Rosen S.D., 2006, Improved Environmental Loading Design Criteria For Nearshore Structures

8/18/2019 Rosen S.D., 2006, Improved Environmental Loading Design Criteria For Nearshore Structures

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MWWD 2006 - 4th International Conference on Marine Waste Water Dischargesand Coastal Environment

IEMES 2006 – 2nd International Exhibition on Materials Equipment and Servicesfor Coastal Environment Projects

MWWD-IEMES 2006-Antalya, Nov. 06-10, 2006P06_Rosen_Design Criteria for Nearshore-MWWD 2006 Proc - page 1 of 15

Improved Environmental Loading Design Criteria ForNearshore Structures

Dov S. Rosen (1)

Summary

The damages to coastal structures due to the December 2004 Indian Ocean tsunami as well as theSeptember 2005 Katrina hurricane, raised public awareness to need for proper design against hazards. Itappears that in most parts of the world, including Europe and the Mediterranean, coastal structures areroutinely not designed to withstand loading due to tsunamis, sea-level rise and meteo-marine climatechange. The design standards of nearshore structures in respect to environmental loading are defined byvarious standards and design manuals. These usually refer to loading induced by currents, waves, wind andwater level. Usually such design of marine structures, including those of intakes and outfalls locatednearshore, refer to two critical loading conditions. The first referes to the ultimate service state and the

second to the ultimate survival state. The first design state appears to be properly defined. However, forstructures near the surf zone edge, particularly for these close but beyond the "normal" surf zone, presentstandards of survival design state seem to lack proper consideration of an important loading factor, evenwithout tsunamis. The insufficient definition of design loading criteria due to longshore current in a numberof international standards will be shown and an assessment of the resulting loading in the ultimate designstate without tsunamis will be presented. For such state, simultaneous loading by extreme wind inducedcurrents, near-breaking/breaking waves and large longshore currents (structures now near/in breakerszone) results in significantly increased velocities and accelerations, hence increased loads. Finally, theimpact of wave climate change, sea level rise and tsunamis will be addressed.

Keywords

d e s i g n c r i t e r i a , u l t i m a t e s t a t e , l o n g s h o r e c u r r e n t , s e a l e v e l r i s e , w a v e s , t s u n am i ,

i n t a k e s , e x t r em e s t a t e s , n e ar s h o r e s t r u c t u r e s

IntroductionRecent damages to coastal structures encountered in December 2004 in the Indian Ocean due tothe Sumatra earthquake generated tsunami event and those encountered in September 2005 inthe New Orleans and its neighbourhoods by the hurricane Katrina, raised the public awareness tothe necessity for proper design against hazards.

It appears that in most parts of the world, including Europe and the Mediterranean, coastalstructures have usually not been designed to withstand loading due to tsunamis, forecasted sea-level rise and meteo-marine climate change. In this presentation it is our intention to draw

attention to loadings due to the above mentioned environmental parameters which seem that areusually dismissed or not properly accounted for, potentially exposing the designed structures toloadings beyond the operational and/or survivability designed capacity. We will addressseparately the aspects of sea level rise, tsunamis and combined wave and wind loading. Thelatter two are addressed both in respect to the impact of climate change on the extreme statisticsof winds and waves, as well as in respect to the adequate assessment of the loading induced inthe survival design conditions. For the latter subject, a case study uses as example for theassessment of the integral extreme loading.

1 Dov S. Rosen, Mr., MSc. P.E., Sea-Shore-Rosen Ltd., 2 Hess St., Haifa 33398 IsraelTel:+972.52.2844174, Fax:+972.4.8374915 - [email protected]

mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Design aspects due to global warming and its effects

In the 3rd assessment report (TAR) of the Intergovernmental Panel for Climate Change (IPCC),established by UNEP and WMO (Church et al. [1], McCarthy et al. [2], Metz et al. [3]), it ispresented a forecast of the global climate changes and of the influences of climate change basedon various scenarios for the 21th century and beyond due to global warming induced by the"greenhouse effect" and other natural and anthropogenic activities in the past, present and in thefuture.

