2d Session.

No. 114.

SURVEY OF THE MOUTH OF THE MISSISSIPPI RIVER.

LETTER

FROM

THE SECRETARY OF WAR,

RELATIVE TO

The act of June 23, 1874, for the purpose of making a survey of the mouth of the Mississippi River.

JANUARY 19, 1875.-Referred to the Committee on Railways and Canals and ordered to be printed.

WAR DEPARTMENT, January 18, 1875. The Secretary of War has the honor to transmit to the House of Rep resentatives, for the information of Congress, the report of the board. of engineers appointed under the provisions of section 3 of the act of Congress approved June 23, 1874, (chapter 457,) for the purpose of making "a survey of the mouth of the Mississippi River, with a view to determine the best method of obtaining and maintaining a depth of water sufficient for the purposes of commerce, either by a canal from said river to the waters of the Gulf, or by deepening one or more of the natural outlets of said river.”

WM. W. BELKNAP,
Secretary of War.

NEW YORK, January 14, 1875.

SIR: I have the honor to transmit herewith the report of the board of engineers convened by General Orders No. 74, of July 2, 1874, to prepare plans and estimates for the deepening of one or more of the natural outlets of the Mississippi River, and also for a canal from the river to the Gulf.

The drawings and estimates, which are not yet ready, will be forwarded as soon as completed.

Very respectfully, your most obedient,

H. G. WRIGHT,

Lieut. Col. of Engineers, Brt. Maj. Gen., President of Board.

Hon. W. W. BELKNAP,

Secretary of War, Washington, D. C.

[General Orders No. 74.]

WAR DEPARTMENT, ADJUTANT-GENERAL'S OFFICE, Washington, July 2, 1874. Lieut. Col. H. G. Wright, Lieut. Col. B. S. Alexander, and Maj. C. B. Comstock, Corps of Engineers United States Army; Professor Henry Mitchell, United States Coast Survey; T. E. Sickles, W. Milnor Roberts, and H. D. Whitcomb, are, by the President, hereby appointed a board of engineers under the provisions of section three of the act approved June 23, 1874, entitled "An act making appropriations for the repair, preservation, and completion of certain public works on rivers and harbors, and for other purposes," for the purpose of making "a survey of the mouth of the Mississippi River with a view to determine the best method of obtaining and maintaining a depth of water sufficient for the purposes of commerce, either by a canal from said river to the waters of the Gulf, or by deepening one or more of the natural outlets of said river.' They are hereby directed to assemble at New York City, N. Y., on the 20th instant, or as soon thereafter as practicable, for the purpose of organizing and entering upon the performance of their duties.

Lieut. Col. Wright is designated as president of the board.

H. D. Whitcomb is designated to disburse, under the direction of the board, the money appropriated to defray the cost of the survey.

The following is the section of the act of Congress above referred to:

"SEC. 3. That a board of engineers, to be composed of three from the Army, one from the Coast Survey, and three from civil life, be appointed by the President; which said board shall make a survey of the mouth of the Mississippi River, with a view to determine the best method of obtaining and maintaining a depth of water sufficient for the purposes of commerce, either by a canal from said river to the waters of the Gulf, or by deepening one or more of the natural outlets of said river; and said board shall make a full and detailed estimate and statement of the cost of each of said plans, and shall report the same, together with their opinion thereon, showing which of all said plans they deem preferable, giving their reasons therefor, to the Secretary of Wa r, to be presented at the commencement of the second session of the Forty-third Congress; and that the sum of twenty-five thousand dollars, or so much thereof as may be ue cessary, is hereby appropriated, ont of any funds in the Treasury not otherwis appropriated, to defray the cost of said survey."

Approved June 23, 1874.

By order of the Secretary of War:

THOMAS M. VINCENT,

Assistant Adjutant-General.

NEW YORK, January 13, 1875.

SIR: The board of engineers constituted under section 3 of an act of Congress approved June 23, 1874, having completed its labors, has the honor to submit the following report:

The act of Congress referred to required that the board should make plans and estimates for the cost of obtaining and maintaining a depth of water sufficient for purposes of commerce at the mouth of the Mississippi River, either by a canal from said river to the waters of the Gulf, or by deepening one or more of the natural outlets of said river, and to report the same, together with their opinion thereon, showing which of all said plans they deem preferable.

The board was ordered by the Secretary of War to meet on July 20, 1874, at New York City. It met on that day and commenced its duties. As there has been no example in this country of the improvement of a large river-mouth so as to give an outlet to the sea with much deeper water than naturally exists on its bar, the board, in its consideration of what experience has elsewhere shown to be practicable in such a case, has found its only examples in the mouths of European rivers.

