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3DApproach for eco-morphological modelling ofmega-nourishments alongthe Holland coast

Approach for eco-morphologicalmodelling of mega-nourishmentsalong the Holland coastAssessment of tools and approach for multi-scale modelling

HK4.1 Long-term sustainable development Holland Coast

1200893-000

Deltares, 2010

Bas HuismanArjen Luijendijk

TitleApproach for eco-morphological modelling of mega-nourishments along the Holland coast

ClientEcoshape

Project1200893-000

Reference1200893-000-HYE-0005

Pages44

QueriesBuilding with Nature, Model comparison, Mega nourishment, Long-term modelling, eco-morphology

SummaryIn this document the suitability for long-term eco-morphological simulations is assessed for anumber of numerical models. Specifications for the review of these models were made, whichwere based on the differing requirements for distinct spatial regions along the coast ofHolland (like uniform beaches). It was found that no model can handle all the requirementsthat were defined. Instead a number of models can cover most issues. Although there is lackof aeolian sediment transport models and ecological models that include feedback betweenecology and hydrodynamics/morphology. Furthermore, more knowledge should becomeavailable on ecological processes. Besides the capabilities of the models there are somepractical aspects that are of importance for long-term modelling, like computational effort,robustness of models, difficulties to couple models and availability of trained staff.

From the evaluation of the models and discussions with specialists on numericalmorphological modelling, it was decided to develop a system of coupled models enabling anintegrated, flexible technique to cover the effects of small-scale interventions on a largerspatial and time scale. The core of the coupled system will be Delft3D, as a mega-nourishment is initially an intervention with a complex geometry along the coast. A coastlinemodel (UNIBEST-CL+) will be coupled in areas with uniform beaches to cover larger spatialand temporal scales, while on the other hand XBeach can provide bed changes after a stormperiod. The long-term impact of measures on the intertidal flats in the Marsdiep estuary canbe modelled with the ASMITA model.

References

Versie Datum Auteur Paraaf Review Paraaf Goedkeuring Paraafdec. 2010 Bas Huisman Z.B. Wang T. Minns

Arjen Luijendijk

StatedraftThis is a draft report, intended for discussion purposes only. No part of this report may berelied upon by either principals or third parties.

1200893-000-HYE-0005, 2 December 2010, draft

Approach for eco-morphological modelling of mega-nourishments along the Holland coast i

Contents

1 Introduction 11.1 Introduction 11.2 Readers guide 1

2 Specifications 32.1 Introduction 32.2 Characteristic areas 3

2.2.1 Open coast 42.2.2 Estuaries and tidal lagoons 72.2.3 Structures 9

2.3 Model related specifications 10

3 Overview of tools and models 133.1 Model description and evaluation 13

3.1.1 ASMITA model 133.1.2 Delft3D 143.1.3 DUROS 153.1.4 DUROSTA 163.1.5 ESTMORF 173.1.6 Habitat 173.1.7 PONTOS 183.1.8 UNIBEST-TC 203.1.9 XBeach 203.1.10 Design formulae and expert judgement 21

4 Comparison of models 234.1 Introduction 234.2 Model evaluation 234.3 Model comparison 25

5 Modelling Approach for Eco-morphological modelling 295.1 Application of models 295.2 Spatial coupling 305.3 Temporal coupling 305.4 Relevant evaluation parameters to consider 32

6 Conclusions and recommendations 336.1 Findings of the model evaluation 336.2 Conclusion 346.3 Activities 2011 34

References 35

Appendix A : Model characteristics 37


Approach for eco-morphological modelling of mega-nourishments along the Holland coast 1 of 44

1 Introduction

1.1 IntroductionWithin the framework of the project Building with Nature (abbreviated as BwN), a study isperformed in which available morphological and ecological numerical models are evaluatedand assessed. The main objective of this assessment is to identify the potential andweaknesses of the considered numerical models to cope with long-term morphologicalchanges along the Dutch coast.

The current study will provide a starting point for the development of an aggregatedmorphological model of the Holland Coast (work packages HK2, HK3.3 and NTW3.2). Thismodel is aimed at enabling the analysis of large-scale morphological developments anddifferent maintenance strategies for large scale nourishments and sand mining, which willbe studied in work packages EDD1 and HK3.1. Another aspect of BwN that is closely relatedto the current study is the aim to develop a habitat- and vegetation model, enabling thetranslation of large scale morphological model forecasts into (ecological) habitat effects andincorporation of vegetation feedbacks on morphological developments at different time scales(linked to the work package HK2.4 and HK3.3). This report has the following aims:

to provide insight in the relevant physical phenomena along the Holland coast and makea categorization of the different types of coast (Chapter 2).

Assess the available coastal morphological and ecological modelling tools for each ofthese regions (Chapter 3).

Discuss the match/mismatch between the available tools and the required tools andprovide insight into suitable modelling approaches (Chapter 4).

1.2 Readers guideSpecifications of the (numerical) morphological models are described in Chapter 2. Chapter 3presents the relevant numerical models briefly and compares them on the basis of thespecifications. An approach is then suggested for the set up of a long-term morphologic andecologic model for the Dutch coast (Chapter 4).



2 Specifications

2.1 IntroductionThe area for which models are applied needs to be known before an evaluation of thenumerical models can be made. An approach was therefore chosen that assesses the typicalmorphological and ecological characteristics along the coast of Holland (Section 2.2). On thebasis of these characteristics, typical model specifications could be derived. Section 2.3 thenidentifies more general specifications, which are relevant beside the area specific (process-based) specifications. The specifications are then used to assess the suitability of availablemodels for the specific regions (Chapter 3).

2.2 Characteristic areasThe morphological models that are assessed should be able to handle a wide range ofmorphological situations along the coast of Holland. For this purpose, an overview is made oftypical features along the coast of Holland. This overview is presented in Figure 2.1. Thefollowing characteristic areas can be distinguished:

Open coast Estuary / Tidal lagoons Structures and interventions

Figure 2.1 Typical morphological features along the coast of Holland.

Open coast

Estuary / Tidal lagoons

Structures and interventions

4 of 44 Approach for eco-morphological modelling of mega-nourishments along the Holland coast


2.2.1 Open coastThe coastal areas adjacent to the North sea typically have sandy sediment and high energeticconditions due to wave forcing and tidal flow. A distinction between areas can be made bycategorising areas on the basis of the complexity of the processes in these areas and theability of models to cope with the local conditions. The following areas are identified (seeFigure 2.2):

Uniform beach with dunes, Offshore sand bars (or foreshore suppletions), Headlands of islands (note that these are not present for Holland coast), Complex coastal features (like a Zandmotor), Borrow areas.

Figure 2.2 Illustration of typical areas of the open coast with distinct complexity of the physicalprocesses.

The areas with least complicated processes are the beaches with dunes that are not too nearto tidal inlets or large interruptions of the coast. These areas are referred to as uniformbeaches. The main processes that are relevant are sediment transport along the coast innormal conditions and dune erosion during extreme events (erosion only suffices as this isonly used as a one-way check for the safety of the dunes). Besides these items, theswimming safety might be a relevant parameter. Ecologically the uniform beach can becharacterised by features in cross-shore direction (see Figure 2.3) while the ecologicalregions are quite uniform in longshore direction. In general, the ecology is hardly influencedby coastline changes in the high energetic area of the beach. On the foreshore, however,there may be a significant influence of the Benthos on the (near bed) hydrodynamics andmorphology (Borsje et al, 2008).

Quite some knowledge is available on the terrestrial ecology and the ecology of the beachand offshore zone. For the foreshore zone, however, only limited information is available onthe relation between the communities (e.g. fish or bottom fauna like worms and shells) thatlive there and the environmental parameters. This has a practical cause as this area isdifficult to survey from land as well as from the sea. In recent years, however, more data havebecome available which are mainly stored at Imares. Knowledge is currently developed onthe effect of vegetation, mussel beds and worms on the hydrodynamics (bed roughness).

borrow area

Uniform beach Complex feature sand bar Uniform beach



Figure 2.3 Typical ecological zones for a uniform coast.

