SWAN 41.31用户手册：第三代海浪模型详解与新手指南

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SWAN (Simulating Waves Nearshore) 是一款著名的第三代海浪模拟软件，专为海洋工程、海洋学研究以及海岸线管理等领域设计。本用户手册（版本41.31A）详细介绍了这款软件的各个方面，旨在帮助新手用户全面理解和掌握其功能与应用。 1. **关于本手册** 该手册提供了SWAN软件的基础信息，包括版权归属（Delft University of Technology），联系方式以及许可证协议（GNU Free Documentation License）。用户可以自由复制、分发和修改文档，但需遵循相应的授权条件。 2. **概述与使用指导** - **介绍**：SWAN的核心目标是模拟海浪在近岸区域的行为，包括波浪的生成、传播、破碎和与海岸地形的相互作用。它广泛应用于风浪预测、港口设计、海洋环境保护等场景。 - **局限性**：手册揭示了SWAN可能存在的限制，例如特定条件下的计算精度、处理复杂地形的能力，以及潜在的编程错误或未解决的问题。理解这些限制有助于用户选择适当的使用场景和调整参数设置。 - **内部机制与问题**：这部分深入解析了模型的工作原理，包括内部处理的场景设定、可能的限制器和短板，以及已知的编码bug。这有助于用户理解模型运作的细节，并在遇到问题时进行排查。 3. **与其他模型的比较** SWAN与其他知名海浪模拟工具如WAM (Wave Attenuation Model) 和 WAVEWATCH III 进行了对比，帮助用户了解各自的优势和适用性。了解这些模型间的异同有助于用户根据实际需求选择最合适的工具。 4. **单位与坐标系统** 用户需要熟悉SWAN采用的单位标准（如米、秒、秒方等）和坐标系统定义，这对于输入数据和解读结果至关重要。正确处理单位和坐标转换是确保模拟准确性的重要步骤。 5. **网格选择、时间窗口及边界条件** - **网格**：手册详细指导如何选择合适的网格类型（如等高线网格、经纬度网格等），网格分辨率对模拟效果有直接影响。用户应根据所研究区域的特点和需求来优化网格设置。 - **时间窗口**：确定模拟的时间长度和时间步长，这对捕捉短期波浪变化和长期波浪动态至关重要。 - **边界条件**：初始、边界和第一猜测条件的选择影响着模拟的起始状态和外部干扰，用户需根据实际情况设定合理的条件。 SWAN用户手册版41.31A是理解和使用这款高级海浪模拟工具的关键指南，通过学习手册中的内容，新用户能够更好地掌握SWAN的功能，优化模型设置，以获得准确且适用的海浪预测结果。

8 Chapter 2

(W). Consequently, the wave height and water depth are in m, wave period in s, etc. For

wind and wave direction

both the Cartesian an d a nautical convention can be used (see

below). Directi on s an d sp h er i cal coordinates are in degrees (

) a n d n ot in rad i ans.

For the ou t p u t of wave energy the user can choos e between variance ( m

) or ene r gy (spa-

tial) density (Joule/m

, i . e. energ y per unit sea surface) and the equivalents in cas e of

energy tran sport (m

/s or W/m, i.e. energy transport per unit length) and spectr a l energy

density (m

/Hz/Degr or Js/m

/rad, i.e. energy per unit frequency and direction per unit

sea surface area). The wave−induced stress component s (obtained as spatial derivatives of

wave-induced radiation stress) are always expressed i n N/m

even if the wave energy is in

terms of variance. Note that the energy density is al so in Joule/m

in the case of spherical

coordinates.