One of the influences of climate change is the forecasted sea level rise, resulting due to theglobal temperature rise. The global warming is assessed to accelerate in the 21 th and comingcenturies. The temperature rise induces sea level rise first by the steric effect of oceans' watervolume expansion, and secondly by melting of the ice caps and glaciers.

According to McCarthy et al. [2], for a business as usual scenario, the assessed average globalsea-level rise due to global warming induced by the "greenhouse effect”, for year 2025 isbetween 3 and 14cm, for 2050 between 5 and 35 cm and for 2100 between 9 and 88 cm, seeFig. 1. Similar estimates were reported by Douglas et al. [4]. According to IPCC reportsmentioned, even if the amount of damaging gases released (CO2, methane, etc.) will becompletely stopped, the greenhouse effect will continue for many centuries. In spite of theforecasted global average sea level rise, the regional and/or local relative sea level change will bedifferent at different places on earth, as one has to account the place position on earth as well asthe regional plate tectonic movements, groundwater withdrawal, glacial rebound, sedimentationor erosion loading, glacier melting. Hence, it is agreed by the professional bodies involved withthis assessment that regional sea-level rise can differ from the above depending to the locationon the globe. Confirmation of sea-level rise due to global warming is yet difficult, being maskedby other factors like seasonal warming and cooling of the sea-water (steric effect), wind inducedsea-level rise during storms (storm surge), wave induced sea-level set-up in the surf zone,atmospheric loading by passing of high and low atmospheric systems.

Figure 1. Sea level rise for business as usual scenarios from 6 models by IPCC TAR Group 1

In a recent position paper, Woodworth et al. [5], state "We report on potential changes inextreme sea level over the coming decades. Changes in the number, path and strength ofatmospheric cyclonic storms, may alter the formation and evolution of storm surges. Extreme

2025: +3cm to 14cm2050: +5cm to 35cm2100: +9cm to 88cm


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water levels will also increase as time-average sea levels rise in the future, so predictions of theregional change in time-mean sea level will also be necessary for predicting changes in extremewater levels a few decades ahead."

While the above quotation covers major aspects of climate change, there are additionalinterrelated aspects to be dealt with. These involve not only long term sea level rise and changes

in extremes but also additional hazards such as extreme wave setup, storm surge, and tsunamis.

Following the tragic outcome of the December 2004 earthquake, the question of the vulnerabilityof other areas of the world to tsunamis has been posed loudly. Based on past history, theMediterranean (including the Black sea) is considered to face a 25% risk of tsunami encounteron the global scale, compared to the about 1% for the North-East Atlantic. Recognizing thishazard,in July 2005, the UNESCO General Assembly formed 3 new regional InternationalCoordination Groups (ICG) responsible to establish early tsunami warning systems in the newrecognized tsunami prone areas, namely the Indian Ocean, the Carribean and the North-EastAtlantic, Mediterranean and connected seas. The latter ICG started working November 2005 toestablish a prototype tsunami early warning system for this region by the end of 2007.

i. Aspects related to sea level change (rise)Since the sea level is an important parameter for the design of nearshore structures, it has to beproperly established, both for normal operational conditions and for ultimate survival stateconditions. It appears however, (as much as we could find), that in the standards we have seen

Figure 2. Sea level changes 1992-2000 based on Topex-Poseidon satellites altimeters (afterFenoglio-Marc, 2002)

(DNV, API, EUROCODE, etc.) there is no account recommendations related to the expected sealevel rise, not even in the new Draft International Standard ISO/DIS 216 regarding "Actions fromwaves and currents on coastall structures" [6]. Even the assessment for increased sea level dueto storm surge in the existing standards is recommended to be based on past data and extremestatistics without requirement for consideration of possible sea level rise due to global warming,


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possible changes in storm surge pattern and in extreme wave statistics affecting wave inducedsetup.

