Page images
PDF
EPUB

The depth of the water in the channel across the tidal bar at this time is only about thirteen feet, but vessels drawing fifteen feet can pass the bar with some difficulty, by dragging their kecls through soft mud at the bottom, about two feet deep. (With regard to the numerous passes above mentioned, and various others that have not been noticed, there is no doubt that each in its turn has once been the most favorable channel for ship navigation, and it may with equal assurance be predicted that the channels now most favorable for navigation will sooner or later give place to others somewhere else, whose future position cannot now be defined.)

NATURE AND CHARACTER OF THE OBSTRUCTIONS.

The tidal bar which now exists and has probably always existed at the mouths of the Mississippi is the main impediment in the way of ship navigation between the river and the gulf. This bar is composed of the alluvious depositions that invariably occur at the points where the fresh water currents of the river meet and are intercepted by the salt waters of the gulf, and extends entirely around the delta formations that constitute the grounds over and through which the river pours its waters into the gulf by its numerous outlets.

The bar at first is composed of recent alluvion in a semi-fluid state, and consisting of particles exceedingly comminuted and unctuous. In time, and by the agency of the gulf billows, it becomes more or less solidified or indurated, and resolves itself into beds and bars, imposing serious impediments in the way of navigation.

From causes hitherto unexplained, portions of the bar of greater or less magnitude, and in innumerable instances, are acted upon by upheaving forces from beneath, and rise vertically in various forms, mostly conical, to the height of several feet above their bases, (the greatest height observed being about 12 feet.)

The cones or mounds thus formed, in the language of the pilots, are called "Mud Lumps," and occur not only at the bottom and sides of the channels across the bar, but on the surface of the swamps, rising in some instances to the height before mentioned above that surface. The mounds on the swamps present circular orifices at their summits many inches in diameter, and extending vertically downward to a very great but unknown depth. From these orifices are emitted streams of salt water more or less copious, accompanied by ebullitions of inflammable gas.

The submarine upheavings or lumps in the passes are supposed to be analagous in their character and formation to those on the swamps, and in both cases they are supposed to arise and subside more or less frequently as the causes producing them continue operation during periods of greater or less duration.

In some instances they appear to have a permanent existence, while in others they subside to their original level after the lapse of a few weeks, which last is true with respect to all localities removed from the tidal bar.

The lumps in very many places near the tidal bar rise to the surface of high water and become nuclei, around which the more recent

deposits of the river accumulate, till at length the swamp lands are formed into delta-shaped islands, and cover the entire areas previously occupied by the lumps at the sides of the outlet channels.

The mud lumps that rise in the channels are covered with a thick coating of fine, adhesive clay, smooth and hard upon the surface, yielding slowly to the action of the strongest currents, and impracticable of removal except by the application of mechanical force; strong iron rakes, harrows or scrapers, dragged across them by steamers, have heretofore proved efficacious in reducing and removing the lumps.

The bars in the deeper channels occasionally contain embedded logs and trees, difficult of removal; but obstacles of this character are far more numerous on the shoals and bars at the sides of the channels. No formidable snags are presented in connexion with any of the passes.

With respect to the existence of reversed currents or "under tows" of salt water beneath the fresh water volumes or sheets that pass over the tidal bar, and supposed by some to be mainly instrumental in forming the bar, Professor Forshay, late of New Orleans, has shown satisfactorily that no such under tows have any existence at any stage either of flood or ebb tide.

The surveys and investigations made under the direction of Captain A. Talcott have led to results of the highest importance in reference to the character and condition of the passes and to the obstructions in the way of their navigation.

The developments subsequently brought to light in connexion with the coast survey, under the direction of Professor Bache, abound in accurate details in relation to the geography and hydrography of the gulf coast in the vicinity of the passes.

