into the Gulf has been for the last one hundred and twenty years the same as for the last thirty-six years, (about 100 feet per annum,) that one hundred and twenty years ago the outer crest of the South Pass bar was where now there is a deep channel. As the bars move out to sea, the river is, then, all the time eroding a channel of the characteristic deep-water cross-section behind and through them. The object of jettees is to aid and hasten this erosion. If, starting from a point in a pass above its bar, where there are now 30 feet of water, we build jettees which so confine the pass that it shall have the width all the way to deep-water it now has at the starting point, we shall be helping the pass to assume the deep-water cross-section it would ultimately take, and by aiding it, if necessary, by dredging, should be able to reduce at pleasure the time required for the process. This plan is then adopted for the improvement of a natural outlet, namely, to begin parallel dikes at the banks of a pass where there are now 30 feet of water in the middle, and carry them over the bar to 30 feet water ontside, (unless the depth is obtained before the dikes have reached the 30-foot curve,) allowing the river to erode the bottom between the dikes till the water-way between them everywhere has the same cross-section as at their beginning, aiding the erosion by dredging or stirring if it is not rapid enough without. The board considered the question of limiting the water-way to the cross-section of 30 feet maximum depth, by converging jettees on the bar, and by spur-dikes in the pass above, instead of by parallel dikes. In view of the lack of experience in such work in this country, and of the danger of excessive scour around the ends of spur-dikes, it was deemed advisable to adopt parallel dikes as offering fewer contingencies and less difficulty of construction. The depth of 30 feet has been chosen, in order that some time may elapse before the bar, which will form at the sea-end of the jettees, can have less than 25 feet at mean low water upon it, that being the minimum depth which it is desired to maintain. Having adopted a general plan for the improvement of one of the natural outlets, it remains to fix on that one. As the improvement of any will be costly, but one should be improved, and that should be made adequate for all purposes. The passes which have been most carefully considered are the South and Southwest. In comparing these passes, it is seen that while the average width of the body of the South Pass is 700 feet, that of Southwest Pass is about 1,400. The greater width is more favorable to navigation; but, in the opinion of the board, the South Pass, when improved, will be ade quate to the present and prospective wants of commerce. The estimated sum required for the construction and maintenance of the works for the improvement of the South Pass is $7,942,110, and for Southwest Pass is $16,053,124. It is assumed that the Southwest Pass bar advances about three times as fast as the South Pass bar. The South Pass being entirely adequate, the much greater cost of improving the larger pass would not, in the opinion of the board, be warranted either by the somewhat greater ease of entering it in storms, or of navigating it when once inside. The former is 12.9 miles long, the latter being 18 miles, and is lengthened only about one-third as fast as the latter. The cost of improving Pass à Loutre would also be far greater than for South Pass, without advantages sufficient to justify the increased cost. The South Pass has the advantage that the works for its im provement, which would require at least two or three years for their execution, would in no way interfere with commerce. The board is therefore of opinion that if any natural outlet is improved, it should be the South Pass. The South Pass of the Mississippi is 12.9 miles long, has an average width of 730 feet, and a minimum interior channel depth of 29 feet. It is 11,900 feet from the 30-foot curve inside the pass across the bar to 30 feet outside. The minimum depth on the bar is 7 feet. It discharges at its mouth about 57,000 cubic feet of water per second, and about 22,000,000 cubic yards of sediment in suspension per annum. It has a shoal at its head, with a minimum depth on it in channel of 17 feet. For the improvement of the South Pass, the board recommends parallel dikes or jettees, constructed of brush, fascines, and stone, in the same general way as those used by Mr. Caland at the mouth of the Maas. These dikes should begin at the two banks of the pass, about 1,650 feet below the South Pass light-house, where the river has a width of nearly 900 feet and a maximum depth of 30 feet. They should run in straight lines, parallel to each other, in the direction of the pass, to where the water is 30 feet deep outside of the bar, provided it should be necessary to carry them so far to secure 30 feet depth. The dikes for the first 7,100 feet should be 10 feet wide on top; should then widen gradually to 20 feet in a further distance