Responsible studies, in particular for large structures and/or with a long economic lifetime shouldaccount for these changes, for example in regards to storm surge by using input from simulationsof global climate models and in regards to sea level rise from global warming by sensitivity

testing with the range of changes assessed by IPCC or more regional sea level rise models.Cazenave et al. [7] based on satellite altimetry data assessed a present rate of sea level rise inthe Mediterranean of +2.6mm/year ±0.4mm/year. Figure 2 taken from Fenoglio-Marc [8] showsthe sea level rate of change in the mentioned period over the Mediterranean and Figure 3 fromRosen [9] shows the sea level changes measured at the GLOSS station 80 (Hadera, centralIsraeli coast) since 1992, confirming the results of Fenoglio-Marc [8]. The sea levelmeasurements in Figure 3 as well as those from satellite altimetry measurements indicate thatthe sea levels in the Eastern Mediterranean rose at a rate of about 1cm/year in the last 13 years,while a lowering has been measured in the Ionian Sea during this period. The changes seem tobe due not only to the global warming effect but also due to fluctuations in the formation andcirculation of the deep Levantine basin waters between the Adriatic and the Aegean during thisperiod, which apparently are related to the North Atlantic Oscillation (NAO) with decadal scalefluctuations.

Figure 3. Recent sea level rise measured at Hadera, central Israeli coast

ii. Aspects related to change of extreme wave and wind statisticsWoodworth et al. [5] discuss also available information on changing wind and wave statisticswhich caused by climate change. Among these are changes in the frequency of extreme storms,and potential change in the directional distribution which may induce changes in the sedimenttransport and coastal erosion or deposition rates and trends. Increased wave heights were clearlyconfirmed around the coasts of UK in the last 2 decades. Woodworth et al. [5] indicate that therecent increase in wave height is highly correlated (>0.8) with an increase in the positive phaseof the NAO for a large area west of the Hebrides (Woolf et al.[10]).Wang et al. [11]) assumed therelationship will continue to hold and suggested that the occurrence of the positive phase of theNAO will be more frequent under global warming. Caires et al.[12] provided return value


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estimates of significant wave height up to the end of the 21st

century using projections of the sealevel pressure under three different forcing scenarios by the Canadian coupled climate model.Their methodology employed regression methods linking climate model pressure simulations towave heights. In all forcing scenarios, significant changes are forecasted in different regions ofthe globe, with the larger and more significant changes occurring under the more severegreenhouse gases emission scenarios. In all future scenarios considered, the forecasts showed

significant positive trends in the North Pacific. Similar patterns were found in a study usingdifferent climate models (Wang and Swail, [13]). In another recent study, Wolf and Woolf [14]used a dynamic wave model approach to show how different climate change effects (e.g. increasein wind speed or change in wind direction) are likely to alter wave conditions in the water aroundthe United Kingdom.

Again, the existing standards, including the most recent draft version of the standard developedby ISO [6] does not include recommendations to account for wave statistics changes due toclimate change. The usual recommendation for the selection of the design wave height is for thatcorresponding to an average return period of 100 years (e.g. DNV [15,16], ISO [6]). However, itis well known that proper design must account for the economic lifetime of the designed structureand for the risk percentage to encounter the design wave height or larger during the economical

lifetime.

Because that for an average return period R (expressed in years) of the design wave height equalto the lifetime of the structure (L in years) there is a risk (r in %) that the design wave heightwill be exceeded during the economical lifetime of the structure of about 64% (unacceptable),one has to select the design wave height with a much lower risk of encounter. This is possible tocompute by formula [1]. Table 1 presents a number of cases for various r, L and R values.

)1(1

1)(

1r

years R L−−

= (1)

Table 1 Design wave height versus encounter risk and economical lifetime of the marine structure

Risk to Encounter of Design Wave Economical Life Time of Structure (years)

[percentages] 2 4 6 8 10 15 20

Average Return Period to be Used

1 200 398 597 796 995 -- --

5 39 78 117 156 195 293 390

10 19 38 57 76 95 143 190

20 10 18 27 36 45 68 90

50 4 6 9 12 15 22 29

64 2 4 6 8 10 15 20

In spite of this classic approach, the standards do not recommend it in general but are satisfiedwith the design wave for a 100 year average return period. One reason is of course the lack ofsufficiently large wave data sample, leading to extrapolation of the 100 year value based of asmall sample (ISO[6] recommends at least 15 years of data). Nevertheless, in view of theforecasted global change impact on wave statistics, the selection should be done taking globalclimate impact into account as well as gathering a largely enough data sample via numericalwave model hindcasting (now available in many places over the globe).

