The top of the jetties must be held low, not higher than the banks from which they extend, because additional height, while adding to their cost, would not induce the passage of more water between them so long as the banks of the river above are at a lower level. An elevation of the jetties above the banks from which they spring would, in fact, endanger the latter in the presence of a rise overtopping them, especially at the points where the jetties and banks unite.

The debouch of Pass à Loutre by two mouths makes it necessary to close one of them, and this operation is supposed to be performed by the north jetty, constructed across the northern mouth.

An inspection of the map of the pass, to fix in the mind the necessary course of the northern jetty, will show that the present direction of the running waters will be deflected by this work, which forms a concave bend to receive them, and a considerable scour of the bank must necessarily ensue, causing the foundations of the jetty to be undermined, unless effective measures are taken to prevent such catastrophe.

The board are unable at this time to suggest any remedy except to sink the foundations deep enough to be out of reach of these influences. As to how great this depth should be to insure safety, the board have no certain means of judging, but it may be 25 feet, or even more.

The closing of the northern mouth, which, following the line of jetty, would be a work operating to deflect the present direction of the currents, and over 7,000 feet in length, is an undertaking of great delicacy, the cost of which, in a soil of the character pertaining to this locality, might prove to be excessive. Success in this operation is, however, necessary to the application of the jetty-system to the pass under consideration, and must be sought at whatever cost, in order to accomplish the desired improvement of navigation. (Note A.)

It is important to say that the advance of the jetties, step by step, will cause deep holes to form at their extremities, due to the escape of the waters as soon as released, and a consequent excavation of the loose soil, which will much increase the depth and cost of these works.

The dislodgment, by the operation of the jetties, of the immense quantities of material from the sides and bottom of the channel would bring the scouring force into contact with the interior of the banks and shoals, which consist generally of soil inferior in hardness and firmness; and it would be impossible so to fix the limits of this disturbing action that it might not often reach the jetties themselves.

The long, low banks and the shoals of the delta do not owe their existence or permanence to anything inherent in the strength and consistence of the soil composing them-for on these points all testimony agrees-but upon the action of the waves and currents, constituting an area of equilibrium, in which the particles are deposited and retained.

But as these forces are not always as to effect, but only periodically, in equilibrium, it necessarily follows that changes in the shoals and banks are constantly occurring, not enough, indeed, to interfere with the general development of the delta, which appears to advance by virtue of uniform laws, but quite sufficient to endanger and even destroy the most skillfully-designed works. (Note B.)

This consideration of the unstable and treacherous nature of the shoals and banks is necessary in order to fix the mind upon the cost and risk as well as upon the disappointment which would likely attend an attempt, upon such foundations, to construct works to coerce or control the currents of the passes.

An estimate has been prepared by Captain Howell, engineer in charge

of the jetties described in this report, supposing them to rest upon the natural bottom, without settlement, as follows:

Fascines and ballast, at $5 per cubic yard.

Riprap stone, $7 per ton

Total

$2,545,220 00 2,241,097 60

4,786,317 60

If settlement and the other probabilities enhancing the cost of this work, as already discussed, be considered, it appears entirely within. limits to state that the above estimate should be doubled.

Assuming that it will take about four years to complete the jetties to the present 25-foot curve outside the bar, and estimating the least yearly advance of the bar at 250 feet, it would be proper to add to the estimate already the cost of 2,000 linear feet, equal to $68,888.

There is, besides, the estimate for future annual extensions to keep pace with an increased advance of the bar, which by the same authority would be $1,613 per linear foot of jetty.

The next step in order is to consider the effects of these jetties, supposed to terminate at the curve of 25 feet outside the bar, upon the depth of water in the channel and upon the bar; and it will be first supposed that the jetties, if projected too far apart, should near the bar be brought together sufficiently close to insure the desired scouring effect upon the bar.

Would this state of things, thus produced, endure for a considerable time, or for a period sufficient to fill up the deep space ahead in the Gulf to a distance equal to the present interval between the termination of the 25-foot curve in the channel and the outer crest of the bar? The principles upon which a reply to these questions depends have been exhaustively treated in Chapter VIII of Humphreys and Abbot's Report on the Physics and Hydraulics of the Mississippi River; and there is nothing more to add, except the conclusions which follow from that report.

