Crescent City Harbor Information

8/12/2019 Crescent City Harbor Information

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PREFACE

The Division Engineer South Pacific Division orps of Engineersinitiated a model investigation to determine the optimum design otetrapod cover layer to stabilize a breakwater section at Crescent CityHarbor California, in multiple l tt r dated 3 February 1953. The Chiefof Engineers authorized the Waterways Experiment Station to conduct thetests in a letter dated 29 July 953Q _ se tests were performed as apart of Civil Works Investigation o. 815 Stability of Rubble-moundBreakwaters. Model investigationthe Crescent City Harbor breakwaterwas conducted during the period August 1953 to December 1953 and testresults were forwarded to the Office Chief of Engineers, in third in-dorsement dated 31 December 1953 to letter from the District Engineer,San Francisco District, dated 11 December 1953 subject: Repair ofCrescent City Breakwater with Tetrapods.


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iiCCNYI 3TINTS

PREF ACE . . . . . . . . . . . . . . . . . . . . . . . *. .

PART I: INTRODUCTION ...... ..... . . .Description of Harbor . . * . **History of' Breakwater Construction and Repair .i . . . .The Problem . . ..... . . . t *f * * * *P'urpose ofModelStudy y . ,

PART II: THE MODEL . . . . . . . . . . . . . . *.Description . . . . . . . . . . . . . .PART III THE TESTI GOG R M . , ...Testing Procedurxe . . ,..... .. . . . PARTly: RESULTS OF TESTS .........PART V : CONCLUSIONS AI~D BECONiENDATIONS . . . . . t . . . .

Conclusions ... . * 9 9

Pagei

3

78810111212


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iii

SUMMARY

he investig tion reported herein was performed to determine theoptimum design of a tetrapod cover layer for a breakwater section pro-posed for construction between sta 34 00 and 37 00 of the Crescent CityHarbor breakwater. Quarried rock of sufficient size to insure stabilityof the breakwater was not available locally; therefore it was decidedto investigate the possibility of using concrete tetrapods for cap rockin constructing the breakwater section. The investigation was conductedin a wave flume 12 ft long 5 ft deep, and 5 ft wide using a 1:55 scalemodel of a section of the breakwater.

It was concluded from the results of the t sts that:a The use of tetrapods to stabilize the damaged portion of

the Crescent City Harbor breakwater is feasible.b. Two layers of tetrapods are sufficient to provide ade-

quate protection to the existing rubble mound.


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DESIGN OF TETRAPOD COVER LAYER FOR A RUBBLE MOUND BREAKWATER

RES ENT CITY HARBOR CRESCENT CITY CALIFORNIA

Model Investigation

PART I INTRODUCTION

Description of Harbor

1. Crescent City Harbor, California, is located on the PacificCoast of the United States, about 17 miles south of the Oregon-CaliforniaState line see plate 1 . The nearest improved harbors are a small-craftharbor at the mouth of the Coquille River, Oregon, and the harbor atHumboldt Bay, California. The mouth of Coquille River is about 106 milesnorth of Crescent City, and Humboldt Bay is about 70 miles south ofCrescent City.

2. Existing improvements at Crescent City Harbor shown in plate 1


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outer breakwater along its present alignment for approximately 2700 ft sta 30+00 to sta 57+00) to Round Rock. A Definite Project Report con-cerning this extension of the breakwater to Round Rock was prepared inApril 1946. Construction of the extension was undertaken in 1947, and bythe end of the 1948 construction season from about 1 May to 15 October)the extension was completed from sta 30+00 to slightly seaward of sta40+00. This section was moderately damaged by waves during the winter of1948-1949, which resulted in the design of the breakwater extension be-tween sta 30+00 and sta 57+00 being revised. This revision entailedflattening the slopes from the crown of the breakwater to mllw. Slopeson the ocean and harbor sides were flattened from 1 on 1 1/2 to 1 on1 3/4 and from 1 on 1-1/4 to on 1-1/2, respectively. This work wasbegun in 1949, and at the end of the 1949 construction season reconstruc-tion of about 1200 ft of the extension from sta 30+00 to sta 42+00)hadbeen completed. The reconstructed breakwater extension was damaged con-siderably by storm waves during the 1949-1950 winter season. Also, con-


