Technical NoteJanuary 2002

MIKE 21 NSW

On Model Validation

Agern Allé 11

DK-2970 Hørsholm, Denmark

Tel: +45 4516 9200

Fax: +45 4516 9292

Dept. fax: 45 16 89 59

e-mail: dhi@dhi.dk

Web: www.dhi.dk

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MIKE 21 NSW is used ex-tensively for calculation ofthe wave transformationfrom deep-water to theshoreline and wind-wavegeneration in local areas.

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CONTENTS

1 INTRODUCTION...........................................................................................................1

2 SHORT DESCRIPTION OF MIKE 21 NSW...................................................................2

3 ON MODEL VALIDATION.............................................................................................43.1 Basic Validation.............................................................................................................43.2 Validation on Growth of Wind-waves in Shallow and Fetch-restricted Water .................43.3 Validation on Wave Transformation...............................................................................4

4 REFERENCES..............................................................................................................6

APPENDICES

A On Modelling Wind-Waves in Shallow and Fetch Limited Areas Using the Method ofHolthuijsen, Booij and Herbers by H.K Johnson

B Calibration/validation of MIKE 21 NSW Model for the Nourtec site by H.K Johnson,December 2000

C Calibration/validation of MIKE 21 NSW Model for the CERC site (SandyDuck' 97)by H.K Johnson, June 2001

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1 INTRODUCTION

The purpose of this short note is to provide validation results of DHI's Nearshore Spec-tral Wind-Wave Model, MIKE 21 NSW.

MIKE 21 NSW is a part of DHI's MIKE 21 numerical modelling package for 2D freesurface flows, waves, sediment transport and environmental processes.

MIKE 21 NSW is one of the most popular models for wave transformation in coastaland shallow water environment. The model is used extensively all over the world also atDHI's main office and in many of our subsidiaries. The combination of an advancedgraphical user interface and efficient computational engine has made this tool very at-tractive for professional coastal and harbour engineers around the world.

The US Federal Emergency Management Agency (FEMA) has per May 2001 officiallyapproved MIKE 21 NSW for use in coastal Flood Insurance Studies. This means MIKE21 NSW has been accepted for prediction of coastal wave heights and coastal wave ef-fect usage. The approval has been granted after a comprehensive review process, whichhas included testing as well as examination of the quality of the documentation of theMIKE 21 package. More information on the FEMA approval can be seen atwww.fema.gov/nfip.

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2 SHORT DESCRIPTION OF MIKE 21 NSW

MIKE 21 NSW is a wind-wave model that describes the propagation, growth and decayof short-period and short-crested waves in near-shore areas. The model takes into ac-count the effects of refraction and shoaling due to varying depth, local wind generationand energy dissipation due to bottom friction and wave breaking. The model also takesinto account the effect of wave-current interaction.

MIKE 21 NSW is a stationary, directionally decoupled parametric model based on theapproach proposed in by Holthuijsen et al. (1989), /1/. To take into account the effect ofcurrent the basic equations in the model are derived from the conservation equation forthe spectral wave action density. A parameterization of the conservation equation in thefrequency domain is performed by introducing the zeroth and the first moment of theaction spectrum as dependent variables.

The frequency spectrum is assumed single-peaked which means that mixed seas (e.g.wind waves and swell) cannot be simulated.

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The basic equations are solved using an Eulerian finite difference technique. The zerothand the first moment of the action spectrum are calculated on a rectangular grid for anumber of discrete directions. A once-through marching procedure is applied in the pre-dominant direction of wave propagation.

MIKE 21 NSW is used ex-tensively for calculation ofthe wave transformationfrom deep-water to theshoreline and wind-wavegeneration in local areas.

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The basic output from the model is integral wave parameters such as significant waveheight, mean wave period, mean wave direction, directional standard deviation and ra-diation stresses. In addition, spectral output data in the form of the distribution of waveenergy with direction at a number of user-selected points can also be obtained.

MIKE 21 NSW can be applied to the study of wave disturbance in coastal areas. The as-sessment of the wave conditions - wave heights, wave periods and wave directions - isessential for the estimation of the wave forces at a shoreline. An important problem incoastal engineering is the calculation of the sediment transport, which for a large part isdetermined by wave conditions and associated wave-induced currents. The wave-induced current is generated by the gradients in radiation stresses that occur in the surfzone. MIKE 21 NSW can be used to calculate the wave conditions and associated ra-diation stresses.