of 2,050 feet; should then gradually widen to 50 feet, which is to be the width in 30 feet of water. At present, this last length would be about 2,750 feet, and the total length of each dike 11,990 feet. The first 7,100 feet of the dikes to have side-slopes of 2, (two vertical to three horizontal,) the rest to have side slopes of down to 15 feet below water, and beneath that depth slopes of . The top of the dikes to be rounded and paved, the crown rising to high water of spring-tides. The question of the average annual expense of prolonging the jettees is a very serious one; it depends on the annual advance of the 25-foot curve, that depth being required. At present, the muddy water issuing from the South Pass spreads out in somewhat of a fan-shape, the handle of the fan being at the mouth of the pass and the ribs several miles in length. If the proposed jettees were instantly completed, and the new channel scoured out, essentially the same amount of sediment would be spread out in fan-shape, but, from the greater velocity of the issuing water, the ribs of the fan would be longer, while the handle would be narrower. More of the sediment would at first be deposited far out in the Galf than before. But with the present rate of advance, the 25-foot curve one hundred and twenty years ago was about 12,000 feet above its present position: and if the volume of water carried by the pass is kept the same, neglecting the slight difference in slope of the Gulf bottom outside the present bar, in about 120 years a new end for the pass will probably be formed of the same general shape as the lower 12,000 feet of the present pass. It makes little difference, in the whole time required to accomplish the work, whether the same volume of water flows out at starting over the present shallow bar or from between two dikes which force the water to take a depth of 30 feet. In an average of many years, the rate of progress must be about the same as now, namely, 100 feet per annum, the volume of water being kept as at present; and it is on this basis that the average annual cost of extension, namely, $130,000, has been computed. It has already been stated that it is proposed to obtain a depth of 30 feet between the jettees, in order that some years may elapse before the shoal which will form beyond the jettees can have on it less than the required depth of 25 feet in the channel through it. There are no precise data for estimating this period. Going seaward from the upper end of the proposed dikes, the slope of the bottom of the South Pass is about. This slope doubtless depends mainly on the velocity of the water flowing through it and on the lifting of the fresh water by the salt. As the causes remain essentially the same, it would seem natural that the new end of the South Pass to be formed by the sediment passing through the jettees should at least have the same bottom slope. If this assumption were true, the bottom would at last shoal from thirty to twenty-five feet in a distance of 5 × 440 = 2,200 feet, and the time required would be about twenty-two years. This time would be shortened by two causes: First, there are about three million of cubic yards of material to be scoured out between the jettees, thus increasing the general bar accretion by that amount and hastening the advance of the pass. As the scour would be distributed over several years, and as the South Pass carries about 22,000,000 cubic yards of sediment in suspension annually to the Gulf, the effect of this 3,000,000 yards cannot be relatively large. Second. at and below the point where it is proposed to begin the jettees, the river-velocity now diminishes very slowly, as it is confined by a slowly-widening channel, while, when the jettees are completed and the channel scoured out, the water issuing from them will, having at first no banks to confine it, spread out more rapidly, thus, perhaps, losing velocity more rapidly and forming a steeper seaward slope on the bottom than now exists at the upper end of the proposed dikes. This steeper slope seaward from the 30 feet of water between the jettees would give a shoal of 25 feet at a distance of less than 2,200 feet and in a period of less than twenty-two years. The period is uncertain; experience alone can determine the precise time. Different estimates made by this method, and others by different members of the board, vary largely, and ten years have been assumed for the purpose of estimate. In ten years, then, it is assumed that the jettees will have to be lengthened 1,000 feet. As shoals will have formed at the ends of the jettees, it has also been assumed that the extension will be in water averaging 15 feet in depth. It has been stated that there is a shoal at the head of South Pass, with but 17 feet of water on it. At present, this shoal is scouring out. Should that scouring not give a depth as great as at the shoalest point below in the pass, the construction of a dike to deflect more water into it would become necessary. Should the South Pass increase much beyond its present size, it might become necessary to put an apron on the bottom and sides of the pass, near its head, to stop that increase. Estimates of the cost of the jettees and of the works which may be needed at the head of the pass are given in Appendix B. First cost of jettees at mouth of South pass, of dredging, $5, 342, 110 00 2,600,090 00 Cost of construction and maintenance of improvement.. 