For some parts of the beaches the cross-shore profile changes can have a significantinfluence. This holds especially for sites with offshore sand bars (or foreshore suppletions).For these situations the behaviour of the cross-shore profile should also be modelled. Boththe erosive and accretive processes need to be included in the cross-shore profile model. Insome situations it is possible to use separate models for the analysis of cross-shore andlongshore sediment transport. Swimming safety is also important for such areas, as sandbars and offshore suppletions may trigger rip-currents.

The processes are more complicated at the headlands of islands, where the beach gets closeto a tidal inlet. At these locations the sediment transport is not anymore predominantly inlongshore direction or cross-shore direction. A 2D approach is therefore required if detailedinformation on the behaviour of these island heads is needed. Swimming safety is not anissue, as it is never safe to swim close to tidal channels. It is noted that no headlands withbeaches are present for the Holland Coast.

For complex coastal features, for example due to very large suppletions (e.g. Zandmotor), itwill also be necessary to use detailed 2D approach that can determine the migration rates ofsuch coastal features. A process-based approach is required when the complex coastalfeature is the result of large suppletions, as empirical relations on coastline behaviour can notbe used if the equilibrium situation is not known. Such that these areas become significantlydifferent from the current situation. For example, it is possible that a dry area with pioneervegetation develops on top of the suppletion (see Figure 2.4), provided that sufficient time isavailable for the vegetation to develop. The morphological development of the dry zone ontop of the Zandmotor is affected by the vegetation. This means that a realistic model willrequire feedback from the morphological model to the ecological model and vice versa.Another aspect that is expected to be very important for the dry area is the aeolian transport.Currently, the aeolian transport is not included in any coastal morphological model. Aknowledge gap exists with respect to the aeolian transport of sediment at the interface of dryand wet areas.

It is furthermore expected that very large suppletions (Zandmotor) may result inaccumulation of fine sediments at the leeward side of the suppletion (see Figure 2.4). As aresult of this, a new ecological zone may develop along the coast. Whether the actualconditions at the leeward side of a Zandmotor are such that a muddy area can occur is,however, not yet thoroughly investigated. Forecasting such a development requires the use of

Dune Beach Foreshore (banks) Offshore

Terrestrialecology

Low diversity and biomass- beach fauna- birds

Moderate diversity- mussel beds- some fish- worms

High diversity &and high biomass- mussel beds- Worms- fish

NAP-20m

NAP-8m



Land

Sea Muddy area with salt marshvegetation and species

Dynamic dry sandy area which is lowon nutrients

modelling tools that can handle fine sediment fractions and support feedback from theecology to the hydrodynamics and vice versa. Models with interaction between ecology andhydrodynamics are not yet available. Furthermore, other functions need to be evaluated likescenery properties and the effect of the complex coastal feature on swimming safety.

Figure 2.4 Indicative sketch of possibly relevant ecological zones at a Zandmotor.

Borrow areas, where sediment for suppletions is taken from, are located outside theKustfundament (beyond the NAP-20m contour). These areas may have an impact on thelocal ecology or other functions like fishery. The most relevant aspect for these areas is the fillup rate of the borrow pit and the morphological changes due to the sand mining. Furthermore,migration speed of erosion can be of interest. These areas can be studied in detail with fieldmodels (2D) or more general with empirical relations and analytical models. The requirementsfor modelling distinct parts of the open coast are summarised in Table 2.1.

Categories Technical specifications

Uniform beach

Longshore sediment transport (Coastline position / MKL) for normalconditions.

Cross-shore sediment transport for extreme situations (Dune erosion)to check safety of the dunes.

Sandy sediment

Sand bars

Longshore and cross-shore sediment transport (coastline position /MKL and profile development) for normal conditions.

Cross-shore sediment transport for extreme situations (Dune erosion) Sandy sediment Impact of the fauna (Benthos) at the sand bars on sediment transport. Swimming safety

Headlands

Complex two dimensional effects on sediment transport (no distinctlongshore and crosshore components) for normal and extremeconditions (dune erosion)

Sandy sediment

Complex coastal features

Complex two dimensional effects on sediment transport (no distinctlongshore and crosshore components) for normal and extremeconditions

Sandy and fine sediments (mud)



Interaction between ecology and hydrodynamics Wind transport (Aeolian transport) Swimming safety Scenery properties

Sand mining pit Morphological development / filling up and shifting (two dimensional) Disturbance to environment (Benthos and fish)

Table 2.1 Relevant technical specifications for parts of the open coast

2.2.2 Estuaries and tidal lagoonsEstuaries and tidal lagoons can be found in the Waddenzee and South-Western delta of theNetherlands. These areas are sheltered from the open sea by Islands or shoals, whichreduces the influence of wave energy. However, the tide is very important as the estuaries fillup and empty through tidal channels, in which large flow velocities can occur in theentrances. The sediment inside the estuaries ranges from coarse sand in the tidal channels tomedium sized sand and mud on the inter-tidal flats. The following typical areas are pinpointedif a distinction is made on the basis of the complexity of the processes (like in the previoussection). The following areas are identified (see Figure 2.5):

Ebb tidal delta, Tidal channels, Sandy inter-tidal flats (zand platen), Muddy inter-tidal flats (slikken), Salt marshes (schorren or kwelders),

Figure 2.5 Illustration of typical areas inside estuaries with distinct complexity of the physicalprocesses.

Ebb tidal delta

tidal channelsSalt marshes

Sandy flats

Muddy flats



The ebb tidal delta is the part of the estuary that is located seaward from the tidal inlet. Thesize of the ebb tidal delta is directly related to the tidal prism (amount of water going in andout of the system in every tidal cycle) which is determined by the size of the tidal lagoon. Ebbtidal deltas consist of sandy inter-tidal flats or sand bars and a submerged bar at the outsideof the ebb tidal delta, which functions as a wave shield. The system is very dynamic and sandbars often migrate from one side to the other over long periods of time (tens to hundreds ofyears. The sediment of the ebb tidal delta predominantly consists of sand.

The ebb tidal delta is connected to the inter tidal flats inside the estuary by means of tidalchannels. The size and shape of these channels is determined by the length of the estuaryand the tidal prism. Therefore the whole estuary should be considered even if only the tidalchannels are of interest. Some estuaries that have a reduced tidal prism (due to partialclosure or obstructions of the tidal flow) are not in equilibrium, as they have tidal channels thatare too deep compared to the current tidal prism. This wet volume of the channels is animportant aggragated parameter. On a more detailed level the migration of the channels andlocal sedimentation or erosion can be studied, although this is difficult to do even with verydetailed models.

The channels link to the inter-tidal flats inside the tidal lagoon. These inter-tidal flats caneither consist of sandy or muddy sediment. The local ecology (flora and fauna) can have asignificant influence on the morphological changes of the tidal flats. This depends on the localsediment (muddy or sandy) and hydrodynamic conditions. Local fauna (Benthos) can be ofimportance on the morphological development of all sandy and muddy inter-tidal flats. Whilefeedback from the ecology to the hydrodynamics and vica versa is needed for salt marshes(i.e. inter tidal flats with vegetation). These salt marshes occur mainly at relatively high muddyflats (near or adjacent to the coast).

Large process-based field models are required to forecast the detailed processes inestuaries, like the migration or building up and erosion of the flats, which makes them verycomputationally intensive. An aspect that complicates the assessment with process-basedfield models is that the gross changes are large compared to the net changes. Especially theinfluence of flora and fauna makes the assessment of morphologic changes very difficult,although formulations for some species have become available now.

A way to include more site specific information is to use empirical relations that were derivedfrom measurements of the development of (parts of) the estuary in the past. This makes themodel more robust. In this way the effect of processes that are too difficult to model isreduced. The parameters used in an empirical model are more aggregated, due to whichlocal and temporal disturbances are filtered out of the results (which can make interpretationeasier). It is also possible to combine a process-based model with empirical models as isillustrated in the model software overview in Chapter 3.

