jetty design

7/28/2019 Jetty Design

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CHAPTER 26DESIGN AND CONSTRUCTION OF JETTIES

R. E. Hickson and F. W RodolfPortland Distnct, Corps of EngmeersDepartment of the ArmyPortland, Oregon

INTRODUCTIONThe purpose of th is paper is to present a br lef outl ine of the general engineering procedure fo r the s i t ing and design of j e t t i es and the methods of construct ing such s tructures . After a general presentation of the formulae proposed byvarious engineers to determine the size and weight of individual pieces of stone orother m ateria l which should be used under various wave heights, th is paper wil l bedevoted principal ly to the construction of rubble stone j e t t i es . This is the typeprincipal ly used on the Pacific Coast of the United States . In th is paper the

w.>..r

Factors which influence the type of Jetty and design of th e ~ t r ~ ~ n g ] _ u . g ~ :t!1e physical ..ch.K?-cteristiCs of the s t t ~ L ' ! 1 : ~ C l , . A m L . t j ; ; J l ~ J U l Q . s . J J J ; ' ~ , . i t 9 Jliu,g., -Nwes_,currents , and t ides ; poss ibi l i ty of ice damage; meteorological conditions andfheTr effecE em' -;;-ater c ~ ~ d I t i ~ n ~ ';:;:;;'d c ~ r ~ ~ - ~ t s ; s e ~ - ~ ~ ~ d i t ion ; i n ~ l ~ d : i ; ; ' g " the determination of maximum wave which shouldbe-'clesigned :for and l i t to ra l currents 'which~ " - - ' - ' ~ : = - - ' - ' - - , ,--,-.. - , , , ' - ' ~ - - - ~ - ' - ~ - - - ~ - - - ..-, .. . - - - ~ ~ ' . "'._- - - - .. 'may affect ~ ' : ! l ~ d ~ ~ , ! l 1 ~ n t s : h ~ . t ; ~ _ ] . , ~ < ! a l i t y .

The cost of construction and maintenance is usually the controll ing factor indetermining the type. Different types may be pract ical in an y local i ty and thecost of each type, as well as the annual cost of maintenance for different types,should be estimated fo r comparison before f ina l decision is made. The avai labi l i tyof materials wil l great ly influence the cost and, fo r some types, would affect thecost of maintenance. Rubble-mound st ructures require periodic repair of portionsdamaged by storm waves but, even though damaged, the st ructure as a whole s t i l lfunctions, whereas breakage of a ver t ica l face monolith st ructure generally leadsto to ta l destruction of portions of the whole st ructure and high cost of repair .The seven,general types of ~ e t t i e s are b r l e f l y . d e s c r l ~ e d as follows:(1) Random stone: A rubble-mound st ructure is in fact a long mound of randomstone. The larger pieces are placed on the outer face to afford protection fromdestructive waves, and the smaller sized stones are placed in the in ter ior of thes tructure . This type i s adaptable to any depth, may be placed on an y kind ofbottom, and absorbs the wave energy with l i t t l e ref lected wave action. This typerequires re la t ive ly large amounts of materia l . I f not carr ied high enough, stormwaves may sweep ent i re ly over the je t ty and cause a secondary wave action in theprotected area , and i f the voids between the stone are too large a considerableportion of the wave energy may pass through the s tructure . Cross-sections of tworandom stone st ructures are shown in Fig. 1.(2) Stone and concrete type is a combination of rubblestone and concrete.This type ranges from a rubble-mound s tructure , in which the voids in the upperportion of the rubble are f i l l ed with concrete, to massive concrete superstructureon rubble-mound substructure. The mound is used ei ther as a foundation fo r a high

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COASTAL ENGINEERING

- -Sou th NorlhS---j-----i--40

I IMOUTH OF COLUMBIA R.,OREGON &WASHINGTON 20SOUTH JETTY REPAIRS, 1933

PORTLAND OISTRICT CORPS OF ENGINIi iLER.S

Fig. Iconcrete superstructure or as the main structure surmounted by a concrete cap withvert ical , stepped, or inclined face. This type requires less material , and isused where the foundation is soft or subject to scour. The superstructure may beundermined by wave recoi l down the face; rubble foundations require time to becomepermanently stable and should be placed years before the superstructure:---This--type of Je t ty , when properly designed and constructed, gives very satisfactoryservice. Cross-sections of stone and concrete je t t ies are shown in Figs. 2 to 7.

(3 ) Caisson type: The f i r s t caissons were bui l t of iron but today they arebuil of r e i n f o r c ~ ~ e , fJ oated into posi t ion. settJ ed upon a p r e p a ~ e J L D ) J m . d a -t ion, f i l l ed with stone to give s tab i l i ty , then ca2.2ed with cap stones_or _ ~ ( ) _ l ! 2 ; , e t eslab, and, occasionally, parapet walls are added. Some caissons have a reinforcedconcrete bottom which is an in tegral part of the caisson, while others, such asthe ones used in constructing the WeIland Ship Canal, are bottomless and are closedwith a temporary wooden bottom which is removed af te r the caisson is placed on thefoundation. Caissons are sui table fo r depths up to 35 f t . Foundations are ei therrubblestone alone or piling and rubblestone.--Rlprap-or-heavy stone is used alongside to prevent scour, to provide resistance against sl iding, and to prevent weavin g under wave action. On sand bottom, considerable riprap is required. The top

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DESIGN AND CONSTRUCTION OF JETTIES

/0 'LAND SIDt: CHANNEL SID

la '

