combining numerical wave models for efficient design of port layouts ...

PIANC-COPEDEC IX, 2016, Rio de Janeiro, Brazil

COMBINING NUMERICAL WAVE MODELS FOR EFFICIENT DESIGN OF PORT LAYOUTS AND ENTRANCE CHANNELS M.P.C. de Jong1, S.P. Reijmerink2, A. Capel3 and A.J. van der Hout2 Abstract: In this paper we highlight our view on wave penetration calculations for ports. For large port areas developed nowadays diffraction may have a relatively small influence, which makes it possible to use simpler numerical wave models for a majority of the wave conditions. These simpler models will represent the large areas involved efficiently. In this paper we state that – in a practical engineering context – using a simpler wave model and accepting a small (conservative) inaccuracy is preferable in such cases over computationally demanding complex models that for large areas perhaps cannot be applied with optimal settings. This paper presents the rationale behind the proposed approach together with a number of validation and application examples. Keywords: wave shelter, port layout design, breakwater layout design, entrance channels, wave penetration, diffraction, numerical wave modelling. 1 INTRODUCTION Wave penetration into ports is a complex process in which many wave phenomena play a role, including diffraction, refraction, (de)shoaling and reflections. It concerns operational wave conditions, exceeded 1-3% of the time in an average year, and extreme conditions, exceeded on average once in 50 or 100 years. In addition, large new port development schemes often include a dredged channel to provide entrance to deep-drafted ships. Such a channel may significantly influence and complicate wave penetration. Entrance channels are generally known to shelter a port from incoming waves, since refraction causes wave energy to leave the channel before reaching the port. However, wave conditions can vary widely in period, in direction and in directional spreading levels, which means that the consistency of the potential shelter provided by a channel needs to be verified in detail; a relatively low offshore wave height from an unfavourable direction may correspond to the critical wave height at the port and not necessarily the highest wave condition offshore. Moreover, non-linear phenomena may also play a role in this complex wave problem (see e.g. Groeneweg et al., 2014, 2015). This means that calculating wave penetration for these situations is not straightforward. On the other hand, with new ships continuously increasing in size, port developments are also becoming larger and they often include large open mooring basins and manoeuvring areas. Depending on the period of incoming waves from sea, diffraction may play a relatively small role in such major port developments. This is because in those large port layouts diffraction may only have a rather local effect. This would mean that the relevant wave propagation phenomena, at least for certain wave period ranges, may be described with sufficient accuracy by simpler numerical models, such as spectral wave models. At the same time we observe a tendency in engineering practice where, by default, the most detailed and computationally demanding wave models, such as Boussinesq-type models and non-hydrostatic wave-flow models, are applied to calculate wave penetration, regardless of the wave periods and the port dimensions considered. This may be because of the expectation that using the most complex wave model available will always lead to the most accurate wave results or simply because such model types are prescribed by clients in their terms of reference. But also those complex wave models will have their application limitations, particularly when considering large port developments. This is mainly related to the high spatial and temporal resolution required in such detailed models for accurate results. One could argue that a large benefit of using complex models such as Boussinesq-type models is that they can also calculate infragravity waves (low-frequency waves at wave-group scale). Evaluating this type of long waves is indeed critical for many port designs because such waves can trigger resonance of port basins or of moored ships. However, in the intermediate water depths associated with large port development schemes (say, 15-25 m) operational Boussinesq-type models can have limited accuracy for this type of long waves (De Jong et al., 2009, 2011). Furthermore, detailed model types may include all related processes but in engineering practice such models may 1 Harbour, Coastal and Offshore Engineering Department, Deltares, P.O. Box 177, 2600 MH, Delft, The Netherlands, Email: [email protected], Tel. + 31 88 335 8596 2 Harbour, Coastal and Offshore Engineering Department, Deltares, P.O. Box 177, 2600 MH, Delft, The Netherlands 3 Coastal Structures and Waves Department, Deltares, P.O. Box 177, 2600 MH, Delft, The Netherlands 1

