Date Author
Address
June 2009
Huijsmans, R.H.M., M. BijI, A. Reniers, K. Ewans and S. Masterton
Delft University of Technology Ship Hydromechanics Laboratory Mekelweg 2, 2628 CD Deift
Evaluation of a model for estimating
infra-gravity waves in shallow water - Accuracy
and suitability for long-term
climatologies
by
R.H.M. Huijsmans, M. BijI, A. Reniers,
K. Ewans and S. Masterton
Report No. 1622-P
2009Proceedings of the ASME 2009 28th International Confe-rence on Ocean, Offshore and Arctic Engineering, OMAE 2009, May 31June 5, Honolulu, Hawaii, USA, ISBN: 978-0-7918-3844-0, OMAE2009-79925)
TUDeift
DeIft University of Technology
WELCOME FROM THE CONFERENCE CHAIRS
file://E:\data\chair-welcome.htm!
8-6-2009
OMAE2009: Welcome from the Conference Chairs
Page 1 of2
R. Cengiz Ertekin H. Ronald Riggs
Conference Co-Chair Conference Co-Chair
OMAE 2009 OMAE 2009
Aloha!
On behalf of the OMAE 2009 Organizing Committee, it is a pleasure to welcome you to Honolulu,
Hawaii for OMAE 2009, the 28th International Conference on Ocean, Offshore and Arctic
Engineering. This is the first conference with the new name, which reflects the expanded focus of the
OOAE Division and the conference.
OMAE 2009 is dedicated to the memory of Prof. Subrata Chakrabarti, an internationally known offshore
engineer, who passed away suddenly in January. Subrata was the Offshore Technology Symposium
coordinator, and he was also the Technical Program Chair for OMAE 2009. He was involved in the
development of the OMAE series of conferences from the beginning, and his absence will be sorely felt.
OMAE 2009 has set a new record for the number of submitted papers (725), despite an extremely
challenging economic environment. The conference showcases the exciting and challenging
developments occurring in the industry. Program highlights include a special symposium honoring the
important accomplishments of Professor Chiang C. Mei in the fields of wave mechanics and
hydrodynamics and a joint forum of 'Offshore Technology', Structures, Safety and Reliability' and
'Ocean Engineering' Symposia on Shallow Water Waves and Hydrodynamics. We believe the OMAE
2009 program will be one of the best ever. Coupled with our normal Symposia, we will also have
special symposia on:
Ocean Renewable Energy
Offshore Measurement and Data Interpretation
Offshore Geotechnics
Petroleum Technology
We want to acknowledge and thank our distinguished keynote speakers: Robert Ryan, Vice President
-Global Exploration for Chevron; Hawaii Rep. Cynthia Thielen, an environmental attorney who has a
special passion for ocean renewable energy; and John Murray, Director of Technology Development
with FIoaTEC, LLC.
A conference such as this cannot happen without a group of dedicated individuals giving their time and
talents to the conference. In addition to the regular symposia coordinators, the coordinators of the
special symposia deserve many thanks for their efforts to organize new areas for OMAE. We also want
to express our appreciation to Dan Valentine, who stepped into the Technical Program Chair position
OMAE2009: Welcome from the Conference Chairs
Page 2 of 2
on very short notice, following Subrata's passing. We also want to thank Ian Holliday and Carolina
Lopez of Sea to Sky Meeting Management, who have done a great job with the organization. Thanks
also go to Angeline Mendez from ASME for the tremendous job she has done handling the on-line
paper submission and review process.
Honolulu is one of the top destinations in the world. We hope that you and your family will be able to
spend some time pre or post conference enjoying the island of Oahu. Whether you're learning to surf in
legendary Waikiki, hiking through the rich rainforests of Waimea Valley, or watching the brilliant pastels
of dusk fade off of Sunset Beach, you'll find variety at every turn on Oahu.