The requirements for modelling estuaries are summarised in Table 2.2. It is noted that theMarsdiep is the only relevant tidal lagoon for the Holland Coast, as the Holland coast is notexpected to influence the Haringvliet and Eijerlandse gat significantly.



Categories Technical specificationsDetailed approach (process-based) Aggregated approach (empirical)

Ebb tidal delta

Migration rates of shoals Sedimentation/erosion Habitat suitability (Benthos &

Fish)

Sand volume of ebb tidal delta Bypass of sediment Sediment exchange with open

coast

Tidal Channels Migration rates of channels Sedimentation/erosion

Wet volume of the tidalchannels

Import of sediment into theinner part of the estuary

Sandy inter-tidal flats(shoals of ebb-tidal delta)

Sedimentation/erosion (sand) Habitat suitability (Benthos) Influence of ecology on

morphology

Muddy inter-tidal flats(slikken)

Sedimentation/erosion (mud) Habitat suitability (Benthos) Influence of ecology on

morphologySalt marshes (schorren) Sedimentation/erosion (mud)

Habitat suitability (Benthosand vegetation)

Feedback from ecology onmorphology and vica versa

Area of the inter-tidal flats

Table 2.2 Relevant technical specifications for parts of estuaries

2.2.3 StructuresStructures are influenced by the morphology of the coastal system and do sometimes alsoinfluence (parts) of the coastal system. Four kinds of structures are discerned:

Dams and Barriers, Harbour moles and groynes, Revetments / Dikes, Navigation channels.

Figure 2.6 Illustration of typical areas inside estuaries with distinct complexity of the physicalprocesses.

groynes

dikes / revetments

navigation channels

harbour moles



Large dams like the Eastern Scheldt barrier and the Afsluitdijk influence the coastal systemsignificantly, as they influence the behaviour and the tidal prism (volume of exchanged waterin a tidal cycle) of estuaries. Furthermore, these structures themselves and their function (toincrease the safety) is influenced by the development of the coastal system. The structuresare designed for specific hydraulic design conditions which may change if the bathymetry ofthe foreshore changes. The same holds for the revetments and dikes along the coast. Arelevant example for the Holland coast is the Hondsbossche Zeewering which extendsseaward of the coast and should be considered a complex area to assess.

Smaller structures like groynes and harbour moles along the coast only locally affect thecoastal system by blocking part of the sediment transport along the coast. The effectivenessof these structures will be influenced if the coastal system changes. For example, a largeaccretion of the coast will result in more sediment bypass at the harbour moles andsubsequent sedimentation of the harbour and navigation channel. For revetments and dikesthe actual strength can be assessed with design rules (theoretical formulations). Thesedesign rules use the hydraulic design conditions at the structure, which are influenced by thecoastal system.

The assessment of relevant parameters (coastline accretion, bathymetrical changes) for thestructures requires the use of models that are fit for the local type of area. The requirementsfor these models were provided in Table 2.1 and Table 2.2. Summarising, the followingaspects are relevant for structures along the coast (Table 2.3):


Barriers and dams Hydraulic design conditions (e.g. impact of Bathymetrical changes) Impact on tidal prism and equilibrium parameters of estuaries Sediment bypass

Harbour moles andgroynes

Hydraulic design conditions (e.g. impact of Bathymetrical changes) Sediment bypass Coastline changes near breakwaters

Dikes / Revetments Hydraulic design conditions (e.g. impact of Bathymetrical changes) Strength of structures Scour / Interaction hard-soft

Navigation channel Sediment infill rates Hydraulic conditions

Table 2.3 Relevant technical specifications for structures

2.3 Model related specificationsThe previous section identifies specifications with respect to the capability of models tohandle physical situations (see Table 2.1, Table 2.2 and Table 2.3). Besides technicalrequirements, model related aspects (like computational effort and robustness) are ofimportance for the selection of suitable models for long-term eco-morphologicalcomputations. Therefore a number of relevant model related specifications are derived (seeTable 2.4).

Categories Model related specificationsModel handling Computational time?

Model area? Complexity of the modelling effort?



Availability of trained staff?Robustness Accuracy suitable for long-term simulations?

Can it deal with measures that will substantially alter the currentcoastline? (no verification material)

Model has been validated?Model aggregation Coupling with other models?

Batch functionality? Probabilistic computations?

Improvements Ongoing work? (Within other frameworks than BwN) Model concept allows for improvements?

Table 2.4 Model related specifications of the morphodynamic/ecological numerical model



3 Overview of tools and models

3.1 Model description and evaluationIn this memo a limited number of morphological models are evaluated, which are expected tobe representative for the typical modelling approaches that can be used to assess potentialnourishment and sand-mining strategies on different time and spatial scales. These modelsare:

ASMITA Delft3D DUROS DUROSTA ESTMORF Habitat PONTOS UNIBEST-CL+ UNIBEST-TC XBeach Design formulae and expert judgement

In general a distinction can be made between empirical and process-based models. Themodels that have an empirical basis use data to predict the changes over time. Thedevelopments can then be used to assess the impact of future changes like sea level rise. Anempirical (or semi-empricial) model has the advantage of being stable for long-termcomputations, as it is still related to data that was measured in the past. The disadvantage ofsuch a model is that the model is often not very detailed.

Process-based models are made on the basis of relations between physical properties.These models provide detailed results, but may be unstable for long-term simulations. This isdue to the processes inside the model that can not handle all situations. Furthermore,process-based models often require much more computational power, which makes itimpossible to perform detailed computations for very large areas (detail will decrease forlarger areas).

A more detailed description of the considered models can be found in Appendix A.

3.1.1 ASMITA modelThe ASMITA model is a semi-empirical behaviour model (Stive et al. 1998, Stive and Wang2003). In the model a network with an arbitrary number of elements can be defined. Anequilibrium situation should be defined for each of these elements as well as an exchangebetween the elements. These equilibrium relations should be derived from available data orfrom numerical models. On the basis of the relations in the model a forecast can be made ofthe expected behaviour of the tidal lagoon in the future. So, the autonomous development ordevelopment with impact from measures can be estimated.

The ASMITA model is often applied for estuaries and tidal lagoons. Modelling the effects ofsediment import into the Marsdiep estuary (a calibrated model is available) would be a typicalapplication. For such a situation, the model consists of three elements (intertidal flats, tidal



channels and ebb tidal delta) which have a tendency towards an equilibrium volume. Thearea and historical development of the volumes of the elements (e.g. flats, channels anddelta) should be used as input for the model. Besides tidal lagoons, the ASMITA model canbe applied to a variety of other situations. The ASMITA model was, for example, also appliedfor sand pits, for which a coupling with Delft3D was used.

The advantage of the ASMITA model is that it is very fast and can also handle large areasand longer timeframes (centuries). The results clearly show the overall tendencies, filteringout the local and temporal fluctuations that might be present in detailed models . Furthermore,the model code is easily accessible and can be modified. A disadvantage of the model is thatelements can only be modelled if the equilibrium situation can be specified.

Figure 3.1 Schematisation of an estuary in the Asmita model (images from Kragtwijk et al. 2004).

Figure 3.2 Estuaries in the Waddenzee (images from Kragtwijk et al. 2004).

3.1.2 Delft3DThe Delft3D model is a multi-dimensional (2D or 3D) hydrodynamic (and transport) simulationprogram which calculates non-steady flow and transport phenomena resulting from tidal andmeteorological forcing on a curvilinear, boundary fitted grid (Lesser et al. 2001 and 2004).The Delft3D model is capable of handling complex geometries (e.g. non-uniform coasts). Thecurrents (tides, river discharges) are computed directly with the model and the waveconditions by means of the SWAN model. The model also contains modules for the transportof cohesive and non-cohesive sediments and water quality which can be updated throughoutthe hydrodynamic modelling.