SeCTION AT,ST/ lT lON40f25 NORTH JeTTY - SHOWING CONCllET" C/lP

(6 )

TYPICAL SI!.CTION So. WeST J"TTV - STATION 0 1 0 0 fa /5 19 0 -SHOWING CONCRETE CA PFR""PORT H/lRBOR, TeXAS

Fig. 2

r---- 6 'O'.'------ + ~ ~ ~ ~ - 9 ' 3 ' cJ"xtj 'xI6' Sfr ingers

/ - ! "x I6"dr i r ' f bolf eachend of ' sfrlnger3-.3")( I 'p lonks

Sf",el p;;ing fo be burned 'of ' r a r t e r pouring top /i1'1

/2 "x/2"x/8'2-/"x.36M bolfswifh woshersafeach pi/e_

(a)

: I ~ - - t - - - t - H o o k J ' bolfs over! " rodsfa outs ide .sheet pil ing,Vole

, I.... - -- +-2'6'-1

Timber f resr /s fo b9 r e m o v ~ daflfer c o n s . f r u c f / ~ n . Crossbraces in fer rer ing wifhconcrert!l seeflon foberemovedbef 'ore pouring_

Fig. 3. Typical Section, Nome Harbor, Alaska - - West Jet ty229


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I15

M.L.G.

15 'I

COASTAL ENGINEERING10'I

. " -0.," ..10 'I

------------_._-- Mean Low Gulf '

I10

TVPICAL SECTION OF JETTYI5 Io

Fig_ 4

Q

-!Q'

-M'

I ,/0 5f..'

Entrance to Mississippi River, Southwest Pass and South Pass

Plvg voids wi;h n o f i v ~ rockf'lrom ground / i n ~ up 0 $ 0 coreand fa suppor f c o n c r f ! l f ~

Fig_ 5Typical Section of Concrete Capped Jet ty , Lake Worth In let , Florida

230

(a)

(6)


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TYPICAL S ~ ~ 7 " . ' O N AT QU7"ER

E.mbedded bloCKSprot-rude Deyona' u r P o c ~

Fig. 6. North Jet ty -- Humbolt Harbor and Bay, California

I11 .......v-..." .Approximafe 1/fop of ' old II 1/enrocAmenf; 1/ 1/ ........ T. ....:.. ' " " ~

, . . . . . . . , . . r ' . . . " N ~ , - f " ' . . . . . . , ...... r . " ' ~ Jj/4"'\ I .. II i ' IS Fe ; /ON,4T OUTER END

(0)

OUTER ENDFig. 7. North Je t ty - - Umpqua River, Oregon

231

E/eV. /.5.0

(0 )

(b)


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COASTAL ENGINEERINGof the foundation rubble is dressed with crushed stone and leveled by a diver before the caisson is placed. Periods of calm water are necessary to float thecaisson into position and sink on the foundation. I f properly designed and placed,caissons are sat isfactory. Cross-sections of concrete caisson structures areshown in Fig. 8.

(4) Sheet pile types include timber, concrete, and steel sheet ,pile structures. Timber is not suitable where marine borers can exist . Use of concretepiling is restricted by driving l imitations. Steel sheet piling is used in severaltypes of structures such as a single row of pil ing, with or without buttresses;two parallel rows with cross walls, and the cells thus formed f i l led with suitablematerial; and cellular s teel sheet pile structures. The cel lu lar type structureis widely used for breakwaters in the Great Lakes area. The l i fe expectancy ofs teel piling depends upon water conditions at s i te . Steel piling may be used onany foundation where piling can be driven, permits rapid construction, but is subject to damage by sudden and unexpected storms during construction. Details andsections of s teel sheet pile structures are shown in Figs. 9 to 11.

(5) Crib types are bui l t of timber, and some of the compartments are floored.The cribs are floated into position, sett led upon a prepared foundation by loadingthe floored compartments, af ter which a l l compartments are f i l led with stone. Thestructure is then capped with a timber superstructure which is usually replaced byconcrete when the timber decays. Stone-filled cribs can withstand considerable

~ y ~ - - . - - 24' - - - - - - - !TYPICAl. SEerlem OF CAISSON JETTY ON RUBBLE

FOUNDATION

TYPICAL SC TlON OF CAISSON JETTY ONPILE FOUNDA TlON

Fig. 8232

(0 )


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.-

"\)' ~ p t ' ~ - f ( ; z . .1" ~ . " " " "~HEErPIU JFrTYBl8.

.1JrBJ:tfPROTECT/ON iT Fr.He!?EPENSIICOLR cm:,fl.R.

~ ~ ' h Y "o-v.'

Fig. 9. Sheetpile Jetty fo r Beach Protection at Ft: McRee, Pensacola Bay, Fla.

,wc.s &/1I1ehuIpi'If'fHellol ' !'5 GfJd 2 p ' ' ' u ~ T J>ilts,oflU'5'/onf

1 J w ! u . ~ ~ ~ . c a e

.. , . 1...: 0-" ' ;...... n"

I ~( J ~ / : : :

r&G-oy.Dnd--_hom. _cfiar .)2-- . ... . . .