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lead to unacceptably long calculation times or they cannot cover the full extent of the port and the relevant nearshore area, including a sufficiently long section of the entrance channel. Moreover, suboptimal settings (coarse resolution, specific numerical calculation schemes) possibly required in those complex models to cover the large areas involved may cause inaccurate or even unstable results. This means that complex numerical wave modelling tools may not always be the best option for computing all wave conditions for large port development schemes. As a practical solution to this we propose to use a combination of simpler and detailed numerical wave model types to calculate wave penetration and channel effects for large port developments (described in Section 2). Although the individual model types discussed here have been around for quite some time, discussing a combined approach is useful in our view in light of the observations described above. A combination of wave penetration models can be applied reliably in a practical setting, i.e. with a proper balance between accuracy and calculation times. The differences in the selected modelling tools, each modelling a different set of wave propagation phenomena, are also useful for comparing and interpreting the model outcomes when applied for overlapping sets of wave conditions. In the proposed approach it is critical to know the boundaries for up to which point each model type can still be applied reliably and when to switch to the next (more complex) wave model. Deltares knows these practical application limits, since we develop a number of wave model types in-house and because we have ample experience in applying different types of wave models in port consultancy and research projects. Some of those projects also include measurements of wave data from physical scale model tests in our laboratory facilities, which have been used for extensive validation of numerical models. In this paper we illustrate the proposed methodology by presenting calculations Deltares made in recent advice projects. These examples also included validation tasks in which the numerical wave models were compared to either physical scale model tests or field data. The next chapter presents the proposed methodology and the rationale behind it. The main potential limitations of the suggested approach together with their foreseen solutions are discussed in Chapter 3. Chapter 4 presents the project examples and their results. Chapter 5 summarises our findings and includes a brief discussion.

2 METHODOLOGY: A CASCADE OF MODEL TYPES Waves propagating over a navigation channel and into a port will mainly be influenced by refraction, diffraction, (de-)shoaling and, particularly in case of longer wave periods, reflections off bathymetry discontinuities (such as the channel slopes). The methodology proposed here combines different numerical model types to represent all these effects in a ‘cascade of models’. In increasing level of detail, and associated calculation times, the models included in this proposed cascade are: the spectral wave model SWAN (Booij et al., 1999), the mild-slope model PHAROS (Berkhoff, 1972, 1976) and the Boussinesq-type model TRITON (Borsboom et al., 2000). We have most experience with these specific models but they are merely an example of the model type they represent; other brands for the different numerical model types or similar model types could alternatively be included in the cascade. Below we first discuss the different characteristics of the models selected here, after which we will present how these models are combined in the proposed modelling cascade. Table 1 shows an overview of the most relevant phenomena related to wave propagation over an entrance channel and into a port 4 and to what extent these are included in the considered wave model types. The spectral wave model SWAN is the most efficient (limited calculation times) of the models applied here for calculating wave penetration into ports, but also the least complex (complete) in terms of wave propagation. It includes refraction but lacks proper modelling of diffraction (this can only be approximated by a parameterised description because phases of individual waves are not modelled) and it does not model bathymetry-induced reflections inside the considered domain, i.e. reflections from distinct bathymetric features. However, SWAN has been shown within a practical context to provide suitable results for calculating wave penetration into large port schemes for relatively short wave periods when waves propagate more-or-less along the axis of the entrance channel5 and provided that a sufficiently high resolution is used to represent the side slopes in case of an entrance channel. This is because the effects of diffraction, the extent of which is influenced by the wave length, and of reflections off the channel side slopes, influenced by the wave direction (and wave period), are 4 Note that this means that not all wave propagation and wave energy dissipation phenomena are considered here, only those most relevant for wave penetration into ports, with or without an entrance channel. 5 Nearshore wave conditions may often be aligned with the channel orientation because of wave refraction towards the coast and typical orientations of entrance channels (shortest distance, often more-or-less perpendicular to the coastline). 2

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not dominant under those conditions. Furthermore, the relatively high levels of directional spreading generally associated with shorter wave periods aid in compensating for the absence of diffraction in this model type. All these characteristics are beneficial for the methodology, since the shortest wave components require the longest computational times when these would need to be considered in phase-resolving models (such as in a Boussinesq-type model). Another large advantage of using a spectral wave model is that models of this type typically include wave generation due to wind, which particularly in case of larger port developments may become relevant, also within the port area. Those relatively short locally generated waves may have limited influence on large ships, but the relevance of these wind waves cannot be ruled out beforehand and should be verified.