iii. Design aspects due to tsunami eventsThe recognition of tsunami risk must be taken properly into account. Present studies for theassessment of tsunami risk of encounter are available for a number of areas and will becomeavailable for all tsunami prone areas within the next few years. Numerical models of tsunamipropagation and flooding are already available and are being further improved, enabling toassess the risk of encounter and the runup elevation at a particular coastal site. For theMediterranean region, Figure 4 provided by Prof. G. Papadopoulos [17] shows the Mediterranean


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area at risk by tsunamis, and the relative risk of encounter as a color scale.

Figure 4. Tsunami prone areas and the tsunami risk in the Mediterranean (Papadopoulos[17])

This potential encounter of tsunami needs to be accounted in the design. It is worth to commendthat the new ISO standard draft [6] includes recommendations to account for tsunamis at locationswhere it is not negligible and indicates the imacts it may induce. Furtehrmore, implementation ofchanges in design codes are presently discussed by the Intergovernmental Coordination Group ofIOC/UNESCO on tsunami warning and mitigation systems, to be based on new risk assessmentstudies presently underway.

Design aspects related to currents in extreme statesThe present guidance for the design of nearshore structures (ISO [6], DNV [15], DNV[16])recommend to account for the extreme combined loading of currents and waves as follows:

If sufficient information is available on joint probability of waves and current, then thecombined wave and steady current with 100 year recurrence interval should be used. Ifinadequate information is available on the joint probability of waves and current, then thefollowing are suggested for the operational condition:

Waves : 100 year return condition of near bottom wave-induced particle velocity

If waves forces dominates normal to the pipeline Current : 10 year return condition.

Waves : 10 year return condition.If current forces dominates

Current : 100 year return condition.

This recommended practice in our view is not right anymore, in view of the feasibility of numericalhydrodynamic models and of long term atmospheric and wind forcing, which can provide reliablecurrent statistics in most places on the globe.Furthermore, the practice (except ISO [6]) does not yet call for accounting of the wind inducedcurrents during extreme conditions, which in shallow water may be an important loading factor.

Design aspects due to longshore currents in extreme statesAs mentioned previously, the design of marine structures, including those of intakes and outfallslocated nearshore, usually refers to two critical loading conditions. The first one referres to theultimate service state and the second one to the ultimate survival state. The first design state


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seems to be properly defined in the reviewed standards. However, for marine structures (e.g.intake heads and outfall difussers) near the surf zone edge, particularly for those close butbeyond the "normal" surf zone, present standards of survival state lack proper consideration ofan important loading element. It is due to very large currents developing in the ultimate designstate as due to extreme wind and wave induced longshore currents.Here we refer again to the usually recommended desing sea state condition in the standards,

with an average return period of 100 years. The existing guidance is in author's opinioninproperly defined, even though general guidance is available by a number of reputable designmanuals (e.g. Goda, [18], Mei [19], 1992; C.E.M. [20]), but even these are insufficientlydetailed. For such sea state, simultaneous loading by extreme wind induced currents, near-breaking/breaking waves and wave induced large longshore currents if the waves approachobliquely to the coast (as the structures are now near/in breakers zone) result in significantlyincreased velocities and accelerations, hence increased loads.

The design and construction of 3 marine intakes for the world largest reverse osmosisdesalination plant which was recently built at Ashkelon, Israel for OTID Desalination Partnership(Figures 5 and 6), serves to show the importance of consideration of longshore currentsdeveloping during extreme (ultimate) state and to indicate a relatively simple method ofassessment and choice of the design current speed in the ultimate state.

Figure 5. General location map of the Ashkelon desalination plant

• The intake heads were planned and built on the -15m water depth contour off the Ashkelonshore, at the southeastern Mediterranean coast of Israel. The coast has an orientation of Az34deg. Top of the heads are 5m below MSL.

