Principles and practice of harbour construction

Front matter

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Preface

The object of this work is to set forth the general principles which should govern the design and construction of harbours^ and to describe the various types of breakwaters and the methods of constructing them usually adopted in modem practice.

Drawings of different kinds of breakwaters, including some of the more prominent existing works, are given ; and, in addition to these, numerous diagrams and sketches have been introduced wherever it has been considered they would be useful in elucidating the descriptions given in the text.

The chance of successfully opposing an enemy depends much upon knowledge, previously obtained, respecting his strength and mode of attack, and of the means and materials available for purposes of defence. It is, therefore, of importance that the harbour-engineer should be well informed respecting the forces against which he will have to contend, and also that he should possess a^ accurate knowledge of the structure and properties of the different materials which are available for use in harbour construction. The subjects of wind, waves, tides, currents, and materials have therefore been treated of at some length ; and attention has also been directed to the more important physical conditions which affect harbour construction.

"The wide range of subjects which the profession of a civil engineer embraces, renders it imperative on every member of it to avail himself of all the help he can obtain.

" The personal experience of one individual cannot fit him for the exigencies of a profession which is ever extending its

VI PREFACE.

range of subjects^ and is constantly dealing with new and complex phenomena ; consequently, nothing is more certain than that he who wishes to reach the perfection of his art must avail himself of the experience of others as well as of his own, and that he will not unfrequently find both little enough to guide him " (" Presidential Address of the late Sir John Hawkshaw to the Members of the Institution of Civil Engineers/' Min. Proc. Inst C,K, vol. xxL).

Kthis be tnie — as it undoubtedly is— of the profession of civil engineering generally, it is eminently so of that admittedly difficult branch of it which deals with sea-works. I therefore feel that I need offer no apology for having consulted numerous works, or for having collected information from all available sources, in compiling the present volume. It is not, however, a mere compilation of knowledge derived from the writings of others, for it includes the results of my own research, and of experience which I have gained in the course of a practice extending over a period of well-nigh thirty years.

In consequence of my time having been very fully occupied by my professional duties, much of what I have written has been jotted down in a somewhat fragmentary manner; and at times many weeks, and even months, have elapsed without my having been able to write anything at all, or even to put my notes into shape. I therefore beg the indulgence of my readers, and trust they will deal leniently with me for aU my shortcomings.

I have endeavoured, by footnotes or otherwise, to record the various sources from which I have gleaned information, but I fear that in some instances I may have inadvertently omitted to do so ; I now therefore very gratefully acknowledge my indebtedness to all those whose works I have consulted in the preparation of this book, and I also tender my sincere thanks to my professional brethren and others who have kindly furnished me with much valuable information, which could not have been obtained from other sources.

WILLIAM SHIELD.

Petebbbad, N.B., 1895.

The following is a list of the principal books which have been consulted in the preparation of this work : —

" Weather." Aberoiomby.

** The Law of Stonns and of Variable Winds." Beid.

'' The Law of Storms." Dove.

'* The Physical (Geography of the Sea and its Meteorology." Hanry.

"Temple of Serapis." Babbage.

" Tides and Wares." Airy (Ezoerpt '< Enoyolop»dia Metropolitana.")

" Encyolopsdia Britannioa."

''Ohambers's Encyolopsddia."

MinutM of Proeeedingi of ihe InttUiUion of Civil EngineerM.

"Harmonic Analysis of Tidal Observations." Baird and Darwin (Iftntftot of

Proceedings of the Boyai Boeiety^ vols, xxxiz. and xlv.) TranaoGtiofM of the Boyal Society of Ediv^rgh, Minutea of Proceedings of the International Maritime Congress^ 1893. " Notes on British and Foreign Harbours." Bennie. " Harbours and Docks." Haroourt.

** The Design and Oonstmction of Harbours." Thos. Stevenson. The various Blue 'Books containing Beports of Commissions on HarbourSi etc.,

1845-1881 Beport of Committee on the Employment of Convicts in the United Kingdom, 1882. Beport of Committee appointed by the Board of Trade to inquire into the question

of Electrical Communication between Light-vessels and outlying Lighthouses

and the Shore, 1887.

