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See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/269161191 WAVE IMPACTS AT SEA WALLS Conference Paper · April 2005 DOI: 10.1142/9789812701916_0327 CITATIONS 4 READS 1,311 2 authors, including: Some of the authors of this publication are also working on these related projects: International Levee Handbook View project HYDRALAB View project William Allsop The University of Edinburgh 208 PUBLICATIONS 2,595 CITATIONS SEE PROFILE All content following this page was uploaded by William Allsop on 09 July 2015. The user has requested enhancement of the downloaded file.

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  • See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/269161191

    WAVE IMPACTS AT SEA WALLS

    Conference Paper · April 2005

    DOI: 10.1142/9789812701916_0327

    CITATIONS

    4READS

    1,311

    2 authors, including:

    Some of the authors of this publication are also working on these related projects:

    International Levee Handbook View project

    HYDRALAB View project

    William Allsop

    The University of Edinburgh

    208 PUBLICATIONS   2,595 CITATIONS   

    SEE PROFILE

    All content following this page was uploaded by William Allsop on 09 July 2015.

    The user has requested enhancement of the downloaded file.

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  • 1

    WAVE IMPACTS AT SEA WALLS

    GIOVANNI CUOMODepartment of Civil Engineering, University of Roma TRE, Via Vito Volterra, 62

    Roma, 00146, Italy

    WILLIAM ALLSOPTechnical Director, HR Wallingford, Howbery Park, Wallingford OX10 8BA UK

    (e-mail: [email protected])

    This paper reports recent advances in knowledge on impulsive wave loads on vertical /steep walls, based results of experiments in the CIEM / LIM flume at Barcelona underthe VOWS project (Violent Overtopping of Waves at Seawalls). Wave impact forcesfrom the new data set have been compared with predictions by the more realisticmethods available in literature. For most of the methods analyzed, scatter has been foundto be large over the measurement range. A simple and intuitive new prediction formula isintroduced which seems to give better estimation of wave impact forces. To supplementthese improvements, new data are presented on durations of wave impacts, together withthe vertical distribution of pressures up the wall.

    1. Introduction

    Over the last 10 years, improved awareness of wave impact induced failures, hasfocused attention on the need to include dynamic responses to wave impact loadsin analysis of loadings to maritime structures. Recent experimental work hastherefore focused more strongly on recording and analyzing violent waveimpacts. These new data are however only useful if methodologies are availableto evaluate dynamic responses of maritime structures to short-duration loads.

    For seawalls or crown walls, dynamic analysis has been rare, and waveimpact loads are often ignored in design despite their magnitude. A rare exampleof analysis of both impact load and structure response on a small (1m high) wavewall was reported by Allsop (2000), where short–duration impulsive loads ofFimp ≈ 180kN/m gave an effective load of Feff ≈ 42kN/m when dynamic responseof the wall was included. Improvements in these predictions require thedevelopment of more complete wave load models (as suggested by Oumeraci etal., 2000), based on new measurements and experiments.

    This paper reports measurements of impulsive wave loads on a steeplybattered (10:1) wall from tests in the large wave flume at Barcelona under theVOWS project (Violent Overtopping of Waves at Seawalls). These data are

  • 2

    used to support a revised simple prediction formula for wave impact forces onvertical walls.

    The paper also discusses dynamic characteristics of linear single degree offreedom systems to non-stationary excitation. Responses are derived to pulseexcitation similar to those induced by wave impacts for undamped systems.Response to short pulses is dominated by the ratio of impact rise time tr to thenatural period of the structure Tn., so new data on impact durations are given.

    2. Wave forces, pulsating and impact

    Since first experiments were carried out by Bagnold (1939), time histories ofwave loads caused by impulsive breaking against a wall have been recognized asresulting from superposition of the short duration spike on a much less violentquasi-static oscillation. A comprehensive review of the physics behind waterwave impact on walls is given by Peregrine (2003).

    Although this general description fits sufficiently well to many observationson idealied structures, pressure and force time history loads on example maritimestructures are usually highly variable, even for very similar wave conditions andstructural configurations. Detailed descriptions of the behaviour of breakingwaves, and impacts on walls, are given by Hattori et al. (1994).