• 3 HDPE pipelines, outer diameter of 1.6 m serve as intake pipelines from the 3 intake heads tothe pumping sump onshore.

• Maximum total discharge – 35,200 m3 /h, i.e. 308 M m3 /year• Economical lifetime of the intake system - 25 years.• The design and build contract was awarded to and carried out by O.G. Pipeline Partnership

Ashkel on


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Figure 6. Plan of the desalination intake at Ashkelon

The design and construct tender technical specifications required the intakes to be designed forthe most critical conditions arriving for:(a) a design wave height of Hmax associated with a sea state with a deep water characteristic

(significant) wave height sea state with a 100 year average return period or(b) the maximum depth limited wave height (at the site of the structures) with an average

return period of 100 years and identified a deep water significant wave height of 8.7m for a100 year average return period.

A schematic of the intake suction heads at Ashkelon designed by the contractor is shown inFigure 7 together with a schematic of a more usual shape of intake suction head.

Figure 7. Schematic intake head at Ashkelon (left) and schematic of usual intake head (right)


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Statistical data regarding sea level, wind, waves and currents are presented to demonstrate thenormal selection of environmental loading for these intake structures.

i. Sea levelsThe tidal (astronomic) range on the Mediterranean coast of Israel is characteristic of the low-tiderange of the Eastern-Mediterranean basin, being induced by the combined effect of the attraction

forces of the moon and of the sun, and by the location of this area on the globe. The tide usuallyvaries between 0.40m during spring tides (occurring in spring and autumn), and 0.15m duringneap tides (occurring in winter and summer). The tide contribution exhibits the usual semi-diurnal periodicity (twice a day highs and lows) and fortnight (14 days) periodicity. Extreme sealevels may occur in combination with extreme meteorological conditions. Low sea-levels occur inwinter during February-March months, while high sea-levels occur in August-September, with asecond maximum in December.

The average return periods of extreme sea levels (excluding sea level rise) are presented in Table 2.

Table 2. Average recurrence of extreme wind gust velocities at Israel coast

Average ReturnPeriod

Low Sea Level High Sea Level

[years] [m] [m]

1 -0.38 0.6450 -0.74 1.04

100 -0.87 1.10

ii. Wind

~77% of the fresh winds blow from directions W to N through NW.

~77% of the strong winds blow from directions SW to W trough WSW.

Table 3 presents the recurrence of high wind velocities as estimated using the Weibull distribution

while the extreme gusts statistics is shown in Table 4. The directional distribution is shown in Fig. 8.

Table 3. Average recurrence of high wind velocities at Israel the coast

Average recurrence Wind speed (average of 10 highest minutes in 1 hour)

once/year 23.6 m/sec 46 knotsonce/50 years 31.5 m/sec 61 knots

once/100 years 32.6 m/sec 63 knots

Figure 8. Weibull Extreme Directional Statistics of Hourly Wind Speeds


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Table 4. Average recurrence of extreme wind gust velocities at Israel coast

Average recurrence Upper gust wind speed

once/year 34.7 m/sec 67 knots



iii. WavesThe year may also be divided in only two wave seasons with a transitional type beginning andend. In such a case the following division is obtained- extended winter season ranging from mid November through mid April (5 months);- extended-summer season ranging from mid April through mid November (7 months). The winter wave climate is characterized by alternating periods of high sea states (storms) andlow sea states (calms). The storms are in general induced by cyclones passing slowly over theMediterranean from West to East. The strongest storms usually occur in the period between midDecember through the first week of March. Lower sea states occur at the beginning and at theend of that season. The prevailing wave direction is WNW, but the predominant wave direction isW, corresponding to the longest wind fetch.

During the summer season the wave climate is characterised by relatively calm seas with wavesinduced by the weak local winds (mainly by the breeze). Therefore, the waves are usually of"sea" type, which direction usually varies during the day in a clockwise direction from WSW inthe early morning to WNW at noon and to N-NW in the afternoon.The average yearly directional deep water significant wave distribution is characterized by:-All moderate and higher sea states come from WSW to NNW through W-66% of all waves approach from W trough WNW directions.-The highest sea states approach from W direction, but storm development occurs by veeringfrom WSW to NW through W directions.