Table 1: Relevant wave propagation phenomena in the considered numerical model types (green: included; orange: partly included; red: not included). Wave model

SWAN PHAROS TRITON

Refraction

(de)shoaling

Diffraction

Bathymetryinduced reflections

Non-linear processes influencing wave directions

parameterised may be too strong

The mild-slope model PHAROS fully models diffraction and bathymetry-induced reflections throughout the considered domain. The latter effect may be somewhat overestimated in this model type due to the mild-slope assumption. Diffraction is relevant for wave penetration into (smaller) port basins, but also for redistributing wave energy in the cross section of an entrance channel leading into a port, i.e. ‘filling in’ areas with low wave energy left by waves that have refracted out of the channel over the channel side slopes. Modelling this effect may be most critical for longer wave periods, since for these conditions the area influenced by diffraction will be the largest and because of the limited directional spreading associated with these wave periods, which would otherwise partly compensate for the absence of diffraction in the wave model results. The Boussinesq-type model TRITON provides the most complete result of the three numerical models applied here (Table 1), but it is also the most computationally demanding. It describes all the main wave phenomena relevant for wave penetration into a port and via a navigation channel. This also includes modelling specific non-linear processes that can influence wave directions, which can be a critical parameter for wave penetration especially in case of an entrance channel (considered further in Section 3). However, calculation times can become quite long for Boussinesq-type models. This particularly applies for shorter waves, because for accurate results such phase-resolving models require a large number of grid cells per wave length and a sufficiently small time step. Furthermore, any inaccuracy that occurs in a phase resolving wave model – per wave length of distance travelled – will have the largest effect on shorter waves; expressed in number of wave lengths, those short waves travel the longest (relative) distances leading to largest cumulative errors. Furthermore, Boussinesqtype models cannot be used in deeper water (typically kh 15 s). The dark green cells in Table 2 indicate how in the proposed methodology these conditions are distributed over the different numerical model types (the ‘cascade’). Please note that this is a proposed typical categorisation and that exceptions could be made depending on the specifics of a project site (considered also in the examples further below). Other combinations of model types and wave periods may also be possible within the modelling cascade. However, some combinations may be less accurate (longer wave periods in a spectral wave model, indicated in red) or less practical in view of calculation times (indicated in light green, e.g. shorter wave periods in a Boussinesq-type model). The combinations indicated in orange in Table 2 are expected to be suitable as comparison material in addition to the preferred model types 3

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represented by the dark green cells. Comparing results for the same wave conditions from different model types, each including a different (partly overlapping) set of wave phenomena, is very useful for assessing the importance of the different wave propagation phenomena. Also this will be considered in the examples presented in Section 4.

Table 2: Distribution of wave periods over the different types of numerical wave models (colours explained in the main text). Wave model

Shorter wave periods (Tp < 10 s)

Intermediate wave periods (Tp = 10 - 15 s)

Longer wave periods (Tp > 15 s)

SWAN (spectral) PHAROS (mild-slope) TRITON (Boussinesq-type)

3 POTENTIAL LIMITATIONS AND SOLUTIONS The proposed methodology, i.e. the modelling cascade, is seen as a practical approach that makes use of both wave physics and numerical model type characteristics. However, we have also identified a number of potential limitations, which we discuss in this section together with foreseen solutions. The first potential limitation that has been identified is that spectral and mild-slope wave models cannot model non-linear effects that involve complex wave-wave interactions that influence wave directions (Table 1). These interactions can particularly become important for situations with entrance channels; a part of the wave energy approaching the channel that in a linear description would refract away from the channel edge will in reality shift in direction due to these non-linear effects and then may still be able to enter the channel and penetrate the port. Groeneweg et al. (2014, 2015) studied these effects in a project related to flooding protection criteria in The Netherlands for situations including deep channels with steep side slopes in shallow (intertidal) areas. They found that these non-linear effects can have a significant influence in those conditions. However, in a practical port engineering context we expect that these particular non-linear effects may not always have to be considered, mainly since for port applications many of the relevant wave conditions will be fairly limited in height. This is because critical wave conditions related to channel and berth downtime correspond to the yearly average wave climate (1-3% average exceedance). For example, operational wave conditions along the entrance channel will typically need to remain below, say, Hs = 1.5 m because higher wave conditions will limit the ability for tugs to make fast and to be effective in assisting in vessel manoeuvres. Since non-linear effects are proportional to the wave height, this means that nonlinear effects will be limited too during operational conditions. Design wave conditions for port structures (breakwaters) will typically correspond to extreme conditions, e.g. with an average return period of 100 years, and therefore to higher waves. This may correspond to stronger non-linear effects effecting wave directions, which may need to be taken into account, particularly in case of a deep entrance channel. This will not be a problem in the proposed cascade of models, however, because extreme wave conditions will generally also correspond to the longest wave periods and for those we foresee use of a Boussinesq-type model (in our case the model TRITON). That type of numerical wave model typically does model these non-linear wave-wave interactions that influence wave directions. In addition, non-linear effects influencing wave directions will have the largest effect when wave directions are around the critical direction; when wave directions are either far below or far above the critical direction then all wave energy is already (un)able to pass the channel anyway and a fairly small shift in direction will not significantly alter that. A second potential limitation of the proposed methodology is related to the generation of bound and free low-frequency waves, i.e. infragravity waves, which have time scales of the groups of primary waves. These long waves are not explicitly covered in the methodology introduced above. We acknowledge that this type of waves is important for port design because they can lead to resonance of port basins or to resonance of moored ships. Both effects can lead to significant port downtime due to excessive vessel motions while (off)loading (see e.g. De Bont et al., 2010, Mol et al., 1986, Van der Molen, 2006, Van Deyzen et al., 2015, Van der Hout et al., 2015). Of the three model types considered here, the generation of low-frequency waves can only be described by a Boussinesq-type model. This could render obsolete the efficient cascade of wave models proposed above, since then again all wave conditions would need to be considered with that complex type of wave model. However, simulating the generation of low-frequency waves in Boussinesq-type wave models for somewhat deeper water (over 15-20 m) may not always be as accurate as these models are in 4