Mahalo nui ba,
R. Cengiz Ertekin and H. Ronald Riggs, University of Hawaii
OMAE 2009 Conference Co-Chairmen
OMAE2009: Message from the Technical Program Chair
Page 1 of 2
MESSAGE FROM THE TECHNICAL PROGRAM CHAIR
! Welcome to the 28th International Conference on Ocean, Offshore and Arctic
Engineering (OMAE 2009). This is the 28th conference in the OMAE series
guided by and influenced significantly by our friend and colleague, Subrata K.
-
Chakrabarti. It was a shock for me to learn that he had passed away so suddenly;
all involved with this conference express sincere condolence to his family, friends
and colleagues (the sentiments echoed by all of us are eloquently expressed in
the dedication included in this program). It is a great honor for me to have been
asked to continue his work on this conference. I and our community will miss his
leadership and friendship greatly. Although this series of conferences was
formally organized by ASME and the OOAE Division of the International
Petroleum Technology Institute (IPTI), it was Subrata's skill and dedication to this
Daniel T. Valentine
division of ASME that made this series of conferences the success that it has
Technical Program chairOMAE 2009 been and IS today.
The papers published in this CD were presented at OMAE2009 in thirteen
symposia. They are:
SYMP-1: Offshore Technology
SYMP-2: Structures, Safety and Reliability
SYMP-3: Materials Technology
SYMP-4: Pipeline and Riser Technology
SYMP-5: Ocean Space Utilization
SYMP-6: Ocean Engineering
SYMP-7: Polar and Arctic Sciences and Technology
SYMP-8: CFD and VIV
SYMP-9: C.C. Mei Symposium on Wave Mechanics and Hydrodynamics
SYMP-lO: Ocean Renewable Energy
SYMP-1 1: Offshore Measurement and Data Interpretation
SYMP-12: Offshore Geotechnics
SYMP-13: Petroleum Technology
The first eight symposia are the traditional symposia organized by the eight
technical committees of the OOAE Division. The other symposia are specialty
symposia organized and encouraged by members of the technical committees to
focus on topics of current interest. The 9th symposium was organized to
recognize the contributions of Professor C. C. Mei. Symposia 10, 11, 12 and 13
offer papers in the areas of renewable energy, measurements and data
interpretation, geotechnical and petroleum technologies as they relate to ocean,
offshore and polar operations of industry, government and academia.
The first symposium, Symposium 1: Offshore Technology was always Subrata
Chakrabarti's project. It was typically the largest of the symposia at OMAE. His
exemplary work on this symposium provided the experience and guidance for
others to continue to develop the other symposia. Symposium 1 in conjunction
with the OMAE series of conferences is Subrata's legacy. The Executive
Committee has a most difficult yet honorable task of finding a successor to carry
on this important annual symposium in offshore engineering. We are all grateful
OMAE2009: Message from the Technical Program Chair
Page 2 of 2
for the inspiration and encouragement provided to all of us by Subrata.
Please enjoy the papers and presentations of OMAE2009.
Daniel I. Valentine, Clarkson University, Potsdam, New York
OMAE2009 Technical Program Chair
OMAE2009: International Advisory Committee
Page 1 of 1
INTERNATIONAL ADVISORY COMMITTEE
R.V. Ahilan, Noble Denton, UK
R. Basu, ABS Americas, USA
R. (Bob) F. Beck, University of Michigan, USA
Pierre Besse, Bureau Veritas, France
Richard J. Brown, Consultant, Houston, USA
Gang Chen, Shanghai Jiao Tong University, China
Jen-hwa Chen, Chevron Energy Technology Company, USA
Yoo Sang Choo, National University of Singapore, Singapore
Weicheng C. Cui, CSSRC, Wuxi, China
Jan Inge Dalane, Statoil, Norway
R.G. Dean, University of Florida, USA
Mario Dogliani, Registro Italiano Navale, Italy
R. Eatock-Taylor, Oxford University, UK
George Z. Forristall, Shell Global Solutions, USA
Peter K. Gorf, BP, UK
Boo Cheong (B.C.) Khoo, National University of Singapore, Singapore
