BASIC COASTAL ENGINEERING Part 10 docx

To address this question someWeld armor unitshave been instrumented with strain gages to measure in situ tensile stressesHowell, 1985.9.6 Laboratory Investigations Most laboratory invest

traps include containers with one open side facing the oncoming bed loadtransport, and larger open containers with the opening facing upward andhaving its edge at the bed surface The volume of sediment accumulated in agiven time is the bed load transport rate at that point that can be integratedacross the width of the transport zone.

9.5 Coastal Structures

Field investigations of coastal structures typically involve the direct ment of hydrodynamic loadings on rigid structures such as piles and sea walls ormeasurement of the displacement of armor units on rubble mound structures.The hydrodynamic loadings of greatest interest are those caused by waves, whichwould be measured at the structure by one of the wave gages discussed above.For large structures such as seawalls and revetments the loading on a structure

measure-is best measured by installing several pressure transducers into the face of thestructure, measuring the time-dependent pressures, and integrating the pressuredistribution over the face to determine the total force as well as its distribution as

a function of time (e.g., see De Girolamo et al., 1995) Commercially availablepressure transducers consist of a small diaphragm with a strain gage thatmeasures the pressure on the diaphragm by the pressure-induced bending ofthe diaphragm It is important that the pressure transducers be selected withsome knowledge of the frequency and maximum magnitude of the expectedpressure Xuctuations that are to be measured As discussed in Section 7.6,breaking wave-induced pressures can have a relatively large magnitude for avery short duration

For thin structures such as cylindrical piles, the loading on a test section of thepile would be measured This has been done by installing a ring of pressuretransducers around the circumference of the pile at the test section or by building

a load cell into the test section The load cell would have interior strain gagesmounted so that the measured strain is related to the instantaneous load on thetest section

Failure of rubble mound structures occurs when a signiWcant number of armorunits are displaced Thus,Weld monitoring of the behavior of a rubble moundstructure requires a survey of the position of armor units immediately afterconstruction and before and after major storms This can be done by markingpoints on selected units and surveying the position of these points using standardsurveying techniques An innovative improvement on standard surveying hasbeen to employ stereophotogrammetry using photographs taken from the air(Davis and Kendall, 1992) Side scan sonar has been used to investigate thecondition of underwater armor units

Large concrete armor units may fail when wave action causes the units to rock

in place and break There is a related question of whether concrete units should

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be reinforced with steel rebar To address this question someWeld armor unitshave been instrumented with strain gages to measure in situ tensile stresses(Howell, 1985).

9.6 Laboratory Investigations

Most laboratory investigations for coastal engineering are concerned with face waves There have been a few studies of wind-blown sand and coastal duneprocesses, and some laboratory investigations of wind-wave generation pro-cesses Also, steady Xow currents have been added in some tidal Xow modelstudies and instantaneous tidal currentXows have been simulated by steady Xow

sur-in bassur-ins

But surface wave investigation are by far the most common They may begrouped into short wave and long wave studies The former are concerned withthe wind wave portion of the wave spectrum and the latter with long wavephenomena including tides, basin oscillations and tsunami propagation and

eVects

One major advantage of laboratory wave studies is the control that theinvestigator has over input wave conditions Within the limits of the laboratorywave generator being used, any monochromatic or spectral wave condition can

be run for any length of time Owing to their smaller scale and the diYcultiesinvolved inWeld work, laboratory studies can generally be conducted faster and

at lower cost

But laboratory studies can have major drawbacks, namely scale and tory eVects Scale eVects generally arise over diYculties in maintaining viscousand surface tension similarity where necessary At smaller scales in the labora-tory, Reynolds and Webber numbers are typically smaller than in the prototype

labora-If these forces are important in the prototype they are diYcult to simulate in thelaboratory, or they may be unimportant in the prototype but signiWcant in thelaboratory For example, there have been numerous wave tank investigations ofwave loadings on vertical cylindrical piles but the laboratory Reynolds numbersare several orders of magnitude smaller than found in theWeld for storm waveconditions Beach sand grains when scaled down to a laboratory size may be sosmall that intersurface forces dominate whereas they are not important in theWeld

Laboratory eVects may also cause unsurmountable diYculties The wavegenerator employed in the lab may not be able to fully simulate theWeld wavesthat can occur During the early decades of wave tank research, wave generatorscould produce only monochromatic waves Recently, one-dimensional and dir-ectional spectral wave generators have come into use Lateral boundaries onthree-dimensional models may aVect conditions over a signiWcant portion of thelaboratory investigation that is not aVected in the Weld