Delft3D is applied to a wide range of problems at different spatial scales (e.g. harbours, tidallagoons, rivers) for small to intermediate timeframes (up to years). The model is very suitablefor complex locations like a Zandmotor. For these complex situations it is also possible toinclude the effect of vegetation in the Delft3D model. There is, however, no feedback from thehydrodynamics and morphology to the vegetation included in the Delft3D model.

A disadvantage of Delft3D is that it is less suitable for long-term computations (100 years) orfor large domains (like the coast of Holland), as this will result in a large demand ofcomputational power and/or slow computations. To cope with this drawback the model wasapplied creatively with simplified model formulations in previous studies. The model was, forexample, applied with fixed cross-shore profiles along the coast of Holland in the Flylandstudy (WL | Delft Hydraulics, 2002), which studies the effects of an airport on an island somekilometres of the Holland coast. Thus allowing only longshore transport. Another study, inwhich Delft3D was applied, investigated the effects of large scale mining on the North Sea.With a Delft3D model that uses only the initial morphological changes, it was studied whethera large gully along the coast (Zandgoot) might influence the hydrodynamics and morphologyof the coast of Holland (Deltares, 2009).

Figure 3.3 Delft3D application for large scale flow computations (Westerschelde and Holland coast)and for smaller scale phenomena like rip currents

3.1.3 DUROSDUROS is an empirical model that computes dune erosion after a storm for a cross-shoreprofile. The model is based on the principles of the mass balance, which means that theamount of dune erosion (see Figure 3.5) should be equal to the amount of deposited sand (onthe seaward side). For this purpose, the model contains an empirical formulation for theerosion profile (after 5 hours storm), which is computed on the basis of wave parameters(wave height, wave period and water level) and properties of the dune and beach (1D profileshape and sediment diameter). The erosion volume and retreat distance of the dune can bederived from the output.

The DUROS model is used in the assessment of the safety of the uniform beaches of theDutch coast. The major advantage of the model is that it is extremely fast. The drawback ofthe model is that it is not suitable for more complicated areas. This holds for example forareas with very gentle sloping beaches and for coastlines with structures. The model concept



will, for example, not be valid if the computed dune erosion profile is more gentle than theinitial beach slope. There are, however, some specific situations for which research wasperformed to assess correction factors. This holds for example for curved coastlines. Threeversions of the DUROS model are available. The original model that was developed in the1980s is called DUROS. This model was extended in 2006 (WL | Delft Hydraulics, 2006a and2006b) by adding the influence of the wave period component and is referred to as DUROS+.Recently, research was performed on a version of DUROS (referred to as D++) that can beapplied with nearshore wave boundary conditions.

Figure 3.4 Principe van DUROS berekening.

3.1.4 DUROSTAThe DUROSTA model is a process-based model that computes dune erosion for a 1D cross-shore profile on a time scale of a storm (WL | Delft Hydraulics, 1990a and 1990b). For everygrid cell the sediment concentrations and transports are computed on the basis of thehydrodynamic conditions. It was developed with the intention to be used for the assessmentof dune safety. In the current practice it is only used for complicated sections of dune coasts.

The model is quite fast as the domain (cross-shore profile) and timeframe (storm duration) ofthe computation are small. It is, however, not suitable for cross-shore profile computationsduring normal conditions, as streaming and wave asymmetry are not included in the model.So, no shoreward sediment flux is computed.



Figure 3.5 DUROSTA application for a dune

3.1.5 ESTMORFESTMORF is a semi-empirical model that computes morphological changes in estuaries andtidal lagoons (Wang, 2005). It combines a one-dimensional tidal flow model with empiricalrelations, which provide a relation between the morphological equilibrium state and the (tide-integrated) hydrodynamic parameters. This means that equilibrium relations are used tocompute average sediment concentrations in each element of the model, which is thentransported by means of the exchange flow between elements that is computed with anumerical flow model. The basic assumptions of the model are:

A morphological equilibrium state can be defined for each element The sediment is transported mainly as suspended load. The rate of morphological change is dependent on the difference between the actual

and the equilibrium state.

The model is very suitable for complex estuaries, as it is faster than field models, like Delft3D,and includes more details than the ASMITA model. It has also proved to work well for theWestern Scheldt and for the Friesche Zeegat. For example, in the context of the impactstudies of deepening the navigational channel of the Western Scheldt (WL | Delft Hydraulics,2003).

Figure 3.6 ESTMORF model of the Western Scheldt

3.1.6 HabitatThe Habitat model is a spatial-analysis tool for ecological assessments (Duel et al., 1995).The model is a shell around the PC-raster GIS software (property of the University ofUtrecht). Habitat can be used to model the availability and quality of habitats for certainspecies (vegetation and animals). For this purposes relations can be included that relate thephysical and environmental properties to the suitability of a habitat for certain species. Themodel is very flexible and can be used for all kind of assessments and ecological parameters.The performance of the model does, however, depend on the availability of relations betweenenvironmental parameters and response of the species or vegetation.



A database with information on the influence factors for the suitability of habitats for a largenumber of species is available. It contains mainly characteristics of species which arerelevant for the ecology in the Netherlands. There is no particular reason to use any otherecological model tool, as the database with characteristics of the species is the mostimportant aspect.

Figure 3.7 Habitat model structure (left pane) and model result for the Markermeer (right pane)(images from Haasnoot et al. 2005)

3.1.7 PONTOSThe PONTOS model is a multi-layer coastline model that computes coastline changes as aresult of longshore sediment transport separately for 5 depth layers (Steetzel et al., 1998).This allows for different rates of coastline changes at different depth levels. Cross-shoresediment transport is computed on the basis of the profile steepness of each layer, which isrelated to an equilibrium profile steepness. Nourishments can be included in the cross-shorelayers of the models. A model study with a Zandmotor was made by distributing thesediment over some cross-shore layers (see Figure 3.9). For this purpose a PONTOS modelfor the uniform coast was coupled with ASMITA models for the tidal lagoons of theWaddenzee (Steetzel et al., 2000). The model has been applied for the Holland coast andwas validated for the period 1970-1990 and 1990-2003.

The model has the advantage that it is faster than a field model like Delft3D, but it is muchslower than a coastline model like UNIBEST-CL+. A drawback of the model is that thederivation of the equilibrium slopes (for each layer) requires validation data and extensivecalibration.



Figure 3.8 Layer schematisation in the PONTOS model

Figure 3.9 Schematisation of the Holland coast in the PONTOS model

3.2.7 UNIBEST-CL+UNIBEST-CL+ is a 1D coastline model that computes coastline changes as a result of wavedriven longshore sediment transport (single layer in the cross-shore direction) at specificlocations along the coast (WL | Delft Hydraulics, 1994). The sediment transport is thentranslated to shoreline migration, which results in changes of the sediment transport in time.The model is generally used in impact studies for coastal structures, like harbour moles. Theimpact of a nourishment could be included as a source term in the model. The cross-shoreaspects of the nourishment, however, are not taken into account. UNIBEST-CL+ can beapplied for uniform coasts with revetments, groynes and breakwaters.

The major advantage of the model is that it is very fast. Consequently, it is possible toevaluate a large amount of scenarios in a short time. The small computational effort of themodel also allows for a schematisation of the wave conditions that is more detailed than formodels like Delft3D (which require a small set of condition to reduce the computational effort).The model has been applied in a wide range of scientific and consultancy projects.



Figure 3.10 UNIBEST-CL+ coastline model (left pane) and cross-shore distribution of longshoresediment transport (right pane)

3.1.8 UNIBEST-TCUNIBEST-TC is a 1D cross-shore profile model which can simulate cross-shore aspects likebank behaviour and foreshore nourishments (WL | Delft Hydraulics, 1999). The modelcharacteristics are described in Table A.7 (see Appendix) and are summarised here:

UNIBEST-TC can simulate cross-shore profile changes and dune erosion on a time scale ofstorms up to years. The model is fast, which is a consequence of the small computationaldomain (cross-shore profile). The model is suitable for cross-shore profile computationsduring normal conditions, as streaming and wave asymmetry are included in the model (VanRijn et al, 2003). The model is mainly applied in research projects.