Yoshiaki Kodama, National Maritime Research Institute, Japan
Chun Fai (Collin) Leung, National University of Singapore, Singapore
Sehyuk Lee, Samsung Heavy Industries, Japan
Eike Lehmann, TU Hamburg-Harburg, Germany
Henrik 0. Madsen, Det Norske Veritas, Norway
Adi Maimun Technology University of Malaysia, Malaysia
T. Miyazaki, Japan Marine Sci. & Tech Centre, Japan
T. Moan, Norwegian University of Science and Technology, Norway
G. Moe, Norwegian University of Science and Technology, Norway
A.D. Papanikolaou, National Technical University of Athens, Greece
Hans Georg Payer, Germanischer Lloyd, Germany
Preben T. Pedersen, Technical University of Demark, Denmark
George Rodenbusch, Shell IntI, USA
Joachim Schwarz, JS Consulting, Germany
Dennis Seidlitz, ConocoPhillips, USA
Kirsi Tikka, ABS Americas, USA
Chien Ming (CM) Wang, National University of Singapore, Singapore
Jaap-Harm Westhuis, Gusto/SBM Offshore, Netherlands
Ronald W. Yeung, University of California at Berkeley, USA
OMAE2009: Copyright Information
Page 1 of 1
COPYRIGHT INFORMATION
Proceedings of the
ASME 2009 28th International Conference on Ocean, Offshore and Arctic
Engineering (OMAE2009)
May 31
- June 5, 2009. Honolulu, Hawaii, USA
Copyright © 2009 by ASME
All rights reserved.
ISBN 978-0-791 8-3844-0
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Proceedings of the ASME 2009 28th International Conference on Ocean, Offshore and Arctic Engineering OMAE2009 May 31 - June 5, 2009, Honolulu, Hawaii, USA
OMAE2009-79925
EVALUATION OF A MODEL FOR ESTIMATING INFRAGRAVITY WAVES IN SHALLOW WATER
-ACCURACY AND SUITABILITY FOR LONG-TERM CLIMATOLOGIES
Matthijs BijiTU-Deift Deift, The Netherlands
ABSTRACT
With interest in developing shallow-water facilities on
the increase, primarily for offloading LNG, there is
alsogrowing interest in infragravity waves. In particular, it is
recognized that infragravity waves can have an important
influence on the motions of tankers in shallow-water regions
exposed to the open ocean, and therefore they need to be
considered in the design and operation of the moorings and offloading facilities. Accordingly, there is a need to model
infragravity waves both for design calculations and to be able
to estimate the design criteria themselves. This paper is
concerned with the later - setting infragravity wave design
criteria, or more precisely on details involved in establishing an appropriate infragravity wave database from which design criteria can be estimated. The focus is on the accuracy of using the ID linear Surf Beat model (IDSB) for estimating nearshore
infragravity wave heights. The study has focused on field measurements made by U.S. Corp of Army Engineers Field Research Facility location at Duck on the east coast of the
United States, and another location at Baja on the north west coast of Mexico. At the Duck location, the study involved data
recorded in shallow water (8 meters water depth) with a
pressure transducer array, while at the Baja location, data from
a directional Waverider buoy with GPS are used. The "short
wave" directional spectra from the measured data are used as
input to the IDSB model, to compute the total infragravity
response generated by the transformation of the grouped short
waves through the surf zone including bound long waves,
leaky waves and edge waves. The computed root mean square
Ad Reniers
Rosenstiel School of Marine and Atmospheric Science University of Miami USA areniers@rsmas.miami.edu also TU-Delft Delft, The Netherlands
Kevin Ewans
Shell International Exploration and Production
Rijswijk, The Netherlands kevin.ewans@SHELL.com
infragravity wave heights have been compared with measured infragravity waves at the respective sites, and assessment has been made of the accuracy of the predictions. The computed results show good agreement with the measured infragravity waves and provide confidence that the IDSB is a suitable tool for developing a long-term infragravity data set for developing design criteria.