,;-'''"Y'''{ ...f ,--;r!',7

3o"" f i l l I

I

I

Fig. 10. Details of Steel Sheet Pile Jetty233

, ...oW

11111r-!

r--


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COASTAL ENGINEERINGf----____ V . a r i o b / e

4" Asphal f ic c o n c r e f e ~ 1 ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~ = - = l = ~ . ~ . D . /570.05'

TYPICAL SECTION A' A

-,.-----+-- ---A

TYPICAL PLAN

Fig. 11 . Circular Type of Cellular Jet tysettlement and racking without rupture. Such structures are suitable for depthsup to 50 f t . or more. Foundations are the same as for caissons but do not requiresuch careful dressing. Settlement of foundation will bleed stone from the unfloored compartments but arching of stone over a cavity may prevent detection ofstone loss by visual inspection. Timber structures are not suitable for sal twater where marine borers may occur. In fresh water, timber-crib structures givelong and satisfactory: . s e r v i c ~ _ . _ - - -

(6) Sol id-f i l l je t t ies are sometimes required to stop sand movement as wellas direct currents. A core of well-graded stone, having a minimum of voids, witha cover of larger stone and an armour of heavy rubblestone is a common type.Caisson and sheet-pile structures are two other types of sol id-f i l l structures.(7) Asphaltic materials have been used to f i l l the voids of rubblestonestructures above the low-water l ine. The record of such structures is not impressive (see Fig. 12).

FACTORS AFFECTING DESIGNThe physical characteristics of the si te must be determined by hydrographicand topographic surveys and sub-surface investigations. Surveys should extend upand down the coast to provide a complete record of conditions before construction,

for future needs of comparison. Usually the fundamental problem is to create andmaintain a sat isfactory channel across-the bar. Two je t t ies are necessary exceptunder very unusual conditions. In determining the area of sectIon between theje t t ies , requirements of navigation as well as t idal and stream flow must be con-234


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TYPICAL SeCTION -STONE AND ASPHALTIC CONCRETE JETTYLAKE WORTH INLt.T, {"LORIDA

10.0 Fl IT "UILT AT OUTIR /10 0, NORTH JETTY

TYPICAL SECT/Ot{ O,. /NNER f)oRTIOJII 0,. SOUTH JeTTYGALVESTON HARBOA. TEAAS

Fig. 12considered. On small streams or bays, navigation may require channels considerablyin excess of the ideal hydraulic section. The natural movement of sand along thecoast requires thorough study, as do wave forces developed a t the si te and the i rdirect ion of approach. These subjects have been discussed a t this conference andwill not receive fur ther consideration in this paper. Model studies of ocean entrances have not consistently given rel iable results and, while the technic ofmaking such studies is improving, their results must be accepted with caution.Selection of the exact s i te for the je t t ies i s a big problem, often not given suff ic ient study. The cost of construction should not be the sole cri terion of je t tylocation. Local sources of materials is a factor in design, often a controll ingone.

DESIGN AND LAYOUT OF JETTIESDesign of a je t t ied entrance resolves i t se l f into two p r o b l e m ~ ; namely, f ~the fundamental one of si t ing the structures_so as to create and maintain a sat i s -factory channel't'hroughtheC'entra:nc"e';-'and"second, to res is t the forces _created byocean storms with a minimumamouiiCof maintenance. B o t h p r 0 1 5 1 e ~ r n v o l v e many-factors diff icul t to evaluate. As a preliminary step in making a layout for je t -t ies a t the entrance to any harbor, a careful study with f ie ld observations shouldbe made of direction of t ida l flow, both flood and ebb. On the north PacificCoast where there is a wide dirunal inequality in the t idal range, the ebb is thestronger and predomipant influence. The direction of storm waves should also beconsidered. I n _ g ~ p e r a l , works should be la id out to conform with the main ebbcurrent, rather than to oppose th is , with a view perhaps to reducing wave actioni n ~ t h e channel. ' ,, ' ---- , ,,'- ------- ,----- - - - - , - - - - - - - - - - - - - - , - - - ~ - - - -Several attempts have been made to determine th e relation between the t idalprism and t h ~ e c t l Q D a l a r ~ ~ Qf toe e ~ n G e c b a n ~ l for natural m s l n t ~ n a n c e o ~

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COASTAL ENGINEERINGthe channel without excessive currents. In a study of the relation of t ida l prismsto channel areas of waterways on the Pacific Coast O'Brien (1931) proposed theformula:

A = 1000 V0.85 (1)-where: V = volume of the t idal prism between MLLW and MHHW insquare mile-feet.A = area of the entrance channel section below mid-tide insquare feet .

A study of the Sacramento, San Joaquin, and Kern Rivers in California (U.S. Congress, 1934) yielded a rat io of 1.08 square feet of sect ional area per acre-footof mean t idal prism for unrestr ic ted es tuaries and a value of 0.82 fo r restr ictedestuaries . Other values have been suggested for the ratios between the sectionalarea of discharge channel and acre-foot of t idal prism, but no one value is general ly applicable to a l l entrances and results are often disappointing. As pointedout by M. A. Mason in a report of the Committee on Tidal Hydraulics (1950), "Studyof the je t t ied entrances of the United States shows that the predicted results ofthe improvement as to entrance channel conditions were actually achieved in veryfew cases, either with respect to concentration of flow or protection."

Where a natural section can be found near the entrance, as is generally thecase, more dependence should be placed in this than in theoret ical or computedsect ions. The section produced by nature has been developed and maintained throughout the years and represents the effect of a summation of a l l the various influences, both favorable and unfavorable, which are at work in the vicin i ty . Themain ebb velocity needed for the maintenance will be found to be much higher thanis required for channel maintenance in non-tidal waters.