" Building Construction," Part III.

" Chambers's Journal."

"* Civil Engineering." Bankine.

" Voyage of the ChaUenger.** Thomson.

" Blasting and Quarrying of Stone.** Burgoyne. '' The Ice Age in North America." Wright. " Engineering."

Contents

OHAPTEB L

Wind the generator of sea-waves — Working season— Wind diagrams— Dixeotion and velocity of wind— Gyolones—Anti-oyolones— Extent of storm area»— Pzogzess of storm»— Wind-ganges— Belation of windvelocity to Pressnxe— Relative frequency of gales on the British coasts- Forecasting gales— Monsoons— Significance of olonds, etc — ^Lnnar rainbows — Halos — BoUers arriving before gale — Harbonr entrances and diieotion of littoral drift affected by wind— Arresting wind-blown sand .

GHAPTEB IL

Chapter I. Wind

Wind the generator of sea- waves— Working season — Wind diagrams— Direction and velocity of wind— Cyclones — Anti-cyclones — Extent of storm areas— Progress of storms^Wind-gaugee— Relation of wind velocity to pressure — Relative frequency of gales on the British coasts— Forecasting gales— Monsoons- Significance of clouds, etc^Lunar rainbows — Halos— Boilers arriving before gale— Harbour entrances and direction of littoral drift affected by wind — Arresting wind-blown sand.

Since wind is the generator of ordinary sea-waves, almost all the troubles which beset the construction of sea-works are attributable to it.

It is not possible to foretell the advent of heavy seas, except by forecasting the wind which produces them ; ^ those in charge of sea-works will therefore do well to anticipate gales, to the best of their ability, and by taking such precautionary measures as may be deemed necessary, or as may be within their power to adopt, prepare in time for the heavy seas which they know the wind will bring up. Work should also be arranged so as to take advantage of what is usually termed the " working season," being that part of the year during which gales are least frequent and the sea smoothest.

This period will be found to vary much on different coasts, according to the extent and direction of their exposure ; and in different parts of the world, according to the season of the year.

The direction of littoral drift is, in the great majority of cases, determined almost solely by that of the prevailing winds ; and there are other considerations, to which reference will be made in the sequel, which render a knowledge of the general direction, strength, duration, relative frequency and sequence of the winds prevailing in any locality where the construction of sea- works is contemplated, very necessary.

* In regard to sweU Bometimes preceding storms, see pp. 19, 20.

B

I propose, therefore, in this chapter, to direct the reader's attention to that section of meteorology which treats of wind, as having a direct and important bearing upon the main subject of this work.

Diagrams, constructed so as to show at a glance the prevailing daily direction and force of winds, extending over as long a period as possible — which should, at the least, embrace one whole year — will be found useful in considering any design for sea-works. The patterns of such diagrams are multiform, but the following (Figs. 1 and 2) are given as examples.

SCALE OF DAYS 5 10 15

\ part qf an inch, measured on tfu radial lines, equals one day. Tkust dufTHg the months of April, May, and June, the directum of the wind vmu due north on 6 days, and due south on 19 daj^s.

Lines thus indictUe observations /or /anuetryt February ^ March, f, ,, April, May, June.

„ „ July, August, September.

„ „ October, November, December.

Fig. I.— Wind diagr«m for Peterhead, year 1889.

WIND. 3

The form of diagram represented in Fig. 1 shows, fairly well, the direction and relative frequency of prevailing winds at different periods of the year ; but it lacks one most essential feature, inasmuch as it gives no indication of the force of the wind. Thus, it may show that, say, southerly winds are the most prevalent in any given locality, while it altogether fails

Beaufort Scale

Force op

Wind (sec p. ii).

Frcih

\MtnitfAtt iHiU.

f\rfyft

Obsesvations taken daily

AT 9 A.M.

