaccretion)In the sections RSP km 47-50 and 60-68 erosion caused by the (extension) of theharbour moles of IJmuiden can be observed.On both sides of the IJmuiden harbour moles (RSP km 50-60) structural erosion may beobserved.The section from RSP km 68-92 shows a mild accretive trend.From RSP km 92 to 98 structural erosion may be observed correlated to the transition ofa groin protected coast to a clean sandy coastBetween RSP km 98 and 105 the coast is relatively stable; south of km 105 structuralerosion may be observed caused by the harbour moles of Hoek van Holland.



Besides an overview of the natural patterns of erosion and sedimentation it is important tohave insight in the order of magnitude of the along shore transport as a function ofalongshore distance. Based on a comparison of existing transport studies (incl. adjustmentsfor recent measurements) Van de Pol (2004) puts forward the along shore sediment transportcurve depicted in Figure 4.1:

Figure 4.1: Yearly averaged alongshore sediment transport, in the zone NAP +3/-8m (sourceVan de Pol, 2004).

The combination of dominant waves from directions varying from SW to NNW, relativelystrong northbound tidal current during floodtide and a NNE coastline orientation all giverise to a net northward directed sediment transport along the Holland Coast. Moving northfrom the harbour moles of Hoek van Holland sediment transport gradually increases due tothe gradually decreased sheltering influence of the Noorderdam. Along the Delfland coastgroins keep the transport relatively constant. North of RSP km 98, in the unprotected sandyarea, transports gradually increase only decrease again due to the reorientation of thecoastline. Just south of IJmuiden the northward directed sediment transport increases againdue to sheltering from the northern wave directions. Reversely transports north of IJmuidenare directed southward due to the sheltering of southern wave directions. Near Egmondtransports turn north again, gradually increasing to an estimated 400.000 m3/year near DenHelder. NB: large uncertainties regarding the net transports exist especially in the so calledKop van Noord-Holland due to the higly complex interactions with the Waddensea ingeneral and the Marsdiep ebb-delta in particular.

4.2 Analysis of the Holland Coast

To increase our understanding of the Holland coast and its interactions with the Wadden Seaand Voordelta a sand balance analysis of the Holland coast was set up similar to that of theWadden Sea. Even though, as became apparent from the literature review, a lot is alreadyknown about the erosion, accretion and sediment transport patterns along the Holland coast,additional analysis is worthwhile for a number of reasons. First of all new data has becomeavailable since many of the above mentioned studies became available. Secondly, neverbefore have we had so many different data sets available in one analysis environment. Andthird, to our knowledge not many sediment budget analyses of the entire Dutch coastalsystem and its constituent components have ever been executed with one consistent



approach with this level of detail. Furthermore, a smoothed NAP-20 m depth line is used asseaward boundary in the present analysis, covering a larger area.

4.2.1 Fixed-map grid data: Vaklodingen data

To obtain a first insight in the volume development of the Holland coast, the analysisapproach outlined in Chapter 2 was applied to the fixed-map grid Vaklodingen data. For thispurpose the Holland coast was subdivided into the four areas suggested by Nederbragt(2006) (viz. Noord-, Zuid-Holland, IJmuiden and Nieuwe waterweg). Those areas werefurther subdivided into 3 zones with depth ranges from NAP +3m to NAP -2m, from NAP -2m to NAP -5m and NAP -5m to NAP -20m.

However, analysis based on fixed-map grid Vaklodingen data proved problematic as(semi)complete coverage for the four proposed areas of the Holland coast is not availableprior to 1990 and is available for a limited number of years after 1990 only (see Table 7-1).Despite the limited amount of available data the UCIT method described in Chapter 3 wasapplied, using the year 1999 as reference year.

Table 4-3. Available fixed-map grid data of ‘Vaklodingen’ data with (semi-)completecoverage for the four selected sub areas of the Holland Coast since 1990sub-area available years

Noord-Holland 1990 - 1996 1999 2001 - - 2006

Zuid-Holland 1990 1993 1996 1999 - - 2005 -

IJmuiden 1990 - 1996 1999 2001 - 2005 -

Nieuwe Waterweg - 1993 1996 1999 - 2004 - -

The Holland coast as a whole

Figure 4-2. Sediment budget of the Holland Coast (the vertical axis shows cumulativesedimentation in Mm3).



Figure 4-2 presents results using fixed-map grid data for the 4 main areas of the HollandCoast. For the large scale volume evolution obviously the larger polygons of Noord- andZuid-Holland dominate the overall volume changes. The volumes in this area, however, alsoshow large variability. As yet this variability is not very well understood. Figures 4-3 to 4-6present separate results for each of the polygons where a further subdivision into three depthranges is made. In these figures the black line shows the sediment budget trend (cumulativesedimentation in Mm3) for the total area, the blue for the depth range NAP -5m to NAP -20m, the purple line for the depth range NAP-2m to NAP -5m and the green line shows thesediment budget trend for the depth range NAP +3m, to NAP-2m.). It can be seen fromFigure 4-6 that the Nieuwe Waterweg as a whole is deeper than NAP -5m and therefore onlyshows the sediment budget trend for the total area. It must be noted that the polygonscovering the three depth ranges were in most cases not filled with data completely; exceptfor 1999 which has a good coverage.

Sub area Noord Holland

Figure 4-3. Sediment budget of three depth ranges of Noord-Holland (black line shows totalarea, blue is the depth range NAP +3m to NAP -2m, purple is the depth range NAP-2m toNAP -5m and the red dotted line is the depth range NAP -5m, to NAP-20m.).

Looking at Figure 4-3 the same signature can be observed that was seen in Figure 4-2. Theshallower regions, which have a more consistent data coverage, show a much more stabletrend. Main observables from Figure 4-3 are that:

The depth ranges of NAP +3m to NAP -2m and NAP -2m to NAP -5m mostly showsedimentation ( V= ±4 Mm3 and ±5 Mm3). The cumulative sedimentation in this arealies in the order of 9 Mm3 over a period of 15 years (app. +0.6 Mm3/year since 1990).The depth range of NAP -5m to NAP -20m shows unexpected ‘extreme’ variability. Thesurface area of this depth range lies in the same order of magnitude as the total surfacearea, which consequently dominates the estimated volume changes of the total area. Thevariability triggers questions as to what happens in this area.



Sub area Zuid Holland

Figure 4-4. Sediment budget of three depth ranges of Zuid-Holland (the black line showstotal area, blue is the depth range NAP +3m to NAP -2m, purple is the depth range NAP-2mto NAP -5m and the red dotted line is the depth range NAP -5m, to NAP-20m.).

Looking at Figure 4-4 the same signature can be observed that was seen in Figure 4-2 forthis sub-area. The shallower regions, which have a more consistent data coverage, show amuch more stable trend here as well. Main observables from Figure 4-4 are that:

The depth ranges of NAP +3m to NAP -2m and NAP -2m to NAP -5m mostly showsedimentation ( V= ±10 Mm3 and ± 4 Mm3). The cumulative sedimentation in this arealies in the order of 15 Mm3 over a period of 15 years (app. +1 Mm3/year since 1990).Also for Zuid-Holland the depth range of NAP -5m to NAP -20m shows unexpectedvariability. As, here as well, the surface area of this depth range is of the same order ofmagnitude as the total surface area, the total volume changes are dominated by thechanges is this sub-area. The variability triggers questions as to what happens in thisarea. Figure 4-4 also shows incomplete data coverage of this area as mentioned above.



Sub area IJmuiden

Figure 4-5. Sediment budget of three depth ranges of IJmuiden (the black line shows totalarea, blue is the depth range NAP +3m to NAP -2m, purple is the depth range NAP-2m toNAP -5m and the red dotted line is the depth range NAP -5m, to NAP-20m.).

Figure 4-5 shows that:The depth range of NAP -5m to NAP -20m dominates the trend of the total area.Looking at the trend in the results of the NAP -5m to NAP -20m range, somesedimentation was found prior to 1996 ( V= ±2 Mm3) and deepening was found after1996 ( V= ±4 Mm3) which is due to dredging (maintenance and/or sand mining).The depth ranges of NAP +3m to NAP -2m and NAP -2m to NAP -5m show negligiblevolume changes.



Sub area Nieuwe waterweg

Figure 4-6. Sediment budget of the Nieuwe Waterweg

The Nieuwe Waterweg as a whole is deeper than NAP -5m and therefore only shows thesediment budget trend for the total area. Furthermore, it must be noted that 1999 has a baddata coverage while 2004 has a good data coverage. Best conclusion to be drawn fromFigure 4-6 is that too few data points are available to draw a convincing trend. Should thepoorly covered year 1999 be ignored, a best estimate for a trend would be an averageddeepening in that sub-area of 0.14 Mm3/year, probably due to dredging (maintenance andsand mining).

4.2.2 Fixed-map grid data: JARKUS data

Even though some information could be extracted from the Vaklodingen data, a properanalysis was inhibited by the poor data coverage in time as well as in space. A different datasource that has a better temporal resolution is the gridded JARKUS data. Downside to thatdata set is that it does not provide complete coverage (up to the NAP -20m) of the coastalfoundation. However, as more data points are available most probably a more convincingimage of at least the near shore volume evolution can be produced.

Figure 4-7 shows the results of a budget analysis on fixed-map grid gridded JARKUS data,using the same polygons for Noord-Holland and Zuid-Holland as in the analysis in section4.2.1. NOTE: in this case the volume changes have also been corrected for humaninterventions (nourishments). This analysis was mainly performed to enable comparison ofthe orders of magnitude changes suggested by the Vaklodingen data vs those suggested bythe gridded JARKUS data.



The Vaklodingen data cover a period from app. 1990 to 2004, yielding app. 10 and 15 Mm3

of uncorrected total volume change for Noord-Holland and Zuid-Holland respectively overa period of 14 years. When looking at the same period in the gridded JARKUS data we seeapp. 10 and 17 Mm3 of uncorrected total volume change for Noord-Holland and Zuid-Holland respectively. This gives us some confidence of the consistency of our analysis andindeed in the consistency of the various datasets we use.

Figure 4-7. Cumulative sedimentation of sub areas Noord- en Zuid-Holland with (solid line)and without nourishments (dashed line).

4.3 Analysis of sub-areas of the Holland coast

Figure 4-8 to 4-11 show cumulative sedimentation for a range of sub-areas of Noord- andZuid Holland based on fixed-map grid gridded JARKUS data with (solid line) and withoutnourishments (dashed line). The coming plots may be read as a journey from north to south.

