190 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[JUNB, 



■were then elevated above low water mark ; the results of these ob- 

 servations we will now detail, merely premising first such brief ac- 

 count of the work itself as seems necessary to a full understanding of 

 the subject. 



The Delaware Breakwater was located and begun in 1829; it will, 

 when finished,* consist of two detached dikes of rock, the Breakivaier 

 proper, a work of 1200 yards in extent, running in a line tangent to the 

 seaward extremity of Cape Henlopen, and commencing at .300 yards 

 distant therefrom; the tce-brcaker, a wo k 5oO yards long, lying 

 obliquely across the prolongation of the line of the Breakwater, and 

 distant from its western end 350 yards, at the nearest point; the 

 Breakwater lies nearly in the original course of the ebb tide, trending 

 E. S. E. and W. N. W., and will cover the harbour from the northern 

 and eastern ninds; whilst the ice-breaker (mainly designed to shelter 

 the anchorage from ice drifting on the ebb,) bears E. by N. and W. 

 by S., and will, at the same time, protect the interior from the winds 

 of the north-west quarter; the contained angle between the horizontal 

 projections of the two works being thirteen of the southern points of 

 the compass, or 146i°. 



This artificial harbour is located in Cape Henlopen road, just within 

 the pitch of the Cape, and its site is fully exposed to the sea winds 

 which blov? directly in from the Atlantic ocean, over the whole arc 

 comprised between E. S. E. and N. E. by N. around by the East, mea- 

 suring seven compass points, or 781°; whilst on the other hand, it is 

 also exposed to winds sweeping an extent, for the most part, of deep 

 water, upon lines of twenty miles long in the Delaware Bay, and in- 

 cluding, in their field of action, the whole arc contained between the 

 N. E. by N. and W. S. W., around by the West, being thirteen points, 

 or 14t;i^; from the W. S. W. around by the South to £. S.E., an arc 

 of twelve points, or ISS"^ ; the roadstead is completely landlocked by 

 the shoals of Broadkill, and the main strand in front of the town of 

 Lewes, Sussex county, Delaware. 



From the above statement it is evident that the roadstead under 

 Cape Henlopen is wholly exposed to the winds of the north-east quar- 

 ter, the severest that blow upon the American coast, and which fling 

 upon the strand within the Cape those tremendous billows which are 

 raised by the sweep of the north-east gales over a free range of 

 ocean; whoever has witnessed the fury of these storms upon our sea- 

 coast, will, from these remarks, appreciate the violence of the seas 

 which sweep that perilous road, wherein the writer has himself seen 

 fourteen sail of vessels stranded at the same time, and with the loss 

 of several valuable lives. 



The average rise and fall of the tide in Cape Henlopen road is five 

 feet, the highest spring tides, in moderate weather, seven feet, whilst 

 the highest storm tides have been known to attain an altitude of ten 

 feet and more above the plane of low water. . 



In the case of breakwaters, rising abruptly as they do from the 

 bottom of the sea, it seems probable that the augmentation of altitude 

 which waves receive in running upon an inclined coast, is not pro- 

 duced by them, or if at all, in but a slight degree. 



The stones forming the summits of breakwaters, when assailed by 

 the waves of the sea, are solicited by two forces, one acting vertically 

 upward, with the force due to the hydrostatic pressure of a column of 

 water, equal in altitude to the maximum wave which assails the 

 work, and which force we can estimate ; the other is the horizontal 

 force due to the progressive motion of the billows, the extent of which 

 we have no means of determining. 



We may make a practical application of these views as follows ; 

 the heaviest stone used in the construction of the Delaware Break- 

 water, consisting of trap rock, from the Palisades on the Hudson river, 

 weighed 1S4 lb. to the cubic foot ; whilst the lightest, being the Horn- 

 blende rock, from Quarryville, in the northern part of the State of 

 Delaware, weighed but 165 lb. to the cubic foot;t the average weight 

 of the materials being 175 lb. per cubic foot, or 2iV times heavier than 

 sea water. 



• If it be ccmpleted upon the plan prujected by Com. Rodgers, Gen. Ber- 

 nard, and W. Strickland, Esq.. Ccmmissioners, and approved by President 

 J. y. Adams, on the 27th of February. 182y. 



t The Hornblende rock referred to, as weighing 165 lb. to the cubic foot, 

 was the Greenstone from Jaque's quarry. A specimen of Gray Gneiss, from 

 one of Leiper and Crosby's quarries, was found at the same time to weigh 

 168 lb. to the cubic foot ; and some Black Gneiss from Hill's quarry, on 

 Cnim Creek, weighed 177 lb. per cubic foot. All the specimens of the above 

 rocks, of which the specific gravity was tried by the writer, were broken off 

 from masses of stone delivered at the Delaware Breakwater, in the construc- 

 tion of which all these varieties were then used. The specific gravities of 

 some specimens of similar rocks, from the same quarries, were subsequently 

 found, by a committee of the Franklin Institute, to be somewh»t greater 

 than is above stated. 