Let us suppose, as the first effects of the jetties, the 25-foot curve to have advanced to the original outer crest of the bar. It will be found that the position of the crest has already advanced, due to the large amount excavated from the sides and bottom of the channel, and the ordinary supply of materials which are rolled on the bottom and deposited on the outer slope; and it is not certain that there would be a full depth of 25 feet at the new crest, on account of the tendency to form the upper surface of this deposit coinciding with the angle at which the river-waters emerging from between the pier-heads would be deflected upward by the waters of the Gulf, an effect which the spread of the river-waters, after their release from the confinement of the jetties, would increase. The succeeding flood, while advancing the bar, should, upon the same principles, still further decrease the depth over its outer crest; and every advance of the bar would be followed by a similar result. Hence the jetties, in order to retain the depth gained, should keep pace in their extension with the progress of the bar. At highwater of river, the deposits are made exteriorly; at low-water, interiorly During the changes from high to low water, the deposits are made between these two, or on what is ordinarily considered the bar.

A condition of things likely to occur periodically, whereby a medium stage of the river, without high floods, might be maintained, would cause unusual deposits upon the bar; and hence an additional reason for the conclusion, apparent already from the first portion of this discussion upon the bar, that in order to secure, at all times, a depth of

25 feet, provision should be made in the arrangement of the jetties to excavate to a depth greater than that. (Note C.)

As a case in point, Major Stokes, royal engineers, in his paper upon the improvement of the Sulina mouth of the Danube, states that it 1863, owing to the absence of floods in the river, a bank formed within the pier-heads almost in the position of the old bar, greatly contracting the channel, though not actually barring it.

If it is not already apparent that the deep space ahead will not of itself prevent the restoration of a shoal depth to the bar after once deepening it, reference may be had to the fact that a shoal bar, for over one hundred years, has been advancing at Pass à Loutre, over a deep space ahead, and at an average rate of about 300 feet a year.

In proportion as the cross-section of discharge on the outer crest of the deposit widens, its progress into the Gulf will become slower, and the depth of water upon it will constantly decrease.

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(Humphreys and Abbot, pp. 446, 447.) On the other hand, if the cross-section be narrowed, the progress into the Gulf of the deposits will become more rapid.

This rapid extension of the pass into the Gulf would tend to increase the volumes of the shorter passes at the expense of its own, and it would eventually be necessary to resort to another pass for the continuance of the plan. (Ibid., p. 456.)

This yearly progress of the bar demands a corresponding extension of the jetties into deep water exposed to the severe storms of the Gulf, and consequently of great cost.

The difficulties at the mouth of the Mississippi, so far as concerns the improvement by jetties, resolve themselves into three sources: 1. The absence of a littoral current.

2. The yielding nature of the banks and shoals.

3. The abundance of deposits.

The first and third combine in the yearly and rapid extension of the bar, and compel the works of improvement to continue at a heavy annual cost until their entire abandonment.

The second makes their construction difficult and their maintenance improbable, unless deeply founded at a very heavy expense.

All the principal objections to the improvement of Pass à Loutre necessarily apply to the Southwest Pass.

But the board does not clearly understand why Pass à Loutre has been preferred for improvement by jetties, its exposure to the storms and storm-tides of the Gulf being much greater than that of the Southwest Pass; and it may be added that the direction of jetties at the Southwest Pass would be straight, while at the other pass they would be inclined to the direction of the current, which is objectionable.

Pass à Loutre, however, has the advantage of being directly in the track of vessels bound to and from the East. The length of both jetties at Southwest Pass, designed for the same objects as at Pass à Loutre, would amount to 54,000 linear feet. It is proper to state that these lengths were taken from a Coast Survey map, of a scale smaller than that of the map of Pass à Loutre, made by the engineer in charge for the operations of the dredging-machine, and upon which the improvement of Pass à Loutre was discussed by the board.

In the study of improvements of this character it is well to refer to instances where trial has been made, holding in view always the sound principle that the fact of work having proved successful, or having failed, at any river-mouth, by no means insures that the same kind of works will succeed or fail at any other river-mouth unless the very same conditions exist.

The board is indebted to the article (Vol. XIII, Professional Papers) of Major Stokes, R. E., British commissioner for the improvement of

the mouths of the Danube, for much of the matter in the present discussion immediately following.

From 1594 to 1682 attempts were made to improve the Vistula by extending piers seaward from its mouth. "A breach in the root of this pier, through which the river cut itself a lateral communication with the gulf 10 or 12 feet deep, suggested the idea of obtaining a permanent channel independent of the mouth."