There has been great difference of opinion among engineers, not only in this country, but in Europe, as to whether the best method of securing a deep water outlet from a sediment-bearing river which empties, like the Mississippi, the Vistula, the Danube, and the Rhone, into a nearly tideless sea, is by jettees or by a lateral canal.

At the Vistula, after vain attempts for many years to improve the natural mouth, the river formed a new outlet, and the old one, turned into a lateral canal, has long given the needed water-way to the important port of Dantzic.

At the mouth of the Danube, after the canal system and the jetteesystem had each been proposed by eminent authorities, and neither adopted, as a final plan it was decided, in order to do something, to try feeble jettees as a provisional scheme. The results were far better than the engineer expected, and the jettees, made permanent, give an admirable outlet to the Danube to-day.

At the mouth of the Rhone, the engineers, after trying for many years to secure a good outlet by dikes, which, however, never reached the bar, abandoned that plan and built a lateral canal, which is more than adequate to the wants of commerce.

While the results were so contradictory, the information at the command of the board about the above-named foreign rivers was, except for the Danube, not recent and very meager.

The interests involved in the proper improvement of the mouth of the Mississippi are so great, and the work so costly, that it is imperative to obtain the best possible knowledge and judgment as to the method to be adopted. Accordingly, in order that the opinion of the board should be based on a full knowledge of what has been done, and of the latest results obtained elsewhere, in this most difficult branch of engineering, it was decided to visit the mouths of the rivers mentioned, and, as bearing on a canal and harbor, also the North Sea and Suez Canal.

The result of these examinations abroad (while additional information has been gained on almost every point relating to the problem before them) has been to largely strengthen their estimate of the value for jettees, at the mouth of the Mississippi River, of the fascine-dikes used in Holland by Caland in the improvement of the Maas mouth of the Rhine.

After returning from Europe, the board met in New Orleans, spending about three weeks, of which eight days were employed in examinations at the months of the Mississippi River, and at the site for a canal proposed by Captain Howell.

After this history of the proceedings of the board, the conclusions reached will be given.

I.-CANAL.

A number of locations for a canal were considered by the board, among which may be mentioned the one proposed by Captain Howell, near Fort Saint Philip; one leaving Pass à Loutre, about six miles inside its bar, and running north into deep water; one leaving Southwest Pass, about six miles inside of its bar, and running east into deep water; and one obtained by closing the South Pass at its head by a dam and entering it by a canal and lock from the Southwest Pass, and opening its mouth by dredging, the mouth being protected by jettees.

The advantages of the sites at Southwest Pass and Pass à Loutre are, that the difference of water-level at the two ends of the canal will rarely exceed a foot, and may often be so small that the use of the locklift would be unnecessary; that the canals would be only about three miles long, and, being near navigable passes, all light-draught vessels would go through the passes; and that these canals would debouch into deep water at once, while at a distance of six miles from the Gulf

outlet of the Fort Saint Philip Canal there are only 28 feet of water at low tide.

The objection to the site at the Southwest Pass is, that its débouché would gradually silt up in the advance of the delta, and that the cost of the harbor covering its entrance would be excessive. The same objection of excessive cost for protection of entrance applies to the site at Pass à Loutre, while the proposed outlet of the Fort Saint Philip Canal is largely protected by Sable Island, and does not need expensive works to cover its entrance. The sites below the Head of the Passes do not give the same promise of permanence as that near Fort Saint Philip. The plan of canalizing the South Pass would involve the difficulty of opening and keeping open its mouth, which is the main difficulty should jettees be applied.*