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4. In view of the high cost of maintaining the breakwater and theunsatisfactory wave-action conditions in the harbor, the Chief of Engi-neers directed that a field conference be held in Crescent City to dis-cuss means for strengthening the breakwater and improving wave conditionsin the harbor. A l tt r report submitted by the District Engineer, SanFrancisco District, to the Chief of Engineers, File 825.4 Crescent CityHarbor), dated 26 June 1951, subject: Recommendations on Crescent CityHarbor Project, presented a general summary of this field conferencewhich was held on 4-7 June 1951. A Definite Project Plan for strength-ening the breakwater was presented in a report entitled Brief DefiniteProject Report on Work Remaining at Crescent City Harbor, California,Corps of Engineers, U S. Army, San Francisco District, 15 January 1953,Serial No 9. This report outlined the improvements considered necessaryto afford the harbor proper protection from wave action, and was in gen-eral agreement with the conclusions and recommendations arrived at duringthe conference of 4-7 June 1951. The report recommended that a) th e


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severe storm at Crescent City in the past 20 years resulted in waves es-timated to be 26 ft high in deep water. It was also estimated that waves20 ft high lasted approximately 34 hours during that storm. Results of arefraction-diagram analysis indicate that waves with a maximum height ofabout 33 ft could occur at the breakwater site. The following additionalfactors pertinent to the problem of designing a revised breakwater atCrescent City, which would be stable under the attack of storm waves,were l isted in the above-mentioned Definite Project Report: a) themaximum weight of stone that can be quarried economically in the CrescentCity area is about 12 tons; (b) the use of floating plant for breakwaterconstruction at Crescent City is not practical; therefore, all stonemust be placed by equipment operating from the breakwater crown (elev +20ft mllw); (c) the available equipment is capable of placing stone a max-imum distance of 120 ft from the center line of the breakwater; and(d) the water depth at the breakwater site between sta 34+00 and 37+00averages about 35 ft at high tide.


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7. Because of the above-described situation, it was desired todetermine the feasibility of using concrete tetrapods to form a pro-tective cover layer for the existing break;cater between et 34 00 and37 00. According to Mr. P. Danel3, Grenoble, France in whose labora-tory tetrapods were developed, concrete tetrapods are much superior toeither concrete blocks or quarried rock.

Purpose of Model Study

8. The model study was conducted to determine the optimum designof a tetrapod cover layer for the Crescent City breakwater. Specifi-cally, it was desired to obtain information concerning a) the optimumnumber of layers in protective cover layer composed of tetrapods andb) the size tetrapod required to insure the stability of the breakwaterfor different slopes and design-wave heights.


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PART II: THE MODEL

Description

9. The model was constructed of concrete sized rock and moldedconcrete tetrapods in an existing concrete wave flume using linearscale of 1:55 model to prototype. The flume in which these tests wereconducted is one of two constructed within concrete tank with commonsump between them. The tank is 18 ft wide ft deep and 120 ft long.Each wave flume is 4 ft deep ft wide and 90 ft in length measuredfrom wave machine to glass viewing window. Photograph 5 is view of oneof the flumes showing the sump and the companion flume on the left theglass viewing window in the right foreground and the wave machine in thefar end of the flume. The model scale was selected after considerationof the dimensions of the flume and the capabilities of the wave machinewith respect to the dimensions of the prototype breakwater and the size


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balancing circuit Brush Universal analyzer, and Brush nRketic oscillo-graph (see photograph 7 . Recordings of water-surface elevations (waveheights) with respect t time were obtared by means of a Brush magneticoscillograph used in connection wit the waveheight gage.

sign

11. After the linear scale had been selected, the model was de-signed and operated in accordance with Froude s model law. Based uponFroude s law, a linear scale of 1:55 Lr) and a specific weight scale7 of 1:1 the followirg model-prototype relationships were derived:

CharacteristicsAreaVolume

Di:ensi ons*

3

Model-prototype ScalesA = =13025r r =L3 =1:166 375 r6,7


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P RT III: TIE TESTING PROGRAM

Testing Procedure

12. Stability tests were conducted in which two three andfour layers of tetrapods were placed on the seaside of an average sec-tion of the existing Crescent City breakwater. The characteristics ofthe prototype breakwater section, obtained from surveys of August 1951and April 1953, are shown on plate 2. The prototype cross sectionsshows a large amount of material on the harborside. This material iscomposed mostly of cap rock displaced from the crown and seaside bystorm waves. In the model this portion of the breakwater section wassimulated by random-size rock of a size approximately the same as class cap rock. The use of this larger-size rock on the harborside slopeprevented erosion due to overtopping waves. Design of the harborsideslope was not the purpose of the model study. Plate 3 shows the ele-


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the breakwater slope below a water depth of -30 ft mllw. The model break-waters were constructed on a sand base in the wave flume. The existingprototype section was first reproduced in the dry flume, and water wasadded until a still water level swl) of 5 ft m w was established. Thesection was then consolidated by the attack of waves about 25 ft inheight. The concrete cap was poured and allowed to set; then the protec-tive layer, from -30 ft mllw to swl, was formed by dumping tetrapods froma bucket held at the water surface. Tetrapods above swl were placed byhand. Photograph 8 shows the model tetrapods used in these tests. Thismethod of constructing the model breakwater was adopted to reproduce, asnearly as possible, a prototype method of side-casting the underwatertetrapods and placing those above water by crane. After each t st thetetrapods were removed and replaced, and the complete t st section wasrepaired to the line and grade of the design t st section.