The extension of the piers from the mouth of this new channel converted into a canal did not avail to secure the requisite draught. "The author was informed by the engineer who, in 1858, had charge of all the Prussian harbor-works, that no efforts were available to keep open a greater depth than 10 feet into the canal before the year 1840. * It was not till after the breaking through of another mouth several miles to the east, in 1840, as before mentioned, that the Prussian engineers could congratulate themselves on having obtained a good entrance to the port of Dantzic. The river was at once shut off from its old course

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by a dam. The old mouth was cut off from the sea by a solid dam. By these means an excellent channel of 17 feet was obtained," (at the mouth of the canal,) "and has since been maintained by constant dredging. As already mentioned, the dredging is carried on under peculiarly favorable circumstances, as the Gulf of Dantzic is land-locked. * In the Gulf of Dantzic there can hardly be said to exist a littoral current. The littoral current of the Baltic, from west to east, passes along the Helas and strikes again the coast, which then immediately trends to the north. The main force of the current is then carried northward, but a portion of it sets into the Gulf of Dantzic from east to west, while a second current, passing round the head of the Helas, sweeps along the shore of the gulf, and, traveling from west to east, meets the main current somewhere opposite the old mouth. No more unfavorable circumstances for the opening of the mouth of the river could be imag ined. The river issuing into slack-water at the meeting of the two currents threw down its deposits at once."

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The two attempts, which were persevered in for more than one hundred and fifty years, to keep a channel open for sea-going vessels at the mouth of the river or of the canal, failed, and it was not until the fortuitous opening of another mouth, five miles away from the old mouth, which removed the deposits to a distance, that success was obtained. The causes of non-success were:

1. Absence or neutralization of littoral currents.

2. Abundance of deposits.

The causes of final success were:

1. A stoppage of deposits.

2. Dredging in a sheltered gulf.

The character of the piers, which were chiefly built of riprap, as shown in the plates accompanying this article, indicates that the foundations were not of the yielding nature of the Mississippi deposits, and as there is no mention made of difficulties arising from the nature of the bed, it is assumed to have been ordinarily good.

The Board passes to another instance of improvement, cited from the same author, of the mouths of rivers in tideless seas, viz, the Sulina mouth of the Danube.

The improvement was made by the construction of parallel piers. The north pier is a continuation of the left bank of the river. It is 4,640 feet in length, and is carried out to what was the 16-foot line before the work was even begun.

*

The south pier approaches the north pier on a curve, and then runs parallel with it, terminating 600 feet short of the other pier.

The construction was of an outer line of sheet-piling, stayed by a framing of piles and timbers, the foot of the sheet-piling being protected on both sides by a large deposit or bank of stone, rising to the level of the water. At intervals there was an inner row of sheet-piling, with cross-lines to the outer row, the space or box thus formed being filled with stone, and both rows protected on their exterior by a deposit of stone.

Since the construction of piers the depth has never been less than 16 feet, and is usually 17 feet, there having been at times a good channel of 17 feet.

The piers were finished in 1861, and in 1863, the date of the article, the experience was that the depth varied at times from 16 to 17 feet, the latter belonging to floods of "unusual violence."

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Spring floods of that year had formed a menacing bank on the continuation of the north pier and about 2,500 feet to seaward of it, but this bank was speedily broken down by the spring gales and carried away by the littoral current. In 1862 the floods threw down a similar bank, but without obstructing the navigation, and that bank was again removed by the action of storms and currents. The rapidity with which they were removed seems to favor the supposition that the bar will only creep across the front of the piers when the general advance of the delta should have pushed the littoral current farther away from them, and thus have caused a double effect dangerous to the channel.

In the first place, the river current would not then be turned southward, and would throw down its deposits immediately in front of the piers; and, secondly, the banks thus formed, instead of being broken down by the gales and carried southward, would be driven back on the channel, which they would choke still more.

Since this article was published, it is ascertained that the south pier has been extended as far seaward as the north, and that a depth of 20 feet has been obtained.

The board are not in possession of a paper on the same subject, by Sir Charles Hartley, the distinguished engineer who constructed these piers; but it is impossible to entertain a question as to the causes of the success of the pier system at this place.

An inspection of the map accompanying the article shows a great development of the delta form at the Kilia mouths, and the same formation to a less degree at the St. George mouth, with a consequent projection outward of the shore-line; but at the Sulina mouth no formation of the kind is distinctly traceable, and it is inferred, though the fact is not material, that the quantity of sediment emptied at this outlet is small in comparison with the others.

The construction of the piers indicates a difference in the character of the bed, as to resistance, from that of the Mississippi, it being certain that this description of work would not answer at the latter place. The cases of the Vistula and of the Sulina mouth of the Danube manifest essential points of divergence from the circumstances attending the improvement of the Mississippi, and the results obtained in the former cases constitute no precedent for the employment of the same means at the latter place.

Upon a review of the practical difficulties which the adoption of the jetty system of improvement at the mouth of the Mississippi would entail, and a due consideration of the original cost of construction and of annual extension, entertaining doubts, moreover, of the successful issue of the attempt, the board does not consider it advisable to recom. mend it.

With regard to the cost of this operation, owing to the uncertain nature of the problem, made so by the peculiar risks attending it, the board find it impossible to fix any reliable limits.