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shallow water (say below 5-10 m, De Jong et al., 2009, 2011). But there are efficient alternative model types available that target specifically this type of long waves (e.g. XBeach, Roelvink et al., 2009) and which are able to cover also the typical depths associated with present-day large port developments. Such models only solve the long waves (in time domain) and describe the forcing due to the primary wave energy on wave-group scale only (via the envelope of the short wave energy). In this way the model resolution in space and time can be optimised for the infragravity waves only – for a given number of grid cells per wave length, longer waves allow for a coarser grid resolution – and the model does not need to describe the much shorter individual primary waves. This makes it possible to cover fairly large areas quite efficiently. When significant levels of long waves are expected at a (proposed) port location, then infragravity wave computations should be made for a selection of critical primary wave conditions with such a model type, in addition to the methodology proposed here. Please note that the wave conditions which are critical for this aspect do not necessarily have to correspond to the highest waves offshore (see e.g. Van der Hout et al., 2015); the magnitude of infragravity waves depends on the wave height and on the period of the corresponding primary waves, with higher and/or longer waves leading to the largest infragravity wave heights. Furthermore, the motion response of a (moored) ship also depends on the periods of the generated infragravity waves. This could mean that a smaller wave height at a more unfavourable wave period could lead to a much larger motion response – and downtime – than a larger wave height at a different wave period. These aspects are not discussed further in this paper and should be considered as a separate modelling task for the port design. Van der Hout et al. (2015) and Jaouen et al. (2016) recently described the different calculation steps involved in such analyses as part of a design methodology for infragravity waves. Their methodology focusses on offshore mooring terminals, but will similarly apply to the port developments in intermediate water depths discussed here. The third potential limitation that has been identified is how to deal with locations where both sea and swell waves occur simultaneously. As a practical solution to this, one could compute these different wave conditions separately with their appropriate model type following the methodology proposed above and then combine the outcomes (summing wave energies). The fourth aspect mentioned here is modelling port resonance, or ‘seiching’. This is not necessarily a limitation of the proposed approach – it can be covered by one or more by the models included in the methodology – but it does require a separate series of computations. A spectral wave model such as SWAN does not model port resonance because wave phases, essential information required for computing resonance, are not modelled in that type of model. A Boussinesq-type model could compute these effects but will often correspond to unacceptably long calculation times. As a basic assessment of a new port layout we therefore recommend making a resonance analysis in a mildslope wave model early on in the design process by considering a uniform wave energy supply at a broad range of frequencies and determining relative amplification spectra at key locations within the port area. This is an efficient approach because it isolates the response from the forcing mechanism(s). The former is generally well known, with time scales dictated by geometry and water depth, whereas the local origins of seiches (which specific phenomena and their magnitudes) are often not known (Rabinovich, 2010). When computing potential resonance modes in a mild-slope model, all boundaries inside the port should be set to (almost) fully reflective (95-100%), regardless of structure type, whether quay wall or sloped revetment. This is because also a sloped revetment will have reflective characteristics similar to a vertical quay wall for the long wave periods generally associated with seiching, i.e. typically a few minutes or more for larger port layouts. We frequently encounter reports on wave modelling discussing whether resonance effects could partly explain numerical wave penetration results. A basic eigenmode-assessment (resonance analysis), such as recommended here, would clarify this efficiently. When interpreting the computed amplification spectra from a resonance computation it is important to realise that higher-order seiche-modes will in most cases not occur without also the lower modes being present. This is because amplification in narrow port basins will generally be largest for the lower-order seiche-modes (Rabinovich, 2010). This means that usually when only higher-order modes appear to be present these will correspond to other sources than port resonance. In addition, the outcome of a resonance analysis may even show that the incoming wave periods studied as a potential source of seiching are far off from the main eigenperiods of the port and that resonance will clearly not occur for those incoming wave conditions anyway, only wave penetration and reflections (not amplification). It may even be that the time scales of the resonance modes of the port correspond to completely different forcing mechanisms of seiches that should be considered instead, such as meteorological phenomena (Wilson, 1972, Rabinovich, 2010). Summarising, the above observations indicate that one should take the following steps to calculate all relevant wave conditions for design of large port development schemes: 5