Peak wave periods range between 3 and 16 seconds. During high sea states they range usuallybetween 10 and 13 seconds, and very high sea states have peak periods between 12 and 16seconds.

Extreme sea states and average return periods are presented in Table 5 below:Table 5 Recurrence of extreme deep water significant wave heights at Israel coast

Average Return Period Deep Water Significant Wave Height

[ years ] [ meters ]

2 5.15

4 5.95

5 6.15

6 6.25

8 6.60

10 6.80

15 7.15

20 7.40

50 8.20

100 8.70500 10.15

iv. CurrentsDetailed general current statistics were gathered for a period of 1 year on the -27m contour at10m below surface. It indicated weak currents of 5-10 cm for most of the time. However,measurements carried out off Ashdod (about 15km north to the site) with an ADCP on the -24mcontour and on the -15m contour showed strong currents during wave storms associated withstrong local winds. An extreme value of 1m/s current was recorded at Hadera, some 65 km northto the site on the -27m contour at -11m below surface during a high sea state with deep watersignificant height of about 7.2m. Later on, strong currents of about 0.8m/s were recorded off the


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Tel-Aviv coast with an ADCP during a relatively low sea state, clearly being induced mainly bywind. This record is shown in Figure 12 below.

Figure 12. Record of current speed vertical profile off Tel Aviv in December 2003.

In the following we will try to show the insufficient definition of design loading criteria in anumber of international standards under the ultimate design state. Under such environmentalconditions, simultaneous loading due to extreme wind induced currents in combination with near

breaking/breaking wave loading and with large wave induced longshore currents in certain casessuch as that at Ashkelon where the deep water angle of the Westerly waves with the coast makean angle of 34 degrees (and because the structures are now near or within the breaking zone)lead to significantly increased resultant velocities and accelerations and consequently increasedloading on the structures.

For the Ashkelon intakes the contractor assessment of the wave and current loading was basedon:• The 100 year design wave was specified to have a significant wave height of Hs,o = 8.7 m.• An increased sea level due to sea level rise and storm surge of 1.25m was agreed to be used.• The maximum wave height for this scenario was derived as depth limited of Hmax-15=12.40 m.• The maximum wave height will depend on the angle between wave direction and coast normal.

In the design it was assumed that the wave height is reduced by the square of the cosine of the

angle.• The associated peak wave period was assumed between 10 to 15.3 seconds. It was concluded

that the linear Airy theory is under the present circumstances equally applicable as the non-linear Stokes 5th order theory in determining the wave induced velocity and acceleration fieldaround the intake structure.

• A maximum current speed of 0.75m/s was initially assessed at the intake site, which waschanged later to 2 m/s, following the design review carried by the author, which identified thelack of consideration of the joint wind induced current and wave induced longshore currentdeveloping in the ultimate (design) sea state as the intake heads became near the surf zoneedge.

It should be mentioned that in a recent paper by Kunitsa et al. [21], stating to deal with


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continental shelf wind-driven currents, have assessed at this location a maximum current valueinduced by wind of 1.28m/s for an average return period of 100 years. However, the currentvalues of Kunitsa et al. were obtained without considering important modelling problems of waveand wind hindcasting indicated by Cavalleri and Bertoti [22]. In this way, unaware engineersmay be lead to believe that the extreme currents assessed include also the wave contributions,when in fact these are completely discarded.

The total current during the 100 year average return period sea state for the case study wasassessed by a number of numerical models (Rosen [23], Sladkevitch et al. [24]), leading tospeeds of 2m/s and more, significantly larger than the original value used by the contractor(0.75m/s), or that assessed by Ronaess and Nestergard [25] using DNV[15] and discarding extremewind induced currents, or those assessed by Kunitsa et al. (1.28m/s).