SWAN operates either in a Cartesian coordinate system or in a spherical coordina t e sys-

tem, i.e. in a ﬂat plane or on a spherical Earth. In the Cartesian system, all geographic

locations and orientations in SWAN, e.g. for the bottom gr id or for output points, are

deﬁned in one common Cartesian coordinate system with origin (0,0) by deﬁnition. This

geographic origin may be chosen totally arbitrarily by the user. However, be careful, the

numbers for the origin shou l d not be chosen too large; the user is advised to translate the

coordinates with an oﬀset. In the spherical system, all geographic locati ons and orienta-

tions in SWAN, e.g. for the bottom grid or for output points, are deﬁned in geographic

longitude and latitude. Both coordinate systems are designated in this manual as the

problem coordinate system

In the input and output of SWAN the direction of wind and waves are deﬁned according

to either

• t h e Cartesian convention, i.e. the direction t o where the vector points, measured

counterclockwise from the positive x−axis of this system (in degrees) or

• a nautical convention

(there are more such conventi on s ), i. e. the direction wh er e the

wind or the waves come from

, m easured clockwise from geographic North.

All other d ir ec ti o n s

, such as orientation of grids, are according to the Cartesian convention!

For regular grids, i.e. uniform and rect an g u l ar , Figure 4.1 (in Section 4.5) shows how the

locations of the various gri d s are determ ined with respect to the problem coordinates. All

grid points of curvilin ear and unstructured grids are relat i ve to th e problem coordinate

system.

General description and instructions for use 9

2.6 Choice of grids, time windows and boundary /

initial / ﬁrst guess conditions

2.6.1 Introduction

Sever al types of grids and time window(s) need to be d eﬁ ned: (a) spectral grid, ( b ) spatial

(geographic) grids and time window(s) in case of nonstationary computa t io n s.

The spectral grid that need t o be deﬁned by the user is a computational spectral grid on

which SWAN performs the computations.

SWAN ha s the option to make computations that can be nested in (coarse) SWAN, WAM

or WAVEWATCH III. In such cases, the spectral grid need not be equal to the spectral

grid in the coarse SWAN, WAM or WAVEWATCH III run.

The spatial grids that need to be deﬁned by the user are (if required):

• a computational spatial grid on which SWAN performs the computation s ,

• on e (or more) spatial in p ut

grid(s) for the bottom, current ﬁeld, water level, bottom

friction, veget at i on, mud, sea ice and wind (each i n p ut grid may diﬀ er from the

others) and

• on e (or mo r e) spat i al ou t put

grid(s) on which the user requires output of SWAN.

The wind, bottom friction, vegetation, mud and sea ice do not require a gr i d if they are

uniform over th e area of interest.

For on e- d i m ensional situations, i.e. ∂/∂y ≡ 0, SWAN can be run in 1D mode.

If a uniform, rectangular computational spatial grid i s chosen in SWAN, then all input and

output grids must be uniform and rectangular too, bu t they may all be diﬀerent fr om each

other.

If a curvili n ear computational spatial grid i s chosen in S WAN, then each

input grid should

be either uniform, rectangular or identical to the used curvilinear grid or staggered with

respect to the curvilinear computational g r id.

If an u n st r u ct ured computational spatial grid is chosen in SWAN, then each

input grid

should be either uniform, rectangular or i d entical to the used unst r uctured grid.

SWAN has the option to make compu t at i ons that are nested in (coarse) SWAN, WAM or

WAVEWATCH III. In such runs, S WAN interpolates the spatial boundary of th e SWAN,

WAM or WAVEWATCH III grid to the (user provided) grid of SWAN (that needs to

(nearly) coincide along the grid lines of WAM or WAVEWATCH III or the output nest

grid boundaries of SWAN). Since, the comp u t a ti o n al grids of WAM and WAVEWATCH

10 Chapter 2

III are in spherical coordinates, it is recommended to use spherical coordinates in a nested

SWAN wh en nes t ing i n WAM or WAVEWATCH III.

SWAN using an un st r u ct u r ed mesh

may be nested in SWAN em ployin g a regular grid and

vice ver sa. However, SWAN using an unstructured grid cannot be n est ed in WAM or

WAVEWATCH III.