    2.1. Definitions and examples

    For waves at more complex structures, load time histories may be much morecomplex, and distinguishing between impact and quasi-static components ismuch less straightforward. This is confirmed by recent observations fromphysical model tests of the Turner Contemporary Gallery at HR Wallingford.Wave pressures on the Gallery were measured in a 1:40 3-dimensional modeltested under random waves in a wave basin (see Fig. 1).

    3.01m ODN

    4.90 ODN8.98m ODN

    13.46m ODN

    13

    87

    465

    10923.01m ODN

    4.90 ODN8.98m ODN

    13.46m ODN

    13

    87

    465

    1092

    13

    87

    465

    1092

    764.4 764.6 764.8 765 765.2 765.4 765.6 765.8-50

    0

    50

    100

    150

    200

    250

    Pres

    sure

    (kPa

    )

    Time (s)

    764.4 764.6 764.8 765 765.2 765.4 765.6 765.8-50

    0

    50

    100

    150

    200

    250

    Pres

    sure

    (kPa

    )

    Time (s)

    Fig. 1. (A) Physical model at HR Wallingford for Turner Gallery at Margate, UK.(B) Pressure time-histories from transducers on front face (for 1:10’000 year return period):- top Transducer No. 2 (dotted line), 9 (dashed line) & 10 (solid line)- bottom Transducer No. 7 (dotted line), 8 (dashed line) & 1 (solid line)

    A B

  • 3

    As the overall loading process depends on both the applied loadings andthe dynamic characteristics, the use of very simple definitions of short durationand quasi-static loads in analysis of load time-histories (see e.g. Fig. 1) may beless useful. The dynamic characteristics of the structure should be used indefining the various types of loading.

    For the generality, however, this paper will define as “impacts” or “short-duration loads” all loads that act on the structure for durations comparable withthe natural resonance period of the structure (or element). We define “quasi-static” (also called “pulsating” or “slowly-varying”) any loads that act for longerthan that resonance period. In figures:

    (1)

    where: T0 = period of resonance of the structure for the mode of vibratingcorresponding to the applied load.

    2.2. Previous work

    Although many researchers have focused on these issues since the 1930’s,understanding of wave impact loads on maritime structures is still limited,partially due to difficulties in measuring wave impact pressures at high-speed,and in the sheer volume of data to be stored and analysed. Appropriate data-logging and storage devices have only became available relatively recently.

    Small scale physical model tests (see e.g. Allsop et al. 1996a, Allsop et al.1996b, Cuomo et al. 2003) have recently demonstrated that wave impacts onwalls and suspended deck structures can be much higher then those predicted bystandard prediction methods, suggesting further investigations were needed toachieve a deeper understanding of the complexity of interactions of wave andcoastal structures.

    3. New experimental work – Big VOWS (tests at UPC Barcelona)

    Bearing these uncertainties in mind, it was decided to extend a set of large-scaleexperiments at the CIEM / LIM wave flume at Universitat Politècnica deCatalunya, Spain. These tests were primarily aimed at quantifying scale-effectson overtopping of vertical / battered walls under the Big-VOWS project, seeresults by Pearson et al. (2002), but they were extended by the addition of waveload measurements.

    The LIM wave flume is 100m long, 3m wide along its full length, and hasan operating depth of up to 4m at the wave-absorbing sliding wedge paddle. For

    loads static-quasiimpacts

    22

    0

    0

    ⋅>⋅≤TT

    tr

  • 4

    these experiments, a 1:13 concrete approach beach was constructed up to the teststructure shown in Fig. 2. Each test consisted of approximately 1000 irregularwaves to a JONSWAP spectrum with γ = 3.3. A test matrix of about 30 differentconditions is shown in Table 1 and Fig. 3. Three structural configurations weretested, respectively:

    1. 10:1 Battered wall2. Vertical wall3. Vertical wall with recurve

    Results in this paper refer only to the 10:1 battered wall.Three different water depths h were used (0.53m, 0.83m and 1.28m). Test

    conditions for the full set of experiments are summarized in Table 1.

    Table 1. Summary of test conditions

    Test Series Configuration Nominal waveperiod Tm [s]

    Nominal wave heightHis [m]

    1A & 1B Rc = 1.16m / 1.40mh = 0.83m

    2.563.123.293.641.98

    0.48, 0.45, 0.370.60, 0.56, 0.39

    0.670.600.25

    1C Rc = 1.46mh = 0.53m

    1.982.563.123.293.64

    0.25, 0.220.48, 0.45, 0.37, 0.230.63, 0.60, 0.56, 0.39

    0.670.60

    1D & 1E Rc = 0.71m / 0.95 mh = 1.28m

    1.972.543.123.65

    0.26, 0.230.44, 0.44, 0.35, 0.23

    0.58, 0.50, 0.340.55

    Fig. 2. Experimental set-up: overall view with pressure transducers

  • 5

    Pressures up the wall were measured by mean of a vertical array of 8pressure transducer, logging at a frequency of 2000Hz. More detaileddescriptions of the experimental setup are given in Pearson et al. (2002).

