DSD-INT - SWAN Advanced Course - 02 - Setting up a SWAN computation

SWAN Advanced Course2. Setting up a SWAN computation

Delft Software Days28 October 2014, Delft

Modelling aspects

Setting up a SWAN wave model

• Define the problem, is SWAN the proper tool?

• Define the area of interest (x and y limits) and resolution (dx, dy)

• Define frequency ( ) and direction ( ) limits and resolution (d , d )

• Define the time scales. Stationairy or non-stationairy?

• Define the boundary conditions

• Carry out sensitivity tests

• Calibration and verification: compare results with measurements

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!** model version swan-dcsm-j13-v1!*************************HEADING************************PROJ 'SWAN-DCSM' '001'!********************MODEL INPUT*************************SET NAUTICALSET LEVEL=0MODE NONSTCOORDINATES SPHERICAL CCMCGRID REGULAR -12. 48. 0. 21. 16. 420 480 CIRCLE 45 0.03 0.6INPGRID BOTTOM REGULAR -15. 43. 0. 560 420 0.050 0.050 EXC -9999.READINP BOTTOM 1. '../INP/exc03.BOT' idla=3 FREEINPGRID wind REGULAR -12 48 0.0 210 240 0.10000 0.06667 EXC -99. NONSTAT 20110905.0000 60 MIN 20110907.0300READINP wind 1. SERIES '../fews_wind.inp' idla=3 FREE!************************************* BOUNDARY CONDITIONS **************************************BOUND NEST '../INP/SPECTRA.BND' OPENINIT HOTSTART SING '../INP/DCSMA00b_hot_2011090612'!****************************************** PHYSICS *********************************************GEN3 KOMENWCAP KOMEN delta=0FRIC JONSWAP 0.038BREA CONST 1.0 0.73!************************************ NUMERICS ***************************************PROP BSBTNUM ACCUR npnts=98 NONSTAT mxitns=20!************************************ OUTPUT ***************************************POINTS 'P1' FILE '../INP/POINTS.PNT'BLOCK 'COMPGRID' NOHEAD 'SWAN.MAT' LAYOUT 3 XP YP HSIG HSWELL TMM10 TPS DIR DSPR WATLEV BOTLEV OUT 20110906.1200 2 HRSPECOUT 'P1' SPEC1D ABS 'SPEC_P1.SP1' OUT 20110906.1200 2 HRTABLE 'P1' HEADER 'POINTS.TAB' TIME XP YP DEP HSIG HSWELL TMM10 TM02 TPS DIR DSPR WIND WATLEV OUT 20110906.1200 1 HRTEST 1 0 POINTS XY 3 52 2.5 52 PAR 'TEST.PAR'COMPUTE NONSTAT 20110906.1200 60 MIN 20110906.2000STOP

general

grids

numerics

boundary conditions & initial state

physics

output

compute

E

N

E

Nnautical cartesian

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Modelling aspects

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SWAN grids

• Computational grids:• Geographical space (x,y; cartesian or spherical):

>rectangular>curvilinear>unstructured

• Spectral space ( , ):> logarithmic (frequencies; flow, fhigh, msc)>regular (directions; circle or sector; mdc)

• Resolutions ( x, y, , )

• Resolve relevant spatial and temporal details• Resolution bottom grid ~ resolution computational grid

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Modelling aspects

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Modelling aspects

CGRID REGULAR -12. 48. 0. 21. 16. 420 480

Spherical coordinateslatitudelongitude

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3. SWAN model set up

Grid 1 (DCSM)RectangularArea:1500 km x 1700 kmCell size:3.6 km x 3.6 km

Grid 2 (ZUNO)CurvilinearArea:770 km x 750 kmCell size:200 m - 2 km x200 m - 2 km

8/19

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SWAN grids: Resolution -space

low ~ 0.5 p

high ~ 3 p

E( )

prognostic part diagnostic part

-4low high

wind sea /swell

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Swell 15 s -> 0.07 HzWind sea 3 s -> 0.33 Hz

CGRID REGULAR -12. 48. 0. 21. 16. 420 480 CIRCLE 45 0.03 0.6

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12

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SWAN grids: Directional spreading

ms[-]

one sideddirectional

spreading [°]Type

1 37.5

4 24.9 wind sea

15 14.2

60 7.3

100 5.7 swell

800 2

( ) A cosmD

Directional distribution of incident wave energy D( )

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SWAN grids: Directional spreading

-0,05

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

-90 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90Direction

Swellm = 100

wind seam = 4

The value of is chosen based on the nature of the wave field• swell has little directional spread, so smaller bin sizes necessary• wind sea has large directional spread, so larger bin sizes possible

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Side boundary effects

Directional spreading causes boundary effects: ‘shadow zone’Disturbed zone approximately equal to half-power width of directionalspread from upwave corner point

x-axis

y-axis

computational grid

dir.spread

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Side boundary effects

Disturbed zone

See also exercise 05

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Time scales

1000 km cdeep 1.56 T 22 m/sgroup velocity 0.5 * c 10 m/s 36 km/h20 – 30 hoursNon-stationairy

Wave boundary conditions every 6 hoursWind every 1 hourHydrodynamics every 1 hour

Tests with dt= 15 min / 30 min / 60 min gave similar resultsHalf the time step does not double the computational time

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Wave boundary conditions• what data is available and how to schematize this

Can be obtained from:

• nesting (WAM, Wavewatch, SWAN)

• 1D / 2D spectra (measured or computed)

• parametric spectra in terms of Hm0 and Tp or Tm01

Beware of boundary value problems!