Each line thus representt

one observatum. Its Unftk

indicates the force of the wind,

a scale for ifu measurement of ", — ^^ ->'%

which is given op^site. ' ^ — ^ //. * m ,t »e

Fio. 3.~Wind diagram for Peterhead, year 1888. Each zone embraces observations for three months. Zone A, January, February, March; Zone 6, April, May, June; Zone C, July, August, September ; Zone D, October, November, December.

to show that those possibly from the south-east, although less frequent, blow with the greatest violence. This is an all-important point for the harbour engineer to be correctly informed upon, and, with the view of supplying this deficiency, I, many years ago, designed the form of diagram shown in Fig. 2, which explains itself.

Such diagrams may be amplified by showing the limits of exposure of any site which may be under consideration, the direction of the heaviest waves, and so forth.

If it should be desired ]/o show the winds separately for each month, it will be found convenient to distinguish the several months by means of colours instead of zones. Thus, for instance, the observations for January, February, and March may be shown by three shades of green ; April, May, and June, by three shades of red ; July, August, and September, by three shades of brown ; and October, November, and December, by three shades of grey.

In order to show to how great an extent the relative frequency of certain winds may vary from year to year, and the consequent necessity for obtaining information which embraces as long a period as practicable, I have compiled the foregoing diagrams from observations taken at Peterhead during two different years.

The generally accepted theory of wind is so well known that I shall not occupy the time of the reader by describing it here.^ Suffice it to say that the never-ceasing effort of the atmosphere to equalize temperature and restore lost equilibrium all over the globe, and the rotation of the earth, are the two principal forces which affect the direction and velocity of the main atmospheric currents.

There are, however, many minor forces or influences at work, which produce changes in the direction and velocity of the wind ; such, for example, as the variation of day and night temperature, the distribution and trend of land, mountain chains, clouds, ocean currents, drift ice, and even the contact of one aerial system with another.

Ocean currents, being slow to part with heat, and being often thrown far out of the courses they would naturally pursue, by the irregular distribution of land and other physical conditions, affect, to a very great extent, the temperature of the atmosphere along their course.

A change in the' temperature of the air is synonymous with a change in its pressure ; consequently, if the air in any locality should be warmed, and thus rarefied by contcbct with a warm ocean-current, an upcast aerial current will at once be formed, and the colder air from surrounding districts will flow in to re-establish equilibrium. Thus vapriations in temperature,

' The nibject is very fally treated in a book to which I shaU hereafter refer, viz. "Weather," by Hon. Ralph Abercromby (Kegan Paul & Co.).

WIND, occurring in different localities, create currents, counter-currents, and eddies, which resolve themselves into those cyclonic and anti-cyclonic movements to which the atmosphere seems ever prone. Associated with these we find V-shaped areas of low pressure, " wedges " of high pressure, and so forth.

Since an ascending current is composed of light air, which is therefore indicated by a low barometer, wind always blows in upon a centre of low pressure. This it does, with but few exceptions,^ in a spiral form.

The reverse takes place around areas of high pressure or anti-cyclones.

Cyclones are analogous to upcast shafts, drawing the air inwards at the bottom, and throwing it outwards at the top ;

30 tns.

^Po apo

30 tns.

ZSjMS.

28 ins.

Fig. 3.— DUigrain ' of m«an barometric curve across the centre of the great Calm harrlcane, transveraely to its pUh, October i to 7, 1844. The mean barometric curve on the centre line of the path of the storm appears not to have differed essentially Arom the above.

while anti-cyclones are analogous to downcast shafts, drawing the air inwards at the top, and dispersing it outwards at the bottom.

Cyclones are indicative of bad weather and high winds, and, in the northern hemisphere, revolve in the reverse direction to the hands of a watch; whereas anti-cyclones are associated with fair, calm weather or light winds, and revolve in the opposite direction. In the southern hemisphere, the direction of rotation, both of cyclones and anti-cyclones, is reversed.

In Fig. 3, a barometric section of a storm-basin, or cyclonic depression, is indicated.

The areas of storm-basins are sometimes very great, measuring not unfrequently from 1000 to 1500 miles across, and seldom

* E,g. the trade winds, monsooiiB, electrical disturbanoea, and what are termed *< straight-line gales."

* Copied from Beid's "Law of StormB," p. 312.