INTRODUCTION
Infragravity (IG) waves are long waves with periods of
30 to 300 seconds. They are most apparent in shallow-water
and were first reported by Munk (1949), who coined the term
"surf beat" to describe them. Since then, they have received
much attention from the coastal engineering community, in the
design of coastal structures and in coastal morphology. In addition, the waves can induce significant motions in ships moored in shallow water, such as LNG carriers, and large
associated mooring loads. Accordingly, with increasing
interest in the development of LNG terminals at coastal
locations, IC waves have also become of interest to the
offshore engineering community.
There is thus a need to model IG waves both for design
calculations and to be able to estimate the design criteria themselves. This paper is concerned with setting IC wave
design criteria. The first step in this process is establishing an appropriate IG wave database from which design criteria can
be estimated. For short waves this often involves a hindcast study in which wave fields are estimated with numerical models based on meteorological data over typically tens of
Copyright © 2009 by ASME
Stephen Masterton René Huijsmans
Shell International Exploration and Production TU-Delft Rijswijk, The Netherlands Deift, The Netherlands
years in order to derive extreme wave heights for reliable
design criteria. Such hindcast databases can he used as input to
a coastal model for estimating the shallow-water 10 wave
climate,
upon which design IC wave
criteria can be established.Practically there are only two types of model that are
suitable for modelling IC waves in
the coastal zone
-Boussinesq-type models and so-called surf beat type models.
Boussinesq models are complex and generally require very long computational times; as a result, they do not usually
include complex bathymetry and coastal features, and they are often restricted to moderate water depths. Nevertheless, these
models can be used to study specific sea state conditions (Madsen et al., 1997). Surf beat models have been more
widely used for studying IG waves in the coastal zone. These models compute IC waves by combining a wave driver model, which provides forcing on the scale of the wave groups of the primary waves, and a shallow-water model used for
calculating the generation and the propagation of IC waves.
Phase information is available for the IC waves, but the
individual primary wind waves are described spectrally and
are not phase resolved. Due to their computational efficiency, these models lend themselves to more comprehensive scales,
and for transforming short wave hindcast databases into
shallow-water IG wave databases.
In this paper the focus is on the accuracy of using ID linear Surf Beat model (IDSB) developed by Reniers et al. (2002) for estimating near shore IC wave heights. The study has involved analysis of field measurements made by U.S.
Corp of Army Engineers Field Research Facility (FRF)
location at Duck on the east coast of the United States, and another location at Baja on the north west coast of Mexico.
The "short wave" directional spectra from the measured data are used as input to the IDSB model, to compute the total IG response generated by the transformation of the grouped short
waves through the surf zone including bound long waves,
leaky waves and edge waves. The computed root mean square IG wave heights have been compared with measured IC waves
at the respective sites, and assessment has been made of the
accuracy of the predictions.
DATA
Field data measurements were used from two locations -the U.S. Corp of Army Engineers Field Research Facility at
Duck on the east coast of the United States, and the location at Baja on the north west coast of Mexico (Figure 1).
The data from the Duck location used in the study
consisted of time series of pressures recorded with
the pressure gauge array situated 900 meters offshore in 8 meters of water. The array consists of 15 pressure gauges spread in across configuration with an alongshore length of 250m a cross-shore length of 125m. The two hours and 16 minutes long pressure records from the FRF-array (available every
three hours) have been translated into surface elevation
variance using linear wave theory. To reduce the influence of the noise at the higher frequencies, the transfer function is set
to a maximum of 2.5. The variance densities in the low
frequency range (0.01 Hz - 0.05 Hz) of these spectra are used to derive the root mean square low frequency wave heights at the array location:
0.05
Hrmsio
=
2ñ $ Sq,1df
0.0!
Where S,rn is the variance density spectrum, and J is
the frequency in Hz.
A lower frequency limit of 0.01 Hz is chosen due to the
fact that at frequencies less than 0.01 Hz the correlation
between the observed IG response and the incident wave
conditions drops off (Okihiro and Guza, 1995, Herbers et at.,
1995), suggesting that these motions originate from other
(remote) sources not captured by the present modeling set up.