Figure 4-8 shows the volume analysis of four sub-areas running from Den Helder to Botgat(NH8), from Botgat to just north of the Pettemer zeewering (NH7), from just north of thePettemer zeewering to the start of the Hondsbossche zeewering (NH6), and from theHondsbossche zeewering to Camperduin (NH5).

The most northern sub-area (NH8) is obviously suffering from erosion by Marsdiep. It canbe seen that this area is steadily loosing sediment. One can also see that this erosive trendhas been effectively halted as a result of continous nourishment.

The next subarea (NH7) contains the area of Callantsoog. This is an area that is regularlynourished which is reflected by a significant difference between the plain and the correctedsediment volume.



The next two sub-areas (NH 6 en 5) both cover one of the two transitions from dike to dune.At the end of the last century, these transition zones have regularly been strengthened bynourishment. These nourishments show up in the graphs. All in all one may conclude that allof the sub-areas have over the years suffered some mild erosion with significant erosionnear Den Helder.

Figure 4-8. Cumulative sedimentation northern sub areas of Noord Holland based oninterpolated JARKUS data.



Figure 4-9. Cumulative sedimentation southern sub areas of Noord Holland based oninterpolated JARKUS data.

Figure 4-9 illustrates what happens to the volumes if we continue our journey southward.Sub-area NH4 (covering Egmond and Bergen) shows some significant impact of thenourishments on the total volumes. The two following subareas (NH3 and NH2) can be saidto be more or less stable, where NH2 is mildly erosive as a result of leeside erosion. Finallysub-area NH1, which lies in the shadow zone of the IJmuiden harbour moles clearly showsthe sedimentation pattern which was discussed earlier.



Figure 4-10. Cumulative sedimentation northern sub areas of Zuid Holland based oninterpolated JARKUS data.

Figure 4-11. Cumulative sedimentation southern sub areas of Zuid Holland based oninterpolated JARKUS data.

Figure 4-10 and 4-11 continue the pattern from the previous pictures, depicting the volumechanges for a number of sub-polygons. Again from North to South we first encounter sub-area ZH1. This subarea is clearly accreting. This is as was expected. The total sedimentationvolume in the South appears to be roughly twice that of the subarea in IJmuidens shadow



zone (12.5 and 7 Mm3 over a period of 32 years). Especially when corrected for humanintervention the next four sub-areas (ZH2 – ZH5) are more or less stable in terms ofsedimentation and erosion. Sub-area ZH3 could be considered mildly erosive, which couldthen be attributed to harbour moles of IJmuiden as this sub-area is roughly located at thespot where sediment transports are picking up. Sub-areas ZH4, ZH5 and ZH6, Zandvoort,Noordwijk and Katwijk respectively, all have been significantly nourished. Area ZH7 ismildly accreting, possibly because it contains the harbour moles of Scheveningen, that,much like the IJmuiden harbour moles, catches sediment in its shadow zones. Sub-area ZH8contains the weak link Terheijde, which explains the significant nourishment of that area.Finally sub-area ZH9 lies in the shadow zone of Hoek van Hollands Noorderdam. Thespectacular volumetric jump in the figure is in deed explainable. In the early seventies, alarge sandbuffer (app. 18 Mm3 sand) was placed around that area. Because the JARKUSmeasurements did not always properly contain the dune area, much of this intervention doesnot show up in our analysis polygons.


Looking back on the detailed analysis of the Holland coast we can say that in a broad sensethe findings from the literature review reported in Section 4.1 have been confirmed. Basedon the data analysis the following hypotheses are formulated:

The available data, especially in the deep zone, are too limited in quantity and/or qualityin order to say anything about the development of the coastal foundation.In the coast-line zone (above NAP -5 m) the amount of sediment has been increasingsince 1990 due to the coast nourishment policy. The increase has been higher thanneeded for compensating the sea-level rise.The erosion of the Holland coast as a whole is mainly driven by the sediment-deficit inthe Marsdiep.Nourishment increases natural losses due to erosion. This is probably because that thecoast profiles become too steeper after nourishment. Estimation for the nourishmentdemand, for e.g. maintaining the coastal foundation, can therefore not simply be basedon the historic loss and the sea-level rise rate.



5 The Delta area

This chapter discusses the sand budget analysis of the Delta area. The Delta area is thesouthern part of the Dutch coast south of Hoek van Holland featuring tidal inlets of which anumber have been closed off in the period 1958 to 1986 as part of the Delta Works. Notethat only the ebb tidal deltas, not the tidal basins, are considered here. In Section 5.1 thepresent knowledge of the Delta area is discussed briefly. In Section 5.2 and 5.3 analysis ofthe Delta area and its sub areas is presented. In Section 5.4 a reflection on the analysis isgiven and hypotheses are formulated.

5.1 Present knowledge of the Delta area

The sand balance analysis of the Holland coast was set up similar to that of the Wadden Sea,with 4 main areas (Haringvliet, Grevelingen, Oosterschelde and Westerschelde) and a moredetailed subdivision based on the local bathymetry (channels, shoals) and the sedimentationerosion pattern. Fixed-map grid data (Dutch: kaartblad data) from the WADI database with(semi-)complete coverage for the main areas of the Delta coast has been used. The UCITmethod for fixed-map grid data described in Chapter 2 was used with the year 1998 asreference year, see Table 5-1.

Table 5-1 Used fixed-map grid data with (semi-)complete coverage of the main areas of theDelta coast

sub-area available years

Haringvliet 1992, 1995, 1998, 1999, 2003

Grevelingen 1960, 1992, 1984, 1992, 1995, 1998, 1999, 2001, 2004

Oosterschelde 1960, 1964, 1968, 1972, 1976, 1980, 1984, 1986, 1988, 1989, 1992,1995, 1998, 2001, 2004

Westerschelde 1968, 1972, 1976, 1980, 1984, 1989, 1992, 1998, 2001

In the Haringvliet and Grevelingen the currents are directly related to the tide at the NorthSea, while the pattern of tidal currents in the Eastern Scheldt is much more complex thanthat of the Haringvliet and Grevelingen due to interactions with the water flowing in and outof the estuary. At the Haringvliet the currents may be dominated by river discharges fromthe Haringvlietsluizen. With flood the water enters the Eastern Scheldt along the southernside and leaves along the northern side, with ebb this pattern reverses.

Hereafter, a number of conclusions from the Alkyon report “Kwantitatieve analyse enprognose morfologische ontwikkeling Voordelta – concept, September 2006” aresummarized. The extension of the slufter continues to extend southward and will eventuallyhinder the outflow from the Hindergat, which in time may be abandoned. The north coast ofGoeree accretes due to attraction of sediment from the west. The coast of Voorne is erosiveand requires regular nourishments. The mouth of the Haringvliet becomes shallower fromsupply of sediment to the area. The main developments of the Grevelingen involve the coast



of North Schouwen, which accretes and the coastal profile flattens. The southwestern coastof Goeree erodes. The Beach of the Brouwersdam migrates northward and eventuallyreduces as it reaches the former Hompelchannel.The seaward migration of channels in theEastern Scheldt continues, while sand ridges migrate landward.

The development of the Western Scheldt is dominated by sea level rise and an increasingtidal amplitude. Sea level rise typically leads to a sediment demand, However sea level risecan lead to a change from sediment import to sediment import at Vlissingen due to the evenlarger shortage of sediment in the mouth (Wang, 1997). At first, this leads to sedimentationof the mouth area from increasing import of sediment from the North Sea and adjacentcoasts and reduction of import from the mouth to the estuary or export from estuary tomouth. The tidal amplitude at the North Sea shows a structural increase (of about 3-4% percentury) which leads to a larger tidal prism and erosion of channels; sediment is exportedfrom the estuary. The effects of sea level rise and increasing tidal amplitude will thereforerather strengthen than weaken each other.

5.2 Analysis of the Delta area

Figure 5.2 shows nourishment volumes of the Delta area (Nederbragt, 2006). Figures 5-3and Table 5-2 provide a plan view and completion dates of the Delta Works which havegreatly influenced sedimentation in the inlets.

Figure 5-1 presents the cumulative sedimentation using fixed-map grid data for 4 inlets ofthe Delta coast. The 4 sub areas are discussed separately and in more detail in Section 5.3. Itcan be seen from Figure 5-1 that for both the Eastern and Western Scheldt a peak insedimentation was found in the late 70’s, which was not found for the Grevelingen andHaringvliet (although for the latter no data was available prior to 1984). For both the Easternand Western Scheldt erosion was found from 1980 to the early 90’s. All 4 inlets show a shortperiod of sedimentation in the early 90’s and then show erosion throughout the period ’94-’04. This was also found from literature but needs further research as the loss is too large tobe explained by the gradient of longshore transport, exchange with the basins and loss to thedeepwater zone.

Figure 5-1. Sediment budget of the Delta area.



Figure 5-2. Nourishments volumes Delta coast (Nederbragt, 2006).

Figure 5-3. Plan view Delta Works.

Table 5-2. Start and completion dates of various parts of the Delta works.

Delta works completion start

Veersegatdam 1961

Zandkreekdam 1961

Grevelingendam 1965 1958

Volkerak 1969 1957

Haringvlietsluizen/dam 1971 1956

Brouwersdam 1972

Oosterscheldekering 1986

Philipsdam 1987

Osterdam 1987



5.3 Analysis of sub elements of the Delta area

5.3.1 Haringvliet

Although in general the data coverage of the Haringvliet is good between 1992 and 2003and 1995 and 2003 have complete data coverage, the year 1998 is incomplete. It can be seenfrom Figure 5-1 that the Haringvliet was accretive between 1984 and 1992 and erosive from1992 to 2004. Overall the Haringvliet loses about 8 Mm3 between 1984 and 2004.

The cumulative sedimentation of the Haringvliet between 1984 and 2004 given in Figure 5-1 was compared to that of De Bok (2002) (see Figure 5-4) which is based on data prior to1999. It can be seen that the general trend between 1992 and 1999 is roughly equal (about -2.5Mm3/yr) and that both Figures show a short period of sedimentation in the early 90’s.

Ontwikkeling zandvolume t.o.v. 1964 van subdeelsysteem"Haringvliet"

-20

0

20

40

60

80

1960 1970 1980 1990 2000

jaar

zand

volu

me

vera

nder

ing

[106 m

3 ]

NAP -10 m NAP NAP +3 m

Figure 5-4. Sedimentation rates of the Haringvliet (De Bok, 2002).