Navigation requires the channel to be of suff icient dimensions, proper direct ion , and to be easy to enter and follow. Vessels should not be required to movealong a weather shore when entering or leaving the entrance, and they should nothave a beam sea in a narrow channel. These reqUirements f ix the direction of thechannel in the quadrant of the prevailing heavy storms. The effect upon adjacentbeaches must be carefully studied and, i f possible, the f inal design should in clude features to prevent deteriorat ion of the adjacent beaches. Arti f ic ial feeding of the down dr i f t beach may be necessary. I f the channel axis is approximatelyperpendicular to waves from the h e a v ! . ~ ~ .:LstorIl\!'l. I m ~ j i J f L e : r r . r 9 J . 1 L l ' I : ; t J 1 ~ d l r e c t : t , Q l l - 0 -the tendency of the sand to be driven into the channel will be minimized. Severestorms from different-direct ions wil l require a compromis-e-.--pap-er:sc;-nllie naturaland ar t i f ic ia l movement of sediment have been presented in Part 3 of these proceedings and the subject will not be covered in this paper.Since cost of transporting materials to the s i te is generally a large percentag e of the cost of the work, a thorough search of the loca l i ty should be made formaterials suitable to use, such as natural rock of suff icient hardness and strengthto res i s t wave action.Wave action is the most important source of the forces which a je t ty must re

s i s t . The fundamentals of wave theory and the development of basic design datahave been covered in Part 1 of these proceedings and further treatment of thosesubjects is considered unnecessary. With wave character ist ics determined, thetype of je t ty and the depth of water a t the structure both enter into determiningthe wave force applied against the s tructure . I f the water is deep enough forwaves to reach a ver t ical surface, approximately perpendicular to the i r l ine oft ravel , before breaking, a wave will r i se against the ver t ical surface to abouttwice i ts normal height without breaking and be reflected back against the followin g wave with an apparent cessation of horizontal movement. The resu l t is a standing wave which osci l la tes vert ical ly against the surface with the same time periodas the open-water wave but about double i t s normal height. This phenomenon ofoscil lating waves is known by the term "clapotis ." At the Internat ional NavigationCongress held in Cairo in 1926, the term "standing wave" was adopted for the Frenchterm "clapotis" but since the term "standing wave" is employed to describe the waveof the hydraulic jump in s t i l l ing basins in dam design the term "clapotis" is genera l ly used. Reference is made to Chapters 22, 23, and 24 for a discussion of thedifferent methods of determining the forces of an oscil lating wave.

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DESIGN AND CONSTRUCTION OF JETTIESIn the United states je t t ies are seldom constructed with vert ical walls extending to depths sufficient to avoid breaking waves. I t is common practice inthis country to construct rubble-mound je t t ies and most of those which have anupper section with a vert ical or stepped face are of the stone and concrete typehaving a rubble mound extending from sea bottom to about mean low water with theconcrete Section above that elevation. Such je t t ies have generally given sat i s -factory service, especially i f the rubble mound is able to res is t the wave action.

OccaSionally, the rubble mound may need some repairs but i f such repairs are madewhen necessary, the concrete super-structure will generally not be injured.Most je t t ies are subject to breaking waves in some portion of the ir length.This may not be the case when the je t ty is f i r s t constructed, but due possibly tothe upsetting of natural forces, changes in the bottom configuration may produceconditions which will cause breaking waves. The extreme shock pressure which mayresult from a wave breaking against a vert ical face may be avoided by a steppedface which breaks up the wave by absorbing the wave energy in a series of lesserimpacts which are not simUltaneous. A sloping wall may pass the wave over thesuper-structure and cause injury to the channel slope of rubble.Rubble-mound types of marine structures have been constructed for thousandsof years but there has been a dearth of engineering l i terature dealing with the

design of such structures . Design of rubble marine structures has, in general,been based upon th e behavior of existing structures and the experience of the designing engineer. European engineers have favored f la t side slopes while Americanengineers have consistently used natural slopes ranging from 1 on 1.1 to 1 on 1.5.No attempt was made to theoretically determine the size of stone or the slope tobe used unt i l d ~ ~ ~ ~ ~ ~ ( 1 9 3 3 ~ u b l i s h e d the following formula to determine theweight of individual stones required for stabi l i ty on the side slope of the structure c o n s i d e r i ~ the design wave.

in which:704 H3 s

W = (cot + 1)2 Vcot _ 2/s (s _ 1) 3 (2)W=weight of individual stones, in kilogramsH =wave height, in meterss = specif ic gravity of the stone =angle of side slope with the horizontal.

CJ,. 'r; ~ ~ ~ C ~ ~ \ ' ~ j-n.~ \ C l ' - ' ~

YY) L. " )

de Castro's formula is based u ~ ~ ~ __tle_ f O l l , < : > ! l i ~ _ t h e o r e _ t l c a l considerations andapproximations:(a ) The destructive action of a wave is proportional to i ts energy andassuming as a rough approximation, that storm waves heights are proportional to thei r length, uses H3 as the value of the wave energy.(b) The weight of the stone necessary to withstand the wave energy variesdirectly as the denSity in a i r , and inversely as the cubes Of thei r

density submerged in water or __ __(s-l)3(c ) The stabi l i ty of a stone subject to wave action is inversely proport ional to some geometric function of the slope upon which i t rests .Subsequent to de Castro's work Iribarren (1949) published the followingformula:

in which:= kl H3 sW (Cos _ sin)3 (s _ 1)3

kl =15 for natural rock-f i l l structures, andk2 =19 for ar t i f ic ia l block structures

(3)

and a ll other symbols are as used in de Castro's formula (equation 2). Within thepast few years there has been considerable discussion among coastal engineers regarding the merits of the Iribarren formula.