In addition, we define the IG wave significant wave
height as:
0.05
HsIG
=
$ S,df
0.0!
and the corresponding short-wave spectrum as
Hs= $ Sdf
0.05
Figure 1: Locations of the Duck and Baja field measurement sites
Raw data recorded at 2Hz in continuous sections of approximately 2 hours 50 minutes (20480 points) every 3
hours were segmented into sub series of 2048 points for
spectral analysis resulting in variance density spectra estimates
based on
19 half-overlapped averages with a spectral resolution of 0.000977Hz.Time series of the significant wave height derived from
the 8-meter pressure array for the entire 2005 data set are given in Figure 2. The plots indicate a good data return over
the year and a significant correlation of the IC waves with the
short waves. In this data set the average IG significant wave 45 35 £ 25 2 15 0.16 0.14 0.12 0.1 E 0 08
I
0.06 0.04 0.02 IA \kheight is 0.11 rn with a maximum of 0.75 m.
U''
''
VAt the Baja location, data were available from measurements made with an RDI 300 kHz Workhorse ADCP,
with the waves upgrade (WI-LW), and a Datawell GPS
directional
Waverider buoy (DWRG). The ADCP was
configured to record wave data for 35 minutes every two
hours, and the IC wave measurements were obtained from the
pressure sensor measurements. Although, the measurement survey at the Baja location continued for one year, the study
reported here focused on the first five months, Phases 1 to 4 of
the programme, when the DWRG was
set to sampleI-to total
tiolO total
continuously at 2Hz. In this period the DWRG spectra were
calculated for 2 hours 50 minute records every 3 hours
following the same scheme as used for the Duck pressure transducer array. During this period the ADCP long wave
estimates have inherently more sampling variability than those
from the DWRG, but the DWRG spectral
estimates atfrequencies below 0.01 Hz were compromised by the
100-second upper limit on wave periods measureable by the buoy. A comparison of the IG rms wave height derived from the two instruments, during these five months is given in Figure 3.
Pttase 1
ADCP DWRG
Phase 4
23-JI-2002 12-Aug-2002 01-Sep-2002 21-Sep-2002 11-Oct-2002 31-Oct-2002 20-NOv-2002 10-Oec-2002
Figure 3: Time series of the IG rms wave heights derived from the ADCP (red) and DWRG (green) measurements made in 20m water depth at the Baja location.
3 Copyright © 2009 by ASME O7Feb-2U 29-Ma-r5 18-May-2035 O7.hi2t05
2uq.5
tS-Oct-2W5 O4Dec-2O 23-Jan-i1XFigure 2: Time series of the signiticant wave heights for the short wave spectral hand (blue) and the IG spectral band (red), recorded in 8m water depth at the Duck location.
All else being equal,
the ADCP pressure
sensormeasurements, with a bandwidth extending to zero Hertz,
would be expected to measure more 16 wave energy by
comparison with the DWRQ due to the fundamental limitation
of the DWRG to measure the wave periods longer than 100
seconds. In this sense, the comparative measured values in the
circled areas in the figure where the DWRG is apparently measuring more 16 energy than the ADCP, is unexpected.
However, these time intervals correspond to very small wave
height values and it is likely that the measured long period waves are dominated by noise. In particular, the Datawell DWRG (Datawell, 2008), buoy has a precision of 1-2cm, which is only marginally less than the 16 rms wave height
levels during Phase 1.
In addition, these time intervals were also associated
with low sea state conditions, and it is likely that the measured long period waves predominantly arise from distant sources. In such cases, little correlation would be expected between the
short wave significant wave height and the IC rms wave
height. The scatter plot of IG rms wave height against the
significant
short wave height Hs for the
firstphase of
measurements given in Figure 4.
0.065 0.06 0.055 0.05 .. 0.045 9 0.04
I
0.035 0.03 0.025 0.04 0.6 0.8 1 1.2 1.4 Ho total (m)Figure 4: Scatter plot of JG rms wave height against short wave significant wave height, for Phase I ADCP measurements at the Baja location.
l)uring the Phase I measurement programme, the sea
states were relatively low. On the other hand, during Phase 4, when the sea states were generally larger, there is significantly increased correlation (Figure 5).