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1. The methodology as put forward in Section 2, i.e. the cascade of numerical wave models to calculate penetration of local sea and swell wave conditions. 2. An efficient scan of resonance modes of the foreseen port layout using a mild-slope model, which can be confronted with time scales and energy levels of potential local seichegeneration mechanisms. 3. At sites along an open coast/ocean with high (swell) waves additional calculations with an infragravity wave model should be performed to determine the incoming energy levels of these long waves. Note that already with a few centimetres amplitude infragravity waves can cause significant resonance effects of moored ships leading to unacceptable port downtimes when such waves show a strong persistency.

4 ILLUSTRATION OF PROPOSED METODOLOGY 4.1

INTRODUCTION

When the typical ranges of wave periods categorised in Table 2 are all relevant for a particular port development scheme, then it will be efficient to use the complete cascade of models in the same port project. We have applied the proposed methodology in that way successfully in several recent wave penetration projects, most of which included a deep and long entrance channel. By applying the models for overlapping ranges of wave periods (the light green and orange cells in Table 2) we have been able to consider wave conditions in which one specific type of wave model approached the upper end of its application limit and the next, more complete and complex, wave model could take over. As long as results from both models remained comparable for these overlapping conditions, then this ensured a reliable transition from one model to the next. This also showed that wave propagation phenomena not included in the simpler of the two model types compared apparently did not yet have a critical effect on the resulting wave patterns. Moreover, some of our wave penetration projects included physical scale model tests and field measurements. This has allowed a further verification of the applicability limits of the different numerical model types and of the proposed methodology. Most port development projects we have studied in recent years are confidential, often because of strategic reasons of our clients. That is perfectly understandable, but that does mean that we cannot present results of those projects here. Fortunately, the examples below are available to illustrate the methodology. Both involve one or more of the simpler types of numerical wave models from the methodology. Although this does not illustrate the complete modelling cascade for one particular port layout, it still shows the benefit of not always applying the most complex wave model available. In this way we aim to illustrate, albeit indirectly, the added value of the proposed methodology.

4.2

APPLICATION OF SPECTRAL MODEL, SHORTER WAVE PERIODS

As an example of a large port development project we present here the wave penetration analyses that we have performed for the greenfield port development at Al Faw, in the south of Iraq (Figure 1). The location of Al Faw, along the northern edge of the Persian (Arabian) Gulf, is quite beneficial with respect to storm conditions. Local storm events (‘Shamal’) pass the Gulf approximately from North(west) to South, leading to quite mild critical wave conditions at Al Faw, with fairly low wave heights and all critical wave periods below 9 s. Verification of the influence of the entrance channel was deemed a prerequisite for accurate numerical modelling of wave penetration into this port. Therefore, the Client requested Deltares to aid in selecting the most appropriate numerical model(s). The full port layout was several kilometres in size (Figure 1) and would fill up the complete laboratory basin, even though Deltares has one of the largest wave basins in the world. This would leave no space in the physical model for the channel. Therefore, Deltares proposed to include only the layout of the port entrance and construct as much channel length in the physical scale modelling facility as possible. This allowed realistic representation of the wave sheltering effects provided by the full channel 6. Verifying the heights of the waves propagating through the port entrance would then be an indirect verification of the resulting wave penetration.