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COASTAL ENGINEERINGPresenting th e formula in terms of units of measurement used in the Uniteds ta tes , i t becomes:

WI = (cos _ sin)3 (s _ 1)3in which:

WI = weight of individual stone, in tons of 2,000 poundsk '1 = 0.000468 for natural stonesk '1 0.000593 for ar t i f ic ia l blocksH = design wave height, in feets = specific gravity of the stone or blocks = angle of the slope with the horizontal

In recent years Mathews (1948) of the Los Angeles Distr ic t , Corps of Engineers, submitted for discussion, the following formula:6 w H2 T

WI = (w - 64)3 (cos - 0.75 sin)2in which: T =wave period in seconds

w = unit weight of the stone in lbs. per cu . f t . , anda ll other symbols are as given for equation 4.

( 4)

(5)

From observations on hydraulic placer mine operations Rodolf, co-author ofthis ar t ic le , had concluded that the s tabi l i ty of a stone subjected to the actionof a stream of water from an hydraulic "giant" was approximately inversely proportional to some power of a function of the slope that had been generally accepted for determination of earth pressure back of a wall , namely tan(450 -/2),and applying th e resul ts of his mining-experience, he submitted the followingformula:WI = 600 tan3 (450 _ /2) (s _ 1)3 (6)

This formula is intended to contain a small factor of safety to provide for theoccasional individual wave which is higher than the highest wave.The formulas of de Castro (1933), Iribarren (1949), Mathews (1948), andRodolf are of a common type but show a considerable variation of results . As ameans of verifying his coefficients Iribarren (1949) compared slopes determined byhis formulas with a known example ( I r ibarren and Nogales y Olano, 1950). Therefollows a description of data used for this comparison:

"Unfortunately those necessary detai ls of each particular case, andespecially the damages experienced, are diff icul t to obtain. Therefore, inthis study, we are going to make special mention of one of Singular in teres t .

We refer to the interesting compilation on the part of Argel concerningweights of stones or blocks and the ir corresponding stable slopes at variousdepths, af ter repairs of numerous damages to deficient slopes. This outstanding compilation was made by Messrs. J. Larras and H. Colin in the irar t ic le of December 1947 published in the periodical Travaux (see referencesa t end of chapter).From page 609 of that compilation are obtained the following data, showing the corresponding batters , depths, and weights of blocks or stones.Materials

Artif ic ia l blocksNatural stonesn nQuarry waste

Minimum weight(Metric tons)504

1

238

Maximumbat ter5/43/23/22/1

Minimum Depth,(meters) ,- 5-1 114- 18


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DESIGN AND CONSTRUCTION OF JETTIESThere can also be adopted, as a stable upper surface bat ter , that of 3/1formed by 50-ton concrete blocks, adopted for strengthening the North dikewhich, according to the cited art ic le , has withstood perfectly even theworst storm (3 February 1934) ever suffered by the port of Argel and whichdestroyed a large part of the Mustafa dike. Likewise, we can adopt the toedepth of 35 meters, which this North dike reaches."

To measure the accuracy of the other three formulas given above, slopes forthe conditions used by Iribarren were computed by the formulas of de Castro (1933),Mathews (1948) and Rodolf and in the comparison of the results shown in Table 1,the profiles determined by each formula are arranged in the order of thei r accuracy as determined by comparison with measured profile (Fig. 13).

TABLE 1COMPARISON OF CALCULATED AND MEASURED BREAKWATER SLOPES

Measured1111

slope Depths Rodolf Iribarren Mathews de Castroonononon

3 +5 to -5 1 on 3.18 1 on 3.51 1 on 2.19 11.25 -11 1 on 1.37 1 on 1.83 1 on 1.32 11.51.5

-14 1 on 1.55 1 on 1.53 1 on 1.44 1-18 1 on 2.85 1 on 1.67 1 on 2.11 1

+10

0 ~ - - - - - - ~ - - - - - - + - - - - - - - 4 - - - - - - - ~ ~ ~ ~ ~ - - - - - - - i - - - - - - ~

_ 2 0 L - - - - - - - L - - - - - - - ~ - - - - - - ~ - - - - - - - L - - - - - - ~ - - - - - - ~ - - - - - - ~

+1 0

o

-1 0-1 4 m

-18m-20

a SURFACE WAVE HEIGH T 9 7 METERS

~ ~- 5 m ~ . V- I I mL / " /~ /b SURFACE WAVE HEIGHT 905 METERS

IRIBARRENRODOLFMATHEW

Fig. 13

de CASTROACTUAL

~ V

on 2.22on L18on 1.02on 1.07

Comparison of computed slopes with actual slope of rock dike at Argel, usingformulas proposed by de Castro, Iribarren, Mathews, and Rodolf239


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COASTAL ENGINEERINGThe height of wave breaking on the dike had been reported as 9 meters, butI r ibarren deduces tha t the wave was actual ly 9.7 meters and this height was usedin computing the slopes shown above. Then, making new assumptions, a height of9.05 meters is computed by I r ibarren and he recomputes the slopes. The followingtabulation shows the resul ts from the formulas of both I r ibarren and Rodolf.