After a careful consideration of the several sites, that in the vicinity of Fort Saint Philip was adopted, with one dissenting member. Aside from the question of cost, it offers greater hopes of permanence than the other sites. It does not seem probable that the river will ever deviate very far from its present position at the head of the proposed canal, and a comparison of the 24-foot curves at Sable Island on Talcott's map of 1838 and Captain Howell's map of 1872 show no important change. The board decided, then, that the canal, for which plans and estimates should be made by them, should leave the Mississippi at a point about five and a half miles below Fort Saint Philip, at such an angle with the river that vessels would be able to enter it easily; that about a mile from its beginning there should be a basin of 1,000 feet by 2,500 feet; that before reaching the basin, and near it, there should be a lock, 500 feet long between its miter-sills, of 65 feet clear width, and with 27 feet of water on its miter-sills at mean low tide; the part of the lock in the vicinity of the gates to be of masonry, on a piled foundation, with heavy grillage; the rest of the prism of the lock to be of earth with revetted slopes of one upon one; guard-gates to be placed above and below the lock; the canal to be 200 foot wide on the bottom, 27 feet deep at mean low water, and with side slopes of, (or one vertical on two horizontal,) allowing these to flatten if they will, (the bank of Cubett's gap stands on a slope of nearly one on one to a depth of 60 feet;) leaving the basin, the canal to run in a straight line to the little bay behind Sable Point, beginning to widen when it reaches 6 feet of water in the Gulf, and widening to 1,400 feet at 24 feet of water; beyond the depth of 12 feet in the Gulf, the canal to be without banks. As, in spite of every precaution, accidents might occur which would make it impossible to use the lock for a considerable period of time, a second entrance from the river to the basin should be begun as soon as the canal is completed; this second entrance having its own lock and guard-gates. As there will be a tendency to silt up at the river end of the canal, and also in the cut at the other end, leading to deep water in the Gulf, some dredging will be necessary. Should that at the Gulf end prove large, it might become advisable to build dikes along the sides of the cut. The cost of the dredging and the cost of working the canal have been capitalized, and added to the first cost in the following estimate. The first cost of the canal complete is estimated at. The annual cost of maintenance is estimated at $60, SS5, which, capitalized at 5 per cent......

Giving for the sum needed to build and maintain canal..

$10, 296, 500

1,217, 700

11, 514, 200

* One member is of opinion that the site for a canal between the Southwest Pass and East Bay has a decided advantage over the Fort Saint Philip site.

The canal is about 6 miles long, passing through a country of which no part is more than 3 feet above tide, and much of its route lies through shallow lagoons. At 2 or 3 feet below the surface, the delta at and below the site of the canal is everywhere of bluish mud or clay, into which a pole can be thrust, by hand, from 5 to 15 feet. On the bars, the wave-action has sifted out and left the coarser sand in thin layers, and such strata, perhaps from the same canse, are found elsewhere in the delta.

Borings 100 feet deep show the same clay, becoming more compact, and mixed with occasional sand. At Fort Saint Philip Canal, low water of the river is 0.7 feet above the mean level of the Gulf, and high water 7 feet above the same plane. It is estimated that high water of storms at Sable Point is sometimes 7 feet above mean level of the Gulf. The lock is therefore planned for a lift of 7 feet; but as the water at Sable Point will never be higher than that of the river at the other end of the canal, except for a few hours during violent storms, no provision is made for locking down into the Mississippi. Guard-gates near the lock are provided to prevent currents rushing in either direction through the canal.

For details as to plan of canal and estimates, see Appendix A.

II. DEEPENING NATURAL OUTLET.

The methods which have been proposed for opening a passage through a bar at one of the mouths of the Mississippi are two:

First, by stirring up in some way the material of the bar, so that the current shall sweep it away into deep water;

Second, by confining a mouth of the river by jettees on the bar, so that, aided, if necessary, by dredging or stirring up, the water shall be able to scour a channel through the bar, and to maintain it when once formed.

Judging from the results thus far obtained, the board is of the opinion that no admissible expenditure of money can permanently maintain by the stirring process a depth of 20 feet at mean low water at the mouth of the Mississippi; and as they do not consider this an adequate depth, they deem the first process named inadmissible.

In considering the method by jettees, it may be well to state the facts on which their application at the mouths of the Mississippi is based. On examining a map of the mouths of this river, it will be seen that each pass through the greater part of its length has a nearly constant width, but that it widens toward its bar, so that on its outer crest it has several times its ordinary width, while its depth is less in something like the same proportion. The cause of this change seems to be the following: The river-water as it approaches the bar is somewhat above the level of the Gulf, and so tends to diverge, the stream thus becoming thinner and wider. This thinning and widening is aided by the greater density of the salt-water, which makes the fresh water rise over it on a slope that is steeper as the velocity of the river-water is less. This thinned sheet of water forms its own banks as the bar moves out to sea; bnt, as in this wide, shallow stream the friction is great near the banks, deposits begin there, the stream is narrowed and deepened, and in time that part of the stream obtains the form already assumed by the portion above, which allows the water to pass with least resistance. Today, while there are but 7 feet of water on the outer crest of the South Pass bar, it is yet only about 12,000 feet from the bar to 30 feet of water above the bar, aud it follows if the bar's progress