Let us assume, according to Arnott, the maximum altitude of waves 

 to be twenty feet, then if we take the case of the Delaware Break- 

 water, where the storm tides rise ten feet, let a, in the annexed sketch, 

 Fig. 1, be the top level of the storm tide, b, the plane of low water, c, 

 the dry rubble foundation of tlie work thrown promiscuously from ves- 

 sels into the water, d, a mass of stone lying upon the rubble foundation, 

 with its base coincident with the plane of low water, and e, a wave of 

 20 ft. high, advancing from sea at high water in the direction of the 

 arrow, the crest rising 10 ft. above the high water plane, and the 

 trough falling 10 ft. below it ; then it is manifest that though whilst 

 the wave is upheaving near the margin of the work, it may not act 

 very powerfully upon substances in front of its base, yet the moment 

 it begins to subside, if it happens to be in the position indicated in 

 the sketch, with its fore foot inserted under the mass of stone, d, it 

 will exert upon that mass the upward hydrostatic pressure due to a 

 head of 20 ft., and in an instant after the horizontal force due to the 

 progressive motion of the wave ; and thus, as it were, by successivt 

 pulsations, the waves possess the power of advancing forward immense 

 masses of rock. 



Now the upward hydrostatic pressure of a column of sea water 

 20 ft. high, would equal the gravity of a mass of stone weighing 



20 

 175 lb. to the foot, if the vertical thickness of the latter were = — - 



or near 7i ft.;* consequently, if the depth or thichieas of the mass of 

 stone were less than "tkft., it would be advanced a little to leeward by 

 each successive ware; but if it were of greater vertical thickness it wouli 

 stand fast, and resist the hydrostatic and progressive action of waves, 

 not surpassing 20ft. in height. 



Breakwaters are, however, occasionally subject to augmented vio- 

 lence, from the assault of vpaves during storms, owing to the partial 

 removal of atmospheric pressure; a fact which has recently been 

 observed by James Walker, Esq., F.R.S. &c., President of the British 

 Institution' of Civil Engineers, and which was mentioned by him at 

 a recent meeting of that Institute, (as reported in the Civil Engineer 

 and Architects' Journal for September 1841,) in the following words: — 

 "At the Plymouth Breakwater, during the great storm in the month 

 of February 1838, several of the largest granite blocks, weighing 

 from three to eight tons each, composing the surface or pavement of 



* Corroborative of this calculation, we find it stated in LyeU's Principles 

 of Geology, that " on the Isle of Stenness, in the winter of 1802, a tabular 

 mass of stone, 8^ feet by 7 feet, by 5^ feet in depth, was dislodged from its 

 bed (by the sea), and removed to a distance of from 80 to 90 feet." 



This quotation shows clearly enough that a depth of more than 5i feet of 

 stone is necessary to withstand the action of waves. 



A similar circumstance is stated in the public prints to have occurred in 

 a severe storm recently, upon the coast of Massachusetts, near the Boca 

 Island Lighthouse, where " a huge rock 23 feet long, 16 feet wide, and 6 feet 

 thick," which weighed probably 170 tons, was moved by the waves a con- 

 siderable distance up mi inclined surface ; evincing there that a vertical 

 depth of six feet of solid stone did not possess sufficient stability to resist the 

 force of the sea. 



How much more accurately than the moderns, the ancients appreciated 

 the powerful action of waves against sea works, may be inferred by the fol- 

 lowing paragraph, quoted from Josephus, by Godwin, in the Trans. Instit. 

 Brit. Archts., 1835-6. 



" In order to form a port between Dora and Joppa, Herod, in the fifteenth 

 year of his reign, ordered mighty stones to be cast into the sea at 20 

 fathoms water, to prepare a foundation ; the greater number of them 50 feet 

 in length, 9 feet deep, and 10 feet wide, and some were even larger than 

 these." 



Stone of these prodigious dimensions would be very likely to mamtaia 

 their places in ordinary storms ; and their magnitude is in striking contrast 

 to the comparatively small materials used in modern breakwaters, which, as 

 a necessary consequence, require incessant repairs. 