Rosen [23] used the NMLONG model developed by US Corps of Engineers, part of the CEDASsoftware package using both basically monochromatic waves equivalent to the significant andmaximum wave heights assessed as well as wind induced current (see Figures 8 and 9).

Figure 9 – Wave induced current speeds at intake site without wind for various conditions.

Sladkevitch et al. [24] used both monochromatic and irregular waves derived to correspond tothe Hrms of the design sea state and with wind induced current, see examples in Figure 11a,b.One of the usual problems encountered by numerical models for the assessment of the waveinduced currents is that most of them use the Hrms wave height to assess the average currentsinduced by a sea state. However, the use of Hrms is correct for assessing the average currents(radiation stresses), used to transport sediments or polluting materials. It is incorrect whenassessing extreme wave induced currents and their loading for design at ultimate survival state.Goda [26] published results and empirical formulations for the computation of the distribution ofthe wave induced longshore current due to wave spectra and the position of the maximumlongshore current speed. However, his method does not account for the wind induced currents in


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the extreme sea state, when it was estimated that strong winds will blow over the study area.Nevertheless, using Goda's formulation, similar values of the wave induced current to thoseassessed by the author were obtained. Nevertheless since the Goda formulations were calibratedfor relatively low wave conditions another method for rapid assessment was seek by the author.

Figure 10 – Wind and wave induced current speeds at intake site for various conditions.

Figure 11. Current velocity (left) and Hrms (right) fields at sea bottom; random waves; CAMERI3D HD model. Hs=8.7m (Hrms=6.2m), Tp=16.1s, waves direction 284.6,; Result U=1.7m/s


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It is obvious that the maximum current velocity to be used in the Morison loading formula in thesurvival state is the vectorial summation of the geostrophic current, wave induced current andwind induced current (and tide if relevant).

Thus it is found that assessment of the combined wind induced and wave induced current by therecommended practice in extreme sea states is not accounted. Furthermore, the proper

assessment of the maximum longshore current speed to be used in the survival state is yetdifficult to assess because the usual model assessments based on the Longuett-Higgins formulauses the Hrms height value, which derives an average, not maximum current speed and becausethe Goda formulas were developed and calibrated against low height waves (2m).To correctly assess the total loading one needs to assess resulting current vector and magnitudewhich includes the joint contribution due to the wind induced current and to the longshorecurrent and the orbital velocities. In this case a numerical hydrodynamic model is necessary.However, a simpler assessment of the loading by a different approach is proposed, not involvinga full hydrodynamic model, but merely analytic computations. This method integrates the locallongshore current contributions due to each wave in the refracted waves spectrum, and selectsthe design longshore current speed as that due to the average of the highest 13.6% of allcurrents occurring during the ultimate sea state at the structure site as described shortly below.It is proposed to assess the maximum longshore current velocity at a site located within or nearbut beyond the normal surf zone for loading assessment in the ultimate (survival) state asfollows:(a) Determine local Hs or Hb in the survival state.(b) Use Beta- Rayleigh wave height distribution to assess the various wave heights at the site for

the survival state.(c) Compute the local longshore current speed for each wave height(d) Compute the local maximum longshore current speed as the average of the highest 13.6%

current values, in a similar way to the evaluation of Hs in the Rayleigh distribution.(e) Compute the resulting total current vector and magnitude by vectorial summation of the

wind induced current assessed via NMLONG model using almost zero wave height and theextreme wind speed selected with the extreme wave induced longshore current.

Conclusion

Improved guidance on the selection of the values of the potential loading environmentalparameters (sea level, wave statistics, wind statistics, tsunami and extreme lonshore currents) isprovided. It is expected that some/all of the suggested guidance will be integrated in new versionsof design standards of nearshore structures.

Acknowledgements Part of the data used were gathered during the consultance provided by the author to OTIDDesalination partnership for the Ashkelon desalination plant. Permission to use these data for thepresent paper from OTID Desalination Partnership and the assistance provided by Eng. A. Kaplan,Project Manager of the Ashkelon Desalination plant are acknowledged with thanks.


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Documents

Rosen S.D., 2006, Improved Environmental Loading Design Criteria For Nearshore Structures