Figure 3.11 Cross-shore profile hindcast with the UNIBEST-TC coastline model

3.1.9 XBeachThe XBeach model is a process-based model that can simulate cross-shore profile changes,dune erosion and long shore sediment transport (in 2DH) on a time scale of storms up tomonths (Van Thiel de Vries et al., 2008). It is typically applied for the computation of duneerosion and overwash of dunes. The model includes a wave model that is coupled to a flow



model. These models compute the propagation of short and long (infragravity) waves, as wellas the transfer of energy between the short and the long waves, which is essential for duneerosion. The model was developed by Deltares for the US Army Corps of Engineers after thehurricane Ivan.

Xbeach models are computationally intensive (especially in large 2DH simulations). However,the computational times are often still very acceptable, due to the small domains that areapplied (1D cross-shore profiles). The model has been validated with a series of physicalmodel tests of dune erosion and overwash situations.

Figure 3.12 Dune erosion hindcast with the XBeach coastline model (left: flume; right: 2DH beach)

3.1.10 Design formulae and expert judgementNot all aspects that were defined in Sections 2.2 and 2.3 are covered by numerical models.Therefore some aspects will need to be based on theoretical formulations or expertjudgement. Relevant theoretical or empirical formulations are available for:

Strength of structures (like dikes, revetments) Aeolian transport of sediment (although only limited data is available)

Note that expert judgement is required for other aspects like scenery properties (e.g. sceneryproperties of a Zandmotor).



4 Comparison of models

4.1 IntroductionThis chapter focuses on the suitability of the available models (Chapter 3) for the areas thatwere discerned in Chapter 2. Section 4.3 describes the comparison of the models to therequirements and the match of the models.

4.2 Model evaluationOn the basis of the findings in the previous chapter, this chapter deals with the potentialapproaches that can be chosen to perform long-term eco-morphological modelling of thecoast of Holland. For this purpose, the models where divided in the following categories:

Coastline and field models Dune erosion and cross-shore profile models Empirical models Ecological model

Coastline and field modelsFor a uniform coastline the Delft3D model can be used. This is the most detailed approach tosolve the coastline changes, but the drawback of the Delft3D model is the substantialcomputational effort required. Furthermore, the cross-shore profile changes are not resolvedwell in Delft3D as gradual erosion of the coastline takes place in the model due to theabsence of a good representation of the swash zone. It would therefore be beneficial for long-term computations to use a model which does not resolve all the details of the cross-shoreprofiles throughout the computations. This is the case for coastline models like UNIBEST-CL+and PONTOS. Another option would be to use ASMITA in combination with Delft3D for allareas (specifying equilibrium relations), but specification of the equilibrium relations isexpected to require a lot of effort which makes this a somewhat less suitable option. Complexcoastlines (e.g. Zandmotor) should be assessed with a 2DH or 3D morphological model (likeDelft3D). Aspects that are relevant here (aeolian transport and interaction with ecology) are,however, not included yet in the models. The Delft3D model can also be used to assess theinfill and shifting of sand mining pits and navigation channels as well as the detaileddevelopment of estuaries on short timeframes (max. years). For some aspects (like localdevelopment of the shoals) such a detailed approach can be very useful, although thecomputational effort will be high. Furthermore, the impact on the tidal lagoon will be moredifficult to assess due to the autonomous development of the tidal lagoon and inherent modeluncertainties.

Dune erosion and cross-shore profile modelsAll dune erosion models have their strengths and weaknesses. For example, the Xbeachmodel is the most complex model, but is more computational intensive than the other models.Xbeach includes long waves which are the major cause for dune erosion. but it lacks theability to simulate the behaviour of sand bars that UNIBEST-TC has. Durosta and UNIBEST-TC are well established models which are faster than Xbeach (especially the Durosta model).The Durosta model, however, is only suitable for storm events as it can not simulate cross-shore changes during mild conditions. This should not be an issue as long as only the duneerosion is considered. Using the DUROS model, which is a semi-empirical model, would beanother option for computing dune erosion. The DUROS model is very fast and is regularly



applied for the safety assessments of the dunes along the coast of Holland. Some processesare not modelled in any of the models, which holds for the process of dune growth. This isdue to the fact that it depends strongly on aeolian sediment transport. So, it is not obviouswhich model should be used for dune erosion. Currently, the Xbeach model is the preferredchoice for dune erosion modelling as it can cope with long waves and is developedintensively by a wide group of people.

Empirical modelsThe long-term development of estuaries like the Marsdiep can be modelled with(semi)empirical models like ASMITA and ESTMORF, as a detailed model like Delft3D is onlysuitable for short timeframes. The strong point of the ASMITA and ESTMORF models is thatthey are very fast models. Furthermore, the impact of measures can be assessed directlyfrom the results (easy to interpret). The results are, however, not very detailed (aggregatedparameters for large areas).

Ecological modelHABITAT is the only ecological model that is considered in this report. In fact it post-processes available environmental data to suitability for ecological species. It can be used forany of the areas, but is most useful for complex features (like a Zandmotor) and inter-tidalflats inside estuaries as the nature value is one the most important properties of such areas.Interaction between ecology and morphodynamics is, however, not yet available. For uniformbeaches it is expected that the development of the ecology is more or less independent fromthe development of the coastline, and can therefore be assessed after the morphologicalcomputation.



4.3 Model comparisonAn overview can be made of the functional capabilities and applicability of each of the models(see Table 4.1), for which the information in Appendix A was used.


ASM

ITA

Estm

orf

Hab

itat

Del

ft3D

Pont

os

Uni

ebst

CL+

Uni

best

TC

Dur

os

Dur

osta

Xbea

ch

form

ulae

/Th

eore

tical

Coastline position / MKL *1Uniform beach

Dune erosion *2Coastline position / MKLProfile development *3Dune erosion *2Impact ecology on morphodynamics *4

Sand bars

Swimming safetyCoastline position / shape

HeadlandsDune erosion *2Coastline position / shape *6Sandy and fine sediments (mud)Interaction ecology and morphodynamics *5Wind transport (Aeolian transport)Swimming safety

Complexcoastal features

Scenery propertiesMorphological development (shift / filling) *7

Sand mining pitDisturbance to environmentMigration rates & Sedimentation/erosionFine sediments/mud (inter-tidal flats)Habitat suitabilityImpact ecology on morphodynamics (flats) *4Interaction ecology and morphodynamics (flats)*5Import of sediment *8Sediment bypass (ebb delta) *8

Estuary- ebb delta- tidal channels- inter-tidal flats

Volume/Area of ebb delta, flats and channels *8Impact on tidal prism and estuaries (dams) *9Sediment bypass (barriers)*10Coastline changes (harbour moles) *11Sediment bypass (groynes) *12Scour / Interaction hard-soft (dike/revtm.) *13Sediment infill rates (navigation channel) *14

Structures- Barriers/dams- Harbour moles- Groynes- Dikes- Revetments- Navigation ch. Hydraulic design conditions & strength *15

Table 4.1 Overview of physical features of the considered models

*1 Coastline position can be computed with field and coastline models

*2 Xbeach includes long waves. The DUROS model (here in column formulae) is an empirical dune erosion model.

*3 Unibest TC includes landward transport of sediment.

*4 The impact of ecology on the hydrodynamics can be included by means of roughness for waves and currents. The available

number of species is, however, limited.

*5 Interaction not available as functionality in models, but some simple applications may be possible if model results are passed

from one model to the other which is a complicated procedure.

*6 Pontos can to some extent include the cross-shore profile characteristics

*7 ASMITA in combination with Delft3D



*8 Emprical models like ASMITA and ESTMORF can better predict areas and volumes of flats, channels and ebb-tidal deltas

than a process-based model can. While sediment bypass is a complex and detailed process that is better studied with a field

model.

*9 ASMITA and ESTMORF can be used to study long-term impact of partial closures of a tidal lagoon.

*10 Delft3D can be used to estimate the effects on tidal prism and sediment bypass.

*11 Coastline models can be used for harbour moles and groynes, although partially blocking structures are difficult.