Nesting from a 2D model to a 1D model is possible although is should not be done by

using the commands NGRID and NEST. Instead, deﬁne the boundary point of the 1D model

as an output point (u si n g command POINTS) and write the spectra for that point using

the command SPECout. In the 1D model , this spectra is used as boundary condition using

the BOUNDSPEC command.

Similarly, the wind ﬁelds m ay be available in diﬀerent time windows than t h e current an d

water level ﬁelds and the computations may need to be carried out at oth er times than

these input ﬁelds. For thes e reasons SWAN operates with diﬀerent time windows with

diﬀerent time steps (each may have a diﬀerent start and end time and time step):

• on e comp u t a t io n al t i m e window in which SWAN performs the computations,

• on e (or more) input time window(s)

in which the bottom, current ﬁeld, water level,

bottom friction, vegetation, mud, sea ice and wind ﬁeld (i f present) are given by the

user (each in p ut win d ow may diﬀer form the others) and

• on e (or mo r e) out p ut ti m e window(s)

in which t h e user requ i r es out p u t of SWAN.

In case of nesting, SWAN searches the boundar y conditions in the rel evant output ﬁle of

the previous SWAN, WAM or WAVEWATCH III runs to take the boundary conditions

at the start time of the nested run. It will not take the initial condition (i.e. over the

enti r e computational grid) for the nested run from the previous SWAN, WAM or WAVE-

WATCH III run.

During the com p utation SWAN obtains bottom, current, wat er level, wind, bottom fric-

tion, vegetation, mud and sea ice informatio n by tri-linear interpolation from the given

input grid(s) and time window(s). The output is in turn obtained in SWAN by bi-linear

int er polation in spac e from the com p utational grid; there is no interpolation in time, the

output time is shifted to t h e nearest computational time level. Interpolation error s can

be reduced by taking the grids and wind ows as much as equal to one anothe r as possible

(preferably identical). It is recommended t o choose output times such that they coincide

with computational time levels.

2.6.2 Input grid(s) and time window(s)

The bathymetry, current, water level, bottom friction, vegetation, mud, ice and wind (if

spatially variable) need to be provided to SWAN on so-called input gri ds. It i s best to

General description and instructions for use 11

make an in p u t grid s o lar ge th at it com p l et el y covers the computationa l gri d .

In the region outside the input grid SWAN assumes th at the bottom level, the water level,

bottom friction, vegetation, mud and ice are identical to those at the nearest boundary of

the input grid (lateral shift of that boundary). In the regions not covered by this lateral

shift (i. e. in the outside quad r ants of the corners o f the input grid), a constant ﬁeld equal

to the value at the nearest corner point of the input grid is ta ken. For the current and

wind velocity, SWAN takes 0 m/s for points outsi d e th e inp u t gr id.

In SWAN, the bathymetry, current, water level, wind, bottom fric t io n , vegetation, mud

and sea ice may be time varying. In that case they need to be provided to SWAN in

so-called in p u t time windows (they n eed not be identical with the co m p u t a ti o n al , output

or other inp u t windows). It is best to make an input window larger than the computa-

tional time window. SWAN assumes zero values at times before the earlie st begin time

of th e input parameters (which may be the begin time of any input parameter such as

wind). SWAN assumes constant values (the last values) at times after the end time of each

input parameter . The inpu t windows should start early en ou g h so that the initial state of

SWAN has propagated through the computational area before reliab l e out p u t of SWAN is

expected.