Table 5-3. Estimates sedimentation rates Haringvliet (Nederbragt, 2006)

dumping/dredging

transport rate trend

best estimate, lower , upper

sea level rise total

-0.1 Mm3/yr -1.2 Mm3/yr 0.8 Mm3/yr -0.5 Mm3/yr -0.4 Mm3/yr

From 1991 to 2005 3.18Mm3 was nourished at Goeree and 0.85 Mm3 at Voorne, see Figure5-5 (Nederbragt, 2006). As no data was available prior to 1992 the effect of construction andcompletion of the Volkerakdam between 1958 and 1969 and the Haringvlietsluizen/dambetween 1957 and 1971 cannot be assessed.



Figure 5-5 shows the subdivision of the Haringvliet based on the local bathymetry (channelsand shoals). Results of sediment budget analysis (cumulative sedimentation) of these subareas are given in Figures 5-6 and 5-7. It can be seen that for the Haringvliet as a whole(black line) sedimentation takes place before 1995 and erosion after 1995. Sub area 1 and 3,mostly shoals and land, show a different trend: erosion prior to 1995 and sedimentation after1995. Sub areas 2,4 (channels) show relatively small changes while the largest sub area (5,deep water) also shows the largest changes , with sedimentation before 1995 and erosionafter 1995. It further should be mentioned that significant changes were found in the shortperiod between 1998 and 1999 and that dumping and dredging volumes were not taken intoaccount in the present analysis.

Figure 5-5. Subdivision of the Haringvliet.



Figure 5-6. Sediment budget of sub areas of the Haringvliet with 2003 as reference year (seealso Fig.5.1 for the definition of the sub areas).

Figure 5-7. Cumulative sedimentation per sub area of the Haringvliet with 2003 as referenceyear (definition of the sub-areas is given in Fig.5.1).



5.3.2 Grevelingen

The cumulative sedimentation of the Grevelingen (Figure 5-1) shows a total loss ofsediment of about 30 Mm3 between 1960 and 2001, which was not shown in De Bok (2002),see Figure 5-8. Both Figures show sedimentation from about 1970 to 1992 De Bok reports amuch larger sedimentation volume than what was found from Figure 5-1. It must bementioned that for the Grevelingen both 1960 and 1972 have incomplete data coverage ascan be seen from the error bars in Figure 5-1 and therefore are considered less reliable,furthermore dumping and dredging volumes were not taken into account in the presentanalysis.

Ontw ikkeling zandvolume t.o.v. 1964 van subdeelsysteem" Grevelingen"

-20

0

20

40

60

80

1960 1970 1980 1990 2000

jaar

zand

volu

me

vera

nder

ing

[106 m

3 ]


Figure 5-8. Sedimentation rates of the Grevelingen (De Bok, 2002).

Table 5-4. Estimates sedimentation rates Grevelingen (Nederbragt, 2006)

dumping/dredging




-0.4 Mm3/yr -2.8 Mm3/yr 0.5 Mm3/yr -0.4 Mm3/yr -0.8 Mm3/yr

From 1991 to 2005 3.18Mm3 was nourished at Goeree and 0.85 Mm3 at Voorne, see Figure5-5 (Nederbragt, 2006). Furthermore, the Brouwersdam was completed in 1972 and the tidalinlet was closed off, which can seen from Figure 5-1 from the change from a sand exporting(erosion) to sand importing system (sedimentation); about 10 years after completion thedeeper areas are starting to erode again, while shallower parts and the channel continue toimport sand (sedimentation). The completion of the Grevelingendam cannot directly be seenfrom Figure 5-1.

Figure 5-9 shows the subdivision of the Grevelingen based on the local bathymetry(channels and shoals). Results of sediment budget analysis (cumulative sedimentation) of



these sub areas are given in Figures 5-10 and 5-11. It can be seen that sub area 4 (deepwater) determines the overall total sedimentation of about 132 Mm3 between 1960 and2004. The volume of sub area 1 (channel) slowly decreases after 1972, both sub area 2(shoals) and sub area 3 (shallow water) also show this trend. Sub area 3 shows a muchstronger erosion after 2001, which was also found at deeper water (sub area 1). It can beconcluded that the Grevelingen as a whole erodes as erosion at deep water dominatessedimentation at shallower water.

Figure 5-9. Subdivision of Grevelingen.

Figure 5-10. Sediment budget of sub areas of the Grevelingen.



Figure 5-11. Cumulative sedimentation per sub areas of the Grevelingen..

5.3.3 Eastern Scheldt

The cumulative sedimentation of the Eastern Scheldt (Figure 5-1) shows sedimentation priorto 1980 and erosion after 1980, except for a short period of sedimentation in the early 90’s,which complies with the trends shown in Figure 5-12 (De Bok, 2002). The sedimentation inthe period 1960 to 1980 is somewhat higher (about +90 Mm3) than that of De Bok (about+70 Mm3), De Bok further reports much less erosion after 1992 (about -20Mm3) withrespect to Figure 5-1 (-50Mm3), although dumping and dredging volumes were not takeninto account in the present analysis. From 1991 to 2005 6.24Mm3 was nourished atSchouwen, 1.93Mm3 at Noord-Beveland and 13.95Mm3 at Walcheren, see Figure 5-5(Nederbragt, 2006).



Ontwikkeling zandvolume t.o.v. 1964 van subdeelsysteem" Oosterschelde delta"

-20

0

20

40

60

80

1960 1970 1980 1990 2000

jaar

zand

volu

me

vera

nder

ing

[106 m

3 ]


Figure 5-12. Sedimentation rates of the Eastern Scheldt.

Table 5-5. Estimates sedimentation rates Oosterschelde (Nederbragt, 2006)

dumping/dredging




-1.1 Mm3/yr -3.6 Mm3/yr -0.1 Mm3/yr -0.6 Mm3/yr -1.7 Mm3/yr

Figure 5-13 shows the subdivision of the Eastern schelde based on the local bathymetry(channels and shoals). Results of sediment budget analysis (cumulative sedimentation) ofthese sub areas are given in Figures 5-14 and 5-15. After 1970 most channels erode, whileshoals accrete. After 1980, this pattern is reversed with sedimentation of channels anderosion of shoals, except for the channel of sub area 9, which first shows accretion and then,after 1992 mostly erodes. The effect of construction and completion of the Volkerakdam andOosterscheldekering can be seen from Figures 5-4 and 5-15 with an increased tidal prismand growth of the outer delta after completion of the Volkerakdam and smaller tidal prism,increased wave-effect and erosion after completion of the Oosterscheldekering.



Figure 5-13. Subdivision of the Eastern Scheldt.

Figure 5-14. Sediment budget of sub areas of the Eastern Scheldt.



Figure 5-15. Sediment budget of sub areas of the Scheldt.



5.3.4 Western Scheldt

The cumulative sedimentation of the Western Scheldt (Figure 5-1) shows sedimentationprior to 1976 and erosion after 1976, except for a short period of sedimentation in the early90’s, which complies with the trends shown in Figure 5-19 (De Bok, 2002). Thesedimentation in the period 1968 to 1976 is of the same order (about +70 Mm3) in both



Figures, but De Bok reports much erosion after 1992 (about -10Mm3) with respect to Figure5-1 (-40Mm3), although dumping and dredging volumes were not taken into account in thepresent analysis.

From 1991 to 2005 3.76Mm3 was nourished at Zeeuws-Vlaanderen, see Figure 5-5(Nederbragt, 2006).

Table 5-5. Estimates sedimentation rates Westerschelde (Nederbragt, 2006)

dumping/dredging




-2.7 Mm3/yr -3.3 Mm3/yr -1.5 Mm3/yr -0.8 Mm3/yr -3.5 Mm3/yr

Figure 5-18. Cumulative sand volumes for the Dutch part of the mouth of the WesternScheldt (black is total volume, blue dotted is cumulative volume and red natural volume),After Nederbragt and Like (2004).



Ontwikkeling zandvolume t.o.v. 1964 van subdeelsysteem" Westerschelde delta"

-80

-60

-40

-20

0

20

40

60

80

1960 1970 1980 1990 2000

jaar

zand

volu

me

vera

nder

ing

[106 m

3 ]


Figure 5-19. Sedimentation rates of the Western Scheldt (De Bok, 2002)

Figure 5-20 shows the subdivision of the Western scheldt based on the local bathymetry(channels and shoals). Results of sediment budget analysis (cumulative sedimentation) ofthese sub areas are given in Figures 5-20 to 5-22. It can be seen that the channels (sub area1,3 and 6) show gradual erosion, while sub areas 2, 4 and 5 first show sedimentation in theperiod 1968 to 1976 and then erosion in the period 1976 to 2001



Figure 5-20. Subdivision of the Western Scheldt.

Figure 5-21. Sediment budget of sub areas of the Western Scheldt.



Figure 5-22. Sediment budget of sub areas of the Western Scheldt.


The following hypotheses have been formulated on the basis of the data analysis in the deltacoast area:

All sub-systems in the delta show a trend of erosion since the early nineteen-nineties.The rate of the total loss of sediment from the delta coast is too high (order 10 Millionm3 per year) to be explained by the possibilities (gradient of longshore transport, importto the basins, loss to the deep water zones). The most probable explanation is thereforeinaccuracy in the data / data-analysis.The same applies for the trend of increasing amount of sediment in the WS and ESmouths in the period 1969-1980 (see Figure 5.1).The effect of the Delta Works to the morphological development of the delta coast isstill far from damped out.Exchange between the systems is negligible compared with the changes in the four sub-systems.

Remark: The first hypothesis needs some explanation. The only basin which is exchangingsand with the delta coast is the Western Scheldt. According to the recent insight (see e.g.Nederbragt and Like, 2004, HAECON, 2006) this exchange has become an export (frombasin to delta) since mid-nineties of the last century. This exchange can thus not explain theobserved loss. If it is explained by the gradient of the longshore transport it would mean atleast a transport of 5 million m3 at each of the two boundaries, much too high compared tothe commonly known rate of the longshore transport. Translated to the loss of the at theNAP-20 m line it would mean order 0.3 m2/day averaged over the whole length, much toohigh than expected.