    3.1. Recent observations (Pressure time-histories)

    A snapshot of the pressure time-history during a wave-impact is given in Fig. 4for the nominal (model) condition of: Hs=0.48m,Tm=2.56s.

    The characteristic impulsive load spike is only evident in the time-historyrecorded by transducer No. 5, confirming that the high pressure peak is localizedin both time and space. Variations in time and space (up the wall) of thepulsating components are clearly visible, but relatively smaller.

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    P /

    ρ g

    Hs

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    0 0.25 0.5 0.75 1-5

    0

    5

    10

    15

    20

    25

    t / Tm

    P /

    ρ g

    Hs

    Fig. 4. Snapshot from pressure time-history up the wall. Pressure transducers numbered frombottom (1) to top (8).

    8 7 6 5

    4 3 2 1

    B1.5

    2.0

    2.5

    3.0

    3.5

    4.0

    0.2 0.3 0.4 0.5 0.6 0.7

    H (m)

    T (s

    )

    Fig. 3. Large scale tests at LIM-UPC. Wave matrix (A) and snapshot of a wave impact duringphysical model tests (B).

    A B

  • 6

    4. Comparison with existing methods

    Wave loads measured in these tests (at exceedance level F1/250, i.e. theaverage of the highest four waves out of a 1000-wave test) have been comparedwith a range of methods, including those suggested in the Coastal EngineeringManual (CEM), British Standards BS-6349, and the guidelines fromPROVERBS. Wave impact loads are compared here with predictions by Goda(2000), Blackmore & Hewson (1984), Allsop & Vicinanza (1996) and Oumeraciet al. (2001) in Fig. 5. The scatter is large for all the prediction methods usedover the range of measured forces. Points falling above the 1:1 line represent un-safe predictions.

    Substantial scatter is seen in each graph although departures from thepredictions are reduced by the Goda or Allsop & Vicinanza methods. For thosemethods, ratios of measured / predicted value are plotted against Hs/d in Fig 6.

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0 10 20 30 40 50 60 70 80 90 100

    Predicted (kN) - Goda (CEM)

    Mea

    sure

    d (k

    N)

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0 10 20 30 40 50 60 70 80 90 100

    Predicted (kN) - Blackmore & Howson (BS6349)

    Mea

    sure

    d (k

    N)

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0 10 20 30 40 50 60 70 80 90 100

    Predicted (kN) - PROVERBS

    Mea

    sure

    d (k

    N)

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0 10 20 30 40 50 60 70 80 90 100

    Predicted (kN) - Allsop & Vicinanza

    Mea

    sure

    d (k

    N)

    Fig. 5. Comparison of new data set with existing prediction methods

    0

    1

    2

    3

    4

    5

    6

    0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

    HS / d

    Mea

    sure

    d / P

    redi

    cted

    0

    5

    10

    15

    20

    25

    30

    35

    0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1HS / d

    Mea

    sure

    d / P

    redi

    cted

    Fig. 6. Ratio between measured and predicted values for after Goda and Allsop & Vicinanza.

  • 7

    Predictions by Allsop & Vicinanza are close to measurements for 0.4< Hs/d

  • 8

    of measurements, confirmed by the much lower value of RMS error shown inTable 2.

    Table 2. Goodness of fit in terms of root mean square error

    Prediction method RMSEGoda 23.5

    Minikin 26.7Blackmore & Howson 28.4

    PROVERBS 37.1Allsop & Vicinanza 23.7

    Equation 2 14.9

    6. Wave impacts

    By definition, impulsive wave loads are applied dynamically to maritimestructure. Any loading therefore depends on the dynamic characteristics of thestructure, so describing the impulsive load by its magnitude only cannot besufficient for design purposes.