HARBOUR CONSTRUCTION.

being less than 600 miles. In such systems, however, the inner or more dangerous portion is of much smaller proportions, probably not exceeding 200 to 250 miles in diameter.

A typical cyclone, having its centre over England, is represented in Fig 4.

The rate of the progress of storms, i.e. of the cyclonic areas, varies considerably. At times their forward movement does not ii!iiiM^ilii''iii'V'iiiiVh^'!i^-iii^^^

Fio. 4.— Diagram of typical cyclone. The arrows indicate the direction of the wind. The curved lines are laobani. or contours of eqoal barometric pressures, varying, in ibis case, fix>m 38*8 inches at the centre of the storm to 30 inches at its outer limit.

amount to more than 2 or 3 miles per hour, while at other times they travel at from 40 to 50 miles per hour; 17 to 25 miles per hour seems to be a common rate for European storms.

Tropical cyclonic hurricanes generally travel at a slower speed, but rotate at a much greater velocity than extratropical ones.

The progressive motion does not indicate either the direction

WIND, 7

or velocity of the wind, both of which are quite independent of the course along which, or rate at which, the storm advances.

It would, therefore, be erroneous to imagine that the velocity of the wind during a gale, multiplied by the duration of the gale, would give a correct idea of the extent of such disturbance \ otherwise a gale of, say, three days' duration, with an average velocity of 50 miles per hour, would have travelled a distance of 3600 mUes, whereas the actual distance traversed would probably not exceed ode-third of this, or it might even be very much less.

The direction and velocity of wind are most conveniently recorded by means of graphic wind-gauges, or anemographs, in which a slewing motion, for indicating direction, is combined with the ordinary cup anemometer by means of tangent screws actuated by fans.

The lines indicating direction and velocity are drawn upon a paper attached to a cylinder, which derives its motion from clockwork.

Some of these machines give continuous diagrams, so that the direction and velocity of the wind at any moment during the day or night may be read off at leisure.

Another description of wind-gauge is that in which pressure, instead of velocity, is recorded; but as yet these have not proved at all reliable.

Relation of Wind Velocity to Pressure.

At first sight, it would appear an easy matter to establish a comparison between the velocity of wind and the pressure exerted by it ; nevertheless, up to the present time no satisfactory method of doing this has been devised.

The chief difficulty seems to lie in the fact that the apparent pressure per unit of area varies with almost every size and form of surface against which the wind impinges. This is doubtless in a great measure due to the varying degree of vacuum which the wind current creates in the rear of the objects acted upon. However this may be, all the best authorities on the subject are agreed that, at present, there exists no satisfactory method of establishing a relation between wind velocity and pressure ; indeed, it seems doubtful if such a comparison be possible.

I cannot do better than here quote what Abercromby sayB on this subject in his excellent work before referred to. On p. 202, he states as follows : —

'' The Telocity of wind is a real qaantitj, which is perhaps capable of measurement in the abstract, though we are at present far from being able to gauge it accurately. But it is quite certain that there is no such thing as an absolute force which corresponds to a given velocity.

" According to the theory of stream-lines, when even an inelastic fluid meets an obstacle, if the angles of the obstruction do not break the continuity of the fluid, so as to form eddies or vortices, the same amount of pressure which is imposed on the body by the first deflection of the fluid is given back again as the stream-lines of the fluid close up behind the obstruction. For instance, if a ship is lying at anchor in a current^ the same amount of strain which the current causes on her cable when forced asunder by the bows, is given back when the current closes in behind her; so that the total pressure which she experiences is only that due to the friction of the water on her skin. This is, of course, on the supposition that her lines are so easy that they do not break the stream-lines so as to form little eddies or vortices.

" Now, the same thing holds with wind. If we put up two square plates of different sizes, face to the wind, the pressure on each is not proportional to the area, while in light breezes neither will record anything. The reason is that, in light wind, a thin mobile fluid like air can glide round even the sharp angles of a square without forming eddies, and as there is no vacuum formed behind the plate, there is no pressure recorded. In higher winds the stream-lines are broken, and every shape and every sized plate of the same shape form a different series of eddies round the rim of the obstacle. Then the amount of rarefication behind the various plates is neither identical nor proportional, and therefore every shape and size of anemometer indicate discordantly at every different velocity.