14. The design wave for a particular breakwater test section wasdetermined experimentally by subjecting the model structure to wave


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1

the maximum testing time was five hours (model). he results of t stests confirmed that a one-half hour period of wave attack was sufficient.

est Conditions

17 Stability tests were performed with waves approaching perpedicular to the breakwater alignment (wave crests parallel to thebre awater). The tests were conducted using 1 4.0-sec-period waves and awater depth in the flume of ft. The most critical storm waves innature are those with a period of about 10.0 sec. The relative depthd/L) for a 10.0-sec-period wave in 35 ft of water is 0.113. This isalso the relative depth for a l14.0-sec-period wave in 9 ft of water.Stability tests to determine the design wave for no-damage were conductedusing two, three, and four layers of tetrapods on slopes of 1 on 2 - 1 on3, and on 14 The weight w), specific weight (r), effective coeffi-cient of friction ia) and per cent of voids (vr) in the tetrapods used


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PART IV: RESULTS OF T T

18 The results of this model study are summarized in the followingta ble:=

Number of

242

234

Height of SelectedDesin vesft

23232325277

29

Coefficient Kin Iribarren s ormula

0.00460.oo460 001460 0090o oc8o0 00720.01180.01050 0085

SideSlope1 on 2 on 21 on 21 on

onon

1 onI on1 on


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PART V: CONCLUSIONS AIRCD ECOMMENDATION

Conclusions

19. The following conclusions may be drawn from the results oftests conducted in this investigation

a The use of tetrapods to stabilize the -more exposed por-tions of the Crescent City Harbor breakwater is feasible.

b. Two layers of tetrapods n the protective cover l yer aresufficiento.

c Considerable overtopping of the breakwater will occur forwaves larger than about 23 ft in height and waves largerthan about 25 ft in height will probably result in damagto the harborside slope unless cap rock of adequate sizeis used to form the backside. The effects of overtoppingon the stability of the harborside slope are also in-fluenced by the shape and height of the crown and theslope of the harbor side.


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13SLECTE RENCE

I. Corps off Engineers, Of ice,waves nd wave pressures*.struction, art CXVI chap.

2.

Chieff off Engineers, Hydraulic design-,niernn anua l Civil Works on8E anDxo .. Julfl 952 Y

Waterways Experiment Station. Stab lityX Io Rube-uomreakwaters. Tecnical Memorandum No. 2w 365 Vicksburg, M iss.June 1953.3 Danel ?ierre, Tetraphods P edn fFurh neec a

Coastal En neering Council on Wave Research, The nineeringF'oundation 1954. ltudson Robert Y,, Wave fforces on breakwaters. Transactions SCE,vol 118 pp 6684.73 1953)


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Photograph 1. Crescent City Harbor California 5 January 9


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Photograph 2 ertical aerial view o outer end of breakwater, January 1951, showingdamaged section seaward of sta 37 00


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Photograph 3 uter breakwater from sta 37 00 towards Round Rock 31 August 1951.Note displaced locks of concrete cap


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Photograpa 4 Outer breakwater from approximately st 34 OO looking shoreward along seasidewhere strengthening s required 31 August 1951


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haotograph 5 Wave lum


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Thotograph 7 Wave height measuring r tus


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Photograph 8 Model tetrapods Wm = 0 212 ib; p = 17 63 tons


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Thotograph 9 Selection of design waves t st section after attack y i-sec waves 23 ft high.Tw o layers of tetrapods placed on l-on-2 slope


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Photograph 10. Test section after attack by 14 sec waves 23 ft high. Three layers of tetrapodsplaced on 1 on 2 slope


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Photograph 11 Test section after attack y 1A~ sec w ves 23 t high Four layers of tetrapodsplaced on l. on 2 slope


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I N iSCALE IN MILES0-

LEGEND6 . DEPTH CONTOURS IN FEET REFERRED

TO MEAN LOWER LOW WATERVICINITY MAP

SCALE IN F T1 0 1000ooo 2

03

H


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CONCRETE CAP CONSTRUCTED IN 1950