6 Hypothesis testing by data analysis

6.1 Introduction

In the data analysis as presented in Chapter 3 only the total changes in terms ofsedimentation / erosion per area are considered. No consideration is made concerning thechanges of the morphological units, or in other words no distinction is made e.g. betweenthe changes in the inter-tidal flat and the channels. This chapter describes a further analysisof the field data with the objective to understand the observed developments and to make astart of the test of the hypotheses as formulated in Chapter 3.

The analysis has been carried out on the data of the hypsometry of the 5 tidal basins(Marsdiep, Eierlandsegat, Vlie, Amlanderzeegat en Friesche Zeegat). By hypsometry datawe mean that for each of these 5 basins the horizontal area at and the volume under a certainlevel are given as function of the level. These data are made available by RIKZ. Based onthis data set a series of parameters describing the morphological state and the hydrodynamiccharacteristics are determined for various years in the period since 1932. With theseparameters two kinds of analysis are carried out, viz. (1) how the morphology of each basinhas been approaching the equilibrium state according to the empirical relations, and (2) howthe characteristics determining the tidal asymmetry have been developing in the time.

6.2 Basic data

In the analysis presented here a fixed tidal frame is used per basin, i.e. the high water, meanwater and low water levels are assumed to be constant in time. The used tidal frames in the 5basins are given in Table 6.1. In reality the mean water level as well as the tidal range in abasin are changing in time due to e.g. sea-level rise. However, for the analysis presentedhere it is more convenient to keep the tidal frames constant in time. The small changes ofthe tidal frames are not expected to influence the conclusions drawn very much.

Tabel 6.1 Used tidal frames per tidal basin

Basin HW (m NAP) MW (m NAP) LW (m NAP) Tidal range (m)

Marsdiep 0.825 0 -0.825 1.65

Eierlandsegat 0.825 0 -0.825 1.65

Vlie 0.95 0 -0.95 1.90

Amlanderzeegat 1.075 0 -1.075 2.15

Friesche Zeegat 1.10 0 -1.10 2.20

With theses fixed tidal frame the following basic parameters can readily be derived for eachbasin and these basic parameters are given in Appendix B.



Area and volume at highest level

The hypsometry data per basin include A(z) and V(z), with z=vertical level, A=horizontal(wet) area at level z, and V=(wet) volume under level z. Examples of the supplied data arepresented in Figure 6.1. Details of the data are presented in Appendix B, including all thehypsometric curves and summary of the derived major parameters.

CumOpp in vak 1op 01/01/1933

-4270

-3770

-3270

-2770

-2270

-1770

-1270

-770

-270

230

0 2E+08 4E+08 6E+08

Cumulatief oppervlak (m²)

Hoo

gte

(cm

)

CumVol in vak 1op 01/01/1933

-4270

-3770

-3270

-2770

-2270

-1770

-1270

-770

-270

230

0 2E+09 4E+09 6E+09 8E+09

Cumulatief volume (m³)

Hoo

gte

(cm

)

Figure 6.1a Example of hypsometry data (Marsdiep in 1933), left: horizontal area; right:volume

Hypsometric curve

-3000

-2500

-2000

-1500

-1000

-500

0

500

0 0.2 0.4 0.6 0.8 1

A/Amax

Z (N

AP

cm)

Marsdiep 1933Eierlandsegat 1933Vlie 1933Amlanderzeegat 1926Friesche Zeegat 1927

Hypsometric curve

-200

-150-100

-50

0

50100

150

200

0 0.2 0.4 0.6 0.8 1

A/Amax

Z (N

AP

cm)

Marsdiep 1933Eierlandsegat 1933Vlie 1933Amlanderzeegat 1926Friesche Zeegat 1927

Figure 6.1b hypsometric curves of all five basins, in the first year when data are available,normalized with the maximum horizontal area (the basin area). Left: the whole curves,

Right: detail around the inter-tidal zone.

Both parameters are given for a sufficiently large range of z, i.e. from a low level z0 at whichboth A an V vanish to a highest level zL far above the HW. It is noted that there is a relationbetween the two parameters, per definition:

0

( ) ( )dz

z

V z A z z (6.1)

The area at the highest level zL is further used as the basin area, and the volume changeunder this heist level represent the total sedimentation (decrease) / erosion (increase) in thebasin.



( )( )

b L

tot L

A A zV V z

(6.2)

Area and volume at HW

( )( )

HW

HW

A A aV V a

(6.3)

Herein a = amplitude of tide (see Table 6.1).

Area and volume at MW

(0)(0)

MW

MW

A AV V

(6.4)

Area and volume at LW

( )( )

LW

LW

A A aV V a

(6.5)

6.3 Empirical relations for morphological equilibrium

First we have tested in how far the empirical relations for the morphological equilibrium aresatisfied by the different basins in different periods. As only data within the basin areconsidered and each basin is considered as a whole, two empirical relations can be tested,viz. the relation between the channel volume and the tidal prism and the relation betweenthe inter-tidal flat area and the basin area.

According to the first empirical relation the channel volume in a basin, defined as the wetvolume under the mean water level (NAP), is proportional to the power 3/2 of the tidalprism P (Eysink, 1990):

32

MWV P (6.6)

If the basin is in equilibrium the constant a is equal to 65*10-6 if both VMW and P are in m3.

The tidal prism P is derived as follows:

HW LWP V V (6.7)

The data from all five basins are presented in Figure 6.4 together with empirical relation(6.6) with a=65*10-6. As an overall picture (see the top-left panel in the figure) the field data



seem to agree well with the equilibrium relation (6.6). In fact relation (6.6) with the value ofthe constant a is probably derived from such data set. This means that data from basins inequilibrium as well as basins not in equilibrium have been used for deriving such empiricalrelations. Therefore the value of a as presented in literature should surely not be considered

as sacred. In other words, we need to take into account some uncertainties in the value of awhen such empirical relations are used. As an example, if the data from Marsdiep areexcluded because we know that this basin is not in equilibrium yet, the value of a derivedfrom the remaining data set will be much smaller. Furthermore, we also need to keep inmind that even when a basin is in equilibrium it is then a dynamic equilibrium for the sea-level rise rate of about 18 cm per century.

In Figure 6.2 the data from the various basins are also shown in more detail (see the otherpanels). In these detail pictures it is also indicated using an arrow how the development of abasin in time has been. This development in time is also depicted in Figure 6.3 by showingthe ratio between the channel volume and the 1.5th power of the tidal prism for all theindividual basins.

Relation between Vchannel and P

0

500

1000

1500

2000

2500

3000

0 500 1000

P (Mm3)

Vcha

nnel

(Mm

3) MarsdiepEierlandssgatVlieAmlanderzeegatFriesche ZeegatTheory

Marsdiep and Vlie

1500

1700

1900

2100

2300

2500

2700

2900

1000 1050 1100 1150 1200 1250

P (Mm3)

Vcha

nnel

(Mm

3)

MarsdiepVlieTheory

Eierlandsegat and Friesche Zeegat

0

50

100

150

200

250

300

350

400

450

500

150 200 250 300 350

P (Mm3)

Vch

anne

l (M

m3)

EierlandssgatFriesche ZeegatTheory

Amlanderzeegat

500

550

600

650

700

750

480 490 500 510

P (Mm3)

Vcha

nnel

(Mm

3)

AmlanderzeegatTheory

Figure 6.2 Channel volume in the basins related to the tidal prism. Top-left: all basinstogether: Other panels: details of the various basins.



Ratio between Vchannel and Power 1.5 of P

4.00E-05

4.50E-05

5.00E-05

5.50E-05

6.00E-05

6.50E-05

7.00E-05

7.50E-05

8.00E-05

8.50E-05

1925 1945 1965 1985 2005

Year

Rat

io

MarsdiepEierlandssgatVlieAmlanderzeegatFriesche ZeegatTheory

Figure 6.3 Development of the value of a in time derived from the individual basins.

The following observations per basin are made:Sedimentation has been taking place in Marsdiep during the whole period considered.During the first period, i.e. the first response to the closure of Afsluitdijk, thedevelopment of the basin is fast and it seems to go in the direction of the equilibriumcondition. After about 1965 the basin is still developing, though slower, but theequilibrium relation between the channel volume and the tidal prism seems no more tobe further approached.Sedimentation in Vlie has been decreasing the channel volume as well as the tidal prismduring the whole period. However, the development in the basin does not look like toapproach the equilibrium relation between the two parameters.In the Eierlandsegat the changes of the channel volume and the tidal prism are small.The development is around the equilibrium condition. The ratio between the twoparameters has been increasing and even become higher than the theoretical value,especially due to the decrease of the tidal prism.The data from the Amlanderzeegat do not satisfy the equilibrium relation. The changesof both the channel volume and the tidal prism in this basin are relatively small and donot look like in the direction of approaching the equilibrium condition.The data from the Friesche Zeegat seem to be around the equilibrium condition all thetime, although the response of the Lauwerszee in 1969 can be clearly observed.

It looks like that the behaviours of all the basins are not straight forward but need someexplanations. The observed behaviours of Marsdiep and Vlie can be explained by the factthat the boundary (tidal watershed) between these two basins haven been assumed to befixed whereas in reality it has been moving in the time (see the analysis in Chapter 4). Laterit will become clear that the observed behaviour of Eierlandsegat can be understood byconsidering the fact that the sea-level has been rising instead of fixed as assumed in theanalysis. From the behaviour of the Amlanderzeegat it can only be concluded that theequilibrium value of a is not necessary 65.10-6. The behaviour of the Friesche Zeegat is dueto the fact that it in fact consists of two basins, viz. Pinkegat and the Zoutkamperlaag. InFigure 6.4 the data from this basin is reanalysed by assuming that the Pinkegat has been inequilibrium all the time and the changes in the basin has been dominated by the



development of the Zoutkamperlaag (note that for all years, also before the closure, theLauwerszee is not taken into account in the analysis). The development of theZoutkamperlaag is now more logical: the sedimentation in the basin makes it approachingthe equilibrium condition in time.

Friesche Zeegat considered as two basin

0

50

100

150

200

250

300

350

50 100 150 200 250

P (Mm3)

Vch

anne

l (M

m3)

TheoryZoutkamperlaagPinkegat

Figure 6.4 Relation between channel volume and tidal prism for Zoutkamperlaag.

The second empirical relation concerns the relative area of the inter-tidal flat in a basin(ranger and Partenscky, 1974):

1 0.025fb

b

AA

A(6.8)

Herein Af is the area of inter-tidal flat and Ab is the total area of the basin, both with the unitkm2 (or million m2). For the analysis the area at the highest level has been taken for Ab andAf has been calculated from:

f HW LWA A A (6.9)

The data from the basins are shown in Figure 6.5 together with the empirical equilibriumcondition (6.8). Also this figure reveals that an empirical relation such as (6.8) is probablyderived from field data without making the distinction between basins in equilibrium andbasins out of equilibrium.