*12 Sediment bypass can be assessed only with detailed models. Or alternatively in a coarse way with PONTOS.

*13 The interaction between structures and sandy coasts is difficult. This holds especially for the local scour holes.

*14 Sediment infill rates can only be studied in detail with a field model (Delft3D). Coasltine models can be useful for unprotected

trenches in the active zone (where longshore transport is interrupted).

*15 A detailed model like Delft3D is required to determine design conditions. Formulae for safety assessment.

From Table 4.1 it can be seen that none of the considered models covers all of thespecifications, but the Delft3D model is capable of handling most issues. It is clear that aprocess-based 2D/3D model, like Delft3D, is required to assess the more complicatedfeatures along the coast of Holland. Such a complex model should (ideally) be assisted by adune erosion model (Xbeach, DUROSTA, UNIBEST-TC or DUROS), a model that cananalyse complex cross-shore profile changes like foreshore nourishments (UNIBEST-TC) anda tool to analyse the ecological aspects (HABITAT).

However, besides the capabilities of a model to handle complex situations, other practicalaspects are of importance, like computational efficiency, availability of trained staff andvalidation of the model. A score for each of the models is presented in Table 3.2.


ASM

ITA

Estm

orf

Hab

itat

Del

ft3D

Pont

os

Uni

ebst

CL+

Uni

best

TC

Dur

os

Dur

osta

Xbea

ch

form

ulae

/Th

eore

tical

Computational timeModel area XL XL S-L S-L L L S S S SComplexity of the modelling effort

Model handling

Availability of trained staffSuitable for long-term computationsCan deals with far from equilibrium situationRobustnessModel has been validatedCoupling with other modelsBatch functionalityAggregation

Probabilistic computationsOngoing work

ImprovementsModel concept allows for improvements

Table 4.2 Overview of model related aspects

Table 4.2 shows that the computational effort can differ significantly for the models, which isdirectly related to the complexity of the processes in the model. To avoid excessivecomputational effort, it is worthwhile to consider the use of models with a less detailed ormore aggregated approach for less complex situations. In this respect, the PONTOS orUNIBEST-CL+ models can be used for uniform coastlines instead of Delft3D. Areas with sandbars should, depending on their complexity be assessed with a combination of a longshoreand a cross-shore model (UNIBEST-CL+ and UNIBEST-TC) or with a complex field model



(Delft3D). In practice, however, coupling of models is very difficult and needs to be improvedto perform this on long-term computations. The coupling functions should therefore beimproved. Another option would be to use a modified Delft3D model with less detailedformulations for uniform coastlines. This approach was applied for the Flyland project. TheASMITA model can be used to reduce the computational load for the impact assessment ofchanges in sediment supply for the Marsdiep inlet. A calibrated ASMITA model is alreadyavailable for this tidal lagoon.

Other aspects that need to be taken into account are: The complexity of modelling with Delft3D and PONTOS is somewhat higher than for the

other models. Only a few trained modellers are available for the ESTMORF, PONTOS and UNIBEST-

TC model, which are also less frequently applied than other models. Less complex models (like the UNIBEST-CL+) will perform better than detailed models

(like Delft3D) for long-term computations, because the degree of freedom for the modelis smaller (e.g. cross-shore profile is fixed). It should then be checked whether theassumptions in the simple model are realistic (do the cross-shore profiles changesignificantly over time). Furthermore, models with aggregated parameters (like area ofintertidal flats in ASMITA) are more stable due to the (relatively) smaller changes overthe modelling timeframe.

Aggregated models, however, sometimes have the disadvantage of being moredependent on calibration data. It should be possible to derive the equilibrium situationfrom these models (like ASMITA, ESTMORF and PONTOS).

In order to create one combined long-term model it should be possible to make acoupling between the individual models. Direct couplings that are included in the modelsare, however, hardly available. Some models, like Delft3D and UNIBEST-CL+ have abuilt-in coupling with SWAN, but no couplings to other morphological models. Moreindirect couplings were made between models with batch functionality, like a couplingbetween ASMITA, PONTOS and Delft3D, but it often took a substantial effort to do this.A large amount of coupling routines should then be made.

Even when all of these models are combined, it is not yet possible to realise a long-term eco-morphological model. It was found that there still is a knowledge gap with respect to dunegrowth (by aeolian transport), although wind driven sediment transport models are availablefor other areas (like deserts). What complicates the modelling of the aeolian sedimenttransport is that the transport is very different for dry and wet areas. Formulations for aeoliansediment transport are, however, expected to become available within BwN through PhDwork of Sierd de Vries. These formulations should then still be implemented in the models(e.g. in Delft3D). Another aspect is related to the response of the ecology to environmentalparameters. For sand bars it can be relevant to take into account the impact of Benthos andvegetation on the hydrodynamics and morphology of sand bars. Formulations to assess theimpact of the ecology on the hydrodynamics are currently being developed (Borsje et al.,2008). Furthermore, the ecological development on top of and on the leeward side of complexcoastal features, like a Zandmotor, requires direct interaction between ecology andmorphology. These interaction models are not available, but some developments are started.Furthermore, scenery properties (relevant for recreation) and structural stability are notincluded in the numerical models and should be assessed now by means of theoreticalformulations and expert judgement.



5 Modelling Approach for Eco-morphological modelling

5.1 Application of modelsFor the morphodynamic simulations of mega-nourishments along the Dutch Coast a suitablemodelling approach was discussed with specialists on numerical morphological modelling. Itwas decided to develop a system of coupled models enabling a flexible technique to coverthe effects of small-scale interventions on a larger spatial and time scale. The core of thecoupled system will be Delft3D. On the one hand, output from Delft3d will be provided toUNIBEST as e.g. annual longshore transports, while on the other hand XBeach can providebed changes after a storm period. The impact on tidal lagoons (like the Marsdiep) can beevaluated with ASMITA.

Figure 5.1 sketches the application ranges on the spatial and temporal scale of theconsidered models applied for the mega-nourishment, combined with the corresponding scaleof the Zandmotor.

XBeach can typically be applied on storm scale and for areas in the order of 100 m. Delft3D can typically be applied from a tidal period up to a decade and typically covers

areas from 100 m up to several 10 kms. A coastline model e.g. UNIBEST can typically be applied for seasonal periods up to

centuries covering areas from 1km up to an entire coastal system of 100 km.

Figure 5.1 Application ranges of the considered models applied to the Zandmotor



5.2 Spatial couplingSediment transports around complex geometries can be computed with the area modelDelft3D, while the transports at more longshore uniform geometries can be studied moreefficiently with a coastline model. Hence, at a certain distance from the intervention along thecoast the sediment transports become more uniform and change gradually, which allows forthe application of a simplified model (like a 1D shoreline model). At this location, the areamodel can be substituted by the shoreline model, while the area model provides e.g. annuallongshore transports to the shoreline model.

The deformation of the mega-nourishment can also be influenced by an extreme event. Astorm period with an extreme water level may cause inundation of the mega-nourishment,possibly leading to overwash and erosion. The crest level of the nourishment may hence belowered increasing the vulnerability for subsequent storms. The process of breaching mayalso occur during extreme events. To take into account the effects of such extreme events onthe morphodynamic evolution of the mega-nourishments, an XBeach model can be setup forthe complex geometry of the mega-nourishment. Morphodynamic simulations can beconducted including the long wave effects for storms with different return periods. In this waychanges in the bed due to extreme events will be included in the longer-term development ofthe nourishment.

Sediments for the mega-nourishment will bemined offshore, probably beyond the -20mdepth contour. At such borrow areas, the bedwill be significantly lowered due to theextraction of sand. Such deep and wide pitsmay alter the propagation of waves towardsthe coast as well as the tidal currents; both indirection and magnitude. The geometry ofthe pits may be designed in such a way thatsediment transport towards the coast isenhanced due to natural processes of tidesand waves. Hence, optimal use is made ofnatural processes and such measures highlyaccommodate natural eco-dynamics.