TABLE 2COMPARISON OF IRIBARREN AND RODOLF FORMULAS

Measuredslope Depth I r ibarren Rodolf1 on 3 +5 to -5 1 on 3.04 1 on 2751 on 1.25 -11 1 on 1.68 1 on 1.211 on 1.5 -14 1 on 1.43 1 on 1.291 on 1.5 -1 8 1 on 1.51 1 on 2.04From the above comparison i t appears that both I r ibarren 's and Rodolf's formulasclosely approximate the real prof i le and the following quotation from I r ibarrencan be applied to both formulas:

"In this way we obtain a theoretical profi le , shown in Fig. 3 ' , similarto 3, even more closely approximating the real prof i le ; but the real ly in teresting fact is that both f igures, whose calculated wave heights differ byless than 8 percent, a degree of approximation tha t we consider di f f icu l t fo ranything practical to real ly exceed, are now authentical ly confirmed by thevery important direc t observation from this interesting compilation. I t isalso now confirmed, undoubtedly, through authentic direc t observation, despi te the Simplifications one is forced to introduce into the complex subjects of maritime engineering that the degree of approximation real ly obtainedis superior to tha t of many calculations of engineering on t e r res t r i a l subJects , in which, even legally, are imposed large safety factors , generallygreater than two and frequently approximating three."A formula, similar to the above formulas is deduced by Epstein and Tyrell(1949). Tests now being conducted by the U.S. waterways Experiment Stat ion will

investigate experimentally rubble-mound breakwaters. The formulas should bechecked by the experimental determinations.The following table gives the angle and value of the term (cos - Sin)3 fo rdifferent slopes.

TABLE 3Slope Angle (Cos-sin)3 Slope Angle (cos-sin)3

1 on 1.5 33 0 41' 0.0214 1 on 3 180 26' 0.25301 on 1.75 29045' 0.0515 1 on 3.25 17006' 0.28981 on 2 26034' 0.0894 1 on 3.5 15057' 0.32381 on 2.25 240 58' 0.1308 1 on 4 14002' 0.38531 on 2.5 21048' 0.1729 1 on 4.5 12032' 0.43751 on 2.75 19059' 0.2139 1 on 5 11019' 0.4825

T h ~ r e are several factors affect ing t h e _ R e r m ~ n e n c e or s tabi l i ty of ,rubblemound structures which are not taken into acg_ount ln tge a b o v ~ f 0 I ' l ! ! l : ! l ~ _ ~ ~ Amongthese may be mentioned:~ l . The angle or direction a t which seas str ike the breakwater. Thisvi tal ly affects the slope presented to the sea.

2. The height of the structure.3. The grading of stone sizes.4. The manner of placing.5. Shape of stones and texture or composition.

At the end of a je t ty or breakwater the seas often st r ike in a direct ion a l -most at r ight angles to the-rIne of the j e t ~ i _ a n d ' t ~ r e s u l t is a running seapara l le l to the rock surface and a slope considered in the formulas as zero opposed~ " " o u : ( . \ l \ f J I " ' H ~ J ~ ,1>, B. C" YI'II\ \ V\' ",,"'"

t\fil.1 Po " '1

. - - - - . - - ~ - - ~ - ~ - . . = > - ~ ~ ~ " " " " " " - - - - - - - - -

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DESIGN AND CONSTRUCTION OF JETTIESto t h _ ~ 2 K ~ ~ _ Q f _ sea. In a ll the formulas proposed this calls for the smallersizes of stone. As a-matte-r--or-rac-t-:hDwever;--the largest stone is required in -such a position. At the end of the south jet ty- l i t mouth of Columbia, althoughbUilt-of the heavlesfs tone available, this cross sea caused a r a v e l ~ O ~ - c ~ t i n g

o f f - ~ f th;-;m;er-:"'stru-cture e n r o ~ k r r 1 e - n t -to low--water- le-veT;at- a-rate-Of"about ---30ej-fh_Rf_ jet ty per annum L un.H:C-SUCh act ion-wa-s prevented by construction-of aheavy concrete supeI'structure t - : : . i ! U i ~ ~ : .-.- --- - n __ _ ~ - - - - - - - ~ - ~ - - - - - --- _H _HSimilarly, a t the mouth of the Umpqua River, heavy "wing" blocks of concrete(136 tons) were poured on a level-rubble foundation at low-water elevation to protec t th e footing of a l ,700-ton main block, yet one of the 136-ton blocks 15 f t . x15 f t . x 8 f t . ) was shifted horizontally 20 to 25 f t . during th e f i r s t storm. Theother blocks could not be observed. These experiences show th e fallacy of depending on small or moderate size stone on f la t slopes i f exposed to a sea which canbreak against or across i t , and throws doubt on the use of functions of the slopeof the structure as a principal determining consideration. I f a breakwater orje t ty is not high enough to prevent the seas from crossing over in force, stoneswill be removed from the crest (zero slope) and the back side of the structurealso will be attacked. The back slopes of concrete capped structures of moderateheight, are often attacked by such over wash.I t is , accordingly, evident that while a rel iable formula for size of stonein breakwaters and je t t ies is most desirable, there are so many inflUencing factorsand construction limitations invOlved, which cannot well be taken into account ina formula, that the problem usually resolves i t se l f into the more practical considerations of th e materials available at reasonable cost and the methods whichcan be used for construction; always bearing in mind that large stone of high speci f ic gravity should be used, i f available, for highly exposed structures. Thelarger th e bet ter .In addition to currents caused by waves, l i t tora l or alongshore currents mustbe considered when designing for protection against scour. Where river currentsare present, special precautions may be necessary to guard against excessive scouralong th e toe and a t the end of th e je t ty . I f the proposed structure will obstruct or change the direction of river currents, scour a t and around the end

during construction may and probably will occur. Where original depth on the lineof the structure is not sufficient to furnish protection to the base, the scour bycurrents around the end of the structure during construction may not be objectionable, as material will be removed without cost and thus permit the base of thes t r u c t u r ~ to be placed at a depth assuring bet ter protection, but will necessitateth e use of more rubble than indicated by the original profi le . g ~ ~ e s ~ n _ r e ~ ~ r g __ _show that as a jet ty was constructed outward from the land connection, scouraround the end caused--depths nearly twice those -shown on the original-profi le , andnecessitated tne use of two o r _ ~ h r e e times as much rubble stone as-tne origlnar