Relation relatieve flat area and basin area

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

50 250 450 650

Basin area Ab (Mm2)

As/

Ab

MarsdiepEierlandssgatVlieAmlanderzeegatFriesche ZeegatTheoretical

Figure 6.5 Relation between relative area of inter-tidal flat and the basin area.

The development of the inter-tidal flat areas in the various basins in time is depicted inFigure 6.6 together with the equilibrium value according to the empirical relation (6.8). Itcan be seen that the basins Eierlandsegat and Amlanderzeegat are close to the equilibriumcondition and not much changes in the relative flat area have been taking place in thesebasins. This agrees with the expectation. The flat area in Marsdiep is and seems to remainfar below its equilibrium value, whereas in Vlie the flat area has been increasing and alreadyclearly exceeded the equilibrium value. This reveals again the fact that the boundarybetween these two basins is not fixed in contradiction to as assumed in the analysis. The flatarea in the Friesche Zeegat seems to be developed to its equilibrium value if it is consideredas a single basin. However, if it is considered as two separate basins (Pinkegat andZoukamperlaag) then the equilibrium value is still not achieved. The development inMarsdiep and in the Friesche Zeegat seems to have the common feature that the adjustmentto the equilibrium flat area takes much longer time than the other response to the closures.

Relative area of intertidal flat

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1925 1945 1965 1985 2005

Year

Af/A

b

MarsdiepEierlandssgatVlieAmlanderzeegatFriesche Zeegat


0.1

0.15

0.2

0.25

0.3

0.35

0.4

1925 1945 1965 1985 2005

Year

Af/A

b MarsdiepTheory




0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1925 1945 1965 1985 2005

Year

Af/A

b EierlandssgatTheory


0.30.320.340.360.380.4

0.420.440.460.480.5

1925 1945 1965 1985 2005

Year

Af/A

b VlieTheory


0.30.350.4

0.450.5

0.550.6

0.650.7

0.750.8

1925 1945 1965 1985 2005

Year

Af/A

b AmlanderzeegatTheory


0.3

0.4

0.5

0.6

0.7

0.8

1925 1945 1965 1985 2005

Year

Af/A

b Friesche ZeegatTheoryTheory two basins

Figure 6.6 Development of the relative area of the inter-tidal flat in the various basins

Another parameter indicating the development of the inter-tidal flat is the relative height ofthe flat hf/H, with H=2a=tidal range and hf=height of inter-tidal flat defined as:

f HWf

f f

V A H PhA A

(6.10)

Herein Vf is the flat volume, defined as the volume of sediment in the basin between HWand LW.

The data of the relative flat heights in the various basins are shown in Figure 6.7. It lookslike that in all the basins this parameters approaches a value somewhere between 0.25 and0.3. The only exception is the Friesche Zeegat, in which the flat seems to be higher than inthe other basins. It is not fully clear how the data for this basin will look like if the twobasins Zoutkamperlaag and Pinkegat are considered separately, although the definition ofthis parameter suggests that combination of two basins together still makes sense as long asthe tidal ranges in the two basins do not differ much from each other. The time scale for theadjustment of the flat height seems to be much smaller than that of e.g. the adjustment of theflat area.

Further it is noted that the development in the relatively small basin Eierlandsegat seems tofollow the rising sea-level. Taking this into account it would mean that the tidal prism is notdecreasing in time as suggested in the data presented in Figure 6.2 and Figure 6.3. Thisexplains the development of this basin shown in Figure 6.3, as mentioned earlier. Also theother basins seem to follow the sea-level rise. The only exception is Amlanderzeegat.



Relative flat height

0.1

0.15

0.2

0.25

0.3

0.35

0.4

1925 1945 1965 1985 2005

Year

Hf/T

idal

ran

ge

MarsdiepEierlandssgatVlieAmlanderzeegatFriesche Zeegat

Figure 6.7 Development of the relative flat height in the individual basins.

6.4 Characteristics related to tidal asymmetry

One of the mechanisms responsible for the residual sediment transport, i.e. import to /export from the basins is tidal asymmetry. Theoretically tidal asymmetry is directly relatedto the morphology of the basin. There are also other mechanisms influencing the import /export (Elias, 2006), but they are not always clearly related to the morphologicaldevelopment within the basin. Therefore the parameters influencing the tidal asymmetry aspresented in the literature area analysed here.

Recently Dronkers (2005) presented the g coefficient, defined as

HW LW

HW LW

A A hA A a

(6.11)

Herein h is water depth and calculated according to

LW

LW

Vh aA

(6.12)

According to Dronkers (2005) tidal basin equilibrium corresponds to a value of gdepending on whether or not tidal asymmetry is already present outside the basin due topresence of shelves and the rate of sea-level rise which needs to be balanced by sedimentimport. For the Wadden Sea basins tidal asymmetry is already present out side the basinsdue to the European north-western shelf. For such case the tidal basin equilibrium wouldcorrespond to g=2 without sea-level rise. Because of the sea-level rise the basins need to be

flood-dominant, corresponding to a g value just smaller than 2.



The data for all the basins are presented in Figure 6.8. The value for all the basins, exceptMarsdiep, approaches a value in the range 1.5 to 2. This suggests that Marsdiep is not inequilibrium yet and it is strongly flood-dominant. Note that also here the analysis is basedon a fixed boundary between Marsdiep and Vlie. It is also noted that the Friesche Zeegat isconsidered as a single basin here.

Gamma defined by Dronkers (2005)

0

0.5

1

1.5

2

2.5

1925 1945 1965 1985 2005

Year

gam

a

MarsdiepEierlandsegatVlieAmlanderzeegatFriesche Zeegat

Figure 6.8 Development of the g coefficient defined by Dronkers (2005).

Earlier Dronkers (1998) presented the same theory with another diagram, in which the ratiobetween the surface areas at HW and at LW is related to ratio of the water depth at the sametwo levels. Note that

HW

LW

h h ah h a

(6.13)

Dronker's diagram

11.21.41.61.8

22.2

1 1.5 2 2.5 3

AHW/ALW

h HW

/hLW

MarsdiepEierlandsegatVlieAmlanderzeegatFriesche ZeegatTheory

Figure 6.9 The diagram of Dronkers.

The tidal basin equilibrium in without sea-level rise corresponds to



HW HW

LW LW

h Ah A

(6.14)

This relation is depicted in Figure 6.9 together with the field data from all the five basins.Again only Marsdiep shows “abnormal” behaviour, diverging from the equilibriumcondition instead of approaching it.

Diagram of Friedrichs & Aubrery

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.1 0.2 0.3 0.4 0.5 0.6

a/h

Vs/

Vc

MarsdiepEierlandsegatVlieAmlanderzeegatFriesche Zeegattheory

Figure 6.10 Diagram of Friedrichs and Aubrery.

Figure 6.10 presents the diagram of Friedrichs and Aubrery (1988) together with the datafrom the considered basins. As already noted by Wang et al (1999), this diagram is similaras the one of Dronkers. The conclusion drawn from this figure is also exactly the same: onlyMarsdiep shows divergent behaviour.



6.5 Conclusions

In summary the following conclusions have been drawn from the analysis of the hypsometrydata of the tidal basins:

The empirical relations for the tidal basin equilibrium, as can be found in literature,appear to be based on field data of basins in (dynamic) equilibrium as well as of basinsout of equilibrium. This implies that if these relations are used for determining thesediment demands of the basins, in response to sea-level rise and human interferences,an uncertainty consideration need to be made.Analysis on the empirical relations as well as the analysis on the characteristics relatedto tidal asymmetry indicates that the basin Marsdiep is not yet in equilibrium yet.The analyses confirm that the boundary between the basins Marsdiep and Vlie cannot beconsidered as fixed in time. Therefore these two basins need to be considered togetherinstead of separately.Also the Friesche Zeegat is not yet in equilibrium and it is still responding to the closureof the Lauwerszee. This is especially the case for the area of the inter-tidal flat.The Friesche Zeegat needs to be considered as two separate basins, Pinkegat andZoutkamperlaag, for the data analysis.The development of Marsdiep and of the Friesche Zeegat, both responding to a partialclosure of the basin, indicates that the time scale needed for the adjustment of the areaof the inter-tidal flat is much longer than e.g. that for the adjustment of the height of theflat.



7 Hypothesis testing by process modelling

7.1 Introduction

The closure of the Zuiderzee (1925-1932) is the largest single intervention ever constructedin the Wadden Sea. After the closure, the basin area reduced substantially to an area ofroughly 712 km2 and a length of about 30 km. The closure considerably altered thehydrodynamics and morphodynamics in the remaining active part of the basin (Elias 2006).The tidal characteristics changed from predominantly propagating to a more standing tidalwave resulting in an increase of the tidal range and tidal prism through Marsdiep byapproximately 26%. With these large changes in hydrodynamics and particularly in basingeometry, pronounced changes in the morphology of Texel inlet have taken place. Theclosure separated the back part of the basin (Zuiderzee) that contained a relative largeportion of shoals. In the remaining active part of the basin the remaining shoals area was toosmall compared to the channel area, therefore, a morphologic adjustment of the basin was tobe expected (Eysink, 1990). Large sedimentation was observed, over 200 million (M)m3 ofsediment, during a period of approximately 40 years. Recent research shows that even nowthe sediment import continues at a rate of 5 – 6 Mm3/year.

The corresponding rates of sedimentation in the basin and erosion of the adjacent ebb-tidaldelta and coasts point to sand exchange between these elements (Van Marion, 1999). In thework of Elias (2006) the basic principles of sediment demand, sediment transport capacityand sediment availability were used to hypothesize on the main mechanisms governing thelarge sediment import since closure.

Sediment demand; As a result of the closure both the basin and the ebb-tidal delta requiresediment. The basin needs sediment to restore the distorted balance between channel andshoal areas; due to the Closure the relative shallow Zuiderzee was separated from the deepWadden Sea. Hence, the remaining shoals area is too small relative to the volume ofchannels (e.g. Eysink, 1990). But also the ebb-tidal delta needs sand. Based on empiricalrelations (Walton and Adams, 1976) the ebb-tidal delta sand volume was expected to enlargeas the tidal prism increased after closure.