6 Numerical computations showed that the wave heights reaching the port entrance did not change with increasing the length of the considered section of channel. This confirmed that the channel section as included in the physical scale model was long enough to provide representative wave conditions at the port entrance. 6

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2.5 km

Figure 1: The greenfield port development at Al Faw, Iraq. The black rectangle (bottom right) illustrates the extent of the area covered in the physical scale model tests depicted in Figure 3. Figure 2 presents a technical drawing of the model layout as constructed in the Delta Basin scale model facility of Deltares (outer horizontal dimensions: 50 m x 50 m). The main bathymetric features were included schematically as indicated with the different coloured areas. Since the weight-bearing concrete basin floor cannot be altered, shallower parts were created in the scale model by covering the basin floor in those areas with a layer of sand, fixated at the exact bathymetric level with a thin layer of concrete. A transition slope parallel to the wave make was included from the deep water present at the wave maker towards the local bathymetry. The influence of this transition slope on the generated waves was taken into account in the calibration of the wave heights generated in the physical scale model. Figure 3 presents photographs of details of the physical model. These show the main breakwaters included in the model. The dark grey areas in these photographs are the deeper channel sections with depths at the original basin floor level. The beige/yellow areas are the shallower surrounding areas. In the project for Al Faw port we applied and compared the numerical wave models SWAN (spectral wave model) and PHAROS (mild-slope wave model). Here we show how these less complex models were suitable to cover the relatively short wave periods relevant for this port (up to 9 s). These short design wave periods are quite exceptional for a large port development and in other cases also longer critical wave periods will need to be considered. Nevertheless, the modelling performed for this project makes a good example of how simpler wave models could be used to cover wave periods up to 10 s.

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Figure 2: The entrance channel and port entrance of Al Faw port in the Delta Basin scale model facility of Deltares (the extent of area covered is indicated with the black rectangle in Figure 1).

Figure 3: Photographs of the physical scale model of Al Faw port (Delta Basin of Deltares). Figure 4 shows examples of wave height patterns computed with the spectral wave model SWAN and with our mild-slope wave model PHAROS for the same conditions taken from the local wave climate (run T107: Hs = 2.78 m, Tp = 6.9 s; run T108: Hs = 2.76 m, Tp = 7.9 s; run T109: Hs = 3.44 m, Tp = 8.9 s, all with directional spreading σ = 15º and with a main direction of 150ºN, which is perpendicular from the wave board, see Figure 2). Note the difference in colour scales. All these runs were made for the local average water level.

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SWAN

PHAROS

Figure 4: Left panels: wave heights (m) from SWAN (excluding diffraction); right panels: results from PHAROS (including diffraction) for the same incoming wave condition. The wave height ‘streaks’ included in the PHAROS results (right column panels in Figure 4) are the result of wave phases and correspond to elongated node/anti-node interaction patterns of waves in different, but nearby, directions. These streaks are absent in the results from SWAN since wave phases are not resolved in a spectral wave model type, resulting in a more smooth overall wave height pattern. The areas with increased wave heights adjacent to the entrance of the modelled section of 9

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channel are the result of the transition from deep water in front of the wave maker to the shallower area with the channel, which is represented in the model by the transition slope. After the waves enter the modelled section of channel, excess wave energy refracts out of the channel over a rather short distance, whereas in reality the outer section of channel, not included in the physical scale model, would introduce these effects much more gradually. These side effects of the selected scale model configuration had no influence on the wave heights reaching the port entrance. This was verified with regional SWAN computations (not shown here), which included the complete extent of the channel. The measured values for the presented conditions are plotted in Figure 4 inside the dots representing the probe locations using the same colour scheme as the overall plot; a match in colour inside and directly adjacent to a dot indicates a match between the numerical and the physical model results. The black arrows in this figure indicate measured wave heights and directions and the white arrows computed values. For SWAN computed directions follow directly from the model output; wave directions from PHAROS are derived with a validated dedicated post-processing tool (developed inhouse) that comes with the PHAROS software package (r-DPRA, De Jong and Borsboom, 2012a,b). Figure 4 shows that the agreement between the measured and computed wave conditions is quite good; wave heights7 and wave directions agree rather well. Even though a slightly better match is found for the results with PHAROS, this does show that SWAN is capable of capturing the overall wave penetration, including the influence of the channel. The absence of diffraction in SWAN is not critical in this case and the slight overestimation of wave penetration included in the SWAN results is deemed acceptable in a practical engineering context (conservative, i.e. somewhat on the safe side). Some theoretical cases were added to the measurement schedule of the Al Faw consultancy project to further explore the practical application limits of SWAN. These fictitious conditions used the same relatively short wave periods (up to 9 s) but they included a larger wave angle relative to the channel orientation (up to 30º) and very limited directional spreading (s