    Dynamic analysis of structural response to wave impacts needsparameterisation of the recorded time-histories. Goda (1994) assumed wavepressures to rise linearly from zero to Pmax at time t = tb, and then drop to aconstant value Ps. Oumeraci & Kortenhaus (1994) suggest an equivalenttriangular force defined by peak value Fmax, rise time tr and duration time td.

    0

    10

    20

    30

    40

    50

    60

    70

    80

    90

    100

    0 10 20 30 40 50 60 70 80 90 100

    Predicted (kN)

    Mea

    sure

    d (k

    N)

    Fig. 8. Comparison of wave impact loads from new data sets with prediction by Eq. 2, α = 0.6

  • 9

    Using observations of time-history loads from the Big-VOWS model tests,we suggest here a simple model to give first estimates of impact loads.

    6.1. Variability in time

    The suggested simplified load-history is a single triangular spike characterizedby the maximum signal during loading (Fmax) and time taken to get to Fmax from 0(tr). An example idealised load-history is superimposed on an original signal inFig. 9, where the shaded area in Fig. 9A represents momentum transfer to thestructure during the impact, the impulse. Even though peak pressure (Pmax) andrise time (tr) both exhibit large variations under very similar conditions, theimpulse, derived by integrating the signal over the rise time, is far morerepeatable.

    As the impulse represents a finite quantity, shorter rise time will correspondto more violent impacts and vice versa. This is demonstrated in Fig 9B, wherevalues of Fmax from 1000 wave model tests are plotted against corresponding trexpressed by the following relation between Pmax and tr of general form derivedby (Hattori et al., 1994):

    (3)

    where A and B are empirical coefficients.The large variation in coefficients A and B given in literature (Weggel et

    al., 1970, Blackmore & Hewson, 1984, Kirkgoz, 1990, Hattori et al., 1994) is, atleast partially, due to the difficulties of recording consistent impact pressuresbetween tests.

    Eq. 3 can be rewritten in dimensionless terms as:

    (4)

    0.22 0.2225 0.225 0.2275 0.23 0.2325 0.235-5

    0

    5

    10

    15

    20

    25

    t / Tm

    P /

    ρ g

    Hs

    0.05 0.1 0.15 0.2 0.25

    2

    4

    6

    8

    10

    12

    14

    16

    18

    20

    22

    Fim

    p / F

    qs+(

    1/25

    0)

    tr / Tm

    Fig. 9. (A) Spike-shaped time-history model; (B) Dimensionless impact force versus rise timeextracted out of a 1000-waves test.

    A B

    BrtAP ⋅=max

    B

    m

    r

    qs TtAF

    F

    ⋅=

    + 250

    max

  • 10

    For wave impact on exposed piers / jetties, coefficients A and B in Eq. 4are given in by Cuomo et al. (2003) for different structural elements

    6.1.1. Joint probability of Fmax and tr

    Coefficients A & B are usually chosen so that the maximum impact pressurerecorded within each experiment falls beneath the curve of Eq. 4 which thereforerepresents the envelope of observed values within these experiments.

    If the population of paired values of (Fmax:& tr) is statistically meaningful, itis possible to determine coefficients A and B for Eq. 4 at any given level of jointprobability. This is far more useful for design purposes, especially since recentinternational design guidance are semi-probabilistically oriented.

    Dimensionless wave impact forces extracted from big-VOWS test series1A, 1B and 1C are plotted in Fig. 10A versus relative rise time. The solid line inFig. 10A has been obtained fitting parameters in Eq. 4 to the joint probabilitycontour at 98.8% level, giving: A = 1.45 and B = -0.49.

    6.2. Variability in space

    Analysing pressures meaasured at eight points up the wall allows the descriptionof the vertical distribution of pressures. Maximum wave impact pressures up thewall (P1/250) are plotted in Fig. 10B for test series 1B & 1C in Table 1,confirming the general observations by Hull & Muller (2002).

    7. Structural response

    For monolithic structures like seawalls, structural responses to applied loads maybe similar to that of a single degree of freedom (SDF) system of known mass(m), rigidity (k) and viscous damping (c), subject to a force f(t) arbitrarilyvarying in time. For this simple case the equation of motion gives:

    0 5 10 15 20 250.2

    0.4

    0.6

    0.8

    1

    1.2

    1.4

    1.6

    1.8

    2Impact

    Pmax

    / ρ g Hs

    z/d

    Fig. 10. (A) Dimensionless impact force versus rise time and joint probability contour at 98.8%non-exceedance level. (B) Pressure distribution up the wall. Test series 1B & 1C.