Sediment transport capacity; The large tidal prisms and associated tidal currents arecapable of transporting vast quantities of sediment during flood from the ebb-tidal delta tothe basin and vice versa during ebb. The residual ebb discharges exceed the residual flooddischarges. Ebb flow dominates Marsdiep along the Texel coast and flood dominatesMarsdiep along the North-Holland coastline.

Sediment availability; In principle when a system is forced out of equilibrium theadaptation rate is dependent on the size of the distortion. After closure, on the ebb-tidal deltaan imbalance between flow and bed is observed; e.g. large flow velocities are directed ontoa relative shallow area. This denotes a large scouring capacity of the tidal currents on theebb-tidal delta. In the upper part of the ebb-tidal delta new channels are formed (Molengatand Schulpengat) and during ebb a vast amount of sediment is redistributed seaward and



deposited. Consequently a large quantity of sediment is available for transport into the basinwith the flood tidal currents. Largest (residual) flood transport rates are observed along theNorth-Holland coastline. In the sheltered environment of the basin sediment partly settles.In conclusion, both basin and ebb-tidal delta require sediment. More sediment is availableduring flood to be transported from the ebb-tidal delta into the basin than during ebb. Thespatially uneven sediment availability in combination with the distinct separation of ebb-dominant transport along the Texel coast and flood-dominant transport along the North-Holland coast plausible explains why sediment is imported into the basin despite theresidual export of flow. It is expected that eventually with the ongoing morphologicaladaptation of the ebb-tidal delta the imbalance between flow and bathymetry, and the (tidal)sediment import into the basin diminish.

7.1.1 Research Questions

The statement “eventually with the ongoing morphological adaptation of the ebb-tidal deltathe imbalance between flow and bathymetry, and the (tidal) sediment import into the basindiminish” forms the basis of the research presented in this section.The role of sediment demand in the basin for sediment import through Marsdiep isinvestigated through initial 1-month model simulations (initial as the modelled period issmall compared to the timescale of morphodynamic inlet changes). The focus is on the twosub-questions:

1. What is the importance of sediment exchange between Vlie and Texel inlet?2. What is the effect of bathymetric changes in the Western Wadden Sea for tidal

asymmetry and sediment import through Marsdiep?

The work described in this Chapter is meant to make a start of testing the hypothesesformulated in Chapter 3 by process modelling.

7.2 Method and Model

We follow the method as described in detail by Elias (2006); Chapter 4 pp: 77-108. TheDelft3D-Flow model (Version 3.53.01) was used in depth-averaged mode to solve theinteraction of the alongshore-propagating open-sea tidal wave with the compound inletbathymetry, the cross-inlet currents and the tide-bathymetry and tide-topography influencedflow in the basin. In this study the focus is on tides as we concentrate on the importance ofthe basin for sediment exchange. Tides are assumed to dominate this exchange justifying theneglecting of waves. A major limitation of the present modeling is the absence of estuarinecirculation (see Elias 2006, Chapter 8 for an indication of density effects on inlet flow). As aresult the model results might not be entirely representative for nature, however we feel thisapproach is justified by looking at the relative effects of the different simulations.

Delft3D-FLOW solves the unsteady shallow-water equations, that consist of the continuityequation, the horizontal momentum equations, the transport equation under the shallowwater and Boussinesq assumptions. Vertical accelerations are assumed minor compared togravitational acceleration (shallow water assumption) reducing the vertical momentumequation to the hydrostatic pressure relation. By specifying boundary conditions for bed(quadratic friction law using a Chézy coefficient of 55 m½/s), free surface (no wind stress)



and lateral boundaries (water levels due to tidal forcing on the open-sea boundaries (Fig. 7-1) and closed boundaries with free-slip conditions at the coasts) the equations can be solvedon a staggered grid using an Alternating Direction Implicit method. The specific modelapplication for Texel Inlet (Texel Outer Delta model) uses a well-structured curvilinear grid(38311 points) with a maximum resolution of 80x120 m at the location of the inlet (Fig. 7-2). The Eierlandse Gat and Vlie inlet are included in the model domain to enable thesimulation of the important internal residual volume transport between Vlie and Texel inlet(Ridderinkhof, 1988a).

Depending on the simulation different bathymetries for the basin have been used (Fig. 7-3).The base case uses the recent bathymetric data (1997 and 2000) for nearshore, inlets andbasin. Depth measurements were grid-cell averaged or triangularly interpolated to thecurvilinear grid depending on the resolution of the observations. Depths in the deeper regionare based on Dutch Continental Shelf data supplied by TNO-NITG (Frantsen, 2001). Theinitial bathymetry was smoothened to reduce the small-scale disturbances ( 0-0.1 m) in thebathymetry.

Besides the base case simulation (run 1). Two basic cases are treated. Run 2 focuses on theimportance of the tidal exchange between the inlets by specifying clear tidal divides (drypoints) between the basins. Cases 3 to 6 are used to obtain an estimate of future basinbathymetries we assumed that the patterns of the major tidal channels remained stable. Thisstability has been shown to persist even after closure of the Zuiderzee. As a cut-off valuebetween stable channel and shoal area we used the -5m contour. The enclosed areas weresubsequently raised by 1m (for the range 1 to 4 m). See Table 9-1 for an overview of thedifferent cases.

Table 7-1: Overview model runs and descriptionRun ID Description1 Base Case;

original bathymetry based on 1997-2000 Vaklodingen2 original bathymetry based on 1997-2000 Vaklodingen

dry points added at the estimated location of the tidal divides to separateTexel, Vlie and Eierlandse Gat basin

3 original bathymetry based on 1997-2000 Vaklodingen butthe areas above the NAP-5m contour made 1 m higher

4 original bathymetry based on 1997-2000 Vaklodingenthe areas above the NAP-5m contour made 2 m higher



The North-Holland coastline, the landward coastline in the back-barrier basin, and the islandcoastlines form closed boundaries (free- slip conditions). The northern basin periphery ischosen on the Terschelling tidal divide (as present in 2000) and set as a Neumann boundary.The open-sea boundaries are located 'far away', outside the direct sphere of Texel inlet’sinfluence and prescribed as representative harmonic constituents (frequencies, amplitudesand phases) of the water level elevations. The time step for the flow computations is 60seconds to fulfil the maximum courant number criterion of 15. Simulations were run over a1-month period, which includes two spring-neap tidal cycles (see Fig. 7-1 for a water-level



time series of Den Helder tidal station) and therefore, due to the periodicity of tides,provides a fairly representative depiction of the long-term tidal residual transport patterns.Default settings of 1.0 m2/s and 1.0 m2/s for the uniform horizontal eddy viscosity and eddydiffusivity coefficients have been applied.

Constant discharges (of the long-term averaged values) through Den Oever (325 m3/s) andKornwerderzand (230 m3/s) are specified. Density differences are not accounted for.Computations start from a uniform water level. A two-day spin up and 60 minutessmoothing time prior to the actual computations is sufficient to dissipate the errors inhydrodynamics induced by the discrepancy between boundary conditions and initial state.

Figure 7-1: Tidal water levels for the month of January 1999 at Den Helder tidal station



Figure 7-2: Texel Outer Delta model grid



Figure 7.3: Bathymetric model schematizations (green areas are located above MSL, blueare deep channels and brown inter-tidal areas.



7.3 The effect of basin geometry and bathymetry on flow

7.3.1 Flow and sediment transports

Table 7-2: Tide-averaged flow and sediment transports through Marsdiep from runs 1 to 6<S> (m3)Run ID <Q> (Mm3)

Total Suspended Bed-load1 -50 3705 3594 1102 -37 7987 7786 2003 -43 5363 5198 1654 -32 4526 4361 1655 -32 2606 2480 1266 -32 1656 1557 98

Table 7-2 summarizes the tide-averaged flow and transport results for the varioussimulations. Complete time-series of the 1-month results are shown in the Appendix (Fig. C-1 and C-2), and illustrations of the transport patterns in Figures C-3 to C-6. Comparing run 1and 2 shows the effect of closed “tidal divides” on the residual flow and transport results inMarsdiep. As a result the residual tidal discharges decrease, but the sediment import ratesdrastically increases.

Note that the decrease in residual discharge is underestimated due to the applied modelschematization. The model is forced with the main constituents of the tidal components ofthe flow only. No A0 is taken into account which partly explains the underestimated flowrates in Marsdiep, 50 Mm3/tide versus the measured 100 Mm3/tide.

Cases 3 to 6 summarize the changes in sediment transport rates with increasing bed levels inthe basin. Compared to the base-case simulation, initially, with limited increase in shoalheight the sediment import rates drastically increase (compare run 1 and 3). Plausibly thisincrease is related to the rise in elevations at the tidal divides that therefore reduce theeffectiveness of tidal propagation and exchange between the basins. This result is consistentwith the observations of case 2.

Runs 3 to 6 show the decreasing rates of sediment import with increasing bed-levels. Inthese simulations the residual discharges remain near equal. However the gross dischargesdecrease with as the storage area of the basin decreases.

7.3.2 Tidal asymmetry and tidal propagation characteristics

The observed decrease in residual tidal discharges versus increase in sediment transportsshows that the residual discharge influences the residual sediment transport, but it is not asimple relation between the two. Plausibly this also relates to the changes in tidal(propagation) characteristics between the different model schematizations.The dominant features of tidal distortion in shallow estuaries can be represented by the non-linear growth of the compound constituents and harmonics of the principal tidal components(Aubrey and Speer, 1984; Friedrichs and Aubrey, 1988). In principal the M4/M2 amplituderatio of water levels (a) or velocities (u) can be used as an indicator for the non-linear tidaldistortion (Aubrey and Speer, 1984), expressed as,



4 2 4 24 2 4 2/ / / / /M M M M

M M a a and or M M u u= = (7.1)

which reflects the combined effects of energy transfer from the M2 to M4 and frictionaldissipation of both components. The relative phase difference between the M2 and M4,expressed as,

2 42 42 2M M

M M q q- = - (7.2)

determines the nature of the tidal asymmetry. For water levels, a relative phase difference inthe vertical tide between 0 and 180 degrees indicates that the duration of the falling tideexceeds the rising tide and a flood-dominant flow occurs. Ebb-dominance is observed forrelative phase-differences between -180 and 0 degrees. In a similar manner, for velocities itcan be derived that sediment import occurs for -90 < 2 M2 - M4 < 90 and sedimentexport for 90 < 2 M2 - M4 < 270 .