Further information is available at:

http://www.dhisoftware.com/mike21/Description/m21nsw/NSW_Module.htm

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3 ON MODEL VALIDATION

3.1 Basic Validation

Before the first release of MIKE 21 NSW (October 1991) the model was successfullyapplied to a number of rather basic idealised situations for which the results can becompared with analytical solutions or information from the literature. The basic testscovered fundamental processes such as wave propagation, wind-wave generation anddissipation. The model was applied to the same test cases as reported in /1/, which de-scribes the HISWA model developed by Delft University of Technology, The Nether-lands.

As in /1/ the model was also tested in natural geophysical conditions which are more re-alistic and complicated than in the academic test and laboratory tests mentioned above.The model was applied to a fairly complex situation in the Haringvliet estuary in themouth of the river Rhine, The Netherlands, in which wave breaking and short-crestnessdominate other effects such as refraction. The root-mean-square (rms) errors for the sig-nificant wave height and the mean wave period for this situation was about 10 and 13 %of the mean measured values, respectively.

3.2 Validation on Growth of Wind-waves in Shallow and Fetch-restricted Water

Appendix A includes a copy of a DHI paper addressing computation of local wave gen-eration by use of MIKE 21 NSW. The model is used to compute the growth of wind-waves in the shallow and fetch limited waters of the east coast of Lolland, Denmark.Simulations are carried out to assess the influence of wind-wave growth source-functions, bottom friction, wave breaking and assumed spectral shape. Comparisonsbetween model results and field data show excellent agreement.

3.3 Validation on Wave Transformation

The model was further validated against field measurement data available in followingtwo projects:

• Nourtec II, where the site is located north of Torsminde, Denmark (see photo below)

• SandyDuck’97, where the site is located at the CERC pier, NC, USA (see photobelow)

Preliminary results of the validation are presented in Appendix B and C, respectively.The results will be published in the near future. In both cases the agreement betweenmodel results and measurements is very good using standard MIKE 21 NSW model pa-rameters.

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Results from various commercial applications of MIKE 21 NSW have been comparedwith field measurement as part of the model calibration/verification process. However,most often the data is confidential.

MIKE 21 NSW has also been used in connection with modelling of wave transforma-tion in reef protection coastal areas, /2/, and prediction of the coastal impact caused bywaves generated by high-speed vessels such as fast ferries, /3/. I both cases model re-sults have been compared to field data.

Nourtec site

CERC Pier, NC, USA

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4 REFERENCES

/1/ Holthuijsen, L H, Booij, N & Herbers, T H C: A Prediction Model for Stationary,Short-crested Waves in Shallow Water with Ambient Currents. Coastal Eng. 13,23-54, 1989.

/2/ Gunaratna, P P , Justesen, P and Abeysirigunawardena, D S: Nearshore WaveModelling in a Reef Protected Coastal Area. Proc. Pacific Coasts and Ports' 97,Christchurch, New Zealand, 6 pp, 1997.

/3/ Kofoed-Hansen, H, Jensen, T, Kirkegaard, J and Fuchs, J: Prediction of wakewash from high-speed craft in coastal waters'. Proc International Conference ofHydrodynamics of High-Speed Craft, 24-25 November 1999, RINA Hq, London,UK, 10pp, 1999.

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A P P E N D I C E S

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A P P E N D I X A

On Modelling Wind-Waves in Shallow and Fetch Limited AreasUsing the Method of Holthuijsen, Booij and Herbers

by H.K Johnson

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A P P E N D I X B

Calibration/validation of MIKE 21 NSW Modelfor the Nourtec site by H.K Johnson, December 2000

Nourtec II:Wave Modeling

Calibration/Validation ofMIKE 21 NSW Model for theNourtec site

H. K. Johnson

Decmber 2000

Nourtec site

Nourished:• November 1997.Volume:• 1,136,000 m3Length:• 3600mDepth placed:• about 6.5mDNN

Model area:9600m x 2600m Wave rose

(1992-98)

General Information

4

Location of instruments

• Offshore:Wave riderDepth=17 to18m

• Inshore: S4Depth = ~ 4m

5

Analysis of measured wavedata

• Selected Validation period:• 25/Feb/99 to 04/Mar/99, simulated

every 3 hours.