- for curvilinear grids use POINT and SPECOUT instead of NGRID andNESTOUT

1) SPECOUT 'PZ' SPEC2D ABS 'RES/SPEC_PZ.SP2' OUT 20120104.0000 1 HR

2) BOUND NEST 'RES/SPEC_PZ.SP2' OPEN

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Boundary value problem

increase in wave height

strong initial decrease of mean wave period

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Boundary value problem

growth above peak frequency

explains increase in waveheight and decay of meanperiod (NOT in peak period)

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See also exercise 04b

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Physics

- Wave growth by wind- White capping- Quadruplets- Bottom friction- Triads- Wave breaking- Diffraction- Obstacles

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Numerics

- Propagation scheme (default 1st order upwind)

- Accuracy ACCUR - relative change in Hm0, Tm01- percentage wet grid points- maximum number of iterations

STOPC - absolute change in Hm0, Tm01- relative change in Hm0, Tm01- curvature in Hm0, Tm01

- Numerical schemes for refraction (DIRIMPL) and freq shift (SIGIMPL)default cdd and css=0.5;cdd=0 (central, no diffusion) is more accurate but less stablecdd=1 (1st order upwind) more diffusive, preferable with large gradients

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Convergence criterium

Hs

DHSIGN

90%-conv. crit.

default 98%-conv. crit.

Hs

Example: 2003 experiment NCEX(Levi Gorrell)

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Output

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Additional SWAN output grids

• Types of sets of output points• Frame/Group: set of output location in box

• Curve: set of output locations along a curve

• Ray: set of output locations along depth/bottom contour

• Points: set of isolated output locations

• Ngrid: set of output locations for a nested grid

• Data file output• Table: integral wave parameters

• Specout: 1D/2D energy density spectra

• Nestout: 2D energy density spectra along boundary of nested grid

• Block: spatial distributions of integral wave parameters (also DHSIGN, DRTM01)

• Test: test output (output per iteration, spectral distribution of source terms, etc.)

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SWAN 1 Swan standard spectral file, version$ Data produced by SWAN version 40.91A$ Project: SWAN-DCSM ; run number: 001TIME time-dependent data

1 time coding optionLONLAT locations in longitude, latitude

2 number of locations3.00 52.002.50 52.00

QUANT9 number of quantities in table

Hsig Significant wave heightm unit

-9.000000 exception valueRTm01 Average relative wave periodsec unit

-9.000000 exception valueSwind wind source term (of var. dens.)m2/s unit

-9.000000 exception valueSwcap whitecapping dissipationm2/s unit-9.000000 exception value20110906.133000 date-time

0.4683E+01 0.6620E+01 0.6661E-03 -0.1137E-02 -0.1254E-04 0.0000E+00 0.0000E+00 0.0000E+00 0.4557E-030.4657E+01 0.6569E+01 0.6404E-03 -0.1155E-02 -0.9097E-05 0.0000E+00 0.0000E+00 0.0000E+00 0.4712E-03

20110906.150000 date-time0.4836E+01 0.7002E+01 0.6162E-03 -0.8178E-03 -0.1603E-04 0.0000E+00 0.0000E+00 0.0000E+00 0.3864E-030.4780E+01 0.6941E+01 0.6148E-03 -0.8210E-03 -0.1147E-04 0.0000E+00 0.0000E+00 0.0000E+00 0.4248E-03




For stationary runs the test output gives Hm0per iteration

For non stationary runs the test output does notinclude Hm0 per iteration, only per time step

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Check convergence

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Directions tend to converge more slowly due to weaker forcing

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Sensitivity tests / calibration / validation

resolutiontime stepphysicsnumerics

Calibration: - Including realistic physics or fitting observations?for instance decrease bottom friction to compensatefor lack of proper non-linear interactions?

See also exercise 03: SWAN DCSM

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Accuracy of model results depends on:

• quality of model: input bottomcurrents / water levels / windswaves at boundary (Hs Tp, )

• physics (representation of)

• numerics

Modelling aspects

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Modelling aspects

Setting up a wave model• Define the problem

• Which physical processes are important?• Is the model suitable to investigate the problem?

• Define the area of interest (x and y limits) and resolution (dx, dy)• What features (geographical or morphological) are important?• What are the dimensions of the feature of interest? Recommended to cover the

feature with at least 5 – 10 grid cells• At what location is data available to define boundary conditions?• Take care of boundary effects• Boundaries preferably parallel / perpendicular to contours

• Define frequency ( ) and direction ( ) limits and resolution (d , d )• What wave conditions are prevalent in the area?

• Define the boundary conditions• what data is available and how to schematize this• how many computations are needed / are feasible

• Calibration and verification: compare results with measurements• Sensitivity study numerics (model convergence, accuracy) and sensitivity study

physics (key parameters) to assess the uncertainties of the model predictions35

Land-sea transitions

• Effects largest just after transition,negligible at longer distances;

• For extreme situations smallerdifferences effect of land-seatransitions can be neglected;

• For islands: dip in wind speeddecreases for very narrow islands

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1 km

Lake IJssel FL25

Hm0 obs = 0.32 mHm0 SWAN = 0.46 m

U10 = 18.3 m/s

Beware of land/water transition

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SWAN computations on flow grid

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DSD-INT - SWAN Advanced Course - 02 - Setting up a SWAN computation

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