Some laboratory investigations are studies of basic phenomena such as uring the wave loading on a rigid vertical cylinder for a selected range of incidentwave conditions and water depths Other investigations involve scaled modelstudies of givenWeld sites for selected Weld conditions, such as an investigation ofwave propagation toward the shore and into the lee of a proposed harbor break-water conWguration Numerical models are increasingly being used in the place

meas-of physical models owing to their lower costs and greaterXexibility Some Weldproblems such as coastal storm surge can only reasonably be studied byWeld-calibrated numerical models

Much, but not all, of the instrumentation used in laboratory studies is similar

to the instrumentation used in theWeld (modiWed to smaller laboratory time andspatial scales and less rigerous conditions) For more extensive discussion ofcoastal engineering laboratory investigations see Hughes (1993) and Hudson

et al (1979)

9.7 Wave Investigation Facilities

Wave investigations have primarily been conducted inXumes and basins—tanksholding water with a wave generator and, if necessary, a wave absorber toprevent waves from reXecting back to the area where the investigation is beingconducted A wide range of size and shapeXumes and basins have been used.Flumes having common lengths of 30 to 40 m, widths of a meter or so, and waterdepths of less than a meter are used for two-dimensional investigations Basinswould be signiWcantly wider and used for three-dimensional studies

Wave tanks constructed during the early to middle years of the 1900s had onlymonochromatic wave generators By 1960–1970 irregular or spectral wave gen-erators became increasingly common Figure 9.1 schematically depicts varioustypes of monochromatic wave generators (see Sorensen, 1993 for more detaileddiscussion.) Most common are the piston orXap generators, the former beingbetter for shallow water waves and the latter better for deep water waves Somemore complex wave generators are designed so that they can be modiWed frompiston toXap motion as the desired wave period is changed The frequency ofoscillation of the piston orXap establishes the wave period and the amplitude ofpiston or paddle motion (for a given wave period) establishes the wave ampli-tude

A variety of wave absorbers have been used The ideal absorber would be arough porousXat slope, but this requires a large portion of the wave Xume orbasin, and would not be easy to relocate as studies change Consequently,modiWcations of this ideal have been employed (see Sorensen, 1993)

If, for example, the stability of a proposed rubble mound structure is beinginvestigated, the model structure will cause wave reXection, the reXected wavespropagating back to the wave generator, reXecting from the generator, etc to cause

300 / Basic Coastal Engineering

a very diVerent incident wave condition than that desired In the past this problemwas dealt with by using a long waveXume, generating a burst of a few waves, andstopping the generator between bursts to allow the reXected wave energy todissipate Recently, wave generators have been designed that detect the reXectedwaves and adjust the piston or blade motion to cancel out the reXected waves.During the past few decades irregular wave generators have become common.Figure 9.2 schematically depicts a common type of irregular wave generator Anappropriate electrical input signal is sent to the generator to drive the piston/blade by a hydraulic, pneumatic, or mechanical device The servo senses thepiston motion and sends a proportional voltage feedback to the signal control.The input and feedback signals are continuously compared to adjust the pistonmotion to the desired form A monochromatic wave can be generated by input-ting a sinusoidal signal A nonsinusiodal oscillating signal can be input togenerate better cnoidal or solitary waves For spectral waves the input signal istypically produced in one of three ways:

1 By superimposing a large number of sine waves of diVerent periods andamplitudes with random phasing

2 ByWltering a white noise electrical signal to form the desied irregular waveinput signal spectrum

Pneumatic Plunger

Figure 9.1 Various monochromatic wave generators (Sorensen, 1993.)

3 By creating an input signal that will produce a previously measured orartiWcially constructed surface elevation time history

There have been some eVorts to use wind to generate irregular wave spectra inlaboratories But, because of scaling problems (see Sorensen, 1993) and im-proved mechanical spectral wave generators, waves generated solely by thewind are no longer used Wind has been used over mechanically generatedirregular waves to more realistically steepen the fronts of waves as would happenduring a storm

For some three-dimensional studies it is desirable to generate directional wavespectra This has been done by using a series (e.g., 60 to 80) of individuallyactivated wave generators along a line and facing in the same direction By a verycomplex operation of driving each of the generators with a diVerent period,amplitude and phasing, a directional wave spectrum can be generated

9.8 Scaling of Laboratory Investigations

Laboratory investigations are commonly carried out at signiWcantly reducedscale from the prototype Thus, attention must be paid to appropriate scalingrelationships Wave motion predominantly involves a balance between pressure,gravity, and inertia so Froude similarity dominates But, as discussed above,viscous and surface tension forces may be important For Froude similarity thetime ratio equals the square root of the length ratio, the pressure ratio equals thelength ratio, and the force ratio equals the length ratio to the 2.5 power.Scale diVerences are commonly accounted for in experimental results bypresenting the results on dimensionless plots (e.g., see Figures 2.11, 2.12, 2.15,3.5, 4.9, 4.11, 6.10, and 7.3) Consider Figure 2.15, which gives the results from a

Input signal

control

Hydraulic pneumatic

or mechanical

drive

Blade signal

Figure 9.2 Typical irregular wave generator (Sorensen, 1993.)