+40

HARBOR SIDE

VSEASIDE

ME N LOW R LOWWATER_ ML LW )

8 6 40 20 0 20 40 60 8DISTANCE FROM CENTER LINE IN FEET

VER GE CROSS SECTION BETWEEN ST TIONS 34+00 ND 37+00

BREAKWATER MATERIALSCLASS A STONECLASS B STONECLASS C STONE

PROTOTYPE WEIGHT12 TON (AVERAGE)

2 7 TON LB 2 TONSPECIFIC WEIGHT Y) OF BRE KW TER MATERIALSIS ABOUT 169 LB PER CU FT.

LEGENDPROTOTYPE BREAKWATERSECTION FROM SURVEYS OFAUGUST 1951 AND APRIL 1953ORIGINAL SECTION CONSTRUCTEDIN 1947-1948

CHARACTERISTICS OFPROTOTYPE BREAKWATER

rNI

r B+10

W

2 2

MM

W

JlW 1

300+20cct

+10 Wi-i ww

hi2 0 2

J-30

-40

_ _

+30-t

30


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HARBOR SIDE

TYPICAL TEST SECTION FOREFFECTIVE SIDE SLOPE OF I ON 2

TYPICAL TEST SE TION FOREFFE TIVE SIDE SLOPE OF I ON 3

40 7

+ 0 I-20

-40

-60-80

40

200

-20

-40

-60

0crw0JwZ

w

I-

wU-

w

U

w

40

20

0

-20

-40

-60

-80

SEASIDE


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LESS TH N NO DAMAGE CRITERION

+3 H=23 FT T=14 SEC L=1 FT d/ =0.113

20:P.o

DIRECT ON OF WAVE TRAVEL 10

0

1

2C

0 2 4 60 8 100 120 140 160 180DISTANCE FROM CENTER LINE IN FEET

DESIGN WAVE FOR NO DAMAGEH=25 FT T=14 SEC L=610 FT d =0.113+3C

200..

DIRECT ON OF WAVE TRAVEL10

1

2C

30


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Volume u ft7 14 14 29 28 57 71 43 142 86 214.29 251.94 285.72 357.25 428 58 571 84

Weight tons1.00 2.00 5.00 10.00 15.00 17.63 20.001.07 2.14 5 35 10.70 16.03 18.85 21.381.12 2.23 5.56 11.15 16.70 19.65 22.30

25.00 30.00 426.70 32 10 42 7527.90 33.50 44.60

Thickness of Layers ft

5.5 6.9 9.4 11.8 13.6 14.3 14.97.4 9.3 12.6 15.9 18.2 19.2 20.09 3 11 7 15 8 20 0 22 9 24 2 25 2

16.121.627.2

SpecificWeightslb/cu ft140.0 0.50149.5 0.53156.0 0.56

Number ofLayers

2 4.43 5.94 7 4

Number ofLayers

2 3403 4554 570

212 133 73 46285 179 97 61358 226 122 77

31 2942 3953 49

253342

17.1 18.823 1 25 228.9 31.9

22 182937 31

Dimensions ft

1.11 1.40 1.90 2.400 56 0 70 0 95 1 201 76 2 21 3 01 3 78

1 72 2 18 2 95 3 720 86 1 09 1 48 1 862.36 2.98 4 05 5.100 80 1.01 1.37 1.723 68 4 65 6 30 7 952.23 2.81 3.81 4.801.12 1.41 1.91 2.404.02 5 06 6 85 8 644 48 5 64 7 65 9 62

2 751.384 334 262 135.831 979.105.492.759 89

11 05

2 901 454.574.502.256 152 089.605.802 90

10 4511 65

3.021 514 774 682 346 432 1710.006 o53 0310.8812 14

3.261 635.145 052.536 892 3410.78

6 o3 2511.7213.10

3.461 735 465.362 687.362.1:811.45

6 983 4912.4813.90

3.811.916 005 912 968.102.7212.607.623.81

13 7115.30

ELEVATIONNOTE DATA BASED ON TETRAPODS USED N MODEL TESTS

CONDUCTED AT TH W TERW YS EXPERIMENT STATION.

SECTION AA TETRAPODSVOLUME WEIGHT THICKNESS OFLAYERS AND DIMENSIONS

/

Number f Tetrapods er 1 000 sq ftBOTTOM

Symbol

0 880 441 391 370 691 880 632 931 770.893 183 56

rIII

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Crescent City Harbor Information