~ o f i l e indicated. Possibil i t ies of such scour should be -investigated in aeterm i ~ i n g the a ~ o u n t ~ o f rubble for the foundation. I f advisable to control the scour,a mat of stone spread over the area in advance of je t ty construction, or a rubbleapron extending a long distance ahead of th e main enrockment, are possibil i t iesfor controlling or preventing the scour. I t cannot be stated with certainty thatsuch scour can always be avoided.

Analysis and design of a jet ty structure follow the usual methods and procedures for design of any structure. R e s i s t ~ ~ c ~ ~ ~ Q v ~ ~ t u r n i n g , safety againstsl iding, and maximum pressure against the foundation are --a-il--invest1gafed- iind designs are developed to satisfy th e requirements as for a land structure, such as adam or retaining wall. I t is assumed that the reader is familiar with th e usualdesign procedure and further discussion of the details of design in this paper isnot considered necessary. The factors that must be considered in design of ajet ty, which do not enter into design of a land structure, are: ~ * t 1 p $ _ Q r _ thejet ty to accomplish desired resul ts yet provide for safe navigation, height ofs t r ~ c t u r e to ~ r o v i d e - p r o t e c t i o n to the sheltered area, avoidance of a locationwher..Lth; - ; ;ve i _YI'iiiJ).1!.:l,l!LulLL.and, _ t h . ~ U ! n g . 1 ~ __iilt. 1 . h l . ! : l _ J ; . h e i ! . ! i 1 i 1 L . 1 t t ; i ~ e _ tliJi-----s t r u c t u ! : ~

241

,I.,' I


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COASTAL ENGINEERINGAIDS TO NAVIGATION

The District Commander of th e U.S. Coast Guard should be furnished the following information: (a ) advice as to authorization of a project involving construct ion of a je t ty , (b) the proposed construction schedule, (c ) maps showing thefinal location of the structure. During construction, temporary aids to navigat ion should be maintained, i f necessary. Information as to installation or discontinuance of these aids should be furnished to the Coast Guard so that such information may be included in "Notice to Mariners" published by th e Coast Guard.Changes in depths and location of channels should be reported to the Hydrographer,Hydrographic Office, U.S. Navy, Washington, D.C.CONSTRUCTION OF JETTIES

Railroad t rest les are often used for the construction of rubble je t t ies inareas subject to frequent heavy sea action, where th e use of floating plant is impracticable. A single-track t rest le will suffice for construction of a je t tyhaving up to about 40 feet top width. The t rest le Should be s e V : ~ ! ' . ! i ~ L ~ ! , e ~ ~ abovefinish grade of je t ty to allow free d u m p i ! ! ( L ~ ! l ~ __ l ~ g ) . _ ~ ~ ! ! , ~ ~ ! " t h ~ - t ~ t l e . Smallrock may be dumped to reinforce th e t rest le bents. Larger rock is handled on f la tcars and may be pushed over th e sides by means of a crawler-type shovel, whichmoves from one f la t car to the next, or standard dump cars may be used. Thelargest rocks used as a f inal covering, especially on the seaward side, are dumpedin the same manner and placed in final position by means of a track crane. Forconstruction by trucks the structure is buil t to approximate finished section asi t progresses into the sea and a roadway is maintained on top of the je t ty , or ona t rest le roadway. The large stones are placed by crawler-type crane. Sectionsof a Jetty section showing use of single and double t rest le and track are shown onFigs. 14 and 15 . Fig. 16 shows the detail of a t rest le .

Rubble SfoneTo oPo/a enrockmenf Elev.aq M.L.l.w.Rock r e ' h i ~ i n e d bu roe w:l""on eQcn ~ i d e

PROFILE

MOUTH OF COLUM 81A RIVER,OREGON AND WASHINGTONSOUTH JETTY TERMINAL

Scale in Feetl o o E H = ~ ' ~ o " " M 5 o ~ = = = ~ ' 0 i i i ; 0 i i i i i O i i i i i O = = : l 2 ! O FT.

PORTLAND DISTRICT, CORPS OF ENGINEERS

Fig. 14242


17/19

//I

Cof>f>erdBm

MoLL. 1'/., lev. 0.0

//

;'//

//

///


8'Ul t ! oflrQ;/s, E/ev.+2B.O

Top of' old enrockmenf

SECTION A-A

SECTION B-B

f

MOUTH OF COLUM BIA RIVER,OREGON AND WASHINGTONSOUTH JETTY TERMINAL

SCALE ( ' . 40 'PORTlAND DISTRICT. CORPS OF ENG INEEfLS

Fig . 15

243

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18/19

,.....,I

C') ~AS't4p c f l o n 81k.(Aboul ' 2 ~ O l . D f I t j )

;fx 2* Iron Simp6 ! ~ ' l o n g 01"Wi" Cabk

COASTAL ENGINEERING4'. 8'" /{,'_Od Guo""; R.a11i'"j' o a . ~ Sr/...es. ~ 7 ' x 8 ' x 9 ~ o " 7 i ~ s S.I-S

/ ..... -- - I f /J2f Mgchme Bolt"~ ~ - - - - ...... - -......... . . . . . . - j 'x /S'K32'-(r' S+nnrl"'S

~ ~ ~ ~ ~ - - r - ~ ~ ~ ~ ~~ - - / ' x.30" f)f"lff 8o/fs

12'x/C,'.JI./2:0" CapI ' K Doff Bofls( 2 d '1 rl- b().lt5 VStJ whef"enecesst7l'fj' Hob -/d,. p//d..-,Ils Z" , . , d ~ . . - Size.