One should choose the spatial resolution for the input grid s such that relevant spatial

details in the bathymetry, currents, bottom friction, vegetation, mud, ice and wind are

properly resolved. Special care is required in cases with sharp and shallow ridges (sand

bars, shoals) in the sea bottom and extremely steep bottom slopes. Very inaccurate ba-

thymetry can result in very inac cu r at e refra ct io n comp utations the results of which can

propagate into areas where refraction as such is not signiﬁcant (the results may appear to

be unstable). For instance, waves skirting an island that is poorly resolved may propagate

beyond the i sla n d with entirely wrong directions. In such a case it may even be better to

deactivate the refraction computations (if refraction is irrelevant for the problem at hand

e.g. because the refracted waves will run into the coast anyway and one is not interested

in that pa r t of the coast). In such cases the ridges are vitally important to obtain good

SWAN results (at sea the waves are ’clipped’ by depth-induced breaking over the ridges

which must

therefore represented in SWAN comput a t io n ) . This requir e s not only that

these rid ges should be well represented on the input grid but also after interpolation on

the computational grid. This can be achieved by choosing the grid lines of the inpu t grid

along the ridges (even if this may require some slight ”shifting” of the ridges) and

choosing

the computational grid to be ident i cal to the input grid (otherwise the ridge may be ”lost”

in the int er polation from the bottom input grid to the comp u t at i o n al gr i d ) .

Finally, one should use a time step that is small enou g h th a t t im e variations in the bathy-

metry, current, water level, wind and bottom friction are well resolved.

12 Chapter 2

2.6.3 Computational grids and boundary / initial / ﬁrst guess

conditions

The computational spatial grid must be deﬁned by the user. The orientation (direction)

can be chosen arbitrarily.

The boundar i es of the computational spatial grid in SWAN are either land or water. In the

case of land there is no problem: the land does not generate waves and in SWAN it absorb s

all incoming wave energy. But in the case of a water boundary there may be a problem.

Often no wave conditions are known along such a boundar y and SWAN then assumes

that no waves enter the area and t hat waves can leave the area freely. These assumptions

obvi ou sl y contain errors which propagate int o the model. These boundaries must therefore

be chosen suﬃciently far away from the area where reliable computations are need ed so

that t hey do not aﬀect the computational results there. This is best established by varying

the location of these boundaries a n d inspect the eﬀect on the results. Sometimes the waves

at these boundaries can

be estimated with a certain degr ee of reliability. This is the case if

(a) results of another model run are available (nested computations) or, (b) obser vations

are available. If model results are availa b l e along the boundaries of the co m p u t a t io n al

spatial grid, they are usually from a coarser resolution th an the computational spatial grid

under consid er at i on. This i m p l i es that this coarseness of t h e boundary propagates into

the computational grid. The problem is t h er efor e essentially the same as if no waves are

assumed along the boundary except that now the error may be more acceptable (or the

boundaries are permitted to be closer to th e area of interest). If observations are available,

they can be used as input at the boundaries. Howe ver, t his usually covers only part of the

boundaries so that the rest of the boundaries suﬀer from the same error as above.

A special case occurs near the coast. Her e it is often possible to id entify an up-wave

boundary (with proper wave information) and two lateral boundaries (with no or partial

wave information). The aﬀected areas with errors are typically regions with the apex

at the corners of the water boundary with wave information, spreading towards shore at

an angle of 30

to 45

for wind sea conditions t o either side of the imposed mean wave

direction (less for swell conditions; the angle is essentially the one-sided widt h of the

directional distribution of wave en er g y) . For propagation of short crested waves (wind sea

condtions) an examp l e is given in Figur e 2.1. For this reason th e lateral boundar i es should

be suﬃciently far away from th e area of interest to avoid the propagation of this error

int o this area. Such problems do not occu r if the latera l boundaries contain proper wave

information over th ei r entire len gt h e.g. obtained from a previous S WAN computation or

if t h e late r al boundaries are coast.

When output

is requested along a boundary of the computational grid, it may occur that

this output diﬀers from the boundary con d i t io n s that are imposed by the user. The reason

is that SWAN accepts only the user-imposed incoming

wave components and that i t re-

places the user- i m posed outgoing

wave components with computed outgoing components

(propagating to t he boundary from the interior region). The user is informed by means of

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