0.16 0.14 0.12 0,1 0.08 0.06 0.04 -0.0
Figure 5: Scatter plot of IG rms wave height against short wave significant wave height, for Phase 4 ADCP measurements at the Baja location.
Because the study concerned IG wave motions generated by local sea states, the Baja data used for further analysis were
restricted to the data collected during Phase 4. The IC rms wave height measurements during this period are given in
Figure 6. The figure shows higher values from the ADCP then
the DWRG except for low-level IG wave intensity, such as
during the time interval indicated by the circle on the graph.
4 Copyright © 2009 by ASME
1.5 2.5
0.16 0.14 0.12 0.1 0.08
I
0.06 0.04 0.02 -7 2610/02 31/10102 05/11102 10/11102 15/11102 20/11102 25/11/02 30/11/02 05/12)02 10/12)02 Time Idays]Figure 6: Timeseries of the Hrms of the IG waves measured by the DWRG (green line) and the ADCP (red line) for phase 4.
MODEL SET-UP
The IDSB model uses conventional frequency-direction
short wave spectra as input and generates the low frequency
field in a predefined spatial computation area. The hathymetry
is uniform in alongshore direction. The bathymetry for the
Duck location were obtained from the Field Research Facility
website and has an average bottom slope of approximately 1:100. The bathymetry for the Baja location were available from proprietary field surveys but had an average bottom
slope of approximately 1:20.
The friction parameter, and the parameters to calculate the transformation of the spectral variance density from the
sea boundary to the shore and the accompanying set-up of the
mean water level are kept the same compared to the study
done at Duck by Reniers et al. (2002). Based on the study by
Reniers et al. (2002) a relation between the signiticant wave height and the optimal wave breaking saturation parameter, 7, for the dissipation formulation of Roelvink (1993) has
been derived which has been used in the present IDSB
computations:
y = 0.42 + tanh(0.05 H,,0)
where H,,,0 represents the incident significant wave height.
The outer boundary of the Duck computational grid was
600 m offshore where the water depth was 8 m, and that for the Baja computational grid was 900 m offshore where the
water depth was 22 m.
The boundary conditions for the 10 wave calculations
account for the incoming bound JO waves, the outgoing free
IG waves and edge waves at the sea boundary, and a 100% reflection of IG waves at the shore line, where a minimum computational water depth of 0.1 meters is applied. IDSB
subsequently calculates the variance densities of the combined
bound and unbound, incoming and outgoing IC waves
(including edge waves) from 0.01 - 0.05 Hz, which are used to calculate the root mean square 10 wave height for each
offshore incident wave condition.
RESULTS AT THE DUCK LOCATION
Figure 7 gives a time series of the Hrms of the 10 waves
predicted by the IDSB model (blue line) and the Hrms of the
10 waves measured by the pressure gauge array (red line) at
the Duck location for the whole year of 2005.
06-
05-18-May-2005 07-Ju2005 26-Aug-2005 15-Oct.2005 04-Dec-2005 23-Jan-2006
Figure 7: Timeseries of the Hrms of the IG waves predicted by the IDSB model (blue line) and the Hrms of the IG waves measured by the pressure gauge array (red line) at the Duck location for the whole year of 2005.
A measure of the performance of the model is given by the skill defined (Gallagher et al., 1998) as
-
)2Where Hr,nc is the measured rms wave height, and
is the rms wave height predicted by the model. The skill of the model predictions presented in Figure 7
is 0.82. This is slightly lower than the value of 0.85 as
reported by Groenewegen ci al. (submitted) for the month of
April 2005. While this
overall skillcan be considered
acceptable, it is interesting to assess this parameter, or equivalently the performance of the model for different sea
state conditions. Figure 8 is a scatter plot of the skill versus the
Hs of the short waves for the one-year of observations at the
Duck location. The plot shows clearly that the very low skills
only occur for relatively low values of the significant short
wave height Hs, indicated by the ellipse in the figure. Larger sea states with significant wave heights exceeding 2 m are all associated with relatively high skill values.
3.5 3 0 1,5 05 81 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Skill
Figure 8: Scatter plot of short wave Hs against skill.