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wave tank experiment at reduced scale on wave runup on a plane slope tational eVects are included in the H0 =gT 2

Gravi-o term which is similar to a Froudenumber Surface tension and viscous eVects are not accounted for—it beingimplicit that they are negligible or can be accounted for with an additionalcorrection factor

If the lengths of laboratory waves are greater than about 3 cm surface tensionforces will be negligible as they are at prototype scale Surface tension forces willbecome important in physical models where the reduced scale causes veryshallow depths in some areas of the model To overcome this potential problemsome models employ a distorted scale (i.e., a vertical length ratio that is largerthan the horizontal length ratio)

As discussed earlier, it is often impossible to conduct a laboratory tion at a suYciently large scale to fully eliminate viscous scale eVects in someexperiments such as the measurement of wave forces on a vertical pile Theinvestigator must be aware that these scale eVects exist when considering theresults from such an experiment, and, when comparable near prototype scaledata are available, try to quantitatively account for this eVect

investiga-An important facet of coastal engineering is the response of a sandy beach towave action At a reduced laboratory scale the prototype sand size is reduced tosubsand size so sediment transport processes are not correctly simulated Theusual approach in these experiments is to use a very Wne sand or some othergranular material of lower density to simulate sand in the laboratory Theconduct of wave-sediment transport investigations then becomes more of anart than a science Several investigators (e.g., Noda, 1972; Kamphius and Read-shaw, 1978; Kamphius, 1985; Kreibel et al., 1986) have developed testing pro-cedures and related scaling guidance for these experiments

When a three-dimensional investigation such as a study of the refraction anddiVraction that occurs as waves propagate from deep water to the shore isconducted, space and cost limitations may require that the investigation beconducted with less than optimum lateral basin dimensions An undistortedmodel scale may then lead to very shallow water depths in a portion of thebasin—and consequent viscous and surface tension scale eVects Also, waveheights may be so reduced as to be diYcult to measure with the requiredaccuracy Thus, a distorted scale investigation may be necessary

At a distorted scale, sloped boundaries become steeper which increases theirwave reXection characteristics compared to the Xatter prototype slope Thisproblem can be overcome, for example, by increasing the laboratory boundary’sroughness and porosity to reduce wave reXection The impact of a distorted scale

on wave refraction and diVraction is more complex

For shallow water waves wave celerity depends only on the water depth, sorefraction patterns are unaVected For intermediate depth waves refraction isaVected by scale distortion A distorted scale intermediate depth wave investi-gation can be carried out if appropriate depth ratio and wave length ratios are

used (see Sorensen, 1993) But if signiWcant diVraction also occurs a conXictarises so that it is impossible to correctly scale refraction and diVraction in anintermediate depth wave investigation For shallow water waves it is possible tocorrectly scale both refraction and diVraction at the same time (see Sorensen,1993) Pure diVraction investigations (constant depth) involve no scaling prob-lems when a distorted scale is used.

9.9 Common Types of Investigations

Generally speaking, laboratory investigations are either basic investigations intowave mechanics and the interactions of waves with beaches and structures ormodel investigations for speciWc projects Both are carried out in two-dimensionaltanks and/or three-dimensional basins Some examples are presented below to give

a general sense of the important types of studies that have been conducted.Investigations of basic wave mechanics have included measurement of surfaceproWles and water particle velocity Welds to evaluate the eYcacy of various wavetheories for diVerent ranges of wave height and period and water depth Exten-sive studies of wave breaking, runup, reXection, overtopping rate, and transmis-sion past structures have been conducted—both as basic investigations and asinvestigations for speciWc design projects that, in turn, have added to our generalknowledge of the phenomena involved Basic investigations and model studies ofshort wave refraction, diVraction, and three-dimensional reXection have beenconducted Long wave investigations involving tide and tsunami propagationand basin resonance have been important to our understanding of bay, coastalriver, and harbor hydrodynamics

