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“On the north end of the bar five 12 feet beyond the ends of the logs on borings were made in order to determine either side, the projecting portion being the character of the substrata.
closely confined and bound together so “All the channels which traverse the as to form a kind of mattress. As much bar are, and, so far as we know, always brush should be used as the logs will have been, ebb-tide channels, produced bear up and float. Upon this wooden and maintained by the scour of the ebb- substructure, which will be at least 2 carrent, except Beach (formerly Maffitt's) feet thick, good soundstone of random Channel, the most northerly of them all, sizes and of compact form, but none which lies close to Sullivan's Island. weighing less than 30 pounds, will be This is a flood-tide channel, possessing placed as compactly as possible. The the usual characteristic of such channels, depth of this bed of riprap stone will be that their least depths are always found 27 feet for all that portion of the work near their inner ends, and, therefore, in that constituted a part of the stone sucomparatively quiet water. Another dis- perstructure and lies within the side tinguishing feature of such channels is slopes of the jetty, while a depth of only 1 that from the cross section of shoalest to 1} feet will be required upon that porsoundings inward, toward the harbor, tion on either side which projects as a matthe descent into deep water is sharp tress beyond the foot of the side slopes. and sudden, while outward, toward the « «The total length of the north jetty, ocean, it is gradual and gentle.”
according to the original plan, is about 8,840 feet. This may be increased here
after. “ It having been determined to begin
" "The width of the log platform at the works by laying so much of the apron any point will be regarded as the bottom foundation of the North or Sullivan's width of the jetty at that point. As the Island jetty as the appropriation made
apron progresses the pre-arranged widths June, 1878, would cover, a careful sur
may require a slight increase in consevey for location was made of a strip or beit of the bar about 800 yards wide and The greatest bottom width of platform
quence of scour in advance of the work. 5,000 yards long, extending from Sulli- will be about 92 feet, and the least will van's Island in a southeasterly direction, not be less than 33 feet. so located as to embrace the whole of bottom width may be a little greater or the north jetty, even should it be deemed less than 52 feet, but for the purpose of expedient, in view of the additional in; bidders will be taken at 52 feet.' formation recently obtained, to add
“In November the contractors requestlargely to its length, and make very con- ed a modification of the specifications, siderable changes in its location.
with respect to the method of making
the mattresses, constituting the bottom “The following extract from the speci- portion of the apron. They proposed to fications which formed part of the con- make the log platform 10 feet wider than tract under which the work of 1878 was the contract required, to omit the joints executed, describes in general terms the in the logs, and restrict the projection apron of the north jetty, according to of the brush beyond the ends of the logs the original plan:
to 4 feet. “A design for the apron has been “In forwarding this proposition for prepared in this office, which is deemed the action of the Chief of Engineers, it suitable for the purpose. It consists was stipulated, with the contractors' essentially of a platform of round logs written consent, that the projecting from 11 to 12 inches in diameter, placed brush was to be close and massive, aland held close together, side by side, at though not formed into a mattress as in right angles to the axis of the jetty. the original plan, that the quantity of The logs are to be as straight as can be riprap stone used in the apron was to be procured, and those of about the same the same as specified in the contract, and size must be selected to go together. that the cost of the work to the United This log platform is to be overlaid by a States was not to be increased; also, that compact layer of stout brush to a depth none of the riprap stone was to be placed of 12 to 14 inches, projecting from 10 to upon the projecting brush, all of it being
distributed above and over the area oc- work shall be continued on the straight cupied by the log platform.
line, or on an arc of 24 miles radius par“These proposed changes in the details allel to the original arc, or otherwise. of the plan were approved by the Chief of Engineers, under date November 13, 1878.
“The width of the log platform which “A cross-section of the apron, accord- forms the base of the structure varies ing to the design as amended, is shown from 43 feet to 66 feet. These widths in Fig. 2.
include the 10 feet added in modifying “The comparatively low bottom velo- the design as already noted, and, therecities of the ebb current observed on the fore, exceed by that much the widths on seaward slope of the bar seemed to point which payments have been made. to the necessity of giving the jetties “The riprap stone is required by the greater length than that contemplated contract to be sound stone of random in the original project. It was thought sizes, but none weighing less than 30 best also to adopt a longer radius for the pounds. The maximum sizes were not curved portions of the north jetty, and specified, either by weight or dimensions, to carry the point of tangency between and the contractors assumed and insisted it and the straight portion farther to on the right to exclude heavy stones. seaward.