" From all this, it follows that, though we might say that the pressure on a board one foot square was twenty pounds, and might compare this force with that on another board of the same size and mounting, we should not be justified in saying that the force of the wind was twenty pounds per square foot in the abstract, because a board ten feet square, even if of the same shape, would have given a different number."

As further bearing on this subject, I quote the following from Hurst's " Handbook for Surveyors" (14th edit., p. 140): —

WIND, 9

Proportional numbers.

Gone, vertex towards the wind 126

„ base to the wind 291

Sphere 124

Cylinder, end to wind 285

HemiBpheie, flat side 288

„ ronnd Bide 119

The careful records which were kept at the Forth Bridge works have done much to throw light upon this difficult subject. The results obtained from the wind-gauges at these works during a severe south-west gale, which occurred in November, 1888, are recorded in ChaTnbers' Jownaal of January 26, 1889, from which I quote as follows : —

*' The severity of the gales in November last (1888), as measured by the recording instruments at the Forth Bridge, affords ample evidence of the intensity of one of the most prolonged storms that have been felt in Scotland for many years. The worst of these gales was that which began early on the morning of the 16th of November, and continued with but little intermission into the morning of the 17th. The wind blew from the south-west; and those in charge of the Forth Bridge believed that, though blowing very hard on Friday, the gale was, if anything, more severe in the early hours of the following morning.

" The Forth Bridge wind-gauges, situated on the old Castle of Inchgarvie, are three in number. The large gauge presents an area of three hundred square feet to the wind, and is fixed parallel to the centre line of the Forth Bridge, being specially designed and erected to test the pressure of wind over a larger area than that of the gauges commonly in use. This gauge registered at 9 a.m. on the 17th of November, for the previous twenty-four hours, a maximum pressure of twenty-seven pounds per square foot. The small pivoted gauge, which has an area of only one and a half square foot, and which, by means of a vane, is always made to stand at right angles to the direction of the wind, indicated a pressure on the same day of thirty-five pounds per square foot; whilst the gauge of similar dimensions, but rigidly fixed with its face parallel to the centre line of the bridge, registered on the same date forty- one pounds per square foot, a pressure corresponding to a velocity of wind exceeding ninety miles per hour.

" It will be noted that the pressure per square foot on the large gauge is considerably less than on the smaller gauges. This reading

TO HARBOUR CONSTRUCTION,

corresponds with all previous experience at the Forth Bridge, and would seem to indicate that the pressure on the small gauges mnst be due to ' threads of air ' of limited area and high velocity, which, when integrated over a large surface, produce an average pressure of considerably reduced intensity."

The foregoing agrees closely with information which the resident engineer at the Forth Bridge works (Mr. F. E. Cooper, M. Inst. C.E.) kindly furnished to me.

Notwithstanding the foregoing generally admitted facts, it may be useful to give the pressures which are commonly associated with wind velocities, as it is likely these will continue to be adopted until some better method of computing them than at present exists has been discovered.

For convenience of reference, I have therefore incorporated these in Fig. 5.

The pressures represented by the full curved line are those usually associated with the relative wind velocities. In view of the uncertainty which surrounds this subject, they must, however, only be taken as approximately correct.

The most recent investigations tend to show that the usually accepted wind pressures — at all events, when applied to large areas — are too high, and that those indicated by the dotted curve are probably the more accurate.

The following particulars, relative to the great storm of November 17 and 18, 1893, were issued from the Meteorological Station at Deemess, Orkney ; —

*' A hurricane of unprecedented violence passed over the north of Scotland on the evening of the 17th and morning of the 18th. The hurricane commenced exactly at 6 p.m. like the shot of a gun, and during the first hour reached a velocity of 77 miles — ^the previous hour being only 14 — and attained its maximum velocity from nine to ten at the rate of 95 miles, seven miles stronger than any one previously recorded from Orkney. The weather for the week previous was extremely fine for November, although some indication of the fearful hurricane was given by the gradual falling of the barometer, which on the 11th, at 9 p.m., had reached the unusually high figure of 30-563 corrected,and gradually fell till 1 p.m. on the 17th, when it reached the minimum of 28*599. After that, owing to the approaching north wind, it gradually rose again. The barometric readings corrected are — 11th, 9 p.m., 30-563 ; 16th, 9 p.m., 29-368 ; 17th, 9 a.m., 28747; 17th, 9 p.m., 28757; 17th, 1 p.m., 28-599.