Figure 9 shows a scatter plot of the Hrms of the IDSB
model IG wave predictions against the Hrms of the measured
IG waves for one year of observations at the Duck location.
There is some indication that the IDSB model may be
underestimating the IG waves at relatively high Hrms values
and overestimating lower levels, particularly in the 0.1 to
0.25m range, but confirmation of this effect must await the
availability of a larger data set.
6 Copyright © 2009 by ASME 07 -J USB Measured 4.5 4
skill =
\/(Hr,ns )2
0.5 0.45 0.4 0.35 03 0 25 a I 02 0.15 01 0.05 0.06 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 Huni 10 m.im,.d [m]
Figure 9: Scatter plot of the Hrms of the model predicted IG waves against the Hrnis of the measured IG waves for one year of observations at the Duck location.
0.16 0.14 0.12 0.1 E 0.08
I
0.06 0.04 0.02 05RESULTS AT THE BAJA LOCATION
A time series of the Hrms of the IG waves predicted by
the IDSB model (blue line), measured by both the DWRG,
and measured by the ADCP for Phase 4 Baja measurements is given in Figure 10.
The mean skill between the measured ADCP values and the IDSB predictions for this period is 0.80, and the mean skill between the
measured DWRG values
and the IDSBpredictions is 0.78. This difference may reflect the broader bandwidth of The ADCP for measuring the IG waves by
comparison with the DWRG.
IDSB DWRG ADCP
26?10/02 31/10)02 05/11)02 10/11/02 15/11/02 20/11/02 25/11/02 30/11/02 05/12/02 10112)02 Time (days]
Figure 10: Timeseries of the Hrrns of the IG waves predicted by the IDSB model (blue line), measured by the DWRG (green line), and measured by the ADCP (red line) for two months of observations at the Baja location
To further examine the performance of IDSB, the skill is
plotted as time series, together with the IDSB predicted IG
rms wave height for the Baja location, in Figure 11.
Low skill values are generally associated with low Hrms
values. The most hkely reason for this is that there
is asignificant proportion of the IG wave energy arriving from
distant sources;
whereas the model
isonly capable of
predicting IG waves that are locally generated.
The exception is the occurrence of low skill values
during the relatively
high energy level event around 10
November. A similar result was found at the Duck location (Gronewegen ci al., submitted), in which energy dissipation
due to infragravity wave breaking as outlined by Van
Dongeren ci al. (2007) was postulated as a possible cause.
0.2 018 010 0,14 12 0.1 1008 006 0.04 02 I I c 26-Oct-Jj2 31-Ocl-20J2 05Nov-2O32 tO-Noy-2922 15-No-.1JJ2 2ONov-aJJ2 25-No02 XI-No2(U2 (Dec-2OJ2 IO.Dee-20J2
Time [days]
Figure 11: Time series of the skill (blue line) and the measured Hrms of the IG waves (green line) for two months of observations atthe Baja location. The units of wave height are meters.
A scatter plot of the measured Hrms of the JO waves (ADCP) versus the Hrms of the predicted IG waves by the
IDSB model is given in Figure
12. The IDSB model
overestimates the IG waves at Hrms values from 0.l2m. This is consistent with the findings for the Duck location.
Unfortunately, values
of Hrms above 0.15m were not
observed at the Baja location during the time that
measurements were performed, but the indication is that the IDSB model may he over-estimating the IG wave heights in
some conditions.
Figure 12: Scatter plot of the Hrms of the measured IG waves vs. the Hrms of the IDSB predicted IG waves for the two months of observations at the Baja location
DISCUSSION
The IDSB predictions presented in this paper were made
without taking the tide into account, but an additional run of
-0.8
0 4
0 2
the IDSB, including the tidal water levels at the Baja, did not reveal any significant differences in the predicted JO rms wave heights. A similar exercise for the Duck location
(Groenewegen, et al., subm.) gave similar results. This
insensitivity to changes in the water level is a function of the local water depth, where significant variation in the
infragravity wave heights can be present closer to the water
line due to non-linear shoaling (e.g. Battjes et al., 2004), but
this effect is mostly absent in deeper water. In this respect
both the Duck location in 8 m water depth and the Baja
location in 20 m water depth would be considered "deeper
water".