• Model Setup• Model: MIKE 21 NSW• Bathymetry: Based on 1999.01 survey• Model Area: 7600m x 9600m

• Gridspacings: ∆x=10m, ∆y=40m,∆θ=10o.

• Bottom Friction: Kn=0.625mm, 6mm• Tm from Tz or Tp

• Wave Breaking: α=1, γ1=1, γ2=0.8.• Water level: Filtered time series at

Torsminde• Directional Spreading: Cos^7 used.

Numerical Modeling

7

Wave model grid

• Wavebathymetry

• Wave directions

8

Model results

Measured & Calculated Hm0, Kn=6mmRMS error = 9cm

Measured water levels at TorsmindeError Parameter Hm0 (m)Mean error (Bias) 0.09Minimum Error -0.16Maximum Error 0.25Root mean square error 0.10Correlation coefficient 0.98

9

Model results

Measured & Calculated MWDBias corresponds to 10deg under-estimation in measured data.

Measured & Calculated Tm. RMS error < 1s

Error Parameter Tm (s)Mean error (Bias) -0.22Minimum Error -2.75Maximum Error 4.50Root mean square error 0.78Correlation coefficient 0.85

Error Parameter MWD (o)Mean error (Bias) 8.91Minimum Error -12.39Maximum Error 29.70Root mean square error 7.35Correlation coefficient 0.84

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A P P E N D I X C

Calibration/validation of MIKE 21 NSW Modelfor the CERC site (SandyDuck' 97) by H.K Johnson,

June 2001

SandyDuck’97:Wave Modeling

Calibration/Validation ofMIKE 21 NSW Model

H. K. JohnsonJune 2001

• FRF Data Website -http://www.frf.usace.army.mil/prsnt.stm

• October 1997 Preliminary DataSummary by FRF, US ArmyCorps of Engineers

• C-Map sea chart (Map 12204)

Data Sources:

• 630: depth=17m, Wave rider.• Hm0, Tp, MWD

• 3111: depth=8m, pressure gauge array.• Hm0, Tp, MWD

• 111: depth=8m, pressure gauge .• Hm0, Tp

• 625: depth=8m, Baylor staff gauge.• Hm0, Tp

• 641: depth=2m, pressure gauge• Hm0, Tp

Wave data

• Selected Validation period:⇒13/October/97 to 25/Oct/97, simulated every

3 hours.⇒This period includes storm conditions of 19-

20 October 1997.

• Model Setup⇒Model: MIKE 21 NSW⇒Bathymetry: Based on 16/Sep survey⇒Model Area: 2000m x 2000m

⇒Gridspacings: ∆x=2.5m, ∆y=10m, ∆θ=10o.⇒Bottom Friction: Kn=2mm, 20mm

⇒Wave Breaking: α=1, γ1=1, γ2=0.8.⇒Water level: Time series, MSL⇒Directional Spreading: Cos^8 assumed.

Numerical Modeling

Model Bathymetries

16-Sep-97 bathymetry

Model Results:16/Sep survey, water level timeseries, Standard NSW parameters

• Mean |Error|/Hm0 = 0.13• RMS Error = 0.11

• Mean |Error|/Hm0 = 0.07• RMS Error = 0.09

• Mean |Error|/Hm0 = 0.03• RMS Error = 0.04

Error = Model - Measured

RMS Error = 1/N*∑(Error)2

Model Results:16/Sep survey, water level=MSL,Standard NSW parameters

• Mean |Error|/Hm0 = 0.11• RMS Error = 0.13

• Mean |Error|/Hm0 = 0.08• RMS Error = 0.07

• Mean |Error|/Hm0 = 0.03• RMS Error = 0.04

Model Result: Typical 2D Wave field

• Simulation result with 16/Sep/97 surveybathymetry.

• Contours of wave height(Hm0) and wave directionvectors scaled with Hm0

Summary

• The effect of the pier seems to be mainlyrepresented by the effect of the nearshoretrough.

• Adequate bathymetry resolution is requiredfor reproducing correct refraction effect innearshore troughs.

• Deviations of +/-0.6m in water level from MSLhas only a marginal effect in the waveprediction at a depth of 2m.

• All in all, MIKE 21 NSW is shown to be able topredict significant wave heights reasonablywell at the SandyDuck site.