Previous studies point to the importance of tidal asymmetry for the sediment exchangebetween basin and ebb-tidal delta after closure (Dronkers, 1998; Ligtenberg, 1998). Thefocus is on the M2 and M4 as in literature it is shown that the long-term residual (coarse)sand transport essentially depends on the interaction of the fundamental constituent and theEulerian mean current, and the interaction of the fundamental constituent and its even over-tides (Van de Kreeke and Robaczewska, 1993). Since the M2 is dominant in Texel inlet, tofirst order, tidal sediment transport essentially depends on the M0M2 tidal residual transportand the M2M4 tidal asymmetry.

The tidal model was previously calibrated on the correct representation of the main tidalconstituents for flow at a cross-section between Den Helder and Texel (Elias 2006). Since1998 an ADCP attached to the hull of the ferry from Den Helder to Texel is used to measurecurrent velocities in the inlet gorge (Ridderinkhof et al., 2002). Preliminary results of theongoing measurements have been made available by NIOZ for analysis (Bonekamp et al.,2002). Flow data have been used to calculate time-series of depth-averaged flow at eighteenequidistantly distributed aggregation points between Den Helder and Texel. Harmonicanalysis has been applied to these time series, and to the simulated flow output in the nearestgrid cells, to derive the tidal mean flow conditions and amplitudes and phases of the maintidal flow constituents.

The correspondence in amplitude ratio and the relative phase difference between model andNIOZ ferry observations (Fig. 7-4a top panels compare crosses and dots) indicate that themodel is capable to reproduce the tidal asymmetry for the present situation reasonably well.A Chézy coefficient of 55 (in stead of 61) results in slightly larger deviations as previouslyreported in Elias (2006). Still the correspondence between modelled and observed flowconstituents is still satisfactory for this study.

Comparing Case 1 and case 2 (middle panels of Fig. 7-4) shows that a separation of thebasins does not result in a decrease of the tidal wave amplitudes. Due to the choice oflocation approximately at the tidal divides the basin storage volume has not changeddrastically. Therefore the total tidal fluxes and amplitudes remain fairly constant. The main



difference occurs in the relative phase difference between the M2 and M4 which on averageddecreases by 40 degrees.

An interesting behaviour occurs by increasing the bed levels in the basin. As a result thetidal storage volume decreases, tidal ranges and M2 velocity amplitudes decrease. However,the M4 is generated by bottom friction related energy transfer from the leading constituent tohigher order frequencies. Due to the decreasing depth of the basin bed friction becomesmore important and the M4 amplitudes increase. Also the M4/M2 ratio, which is anindication of the strength of the tidal asymmetry increases. This would imply that sedimentimport due to tidal asymmetry increases with increasing bed-levels (note that the relativephasing changes, but does not shift to an exporting regime).

Based on tidal asymmetry in the inlet gorge we cannot explain the drop in sediment importrates (see also Fig.7-5). Plausibly this discrepancy relates to the dominance of the suspendedload component.

Table 7-3: Characteristics of tidal wave propagation in the centre of the inlet gorge1 2 3 4 5 6

Amp 1.10 1.11 0.98 0.81 0.64 0.53M2velocity Phi -56 -62 -64 -85 -102 -110

Amp 0.74 0.77 0.81 0.80 0.94 0.95M2 waterlevel Phi -17 -16 -21 -27 -32 -34Phase difference -39 -46 -43 -58 -69 -75

Table 7-3 addresses the changes in tidal wave propagation in the inlet gorge in more detailby analysis of the amplitudes and velocities of the leading constituent. An interestingbehaviour is observed. With decreasing tidal storage volume the water levels in the inletgorge increase. This increase however does not result in an enlargement of the tidalvelocities; these reduce drastically by approximately 50 % as the total tidal prism reduces.The reduction in main tidal velocities seem to govern the decreasing rates of sedimenttransport as this is mainly suspended load wherein advection plays a dominant role.

The decreasing velocities despite increasing water levels are also partly related to thechange in tidal wave characteristics as shown by the changes in relative phase differencebetween water level and velocity phasing.

7.4 Conclusions

Initial model simulations of tidal sediment transport in the western Wadden Sea show thedecreasing rates of sediment transport with increasing bed-levels in the basin. Tidal analysisshows that the decreasing transport rates relate to the decrease of the tidal storage in thebasin rather then tidal asymmetry change. Due to the increase in bed-levels tidal asymmetryincreases, this would result in increased sediment import. Sediment import however reducesdue to the smaller tidal prisms in the inlet gorge with decreasing tidal storage volume.Interestingly are the increasing tidal ranges with decreasing storage volume and the changein phase ratio between vertical and horizontal tide. As a result tidal velocities reducedrastically and sediment import diminishes.



Figure 7-4: From left to right, comparison of (a) M2 velocity amplitude, (b) M4 velocityamplitude, (c) M4/M2 amplitude ratio and (d) M4 phase relative to the M2 for the modeledand observed tidal flow in the NIOZ-ferry transect (see Fig. 7-1 for location).



Figure 7-5 Ratio between maximum flood velocity and maximum ebb velocity at a numberof stations around the centre of Marsdiep inlet.



8 Preliminary synthesis

After the work reported in the previous chapters, this chapter contains some prelimarythoughts to synthesise all information generated. To order the thinking process the Dutchcoastal system behaviour is considered at three scale levels:

the Mega scale (the Dutch coastal system as a whole: coastal foundation, Wadden seaand Western Scheldt),the Macro scale (interaction between the coastal foundation elements (Wadden coast,Holland Coast and Delta coast), the Wadden sea and the Western Scheldt).and the Meso scale (internal behaviour of the various coastal elements).

The first scale will be addressed in the next section. The other two scales are consideredtogether in section 8.2. It is recommended that all three scales will be analysed further infuture work on this topic. The synthesis described here is preliminary at the moment becausethe data analysis and in particular the interpretation of the results will continue next year.Therefore the results in this chapter should be regarded as a first, but by no means final, steptowards a better understanding of the Dutch coastal system.

8.1 Mega scale: Behaviour of the Dutch coastal system

Nederbragt (2006) suggests as a first estimation that the total sand nourishment requirementon the scale of the entire Dutch coastal system should be of the same order of magnitude asthe sea level rise times the total area of the region considered. For the current rate of sealevel rise (18 cm /century) this results according to Nederbracht in a nourishment need forthe entire Dutch coastal system of 12.5 Mm3/year (7.5 Mm3/year for the coastal foundation+ 5 Mm3/year for the Wadden sea + 0.5 Mm3/year for the Western Scheldt). For a sea levelrise rate of 20 cm/century he estimates the nourishment demand to be 13.9 Mm3/year.

Based on the results previously discussed in this report we can make a first rough estimationof the current sand budgets for the entire Dutch coastal system. To do so we have looked atFigures 2-7 (lower 2 panels), 4-3 and 4-4 (only the near shore regions), 4-7 and 5-1. For allthese figures we have estimated a trend. For the Wadden data we used the data from 1980 to2005, for the other areas we used the data from 1990 to 2005. By dividing the total volumechange in these periods by the number of years we get an average yearly sandloss/gain persection. It should be noted that these estimates are all including human interventions likedredging and nourishment. A first effort yields the following image (see Figure 8-1):Wadden Sea +12 Mm3/year, Coastal Foundation -16 Mm3/year and Western Scheldt -2.5Mm3/year. It should be noted that for the Western Scheldt we did not perform volumeestimates our selvers. The figure of -2.5 Mm3/year, mainly due to sand mining, is based onan expert opinion resulting from the studies that have been done recently into this area(Nederbragt and Like, 2004, HAECON, 2006).



Figure 8-1: Rough estimates of the annual volume change (in Mm3/year) in the elements ofthe Dutch coastal system: Wadden Sea, Western Scheldt and Coastal Foundation (trendestimates over period 1990 – 2005)

When add these figures we come to a sum total volume change of -6.5 Mm3/year for theentire Dutch coastal system. It must of course be said that these figures are based on a veryrough estimate of the analysis results obtained so far. It must also be emphasised that ourvolume estimates do not include the complete area up to the NAP -20 m (which is theseaward part of the coastal foundation). In Chapter 4, dealing with the Holland Coast thefew data points that were available for the deeper parts shows additional erosion. However,these data points were not trusted enough to include in the estimates here.

The figures used in these estimates are not corrected for human interventions. Nederbragtestimates the annual losses in the Coastal Foundation by sand mining to be -5 Mm3/year. Heestimates the annual gains to be roughly 6 Mm3/year for the period 1991-2000 and 12Mm3/year for the period after 2000. Because our trends are taken for the period 1990-2005we estimated an average annual gain by sand nourishment to be in the order of 9 Mm3/year.The sand mining in the Western Scheldt is about 2.5 Mm3/year. This leads to an estimatedsand gain in the entire Dutch coastal system of in the order of 1.5 Mm3/year due to humaninterferences. The net loss due to natural erosion, averaged over the entire Dutch coastalsystem, is thus about 8 Mm3/year. The numbers generated so far using the UCIT approach,seems not to support the conceptual model of Nederbracht (2006) who ignored the net lossduo to natural erosion for determining the nourishment demand (viz. 12.5 Mm3/year for 18cm/century). According to the present analysis the nourishment demand is order 8 Mm3/yearhigher than what is needed for compensating the sea-level rise. This difference in theoutcome of the two approaches can be explained by the different period considered fordetermining the trends of changes. Nederbragt (2006) uses a much longer period whereas inthe present study only the data after 1980 are used.

8.2 Macro and Meso-scale: Interaction between elements ofthe Dutch Coastal System

When we take a slightly more detailed look at the volume changes in the differentsubsections we get an image like the one sketched in Figure 8-2. The values in the boxes are

12

-2.5

-16

Wadden Sea

CoastalFoundation

WesternScheldt

Dredging -2.5

Dredging: -5Nourishing: 9



the estimated volume change trends. The values next to the boxes of the coastal foundationare estimated annual values of dredging and nourishment activities.

Figure 8-2: Rough estimates of the annual volume change (in Mm3/year) in the elements ofthe Dutch coastal system distinguishing: Wadden Sea, Westernscheldt, Delta coast, SouthHolland, North Holland and Wadden coast.