The largest pieces are such as one man “It was determined to retain the orig- can pick up and throw overboard from inal point of departure from the line of the scows used in conveying the stone ordinary high water on Sullivan's Island, to the jetty, while the smaller sizes run and to carry out the jetty for a length of somewhat under the prescribed limit of 8,840 feet, on an arc of a circle described 30 pounds. As these latter, however, in with a radius of 2} miles, so located that great measure go to fill up the interstices a straight tangent at the sea-end of the between the large pieces, thus increasing arc would have a direction about east 32 the weight of the mass without materidegrees south.
ally adding to its bulk or cost, no objection has been made to their use to a
moderate extent. “The contractors, anticipating great “The frequent inspections that have difficulty in laying their work upon a been made do not show that any of the shoal lying in the line of the north jetty, stone has yet been carried off the apron known as Drunken Dick Shoal, which is by the force of the waves, and perhaps covered with breakers a great portion of no apprehension need be entertained of the time, requested, under date April 21, serious disturbance from this cause to 1879, such a change in the location of any of the work already laid, unless it the line of the jetty as would in effect be on that portion put down in June, divert it to the westward, thereby escap- 1879, on the inner slope of Drunken Dick ing a portion of the shoalest water, and Shoal, where the depth at mean low-tide diminishing the quantity of work to be does not exceed 8 to 84 feet. It seems put down in the breakers.
hardly safe to assume, however, that “The work from the beginning had stone of such small sizes will continuously been laid upon straight ranges, or chords withstand the force of the waves on the of 400 feet in length, and not upon the crest and exterior slope of the bar. Not circular arc of 24 miles radius. By the that there is likely to be any sudden change desired the line of the work, after movement of large masses, but that the completing the eighth chord, 3,200 feet waves produced by the prevailing heavy from ordinary high-water mark on the storms, those from the northeast, will shore, would continue on the straight carry the stones gradually, a few only, prolongation of that chord for a distance and for short distances at any one time, of about 3,000 feet, or until Drunken from the inner slope of the jetty to its Dick Shoal was crossed. This change of crest, and thence over to the western direction was approved for a length of slope. Large stones will have to be used 2,000 feet beyond the eighth chord, leav- for the upper portions of the work, and ing it to be determined, when that limit some device may have to be resorted to shall have been reached, whether the for holding them in place.
“ The design of the apron is such that convexity turned away from the channel, the log platform which forms its base act as training-walls to guide the outwould be exposed to the ravages of the flowing water. The curved jetties, conteredo unless covered with sand or other vex toward each other, being less open material placed upon the ends of the to this objection, are the ones adopted in logs, or accumulated there by the action this project. of the currents, or by the settlement of “ The north jetty starts from a point the structure.
on Sullivan's Island, 1,800 yards east of “An examination made on the 24th of Bowman's jetty. The half next the shore June shows that for a length of about is curved to a radius of about 14 miles, 1,450 feet from the high-water mark on the outer half being very nearly a straight Sullivan's Island, comprising all the line. The total length of this jetty, from work laid before the 4th of February, the C to X, is 7,450 feet, and its general apron is so well embedded in the sand as direction is southeast. to afford a complete protection to the “The south jetty, having a total wood.
length of 11,650 feet, from D to Y, starts “Most of the work laid in February, from Morris Island at a point about 650 and all laid subsequent thereto, is more yards from Cumming's Point, its general or less exposed, and directions were given direction being east. The shore end is near the end of June to cover the ex- curved to a radius of about 3 miles for a posed wood with riprap stone.
little more than one-half its entire length, “No settlement of the work has been while the half next the sea is nearly detected on any part of the line." straight, as in the case of the north jetty.
As set forth in a former report, the “The specified length of the jetties is plan is stated in these words :
taken for purposes of discussion. As “It is proposed to construct two will be seen hereafter, they would not be jetties, one springing from Morris Island able to produce a channel of the reand the other from Sullivan's Island, quisite capacity through certain materials converging toward each other in such which may be encountered in the bar, manner that their outer ends on the although they would be expected to crest of the bar shall be one-half to five- maintain such a channel if once estabeighths of a mile apart. The outer ends lished. of the two jetties will rest respectively “ The outer ends of the two jetties upon the shoals lying to the northward slightly converge toward each other as and southward of what is known as the they approach the crest of the bar, and north channel, that being the middle are intended to act as training-walls for channel of the north group of three a distance, in each case, quite equal to channels, and having its line of deepest half its entire length. These portions water located, more nearly than either lie in the direction of the flood-currents, of the others, upon the prolongation of and may be built to any height without the axis of deep-water flow through the obstructing the inflow. For fully onegorge of the harbor between Cumming's fourth of their entire length the sea ends Point and Fort Moultrie.