The design of stable rubble mound structures and the prediction of induced pressure distributions and forces on piles, seawalls, and large submergedstructures require the evaluation of empirical coeYcients included in the designformulas Much of the guidance in this area comes from laboratory investiga-tions This is also true for the dynamic response ofXoating structures and thewave transmission characteristics ofXoating breakwaters

wave-While, as indicated above, there are often serious scaling problems withthe investigation of beach response to wave attack, some useful basic investiga-tions and model studies for speciWc locations have been carried out This

is particularly true for the investigation of wave-induced scour at coastal tures Some model studies where the bottom geometry is Wxed but a granulartracer is used to indicate potential shoaling and scour patterns have been useful.The vast majority of coastal engineering laboratory investigations focus on thecharacteristics and eVects of short and long period surface gravity waves Butother useful laboratory investigations have been carried out including studies ofinternal waves, coastal and inlet currents, marine waste diVusion, and windloadings on structures

struc-304 / Basic Coastal Engineering

9.10 Summary

Coastal engineering is an atypical branch of civil engineering in that coastalengineering design is less dependent on government or professional societydeveloped design codes (e.g., versus the design of bridges, buildings, highways,and water treatment facilities) It requires a thorough understanding of thecomplex air/water/land environment at the site where a design is to be carriedout, coupled with an understanding of the procedures needed to satisfy designrequirements in this complex environment Both this understanding of thecoastal environment and the development of coastal engineering design proced-ures are strongly dependent onWeld and laboratory investigations–the subject ofthis chapter

9.11 References

Anders, F.J and Byrnes, M.B (1991), ‘‘Accuracy of Shoreline Change Rates as mined from Maps and Aerial Photographs,’’ Journal, American Shore and Beach Preservation Association, January, pp 17–26.

Deter-Birkemeier, W.A and Mason, C (1984), ‘‘The CRAB: A Unique Nearshore Surveying Vehicle,’’ Journal of Surveying Engineering, American Society of Civil Engineers, March,

pp 1–7.

Clausner, J.E., Birkemeier, W.A., and Clark, G.R (1986), ‘‘Field Comparison of Four Nearshore Survey Systems,’’ Miscellaneous Paper CERC 86–6, U.S Army Waterways Experiment Station, Vicksburg, MS.

Cross, R.H (1968), ‘‘Tide Gage Frequency Response,’’ Journal, Waterways and Harbors Division, American Society of Civil Engineers, August, pp 317–330.

Davis, R.B and Kendall, T.R (1992), ‘‘Application of Extremely Low Altitude grammetry for Monitoring Coastal Structures,’’ Proceedings, Coastal Engineering Prac- tice ’92, American Society of Civil Engineers, Long Beach, pp 892–897.

Photo-De Girolamo, P., Noli, A., and Spina, D (1995), ‘‘Field Measurement of Loads Acting On Smooth and Perforated Vertical Walls,’’ Proceedings, Advances in Coastal Structures and Breakwaters Conference (J.E Cli Vord, Editor) Thomas Telford, London, pp 64–76 Grace, R.A (1978), ‘‘Surface Wave Heights from Pressure Records,’’ Coastal Engineering, Vol 2, pp 55–68.

Horikawa, K (1988), Nearshore Dynamics and Coastal Processes–Theory, Measurement and Predictive Model, University of Tokyo Press, Tokyo.

Howell, G.L (1985), ‘‘Crescent City Prototype Dolos Study,’’ Proceedings, Workshop on Measurement and Analysis of Structural Response in Concrete Armor Units, U.S Army Waterways Experiment Station, Vicksburg, MS.

Hsiang, W., Dong-Young, L., and Garcia, A (1986), ‘‘Time Series Surface-Wave ery from Pressure Gage,’’ Coastal Engineering, Vol 10, pp 379–393.

Recov-Hudson, R.L., Herrmann, F.A., Sager, R.A., Whalin, R.W., Keulegan, G.H., Chatham, C.E., and Hales, L.Z (1979), ‘‘Coastal Hydraulic Models,’’ Special Report No 5, U.S Army Waterways Experiment Station, Vicksburg, MS.

Hughes, S.A (1993), Physical Models and Laboratory Techniques in Coastal Engineering, World Scienti Wc, Singapore.

Irish J.L and White, T.E (1997), ‘‘Coastal Engineering Applications of Higher-resolution Lidar Bathymetry,’’ Coastal Engineering (in press).

Kamphius, J.W (1985), ‘‘On Understanding Scale E Vects in Coastal Mobile Bed Models,’’ Physical Modelling in Coastal Engineering, (R.A Dalrymple, Editor), A.A Balkema, Rotterdam, pp 141–162.

Kamphius, J.W and Readshaw, J.S (1978), ‘‘A Model Study of Alongshore Sediment Transport Rate’’ in Proceedings, 16th International Conference on Coastal Engineering, American Society of Civil Engineers, Hamburg, pp 1656–1674.

