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water from the north through the south channel. It moreover contemplates 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.
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
in consequence of their having used piles of inadequate length in the formation of the lower portion of the line, the depth of water on this portion varying from eight to more than nine feet. In repairing the breach they have adopted piles of a much greater length and driven them much deeper into the bed of the Pass. By applying this remedy, and observing similar precautions on other portions of their work still to be done, they feel sanguine no such disaster will again occur. The contractors had now two pile-driving boats in operation, one worked by steam, employed in repairing the breach at the lower end of the line, and the other by manual power, at the upper end of the same line. The length of the line as repaired and extended on the 21st ultimo was about the same as heretofore reported, viz: sixteen hundred feet.
I desired of the contractors a designation of the point at which they proposed to cross the tidal bar and enter the open gulf with the new channel; and in reply was informed that they had fixed on no such. point, nor did they intend to do so until the changes and currents produced by their work should indicate the best position and direction of the channel in question.
TRIANGULATIONS AND SOUNDINGS.
I assumed as a prime base a line 1,000 feet long, carefully measured along the line of piles, the direction being N. 40° E. or S. 40° W. Signal poles bearing red jacks at their tops were erected at the extremities of this base. At each extremity the angles made by lines drawn to the light-house were carefully observed by the use of a box sextant, graduated, by means of the vernier attached, to minutes of a degree.
The distance from the lower end of the assumed base to the lighthouse was carefully computed, and assumed as a secondary or main base for all subsequent triangulations.
The triangulation was extended upwards about two and a half miles to the observatory at the pilot's station, on the easterly side of the Pass, and to the telegraph station on the westerly side, and downward to a high pole and signal on an extensive mud lump of some fifty or sixty acres, called Stake island; also to a large metallic buoy, called the can buoy, situated at the bifurcation of two distinct main channels leading across the tidal bar, through one or the other of which all vessels entering or leaving the Pass must be conveyed.
In addition to the points of triangulation above mentioned, four other points were established and marked by signal poles, surmounted by red flags or jacks, set in mud lumps, rising some eight to ten feet above the surface of the water, and presenting areas varying from about one-eighth to three-fourths of an acre, respectively.
Two of these signals were located at the distance of about two miles apart, on a line crossing all the channels of the Pass, nearly at right angles with the general direction of the currents of the Pass, and about one mile below the lower extremity of the primary base.
The other two signals were placed at the distance of about two and a half miles apart, on a line nearly parallel to that of the two signals
just before mentioned, and at the distance of about three-quarters of a mile below them.
The line last mentioned is nearly coincident in position and direction with those of the crest of the tidal bar, and crosses the main navigable channels leading across the bar.
The selection of the four points indicated as above was made in conformity to the presentation of suitable mud lumps rising above the surface of the water in positions most favorable for lines of soundings, leading across all the channels intended to be affected by the work of Messrs. Craig & Rightor in the Southwest Pass.
The position and extent of the several lines having been determined, soundings are to be made and repeated as often, at least, as once a week on all the lines leading across the channels, for the purpose of showing any changes that may take place in the positions, widths, and depths of the channels.
The manner provided for taking and applying the lines to be sounded provides for the casting of the lead at stated intervals of time, (viz: once in every minute or half minute,) and for noting the depth at every sounding in feet and parts.
With the data thus obtained, (the length of the lines having been previously determined,) a submarine section on each line sounded is to be prepared and delineated, the line sounded being regarded as the base, and the depths of soundings as ordinates, depending therefrom at equal distances asunder, corresponding to the number of soundings made along the line. A waved line, drawn through the lower extremities of the ordinates, will show the inequalities of the bed of the Pass, its channel, &c., at the time and place of each set of soundings. This operation, being repeated once a week on every line proper to be sounded, and corresponding delineations being prepared therefor on every occasion of sounding, will clearly exhibit the nature and extent of the changes that occur from week to week in the depths and positions of the channels.
The operations detailed as above, contemplate the preparation of a sectional diagram for each set of observations, the diagrams being numbered 1, 2, 3, 4, &c., in the order of the preparation, for each line of the triangulation upon which the sounding may have been made, each series of diagrams being referred to its appropriate line of soundings, as marked and designated on the plot of triangulations.
In accordance with the foregoing representations, instructions have been furnished to Wm. Johnson, esq., who has been employed to direct and supervise the operations above considered; a copy of which instructions is as follows:
"SOUTHWEST PASS, April 18, 1857.
"SIR: You are desired to remain in the supervision of the operations of Messrs. Craig and Rightor, and of the results produced thereby so far as they relate to the accomplishment of the objects specified in their contract for the removal of obstructions from this Pass.
"In the discharge of the duties claiming your attention you will observe the following details, viz:
"1st. You will make such observations as are necessary to test the
accuracy of the triangulations already made by us conjointly, carefully rectifying such errors as may be detected therein.
"2d. You will repeat the soundings already made on the several lines sounded by us, and continue the repetition as often at least as once every week, the weather permitting.
"3d. You will plot each line of soundings as above, and all other lines of soundings you may find occasion to make, in the manner explained to you, viz: in separate slips of paper, designated and numbered as directed. The plot of each line will be attended by ordinates, depths, and a waving line, indicating the variable depths of the river channel, after the manner exemplified in our late drawings.
"4th. In all your soundings you will ascertain, as nearly as practicable, the elevation of the surface water above ordinary low water at the time of sounding, and make due allowance therefor in the depth of the soundings.
"5th. You will establish one or more bench-marks on a level of ordinary low water, in the following manner, viz: Fix one or more tide-gauges at suitable points; assume a point about an inch below extreme low water and mark it with 0 or zero; divide the gauge-rod into inches and quarters and number the inches, from zero downward, 1, 2, 3, 4, &c.; observe and note the inches and parts on the rod at extreme ebb tide daily for one entire lunation; add the recorded observations together for one lunation and divide their sum by the number of observations, and the quotient will indicate the bench-mark level for the lunation.
"Repeat the observations during a series of lunations, and divide the sum of the series (for one year if practicable, or six months if otherwise,) by the number of the series, and the quotient will indicate the true level of the bench-mark, which may be fixed by driving a stake at the margin of the water at that level till the head of the stake coincides exactly with the surface of the water.
"6th. Keep a daily register of the observations made upon the tide gauge, from which the level of the bench-mark (or ordinary low water,) may be deduced with accuracy.
"7th. Keep a diary of all your proceedings relating to the triangulations, soundings, tidal observations, &c., &c., prepared with neatness and accuracy, to be submitted in the form of field notes" at the expiration of your period of service.
8th. You are desired to report in general terms your proceedings in all respects as often as once in a month, on or about the end of every month, by letters, to my address at Louisville, Kentucky, unless otherwise directed.
"9th. Whenever Messrs. Craig & Rightor shall give notice that their work in the Southwest Pass is ready for final inspection (or fix the date at which it will be ready for final inspection,) you are desired to apprise me thereof by telegraphic despatch if the result of your previous observations and surveys should render it certain or highly probable that the work is actually ready, or will be ready for final inspection according to contract.
10th. It is understood that the work will be ready for final inspection when a straight channel, 300 feet wide and 20 feet deep, clear of