Regarding the application on the mega-nourishment, the different considered modelscovering a specific area will be coupled in aflexible manner such that information isexchanged at specific times during the eco-morphological simulation. The sketch depicts apossible combination of models (XBeach, Delft3D, UNIBEST, and Durosta) for the areaaround the mega-nourishment.

5.3 Temporal couplingBesides the spatial coupling between the considered models, a temporal coupling is requiredto ensure a smooth development of the bed changes at the mega-nourishment as well as forthe adjacent coasts. Typically, the longshore sediment transports from the Delft3D model areprovided to the adjacent two UNIBEST models at an interval of one year. The XBeach modelcan be called at specific intervals during the morphodynamic Delft3D simulations, dependingon the return period of the considered storms.



Alon

gsho

re d

ista

nce?

Time (yrs)?

XBeach

Delft3D

UniBest

XBeach

Delft3D

UniBest

XBeach

Delft3D

UniBest

XBeach

Delft3D

UniBest

1 2 3

Alon

gsho

re d

ista

nce?

Figure 5.2 Coupling of different models in time and space

To illustrate the coupling intervals during the morphodynamic simulation the different calls aredepicted in the figure above. The orange blocks indicate the moments where XBeach iscalled during the Delft3D computation. UNIBEST is called at a one-year interval andexchanges information with Delt3D. Similarly, the ASMITA model at the Marsdiep (which isnot included in the figure) can be coupled to the output of the UNIBEST model.

Ecological effects are included mainly in Delft3D, as it is expected that ecology has relevantimpact where gradients are large; e.g. vegetation may effect the rates of bed change and thecharacteristics of the morphology. Interaction of ecology and morphology is expected onspatial scales of O(100m) O(1 km) and on temporal scales of seasons to years. Therefore,Delft3D will be the most suited model to include the biogeomorphological interaction.



5.4 Relevant evaluation parameters to considerThe integrated approach for the mega-nourishment, that was discussed in the previoussections, consists of three different models: XBeach, Delft3D and UNIBEST. The effects onthe Marsdiep can be included with the ASMITA model. Table 5.1 presents the parametersthat can be studied with the considered models.

Model Applications

Xbeach Dune erosion / safety Morphological changes of remote areas

Delft3D

Swimmer safety Siltation effects in shadow zones Eco-morphological effects Medium-term morphological changes Sedimentation of port access channels Sand bypassing

UNIBEST-CL+ Interaction between nourishments Alongshore migration of nourishments Transport into Marsdiep tidal lagoon

Wind model Dune growth Sediment losses from emerged areas (e.g. high

areas of mega-nourishments and dune areas)Table 5.1 Overview of parameters that are to be studied with the selected models

Figure 5.3 Relevant evaluation parameters for a mega-nourishment



6 Conclusions and recommendations

6.1 Findings of the model evaluationIn this document the suitability for long-term eco-morphological simulations is assessed for anumber of numerical models. Specifications for the review of these models were made, whichwere based on the differing requirements for distinct spatial regions along the coast ofHolland (like uniform beaches). From this assessment, the following was concluded withrespect to the capabilities of the models and availability of knowledge that is required for long-term modelling:

The Holland Coast can be characterized by a limited number of areas with distinctcomplexity. Ranging from uniform beaches, which are the least complex, to complexcoastal features such as spits/zandmotors and estuaries.

No model can handle all requirements. Instead, a number of models cover a widevariety of situations. However, even a combination of models is often not sufficient

The current ecological model (HABITAT) computes HABITAT suitability, which isinsufficient for parts of the coast with a feedback mechanism between ecology andhydrodynamics/morphology.

There is a lack of ecological process information for some species, which is ofimportance as the ecological models strongly depend on the available database withecological process information.

A model that computes aeolian (wind) sediment transport for coastal applications is notavailable. Such a model could be very valuable for complex coastal features or meganourishments (e.g. zandmotor).

Besides the capabilities of a model to handle complex situations, the following conclusions onpractical aspects are drawn:

Computational time can be excessive if the most complex models are applied fordetailed simulations.

It is should be considered to use less detailed models or more aggregated models (e.gcoastline model) for less complicated areas along the coast of Holland.

For some models (PONTOS, UNIBEST-TC) only a few trained modellers are available. Robustness of the computations (due to physical aspects like cross-shore profile

changes) is an issue for long-term computations with detailed 2DH models like Delft3D.It is also possible that numerical accuracy becomes relevant for detailed models (due tosmall cells and time steps). Aggregated models are more robust.

Aggregated models are more dependent on calibration data than process-basedmodels. This holds especially for ASMITA and PONTOS, which use equilibriumrelations. Extra attention must hence be paid to the calibration data for these models.

Coupling between the models is required, but not yet available.



6.2 ConclusionFrom the evaluation of the models and discussions with specialists on numericalmorphological modelling, it has been decided to develop a system of coupled modelsenabling an integrated, flexible technique to cover the effects of small-scale interventions on alarger spatial and time scale.

The core of the coupled system will be Delft3D as a mega-nourishment is initially anintervention with a complex geometry along the coast. A coastline model (UNIBEST-CL+) willbe coupled in areas with uniform beaches to cover larger spatial and temporal scales, whileon the other hand XBeach can provide bed changes after a storm period. The long-termimpact of measures on the intertidal flats in the Marsdiep estuary can be modelled with theASMITA model.

6.3 Activities 2011To develop an integrated system of coupled models the activities listed below are required.These tasks are scheduled in several work packages (HK4.1, HK3.6, NTW1.3) for 2011 incombination with the work of PhDs and master students.

For the evaluation of strategies of both nourishments and sand mining, an integrated systemof coupled models is required to cover the range in space and time; small-scale, short-terminterventions and impacts up to long-term effects on a large-scale. The evaluation ofstrategies will mainly focus on effects on periods of 1 year 1 decade as in such periodsdistinct differences can still be observed between strategies. Another important fact is that theeffects of ecology can not be assessed on time scales larger than about 5 years. Substantialknowledge and experience is lacking for longer periods.

A. Coupling of modelsAs aforementioned, for strategies consisting of mega-nourishments, an integrated system ofcoupled models is required. Therefore, different models covering a specific area will becoupled in a flexible manner such that information is exchanged at specific times during theeco-morphological simulation. This task will focus on the coupling of XBeach, Delft3D,UNIBEST and ASMITA.

B. Aeolian transport modelWith respect to aeolian transport, it is recommended to perform research on availableliterature, formulations and models for aeolian transport that may be applicable for a mega-nourishment in a coastal zone.

C. Interaction of ecology and morphodynamicsIt is recommended to perform simulations with interaction between ecology (vegetation,benthos) and morphodynamics in order to study the potential effects of ecology on themorphological development and vice versa. This model feature should take into account thefeedback between ecology and hydrodynamics/morphology. Point of attention is that processknowledge on the behaviour of ecological species should be improved and included in themodel.



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WL | Delft Hydraulics, 1990b. DUROSTA : tijdsafhankelijk dwarstransportmodel voor extreme condities.Modelonderzoek duinvoetverdedigingen. H.J. Steetzel, Report nr. H0298 deel III.

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Appendix A : Model characteristicsThis appendix presents the characteristics of the considered model.

ASMITAType of model ? Semi-empirical behaviour model. A network with an arbitrary number of elements can

be defined. These elements can be coupled by means of equilibrium relations.Input data ? Equilibrium situations for model elements (e.g. tidal flats). These are often based on

long-term historical development of the considered model element.? Exchange rates between elements, which can for example be based on the

hydrodynamic characteristics of the tidal lagoon.Output data ? Long-term development of the model elements (e.g. tidal flats) for different scenariosModel handling ? The model is extremely fast as it is dealing with the estuary in a parameterised way.

? The model simulates ten to hundreds of years.? The model area can be very large (estuary / tidal lagoon)? Sufficiently trained staff is available

Robustness ? The model can be used for long-term simulations, but the accuracy depends on thequality of the data.? The model changes should be small compared to the scale of the total system of

elements (otherwise data are not valid anymore).? The model has been validated on various estuaries.