The volumes changes in the Wadden Sea and in the Wadden Coast of the Coastalfoundation, can be well explained. The Wadden Sea is demanding sediment, as a result ofsea level rise and response to human intervention (closure of the Zuiderzee and theLauwersmeer). The total transport into the tidal basins is about 12 Mm3/year, whereas thetotal loss at the Wadden coast of the Coastal Foundation is about 11.3 Mm3/year. Apparentlythe Wadden Sea system shows a more or less closed behaviour.

The volume change rates of Noord Holland and Zuid Holland are based on the near shorevolume changes mainly (JARKUS zone). It could be that more volume loss would be theresult if more data of the deeper parts could have been included in the analysis. Overall thebehaviour of the separate elements of the Holland Coast is well described and reasonablywell understood (see Chapter 4).

An item that is harder to understand is the volume loss suggested by the data from the Deltacoast of the Coastal Foundation (Haringvliet, Grevelingen, Eastern Scheldt and WesternScheldt). Added, the volume loss in this area amounts to a staggering 11.9 Mm3/year. Wheresuch a huge amount of sediment could go remains a mystery. One might think that there is aproblem with the data. However, the data does seem to show a consistent eroding trendalong the Delta coast. Another explanation might be found in the method. Chapter 5,however, shows that other studies also show a switch to a significantly eroding trend after1980 in this area. Additional research is needed to better understand the behaviour suggestedby the data.

1

0.7

-5 2

(-2.5)

Wadden Sea

Coastalfoundation

WesternScheldt

Dredging -2.5

Noord Holland

Zuid Holland

Dredging: -5Nourishing: 9

-0.78.3

1.7 0.7-2

-3 1.3 0

-1.7-1

-3.7

-2.7

HaringvlietGrevelingen

Eastern scheldt

Western scheldt

2.8

- 0.6

- 0.8

2.6

Marsdiep

Eierland

VlieFriescheAmeland



The total picture is thus, although for the entire Dutch coastal system a more or less closedsediment balance is found, the large loss at the Delta coast and the gain of the same order inthe Wadden Sea are difficult to be understood.

Mulder 2000 suggested that the subsystems of the coastal foundation (more or less the samesubsystems investigated in this study) can be regarded as independent on the timescale of 50years. The results of the present study indeed suggest that this is more or less the case on themacro spatial scale (viz. Waddensea and Wadden coast, Holland coast, etc.) and on a shortertime scale. However, the data reported here does not provide enough information to validateit on the meso scale. Especially the addition ‘on the timescale of 50 years makes a firmconclusion complicated.

8.3 Future work

In the framework of VOP II Lange Termijn Kustlijnzorg and WP2 of DC 05.20 Noordzeeand Kust additional work has been suggested for 2007.

Part of the work in 2007 should focus on further detailing of the analyses performed inthis report. More data may be found that can be used (bathymetry data as well as data onhuman interventions). The experience with applying the UCIT methods in this studymay trigger new ways of looking at the data. Additional information from literature maydeepen insights, or trigger new angles for analysis.Now that we seem to have some basic understanding of the volume changes that haveoccurred in the subsystems a next step for 2007 is to get a feel for the sediment transportrates over the various system boundaries. For this purpose some results are expectedfrom the set up of a ‘reservoir model’ to understand the coherence of the entire Dutchcoastal system. Other results are expected from the use of process models on parts of thecoastal system.A final part of the work in 2007 should focus on bringing together the detailed analysesthat have been performed in this and in other frameworks. It seems clear that the effortsto understand the behaviour of the Dutch coastal system present a challenge on manylevels: detailed analysis of individual data sources, combination of new results withresults found in literature, analysis of the wealth of information to formulate hypotheseson system behaviour, the use of advanced models in combination with data to test theformulated hypotheses and improve the understanding of the system. With that it is clearthat the quest for increased system understanding is an effort that should remain on theresearch agenda in order to make progress.



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VAN MARION, B. B. (1999). Zandbalansen van het Zeegat van Texel met het invers sediment transport model(1931 tot 1997) (in Dutch), Report RIKZ/OS - 99.116X. Rijkswater-staat RIKZ, The Hague.

Vessem, P. van and Stolk, A. (1990). Sand budget of the Dutch coast. Proc. of 22nd Int. Conf. on Coastal Eng.,Delft, The Netherlands. ASCE, pp. 1895-1908.

Walburg, A.M., 2001, De Zandbalans van het Zeegat van Texel,Walburg, A.M., 2005, Zandvolumes in het Nederlandse kustsysteem, Werkdocument RIKZ/OS/2004.xxx, RIKZ.WALTON, T. L., and ADAMS, W. D. (1976). "Capacity of inlet outer bars to store Sand" Proc. 15th

International Conference on Coastal Engineering, Honolulu, 303-325.Wang, Z.B.,Jeuken, C. and H.J. De Vriend, 1999, Tidal asymmetry and residual sediment transport in estuaries,

WL | Delft Hydraulics, Report Z2749.Wijnberg, K.M. (1995). Morphologic behaviour of a barred coast over a period of decades. PhD. thesis, Utrecht

University, The Netherlands.


WL | Delft Hydraulics A – 1

A Figures Wadden Sea

Figure A-1: Bathymetry Wadden Sea based on 1926 measurements


A – 2 WL | Delft Hydraulics



WL | Delft Hydraulics A – 3



A – 4 WL | Delft Hydraulics

Figure A-4: Sedimentation – Erosion patterns derived from subtracting the 1926 from the1990 bathymetry


WL | Delft Hydraulics B – 1

B Basic data derived from the hypsometry data Wadden Sea

B.1 Marsdiep

Table B1 Major parameters derived from the hypsometric datadate 01/01/1933 01/01/1951 01/01/1965 01/01/1972 01/01/1977 01/01/1982 01/01/1988 01/01/1991 01/01/1997area at highest level (km2) 688.704 688.704 688.704 688.704 688.704 688.704 688.704 688.704 688.703volume under highest level (M m3) 6893.340 6759.355 6727.086 6670.669 6662.987 6714.508 6702.244 6670.410 6674.386area at fixed HW (km2) 688.704 688.704 688.704 688.704 688.701 688.704 688.704 688.684 688.633Area at fixed LW (km2) 580.922 596.004 591.733 576.935 573.356 581.369 581.529 576.034 579.573Volume under fixed HW (M m3) 3563.736 3429.759 3397.695 3341.232 3333.117 3384.905 3372.364 3340.540 3344.580Volume under fixed LW (M m3) 2452.243 2317.878 2290.243 2244.651 2240.145 2284.173 2275.205 2245.017 2249.134


B – 2 WL | Delft Hydraulics

Marsdiep

-3000-2500-2000-1500-1000-500

0

0 200000000 400000000 600000000

Area (m2)

Z( N

AP+

cm)

01/01/193301/01/195101/01/196501/01/197201/01/197701/01/198201/01/198801/01/199101/01/1997

Figure B.1 Hypsometric curves of the Marsdiep



Marsdiep

-200-150-100-50

050

100150200

350000000 450000000 550000000 650000000

Area (m2)

Z( N

AP+

cm)

01/01/193301/01/195101/01/196501/01/197201/01/197701/01/198201/01/198801/01/199101/01/1997

Figure B.2 Hypsometric curves of the Marsdiep, detail upper part



B.2 Eierlandsegat




Eierlandsegat

-1200-1000-800-600-400-200

0200

0 50000000 100000000 150000000

A (m2)

Z (N

AP

+ cm

)

01/01/193301/01/194901/01/196201/01/197301/01/197601/01/198201/01/198701/01/199301/01/1999

Figure B.3 Hypsometric curves of Eierlandsegat



Eierlandsegat

-200-150-100-50

050

100150200

20000000 70000000 120000000

A (m2)

Z (N

AP

+ cm

)

01/01/193301/01/194901/01/196201/01/197301/01/197601/01/198201/01/198701/01/199301/01/1999

Figure B.4 Hypsometric curves of the Eierlandsegat, detail upper part



B.3 Vlie




Vlie

-2000

-1500

-1000

-500

0

0 200000000 400000000 600000000

A (m2)

Z (N

AP

+ cm

)

01/01/193301/01/195101/01/196501/01/197201/01/197701/01/198201/01/198801/01/199201/01/1998

Figure B.5 Hypsometric curves of theVlie



Vlie

-400

-300

-200

-100

0

100

200

1E+08 2E+08 3E+08 4E+08 5E+08 6E+08 7E+08

A (m2)

Z (N

AP

+ cm

)

01/01/193301/01/195101/01/196501/01/197201/01/197701/01/198201/01/198801/01/199201/01/1998

Figure B.6 Hypsometric curves of the Vlie, detail upper part


B – 1 0 WL | Delft Hydraulics

B.4 Amlanderzeegat



WL | Delft Hydraulics B – 1 1

Amlanderzeegat

-2000

-1500

-1000

-500

0

0 100000000 200000000

A (m2)

Z (N

AP

+ cm

)

01/07/192601/07/195001/07/196701/07/197301/07/197801/07/198401/07/198901/07/199301/07/1999

Figure B.7 Hypsometric curves of the Amlanderzeegat



Amlanderzeegat

-300

-200

-100

0

100

200

40000000 140000000 240000000

A (m2)

Z (N

AP

+ cm

)

01/07/192601/07/195001/07/196701/07/197301/07/197801/07/198401/07/198901/07/199301/07/1999

Figure B.8 Hypsometric curves of the Amlanderzeegat, detail upper part



B.5 Friesche Zeegat




Friesche Zeegat

-2000-1800-1600-1400-1200-1000-800-600-400-200

0200

0 50000000 100000000 150000000

A (m2)

Z (N

AP

+ cm

)

01/01/192701/01/194901/01/196701/01/197001/01/197901/01/198201/01/198701/01/199101/01/1997

Figure B.9 Hypsometric curves of the Friesche Zeegat



Friesche Zeegat

-300

-200

-100

0

100

200

30000000 80000000 130000000 180000000

A (m2)

Z (N

AP

+ cm

)

01/01/192701/01/194901/01/196701/01/197001/01/197901/01/198201/01/198701/01/199101/01/1997

Figure B.10 Hypsometric curves of the Friesche Zeegat, detail upper part


WL | Delft Hydraulics C – 1

C Model results


C – 2 WL | Delft Hydraulics

Figure C-1: Water level, discharge and sediment transport rates in Marsdiep (run 1 versus 2)



Figure C-2: Water level, discharge and sediment transport rates in Marsdiep (run 1, 3,4,5,6)



Figure C-3: Curved vector representation of residual sediment transports (run 1)










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