WIND,

II

This hurricane was preceded by, and accompanied with an excessive rainfall and high thermometer, which is invariably the case with well-marked cyclonic depressions. The rainfall for the 24 honrs from 9 a.m., 17th, to 9 a.m., 18th, was 155 inch, the highest for

BiaufarPs Pressure qf Wind in pounds per sqttare foot, n ifj? ^rernr- \ fl Cr^tfr rf *l ■ ' ^L L

55 . * \ d ^ijJTUT- >I H ^"^ ill L \ 1 ^~ ^ r J h \ T or LL-.zJj. . iX . ._J__^

Fig. 5.— Diagram of relative wind Telocities and pressures.

several years. If we include the 12 hours previous to this, we have 36 hours with the heavy rainfall of 213 inches. The maximum reading of the thermometer on the forenoon of the 17th was 488 — this high figure being caused by'^reduced pressure and atmospheric saturation.

At 9 p.m. it had fallen to 390, and at 9 a.m., 18th, to 30*5 — a variation in temperature of 18 degrees in 24 honrs. The annexed table shows the force and direction of wind as recorded by Robertson's anemograph, with estimated wind pressure in pounds per square foot as calculated by Sir Henry James, Director of Ordnance Survey.

Hoar and date.

Direction of wind.

Velocity.

Pressure.

S.S.E.

12 „

S.E.

2*

1 p.ixi.

S.E.

2 „

S.E.

1^

3 „

E.S.E.

E.S.E.

6 ,.

E.

6 „

N.

7 „

8 „ f»

41*

If

10 „ n n „

12 midnight.

2 „

If

34*

3 „

4 M

5 „

6 „

7 M

«l

8 ,.

9 ,.

Prom the above it will be noticed that, taking the velocity for 15 hours from 6 p.m., 17th, to 9 a.m., 18th, the average rate is 82^ miles, which, for duration and high velocity, has probably not been surpassed in G-reat Britain. The following are dates of gales with velocity over 80 miles experienced in Orkney since 1863 : —

Date. 1869— February 27 1877— November 16

Max. velocity. 81 miles. 83 „

1882— January 6

1884— January 20 1885— February 21 1893— November 17

82 ,, 88 ,, 82 „ 95 „

Attempts have been made to construct empirical formulse, based upon the average of a great number of observations, by which the height of waves may be deduced from the velocity of the wind. It is, however, impossible to establish any reliable comparison between them.

WIND.

At best the results obtained could only be average results, the variations on either side of which must of necessity be very great, inasmuch as, even assuming practically unlimited fetch ^ and depth of water, the duration of the gale, and the variations in its direction, not only at the point of observation, but along the whole line of fetch— or, at all events, along that portion of it traversed by the gale — form most important yet unobtainable factors.

The subject of waves will, however, be considered in the next chapter.

Relative Frequency of Gales on the British Coasts.

In a paper which was read by Mr. R. H. Scott, Secretary to the Meteorological Office, at one of the conferences in connection with the Health Exhibition of 1884, it was shown from an analysis of daily records, which had been taken during fourteen consecutive years, commencing April, 1870, that the total number of severe gales, of the force of 9 and upwards (Beaufort Scale, p. 11), which occurred on the coasts of the British Isles during that period was 379 ; and that they were distributed over the several months in the following proportions.

Month.

Jannary November October March Deoember February September April ...

AngUBt May ...

June ...

July ...

Avenge number

Average number during 14 years.

for each year.

3-98

8-36

8-36

8-28

1-28

1-21

0-79

0-29

0-21

It would thus appear that the periods about the vernal and autumnal equinoxes are not those which are most to be dreaded, as many persons are in the habit of supposing.