    A B

  • 11

    )()()()( tftkutuctum =++ &&& (5)A dynamic amplification factor Φ can be then defined as the ratio between themaximum displacement of the system u(t)max and the displacement u0 of the samesystem due to the static application of the maximum force Fmax:

    (6)

    For linear systems, solution to Eq. 4 at time t can be expressed as the sum of theresponses up to that time by the convolution integral:

    ∫ −=t

    dthftu0

    )()()( τττ (7)

    Where h(t-τ) is the unit input-response function. For a SDF system, Eq.6specializes (Chopra, 2001) in:

    [ ]∫ −= −−t

    Dt

    D

    dtefm

    tu0

    )( )(sin)(1)( 0 ττωτω

    τζω (8)

    with 20 1 ζωω −=D , mk /0 =ω and 02/ ωζ mc= is the dampingratio. Eq.8 is known as Duhamel’s integral, and, together with the assigned initialconditions, provides a general result for evaluating the response of a linear SDFsystem subject to an arbitrarily time-varying force. Example displacement time-histories obtained by solving Eq. 8 for a spike-shaped pulse are shown in Fig10A for the cases of ζ = 0 and ζ = 0.06.

    For an un-damped system subject to pulse excitation, the deformation ofthe system only depends on the pulse shape and on the ratio between the durationof the pulse (tr) and the period of resonance of the system (Tn). For a given shapeof the exciting pulse, the amplification factor of an un-damped system only

    0 0.5 1 1.5 2 2.5-1.5

    -1

    -0.5

    0

    0.5

    1

    1.5

    u(t)

    / u 0

    t / T0

    0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

    0.5

    1

    1.5

    u max

    (t) /

    u0

    tr / T0

    Fig. 10. Dynamic response of a SDF system to spike-shaped pulse excitation.(A) Dimensionless displacement time-history for ζ = 0 (solid line) and ζ = 0.06 (dashed line)superimposed to input spike-shaped pulse time-history load (dotted line).(B) Amplification factor for ζ = 0 (black) and ζ = 0.06 (grey)

    A B

    ktFu

    utu max

    00

    max )()( ==Φ

  • 12

    depends on the ratio tr / T0. Amplification factor for the cases of spike-shapedand triangular pulses are plotted in Fig. 11B as a function of tr / T0.

    When the system is excited by a single pulse, energy dissipated by dampingis much smaller and relative importance of damping on maximum displacementdecreases. As a result, variation of amplification factor with damping is lessimportant than for systems subject to harmonic excitation. (Fig. 11B).

    8. Initial conclusions & further work

    New measurements of wave impact loads at large scale have been compared withhistorical prediction methods. Scatter is large over the range of condition tested.A simple and intuitive new prediction formula has been suggested which uses themost informative parameters and seems to give a better estimate of wave impactloads. A joint-probability approach has been introduced for the estimation ofwave impact durations, together with a simple methodology which takes accountof dynamic response of structure when evaluating design impact loads.

    Further analysis of the whole data set will extend the confidence of the newprediction methods, allowing at the same time to take into account the relativeimportance of different structural configurations on both impact and pulsatingwave loads at sea walls.

    Acknowledgments

    This paper follows from work completed by Universities of Edinburgh, Sheffieldand Southampton, and HR Wallingford under a number of projects. Additionalsupport by Universities of Rome 3, HR Wallingford and the Marie Curieprogramme of the EU (HPMI-CT-1999-00063) are gratefully acknowledged.Previous research data were supported in UK by DTI under contracts PECD7/6/263 & 312, CI 39/5/96, CI 39/5/125 (cc 1821), and PROVERBS (ECcontract MAS3-CT95-0041). The Big-VOWS team of Tom Bruce, Jon Pearson,and Nick Napp, supported by the UK EPSRC (GR/M42312) and XavierGironella and Javier Pineda (LIM UPC Barcelona) supported by EC programmeof Transnational Access to Major Research Infrastructure, Contract nº: HPRI-CT-1999-00066, are thanked for test data on wave forces at large scale. JohnAlderson & Jim Clarke from HR Wallingford are also warmly acknowledged.

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    KEYWORDS – ICCE 2004

    WAVE IMPACTS ON SEA WALLSGiovanni Cuomo, William Allsop569

    Wave impactsWave forcesSeawallsDynamic responseJoint probability

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