"On the north end of the bar five borings were made in order to determine the character of the substrata.

12 feet beyond the ends of the logs on either side, the projecting portion being closely confined and bound together so as to form a kind of mattress. As much brush should be used as the logs will bear up and float. Upon this wooden substructure, which will be at least 2 feet thick, good soundstone of random sizes and of compact form, but none weighing less than 30 pounds, will be placed as compactly as possible. The

"All the channels which traverse the bar are, and, so far as we know, always have been, ebb-tide channels, produced and maintained by the scour of the ebbcurrent, except Beach (formerly Maffitt's) Channel, the most northerly of them all, which lies close to Sullivan's Island. This is a flood-tide channel, possessing the usual characteristic of such channels, depth of this bed of riprap stone will be that their least depths are always found near their inner ends, and, therefore, in comparatively quiet water. Another distinguishing feature of such channels is that from the cross section of shoalest soundings inward, toward the harbor, the descent into deep water is sharp and sudden, while outward, toward the ocean, it is gradual and gentle."

2 feet for all that portion of the work that constituted a part of the stone superstructure and lies within the side slopes of the jetty, while a depth of only 1 to 14 feet will be required upon that portion on either side which projects as a mattress beyond the foot of the side slopes.

"The total length of the north jetty, according to the original plan, is about 8,840 feet. This may be increased hereafter.

"The width of the log platform at any point will be regarded as the bottom width of the jetty at that point. As the apron progresses the pre-arranged widths may require a slight increase in conseof scour in advance of the work. quence

"It having been determined to begin the works by laying so much of the apron foundation of the North or Sullivan's Island jetty as the appropriation made June, 1878, would cover, a careful survey for location was made of a strip or belt of the bar about 800 yards wide and The greatest bottom width of platform 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. The average 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 largely to its length, and make very considerable changes in its location.

"The following extract from the specifications which formed part of the contract under which the work of 1878 was executed, describes in general terms the apron of the north jetty, according to the original plan:

"A design for the apron has been prepared in this office, which is deemed suitable for the purpose. It consists essentially of a platform of round logs from 11 to 12 inches in diameter, placed and held close together, side by side, at right angles to the axis of the jetty. The logs are to be as straight as can be procured, and those of about the same size must be selected to go together. This log platform is to be overlaid by a compact layer of stout brush to a depth of 12 to 14 inches, projecting from 10 to

"In November the contractors requested a modification of the specifications, with respect to the method of making the mattresses, constituting the bottom portion of the apron. They proposed to make the log platform 10 feet wider than the contract required, to omit the joints in the logs, and restrict the projection of the brush beyond the ends of the logs to 4 feet.

"In forwarding this proposition for the action of the Chief of Engineers, it was stipulated, with the contractors' written consent, that the projecting brush was to be close and massive, although not formed into a mattress as in the original plan, that the quantity of riprap stone used in the apron was to be the same as specified in the contract, and that the cost of the work to the United States was not to be increased; also, that none of the riprap stone was to be placed upon the projecting brush, all of it being

distributed above and over the area occupied by the log platform.

"These proposed changes in the details of the plan were approved by the Chief of Engineers, under date November 13,

1878.

"A cross-section of the apron, according to the design as amended, is shown in Fig. 2.

"The comparatively low bottom velocities of the ebb current observed on the seaward slope of the bar seemed to point to the necessity of giving the jetties. greater length than that contemplated in the original project. It was thought best also to adopt a longer radius for the curved portions of the north jetty, and to carry the point of tangency between it and the straight portion farther to seaward.

"It was determined to retain the original point of departure from the line of ordinary high water on Sullivan's Island, and to carry out the jetty for a length of 8,840 feet, on an arc of a circle described with a radius of 2 miles, so located that a straight tangent at the sea-end of the arc would have a direction about east 32 degrees south.

"The contractors, anticipating great difficulty in laying their work upon a shoal lying in the line of the north jetty, known as Drunken Dick Shoal, which is covered with breakers a great portion of the time, requested, under date April 21, 1879, such a change in the location of the line of the jetty as would in effect divert it to the westward, thereby escaping a portion of the shoalest water, and diminishing the quantity of work to be put down in the breakers.

work shall be continued on the straight line, or on an arc of 2 miles radius parallel to the original arc, or otherwise.

"The width of the log platform which forms the base of the structure varies from 43 feet to 66 feet. These widths include the 10 feet added in modifying the design as already noted, and, therefore, exceed by that much the widths on which payments have been made.

"The riprap stone is required by the contract to be sound stone of random sizes, but none weighing less than 30 pounds. The maximum sizes were not specified, either by weight or dimensions, and the contractors assumed and insisted on the right to exclude heavy stones. The largest pieces are such as one man can pick up and throw overboard from the scows used in conveying the stone to the jetty, while the smaller sizes run somewhat under the prescribed limit of 30 pounds. As these latter, however, in great measure go to fill up the interstices between the large pieces, thus increasing the weight of the mass without materially adding to its bulk or cost, no objection has been made to their use to a moderate extent.

"The frequent inspections that have been made do not show that any of the stone has yet been carried off the apron by the force of the waves, and perhaps no apprehension need be entertained of serious disturbance from this cause to any of the work already laid, unless it be on that portion put down in June, 1879, on the inner slope of Drunken Dick Shoal, where the depth at mean low-tide does not exceed 8 to 8 feet. It seems 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 2 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 structure.

Most of the work laid in February, and all laid subsequent thereto, is more or less exposed, and directions were given near the end of June to cover the exposed wood with riprap stone.

"No settlement of the work has been detected on any part of the line." As set forth in a former report, the plan is stated in these words :

"The north jetty starts from a point on Sullivan's Island, 1,800 yards east of Bowman's jetty. The half next the shore is curved to a radius of about 1 miles, the outer half being very nearly a straight line. The total length of this jetty, from C to X, is 7,450 feet, and its general direction is southeast.

"The south jetty, having a total length of 11,650 feet, from D to Y, starts from Morris Island at a point about 650 yards from Cumming's Point, its general direction being east. The shore end is curved to a radius of about 3 miles for a little more than one-half its entire length, while the half next the sea is nearly straight, as in the case of the north jetty.

"The specified length of the jetties is 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 of the others, upon the prolongation of the axis of deep-water flow through the gorge of the harbor between Cumming's Point and Fort Moultrie.

"Assuming, for the purposes of discussion, the sea ends of the jetties to rest respectively at X and Y, it seems, in some measure, immaterial whether they be established upon straight lines, as shown at AX and BY, or upon curved lines; and if curved, whether the convexity be turned toward the central channel, as at CX and DY, or from it, as at EX and FY. In either case, if kept at the proper heights, they will produce an ebb-flow through the gap able to maintain a deep channel through the bar. Neither the straight jetties, however, nor, more especially, those with their

lie in the direction of the flood-currents, and may be built to any height without obstructing the inflow. For fully onefourth of their entire length the sea ends could be carried above the level of highwater, so as to be visible at all stages of the tide. The inner portions, crossing the Beach and main channels, respectively, might perhaps be kept low enough to allow the smaller classes of vessels to pass over them.

"One characteristic feature of the design-that of low jetties-is intended to maintain the bar in its present general location, with such moderate increase of magnitude as may be expected. to result from concentrating, upon a gap one-half to five-eighths of a mile in width, a portion of the water which is now dispersed over a width of 10 miles.