Model aggregation ? The model was coupled to Delft3D and PONTOS in previous situations.? Batch runs can be made.? Probabilistic computations not included directly in model, but it can be used for

probabilistic computationsImprovements ? Currently no improvements are scheduled nor required.

? The model code (FORTRAN or MATLAB) can easily be adjusted to specific demands ina project.

Applicable for: Technical specifications:Sand mining pit Applied in combination with Delft3D to model sediment infill rates with equilibrium

relations for local bed levelEstuary Long-term development of estuaries

Area of intertidal flatsVolume of ebb tidal delta and channelsSediment exchange with the coast

Barriers/Dams Impact of partial closure of a tidal lagoon on the development of estuariesTable A.1 : Overview of model characteristics of the ASMITA model

Delft3DType of model ? Process-based 2D/3D model. The model computes flow hydrodynamics including

turbulence, wave transformation towards the coast (SWAN), water quality (as a result ofadvection and diffusion processes) as well as sediment transport rates and resultingbed level changes.

Output data ? Maps with initial bathymetrical data, sediment characteristics and water quality.? Hydrodynamic boundary conditions like offshore wave data (bouy data), tidal

constituents or time series of water levels.Input data ? Maps with hydrodynamics parameters (e.g. flow velocities, water levels, inundation

frequency), sediment transport rates, water quality parameters and bathymetrical



changes (e.g. cross-shore profile development)Model handling ? Slow computations, depending on the actual size of the considered area, timeframe and

physical processes involved? Delft3D can be applied on short/medium time scales (days to years)? Depending on the application it is suitable for moderately small (e.g. suppletion,

zandmotor) to large areas (tidal inlet)Robustness ? The model has been used for long-term simulations (like the Flyland study and MER

study for the Zandmotor). These simulations lasted for example 20 years.? Large changes should not be a problem for the model. However, calibration is required

to include cross-shore behaviour (bank / foreshore nourishments) in 2DH computations,which is valid for the present situation and probably different for a significantly alteredsystem.? The model has been validated on various estuaries.

Model aggregation ? Couplings are included between the flow, wave, sediment transport and water qualitymodel. This coupling is updated throughout the computations.? A coupling with a 1d coastline model (e.g. UNIBEST) or dune erosion model (XBeach

or Durosta) is not available.? Batch functionality is already applied in projects.? Probabilistic computations are not feasible due to the long simulation time

Improvements ? Ongoing work : the implementation of a simple wave model inside Delft3D, which willmake computations faster.? Ongoing work : modifications will be implemented to improve the accuracy of the

computed longshore transport in a 3D model.

Applicable for: Technical specifications:Uniform BeachSand Bars

Longshore and cross-shore sediment transport (2D) for normal conditions.

Complex coasts Complex (2D) morphological changesDetailed flow features like rip currentsTransport of fine sediments and mudInfluence of vegetation on hydrodynamics (no interaction yet)

Sand mining pit Sediment infill ratesShifting of the location of sand mining pits

Estuary Short term development of estuariesMigration rates of shoals and tidal channelsSediment bypass of estuarine systemSedimentation and erosion on tidal flats (with/without vegetation)

Barriers/Dams Impact on tidal prism in tidal lagoonsSediment bypass at barriersHydraulic design conditions

Navigation channel Sediment infill ratesHarbour moles andgroynes

Coastline changes near to structuresSediment bypass

Dikes andrevtements

Large scale scourHydraulic design conditions

Table A.2 : Overview of model characteristics of the Delft3D model



DUROSType of model ? The DUROS model is an empirical model that computes the dune erosion after a storm

for a cross-shore profile. It assumes that an erosion profile (after 5 hours storm) can befound, which should be more gentle than the initial beach slope. The erosion profile isthen balanced such that the sedimentation (on the seaward side) is in balance with theerosion of the dune.

Input data ? As input the model uses wave parameters (wave height, wave period and water level)and properties of the dune and beach (1D profile shape and sediment diameter)? It is possible to apply corrections for curved coastlines

Output data ? Dune erosion volumes? Dune retreat distances

Model handling ? The model computations are extremely fast (minutes), because it is only a simpleempirical model.

Robustness ? Only dune erosion can be computed.? Areas which have cross-shore profiles that are very gentle are hard to evaluate, as the

erosion profile that is computed with the model is steeper than the initial profile.? It can not be applied in areas with structures

Model aggregation ? No coupling with other models.? It is possible to perform batch computations.? Probabilistic computations not included directly in model, but it has been used for

probabilistic computationsImprovements -

Applicable for: Technical specifications:Uniform Beach Dune erosion volumes and retreat distances for extreme conditions (dune erosion)Table A.3 : Overview of model characteristics of the DUROSTA model

DUROSTAType of model ? Process-based 1D dune erosion model, including transformation of waves towards the

shore with the Battjes-Jansen model. Wave run-up is included in a parameterised wayin the model. Only the erosive processes are modelled. So, no realistic accretion willtake place in the mean time.

Input data ? Data is required on the shape of the cross-shore profile, sediment characteristics, waveboundary conditions and (tidal) flow conditions

Output data ? Cross-shore profile evolution and wave transformation towards the shoreModel handling ? The model computations are fast (minutes), which is due to the small spatial (cross-

shore profile of uniform coast) and temporal (storm duration) scales.Robustness ? The model allows only short term computations.

? Substantial changes to the coast should not be a problem.? The model has been applied in large number of projects, but has never been approved

officially for the safety assessments of the Dutch dune coast.Model aggregation ? No coupling with other models.

? It is possible to perform batch computations.? Probabilistic computations not included directly in model, but it can be used for


Applicable for: Technical specifications:Uniform Beach Cross-shore sediment transport for extreme conditions (dune erosion)



Sand Bars Crude representation of cross-shore profile changes (no streaming or wave asymmetry)Dikes /Revetments

Suitable to include interaction between structures and sandy coast (hard-soft)

Table A.4 : Overview of model characteristics of the DUROSTA model

ESTMORFType of model ? Semi-empirical network model. A network with an arbitrary number of elements (area)

can be defined. For each of the elements an equilibrium concentration is defined. Theexchange between elements is computed with a 1D flow model (SOBEK)

Input data ? Area and bed level of network elements, as well as connections between elements? Equilibrium concentrations for model elements (on the basis of data or computations)? Hydraulic parameters required for 1D flow computations

Output data ? Long-term development of the volume in the model elements for different scenariosModel handling ? The model is quite fast (depends on 1D flow computations).

? The model simulates ten to hundreds of years.? The model area can be very large (estuary / tidal lagoons)? Only a few people have applied the ESTMORF model

Robustness ? The model can be used for long-term simulations? The model changes should be small compared to the scale of the total system of

elements (otherwise data are not valid anymore).? The model has been applied for the Western Scheldt and Friese Zeegat

Model aggregation ? The model itself is a coupling between a 1D flow model and an equilibrium modelImprovements ? No improvements are scheduled.

? The model code (FORTRAN or MATLAB) is available.

Applicable for: Technical specifications:Estuary Long-term development of estuaries (especially if they are tide dominated)

Area, bed level and sediment concentrations of elementsBarriers/Dams Impact of partial closure of a tidal lagoon on the development of estuariesTable A.5 : Overview of model characteristics of the ESTMORF model

HABITATType of model ? Model processes GIS data with environment variables into habitat suitability maps for

certain speciesInput data ? Maps with environment variables (inundation frequency, toxic levels, disturbance/noise)

? Relations between environment variables and habitat suitabilityOutput data ? Habitat suitability mapsModel handling ? The model is very fast, as it only processes available data.

? It can be applied on all time and spatial scales.Robustness ? Model results depend strongly on the input dataModel aggregation ? Coupling tools available to import Delft3D output data

? It is possible to perform batch computations.? Probabilistic computations not included directly in model, but it can be used for


Applicable for: Technical specifications:Sand Bars Suitability for

Documents

Approach for Eco-morphological Modelling