7, 942, 110 00 The board also made an estimate of the cost of improving the Southwest Pass, the result of which is as follows: First cost of improving Southwest Pass by jettees italized at 5 per cent., is..... $8, 253, 124 00 7,800, 000 00 Sum required to improve and maintain Southwest Pass 16, 053, 124 00 III-METHOD RECOMMENDED. The board has now given plans and estimates for improving the mouth of the Mississippi either by a canal or by opening one of the natural outlets. It is also required to give its opinion as to which plan is preferable. Leaving cost aside for the moment, and assuming that one plan can be as easily executed and maintained as the other, there is no question that the improvement of the South Pass would be best. It would give a good sea-entrance about 900 feet wide, and a minimum width in the pass of about 530 feet, while the width of the canal would be about 300 feet. It would offer no locks (liable to do or suffer injury) to delay the passage of vessels through it. It would give an ample unobstructed water-way to commerce in place of a narrow and obstructed one. If the question of cost and maintenance be considered, we have for the canal $11,514,200 against $7,942,110 for the South Pass. The only remaining question is whether the two plans can be executed and maintained at about the estimated costs. For the canal, the difficult points are to maintain a coffer-dam and secure a stable foundation on which to build the masonry. By proper examinations, it is believed that sites can be found where coffer-dams can be maintained; and, although the masses of masonry are heavier than any which have been built in that region without settling, it is believed that, by the plan proposed, injurious settling would be prevented. The board is therefore of opinion that the canal can be built at the estimated cost. In the improvement of the South Pass, the difficult points are the control of the water entering it, the removal of the shoal, and the maintenance of a channel at its head, and the execution and maintenance of that part of the jettees lying outside of the outer crest of the bar. While the proper method of control of the water at the head of the South Pass must be learned mainly by trial, great difficulty is not anticipated, as such control was readily obtained on the Sulina. The construction and maintenance of the jettees beyond the crest of the bar is a difficult work, in which there will be contingencies arising from the action of heavy storms, either on the jettees themselves or on the material on which they rest, and from settling. A liberal allowance has been made for such contingencies, and the board sees no reason to increase its estimate. Indeed, it is of opinion that experience in construction may very probably show that the cross-section of the dikes may be reduced, thus lessening the cost. If the jettees are constructed and maintained, the ends being prolonged as becomes necessary, the board has no doubt that the desired depth will be obtained. It therefore recommends that the South Pass of the Mississippi River be improved by the plan already given. The board concludes its report with the recommendation that, if Congress decides to open one of the passes of the river, the entire sum H. Ex. 114 necessary to accomplish the work be appropriated at once, or in some way be made available. If the mouth of the river is to be improved by jettees, the work, when once begun, should be pushed as rapidly as possible to its entire com pletion. The board is of opinion that the works it proposes for the improvement of the South Pass can be completed in three years. Respectfully submitted. B. S. ALEXANDER. Lieut. Col. of Engineers, Bvt. Brig. Gen. U. S. A. C. B. COMSTOCK, Major of Engineers and Bvt. Brig. Gen. HENRY MITCHELL, U. S. Coast Survey. T. E. SICKLES. W. MILNOR ROBERTS. H. D. WHITCOMB. I concur in so much of the report as refers to the selection of the South Pass for the trial of the jettee-system of improvement, if that system is to be adopted; also, to the plans and estimates for both canal and jettees; but as, in my judgment, the chances of success of an attempted improvement of any one of the natural outlets of the river do not justify the recommendation of the board, I have withheld my signature from the report. If an adequate and permanet channel could be obtained at any one of the passes, it would no doubt be preferable to the proposed canal. As the ship-canal project does, in my judgment, offer reasonable chances of success, I must give it the preference over the jettee-project recommended by the board. Hon. W. W. BELKNAP, H. G. WRIGHT, Lieut. Col. of Engineers, Bvt. Maj. Gen. |