The report of the board of engineers organized by the War Department in 1852, and composed of Captain William R. Latimer, of the navy, and Majors Chase, Barnard and Beauregard, of the corps of engineers, exhibits numerous valuable results derived from instrumental surveys, and abounds in representations and conclusions entitled to the highest consideration.

To the memoirs furnished on these several occasions, I beg leave to refer for more elaborate and reliable information than my opportunities have hitherto enabled me to impart, on the subjects to which my attention has been directed.

METHODS OF IMPROVEMENT.

Various projects and devices for the improvement of the outlets of the passes across the tidal bar have been entertained and put forth as the best method of improvement, two of which only have as yet been tested, of which the following are examples, viz.:

1. The process of dredging, and the removal of the materials dredged into the deep waters of the gulf. This process was adopted in 1838 under the direction of Captain Talcott, and prosecuted with doubtful success. The results produced are represented as having had a tendency to form new bars at the outlets, and as having been

injurious rather than beneficial to the navigation across the bar. With the details of the operation I have had no opportunity to become acquainted.

2. An application of large triangular harrows, armed with strong iron teeth or coulters protruding 15 to 18 inches below the timbers of the harrow, or with a strong iron scraper attached to the aft transom of the harrow, was made in 1853 by the Tow Boat Company of New Orleans. This machine was dragged downwards along the bottom of the channel by a powerful steam-tug, and proved efficacious in cutting away the mud lumps and reducing them to a semi-fluid consistency. Thus reduced, the materials of which they were composed were swept by the current into deep water outside of the bar.

This process of removal was at first attempted in the channel inside of or above the bar, and afterwards with better success at the outlets immediately below the bar, and thence upward in the channels by successive soundings of the tug, the operations of the tug and machine on the bottom being invariably in the direction of the current. On every upward return of the tug for a new operation the harrow was raised from the bottom of the channel by means of a derrick or crane, erected on the stern of the tug for that purpose.

No doubts are entertained with respect to the utility and efficiency of this mode of operation. The work of improving the channel across the bar at the Southwest l'ass was undertaken and contracted for by leading members of the Tow Boat Company in 1853, and was prosecuted with due diligence in that year. The contract provided for the opening of a channel at least 300 feet wide and 18 feet deep. The work having been finished and duly inspected, payment was made therefor, in accordance with the terms of the contract.

The method devised by Messrs. Craig & Rightor, sanctioned by the War Department, and made the subject of contracts with those gentlemen, now claims attention.

The contracts provide for the opening of a straight channel at least 300 feet wide and 20 feet deep, entirely across the tidal bar, at the outlets of the Southwest Pass and the Pas à l'Outre.

The manner of accomplishing the work consists in the application of pile dams, for the purpose of intercepting the discharge of water through numerous outlets on both sides of the respective passes above mentioned.

In addition to the dams above mentioned, the method of the contractors contemplates the construction of jettys of plank piles on both sides of the Southwest Pass, commencing at points about 1 miles above the tidal bar, and extending downward in directions more or less convergent till they arrive at or near the bar, leaving a space of several hundred feet between their lower extremities, the length of the jettys being a mile or more. The volume of water being thus reduced in breadth is expected to move with an accelerated velocity, and sweep the mud lumps, &c., from the channel, and convey them across the bar into the deep waters of the gulf.

With respect to the Pas à l'Outre, the method contemplates the construction of a pile dam about a mile long across the head of the north channel of the pass for the purpose of diverting the flow of

water from the north through the south channel.

It moreover con

templates the construction of pile dams and jettys on the southerly side of the Pass for purposes similar to those described in reference to the Southwest Pass.

The efficacy of this method cannot be predicted with any degree of certainty, but must remain for demonstration till the works have been completed according to contract.

Progress of the work.