Komar, P.D and Inman, D.L (1970), ‘‘Longshore Sand Transport on Beaches,’’ Journal

of Geophysical Research, Vol 75, pp 5914–5927.

Kreibel, D.L., Dally, W.R., and Dean, R.G (1986), ‘‘An Undistorted Froude Model for Surf Zone Sediment Transport,’’ in Proceedings, 20th International Conference on Coastal Engineering, American Society of Civil Engineers, Taipei, Taiwan, pp 1296– 1310.

Langley, T.B (1992), ‘‘Sea Sled Survey Through the Surf Zone,’’ Journal, American Shore and Beach Preservation Association, April, pp 15–19.

National Research Council (1982), ‘‘Proceedings, Workshop on Wave Measurement Technology,’’ NRC Marine Board, Washington, DC.

Noda, E.K (1972), ‘‘Equilibrium Beach Pro Wle Scale-Model Relationships,’’ Journal, Waterways and Harbors Division, American Society of Civil Engineers, November,

pp 511–528.

Ribe, R.L and Russin, E.M (1974), ‘‘Ocean Wave Measuring Instrumentation,’’ ceedings, Conference on Ocean Wave Measurement and Analysis, American Society of Civil Engineers, New Orleans, pp 396–416.

Pro-Schneider, C (1981), ‘‘The Littoral Environment Observation (LEO) Data collection Program,’’ Coastal Engineering Technical Aid 81–5, U.S Army Coastal Engineering Research Center, Ft Belvoir, VA.

Schneider, C and Weggel, J.R (1980), ‘‘Visually Observed Wave Data at Pt Mugu, California,’’ in Proceedings, 17th International Conference on Coastal Engineering, American Society of Civil Engineers, Sydney, pp 381–393.

Seelig, W.N (1977), ‘‘Stilling Well Design for Accurate Water Level Measurement,’’ Technical Paper 77–2, U.S Army Coastal Engineering Research Center, Ft Belvoir, VA.

Soares, C.G (1986), ‘‘Assessment of the Uncertainty in Visual Observations of Wave Heights,’’ Ocean Engineering, Vol 13, pp 37–56.

Sorensen, R.M (1993), Basic Wave Mechanics for Coastal and Ocean Engineers, John Wiley, New York.

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Tucker, M.J (1991), Waves in Ocean Engineering–Measurement, Analysis, Interpretation, Ellis Horwood, New York.

U.S Army Coastal Engineering Research Center (1984), Shore Protection Manual, U.S Government Printing O Yce, Washington, DC.

U.S Naval Weather Service Command (1976), ‘‘Summary of Synoptic Meteorological Observations,’’ National Climate Data Center, Ashville, NC.

Williams, S.J (1982), ‘‘Use of High Resolution Seismic Re Xection and Side-Scan Sonar Equipment for O Vshore Surveys,’’ Coastal Engineering Technical Aid 82–5, U.S Army Coastal Engineering Research Center, Ft Belvoir, VA.

A Notation and Dimensions

A L2 bay surface and channel cross-section area, structure

projected area

ac, at L wave crest amplitude; trough amplitude

ax, az L=T2 horizontal and vertical components of acceleration

B L wave orthogonal spacing, structure crest width

Bo L wave orthogonal spacing in deep water

Co L/T wave celerity in deep water

d L water depth, sediment grain diameter

d0 L setup, setdown of mean water level

db L water depth at point of wave breaking

ds L water depth at structure toe

E, Ek, Ep F total, kinetic, potential energy per unit crest width

E¯ F/L average energy per unit surface area

F L, F,— wind fetch length, freeboard, force acting on a body,

Fs F/L force per unit length

fp 1/T wave frequency at spectral peak

G( f , u) — directional spectrum spreading function

g L=T2 acceleration of gravity

Hmo L signiWcant wave height based on spectral energy

Hn L average height of highest n percent of waves

Hrms L root mean square wave height

Hs L signiWcant wave height based on individual wave

analysis

h L vertical distance from berm crest to depth at which

wave transport of sediment vanishes

hc L structure crest height above the seaXoor

K — coeYcient in sediment transport equation

KD — armor unit stability coeYcient

Ks — shoaling coeYcient, wind stress drag coeYcient

k 1/L,— wave number, inertia coeYcient

k2 — parameter in cnoidal wave theory

Lp L wave length for fpat water depth of interest

M, N — solitary wave theory coeYcients, resonance modes

mn L2=Tn nth moment of a wave spectrum

mo L2 zeroth moment of a wave spectrum