could be carried above the level of high“Assuming, for the purposes of dis- water, so as to be visible at all stages of cussion, the sea ends of the jetties to the tide. The inner portions, crossing rest respectively at X and Y, it seems, in the Beach and main chantiels, respectivesome measure, immaterial whether they ly, might perhaps be kept low enough to be established upon straight lines, as allow the smaller classes of vessels to shown at AX and BY, or upon curved pass over them. lines; and if curved, whether the con “One characteristic feature of the vexity be turned toward the central design—that of low jetties—is intended channel, as at CX and DY, or from it, as to maintain the bar in its present genat EX and FY. In either case, if kept eral location, with such moderate in. at the proper heights, they will produce crease of magnitude as may be expected an ebb-flow through the gap able to to result from concentrating, upon a gap maintain a deep channel through the one-half to five-eighths of a mile in bar. Neither the straight jetties, how- width, a portion of the water which is ever, nor, more especially, those with their now dispersed over a width of 10 miles.
“The complete success of the works is to transfer the gorge of the harbor from believed to depend on three important its present position to the sea ends of the conditions, which they are expected in a jetties and move the shore line out to great measure to satisfy, and which have that point, by causing a filling in of the been kept in view in preparing the exterior angles between the jetties and design, viz. :
the shore. After reaching this stage, a “1. They should not impede the in- drift-and-wave bar would probably be flow to such degree as to prevent the tidal found to the seaward of the present bar, basin being filled, as now, at every influx in front of the jetties, rendering it necesof the tidal wave.
sary to extend them, in order to cut a “To this end the inner half of each passage through it. jetty, more especially its central portion, “It seems essential, therefore, that the located in deep water across the thread agencies which maintain the present bar of the current, is kept several feet below should remain in as full force as possible, the water. The outer half, being nearly consistent with the requisite concentraparallel to the direction of the flow, is tion of outflow between the jetties. built higher, and the sea end, for a dis “The probable effects will be that the tance of several hundred feet, may be bar will be raised somewhat throughout carried high enough to be visible at all its entire length, the waves will break stages of the tide.
upon .it more frequently than now, and “2. They should control the outflow to considerable shoaling will, of course, take such degree, and in such manner, that a place in Beach Channel and in all the channel of the required depth will be southern group of channels. But it is maintained through the bar.
believed that the important condition of “To this end, although a large por- keeping the bar generally in its present tion of the surface flow will spread out position will be secured. over the tops of the jetties and thence “The drift material carried along the over the bar, the central flow, through- coast by surf-currents, as well as the out the entire depth along the axial line sand thrown up by the breakers on the of the gorge between Sullivan's and north and south shoals, instead of lodgMorris Islands, is aided in its natural ing in and filling up the exterior angles tendency to reach the sea along the pro- between the jetties and the shore, as in longation of that line by the opening left the case of high jetties, will be disfor it between the jetties. The bottom posed of in a harmless manner. flow through the gorge of the harbor is “For example, a heavy northeasterly deflected on converging lines by the storm, producing breakers along the jetties, and is, therefore, forced in a north shoal and strong southerly surfmeasure to concentrate itself in, and flow currents along the shores of Long and out through, the gap between them. Sullivan's Islands, would put in motion The outer half of each jetty, and the ad- a large quantity of material, a portion of jacent portion of the shore end act as a which would be carried in by the floodtraining-wall for this flow.
currents over the north jetty and through “3. They should not, to any consider- Beach Channel, coming to rest in the able extent, cause a movement seaward deep water of the main channel. It of the main body of the bar; that is, the would next be taken up by the ebbgeneral position of the bar should be in- current and rolled out to sea between dependent of the effects produced between the jetties. Beyond the jetty-heads it and beyond the heads of the jetties. would encounter the littoral ebb-current,
“It is believed that this condition will moving to the southward with a velocity be secured by making the shore ends of accelerated by the storm, by which it the jetties low for at least one-half their would be again carried in a southwesterly length, or throughout those portions direction, until, finally, left to the action which cross the thread of the current in of the south breakers, it would be either deep water, so as to allow the tide to ebb deposited temporarily upon the south and flow somewhat freely over them. shoal, or carried still farther to the The effect of high jetties, with a cor- sonthward. respondingly wide gap between them to “The action of a southerly storm allow a full influx of the tide, would tend I would be the reverse of this. In either