One of the assumptions of the IDSB model is that there is
no alongshore variation of the coastline. The effect of the
choice of beach bottom profile on the IG wave predictions was investigated at the Baja location by choosing profiles at four
intervals of lOOm along shore and rerunning the JO wave
predictions. Again, as for the tides, there was little change in the predicted 10 wave heights. A similar result was found for the Duck location and is reported elsewhere.
The relatively insignificant effect of the water levels on
the IG wave height estimates indicates that the JO wave
intensity at the water depths studied
is not sensitive torelatively small changes in water level and likely more
sensitive to the characteristics of the local sea state, particularly the significant wave height and likely the length of
the beach cross shore profile over which the second-order
interactions responsible for the local generation of 10 waves
can act. However, while the IG waves at both the Duck and Baja locations are insensitive to moderate changes in water
depth, it should be noted that this is not true for all locations (Thompson et al., 2006).
The wave breaking saturation parameter, 7, is expected to be dependent on different aspects such as the bottom slope,
8 Copyright © 2009 by ASME 0.02 004 0.08 006 0.1 0.12 0.14 ..".dPW 0.16 0.18 02 08-2
'1)
0-04
the significant wave height and the wave steepness. Although the used values of for Duck and Baja in this study are in line
with the values used at similar sites in other studies like in Apotsos et al. (2007), further evaluation of other sites would
need to be undertaken to verify this.
CONCLUSIONS
The IDSB
model has satisfactory performance inpredicting IG spectrally integrated wave height parameters,
such as rms wave height, at the Duck and Baja locations with
a mean skill of 0.82 and 0.80 respectively. The IG wave
heights are strongly correlated to those of the short waves,
indicating that the IG waves are predominantly generated by
the local sea state, and making good IDSB performance
possible. Indeed, at both locations, the IDSB skill is low during quiescent local sea states, an effect likely resulting from a proportionally increase in the free wave component
propagating from the deep ocean, relative to the
locally generated component; IDSB only models thelocally-generated component. However, occasions of low skill have also been observed during high sea states, and we postulate
that this effect may be due to the presence of free waves
released from waves breaking seaward of the outer boundary of the model; such free waves will not be predicted by IDSB.
Energy dissipation due IG wave breaking in high sea state
events as observed by Van l)ongeren et al. (2007) may also be contributing to this effect.
An important component of establishing of IG wave
design criteria, is to have a long-term data base. It has been
shown that surf beat models such as IDSB can have sufficient skill to produce such a data base from a long-term short-wave data set is available. However, it is critical that the models are
validated against good in-situ measurements; the IDSB has
good skill for the Duck and Baja locations, but that cannot be
considered proof that it will perform well for all locations,
where different coastal features, water depths, or beach slopes for example may have significantly affected the local IG wave generation. In this respect it is apparent that instruments with
pressure sensors can be used for such in-situ measurements.
The Datawell DWRG buoy on the other hand appears to have
some noise limitations for
quiescent IG periods and
isfundamentally limited for IG measurements with its lOOs
upper wave period bound.
More IG activity was observed at Duck than at Baja. A possible reason for this is that the Duck location has a less
steep beach cross-shore bottom profile than Baja, which would provide a longer IC wave build-up time. Okihiro et al. (1995) reported higher IG levels offshore of broad sandy beaches than offshore at rocky or cliff coasts; the Duck location is more the former type and Baja the latter. In addition, the IG waves were
measured and predicted
in shallower water for Duck (8
meters) than at Baja (20 meters), and IC waves are known to be depth dependent - increasing with decreasing water depth.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of
Shell International Exploration and Production, and
particularly thank Charles Long of the Duck Field Research
Facility for the Duck raw wave data and Ewoud van I-Iaaften
of Shell Global Solutions for the raw wave data. This work
was undertaken partly through the SAFEOFFLOAD EU
project.
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