;,----"- 4'1/ 10'1< I S ~ O " 8rac.es-"-- f " ~ f orf'" IZ" Bod Sf,&s

4S milt be I?ecess(u"j

Fig. 16 . Detail of Jet ty Trestle, South Je t ty , Columbia River, OregonFloating plant may be used for the construction of a je t ty in waters wherewave action is not too continuous or where the vessels may work in sheltered wateron the lee side of the structure . This method consists of moving the material tothe s i te in vessels and ei ther dumping the material or placing by floating derrick,Quantities of stone used in je t ty construction may be determined by weighing,or by measurement of volume by cross-sectioning and calculation of weight. Theformer method is preferred. I f the structure is bui l t with f loat ing plant theweight of the stone is usually dej;ermined b y _ ~ e i < ~ c e d on thebarges--tomeasuretha._dis-plac.emenLof._the_ves s e 1 when L o a d e J i . _ a n ~ , t J l l L W J ' ~ i E h t ...Qr._the,.lLtQ!l.Li._C,9J cula ted therefrom.

CONCRETING EQUIPMENTThe methods employed in placing the concrete portion of a composite breakwater or je t ty vary with each individual project, being influenced by such factorsas the magnitude of the project , plant readily available, tran sportation fac i l i t i es , and weather and sea conditions. Depending upon the design of the structure,concrete must sometimes be poured below the mean water level, thus necessitatingthe use of extensive protective cofferdam arrangements on a wide stone base.Specifications usually require the continuous pouring of each monolith block tocompletion. This may ca l l for fas t , concentrated ef for t between t ides .In areas of heavy wave action the attack on the structure is often severe between low water and half-t ide level. I t is accordingly necessary to base the concrete superstructure a t elevation of mean-lower-low-water or lower, i f practicable.

244


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DESIGN AND CONSTRUCTION OF JETTIESThe equipment and materials involved in concreting operations should be located conveniently close to the structure s i te . To accomplish the desired proximity, construction of access roads may be necessary. Stock piles of aggregatematerial in various gradations, as well as dry storage fac i l i t ies for cement shouldbe grouped about a central batching plant. On large jobs careful attention shouldbe given to equipment and arrangements for high speed economical operation.For the most efficient control of operations, th e mixer should be reasonably

close to the placing area. For small sections employing a single tramway, a sideplatform supported by piles has been used. For larger sections, with a tramway onboth sides, th e mixer usually can be supported by a platform between th e tramways.On large jobs requiring large daily placement from t res t le work i t may be foundimpracticable to provide sufficient storage space at the si te and mixed concretemay have to be transported a considerable distance (3-1/2 miles on railway cars atmouth of Columbia). The concrete is transported by dump buggies, trucks, or ra i l -way cars, using inclined chutes for placement to avoid a free vert ical drop ofmore than about 5 f t .In closing this paper i t may not be amiss to again quote from Iribarren whenhe states:

"However i t is not logical to apply s t r ic t results , obtained by means ofth e application of theoretical formulas to sea conditions when i t is the general rule to employ ample factors of safety for land conditions."

REFERENCESCorps of Engineers, U.S. Army (1950). Evaluation of present state of knowledge of ~factors affecting t ida l hydraulics and related phenomena: Committee on TidalHydraulics, Report No.1 , p. 35.de Castro, E. (1933). Diques de escollera: Revista de Obras Publicas, April 15,1933.Epstein, H., and Tyrell , F.C. (1949). Design of rubble-mound breakwaters: XVIIInternational Navigation Congress, Sec. I I , Communication 4, Lisbon,pp. 81-98.Iribarren, Cavanilles, R., (1949). A formula for th e calculation of rock f i l ldikes (A t ranslat ion): Bulletin of the Beach Erosion Board, vol. 3, pp. 1-16,Corps of Engineers, Washington, D.C.Iribarren, Cavanilles, R., and C. Nogales y Olano (1950). Generalization of theformula for calculation of rock f i l l dikes, and verification of i ts coefficient: Revista de Obras Publicas (Translation available at Beach Erosion Board,Washington, D.C.).Larras, J . , and Colin, H. (1947-48). Les ouvrages de protection du port d'Algera talus inclines: Travaux, vol. 31, pp. 603-609: vol. 32, pp. 163-168.Mathews, W.J. (1948). A re-evaluation of rock size formulas for rubble breakwaters: Los Angeles Distr ic t , Corps of Engineers, (unpublished).O'Brien, M.P. (1931). Estuary t ida l prisms related to entrance areas: CivilEngineering, vol. 1, pp. 738-739.U.S. Congress (1934). Sacramento, San Joaquin, and Kern rivers , California:House Document No. 191, 73rd Congress, 2nd session.

jetty design

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