The operations of Messrs. Craig and Rightor have hitherto been confined to the Southwest Pass. I visited their work on the 27th ultimo, and had the satisfaction to find that they had formed a line of piles in a direction obliquely downward towards the deepest channel across the tidal bar 1,064 feet long; the depth of water along the line varying from four to eight feet. The line consists of a series of posts or piles about one foot square driven vertically into the bottom to the depth of about 25 feet below its surface, and at the distance of about 15 feet apart. Longitudinal strings or streamers, four by eight inches and 30 feet long, are spiked to the upper sides of these posts a little above the surface of the water. The strings serve as guides to the plank piling, which is composed of planks five inches thick at top and two and a half inches thick at bottom, rebated in the manner specified in the patent. The plank piles vary in width from 12 to 20 inches, are driven 10 to 15 into the ground, and fastened by iron spikes to the upper sides of the streamers. The spikes employed for the fastenings are about half an inch square and nine inches long, and the timber of the structure consists of yellow pine and cypress. The line thus formed presents a substantial and well-built structure, is able to withstand a current of about two miles per hour, and is quite impervious to water. On the upper side of the line the depth of water remains as it was before the piles were driven, while on the lower side sedimentary deposits have been made to the depth of four or five inches, the water on this side having become nearly or quite stagnant.

Since the date of the inspection as above, I have been informed that the line has been prolonged more than 500 feet, making the entire length of the line about 1,600 feet. The daily progress in extending the line, the requisite material being at hand, and the weather favorable, is about 150 feet.

In the present state of the work, no reliable conclusions can be drawn with respect to its final efficiency in producing the desired results.

I propose to inspect the work again for the purpose of instituting a system of triangulation and soundings preparatory to a final inspection when the work shall have been completed.

In conclusion, I take leave to offer some of my convictions in regard to the past, present, and future condition of the outlets of the Mississippi, and their susceptibility of improvement.

The history of their navigation acquaints us with the fact that no material changes have taken place in the depths of the channels

across the tidal bars, although their geographical positions have been continually subject to change.

The greatest depths in the channels across the bar have seldom, probably never, exceeded 19 feet. The prevailing greatest depth for the last half century has been limited to 17 or 18 feet, and such is the limit at the present time.

The opening and maintenance of a permanent channel of a depth exceeding this limit are at least very doubtful, if not utterly impracticable, for a duration exceeding a single year. The frequent changes of the channels, the vast accumulations of sedimentary deposits, and the effects produced upon them by the billows of the gulf, forbid the expectation that any permanent benefits can result from any efforts to effect such an object.

In my opinion, the only feasible remedy that can be found for the evils complained of, consists in the application of powerful rakes, harrows or scrapers, propelled by steam power, for the purpose of scalping and reducing the mud lumps as often as they present themselves at the bottom of the deepest channels at the bar.

For this purpose, an annual appropriation of $100,000 for an indefinite series of years is deemed adequate for its accomplishment. In the mean time the commercial world, so far as it has any connexion with the Mississippi at and below New Orleans, should be notified that vessels drawing more than 17, or at most 18 feet, cannot cross the tidal bar and enter the river without serious hazard to their proprietors.

S. H. LONG,

Lieutenant Colonel Topographical Engineers.

No. 10.

Colonel Long to Colonel Abert.

LOUISVILLE, May 5, 1857.

SIR: In my inspection report of the 6th ultimo I proposed to revisit the work of Messrs. Craig and Rightor, and institute a system. of triangulations and soundings for the purpose of testing, according to their contract, the results produced by their operations in furtherance of the fulfilment of their contract stipulations.

Accordingly I repaired to the site of their operations on the 15th ultimo and found them still employed on the line of piles mentioned in my report of the 6th as having been extended about sixteen hundred feet. The further extension of the line had been prevented by a violent norther which occurred on the 5th ultimo, and was again repeated, but with less violence, on the 12th of the same month. These storms contributed to break up and derange about one-third of the line previously formed, and left the remaining two-thirds in its original position, though somewhat shattered in places.

The contractors are of the opinion that the catastrophe happened

« ՆախորդըՇարունակել »