REPORT OF THE CHIEF OF ENGINEERS.

of the charts, the conclusion must be such as to warrant belief that no greater changes will be observed during the next one hundred years unless new causes for change are introduced.

Let us consider the possibility from new causes.

The dimensions of the pass are determined by the volume and velocity of the tidal currents through it. These are dependent chiefly on the area of the reservoir to be daily supplied and depleted through the pass.

So long as this reservoir (the west end of Breton Island Sound, and the bays, bayous, and lakes connecting with it) retains its present area, there can be no change resulting from natural causes alone in the dimensions of the pass. That this area cannot be materially diminished by silting, is evident from the following:

The ebb-current through the pass being always stronger than the flood-current, is able to return to the Gulf the greater part of what silt may be brought by the flood-current from the eastern outlets of the Mississippi River. These outlets are at a considerable distance from the pass. The present indications are that the nearest is filling, and we know that the others are pushing out farther into the Gulf, increasing their distances from Breton Island Pass. There is no reason to apprehend the formation of new outlets above the present ones. Should there be, at any future time, cause for apprehending breaks in the river-bank that might result in injury to the pass, comparatively inexpensive levees can be built to effectually guard against them.

As before stated, there are no streams emptying into the sound, and the Mississippi River is securely leveed. There can be no silt brought in from that direction, except through a possible crevasse. There can

be no silt brought in from the northeastward through Chandeleur Sound, since the tides through Grand Gosier Pass effectually cut it off. Filling of the sound by the gradual growth of oyster-beds and reefs is barely considerable.

Since existing sources of silt are being blocked up or further removed, and since possible new sources can be easily guarded against, I conclude that the area of Breton Island Sound will remain as now for an indefinite time, and the dimensions of Breton Island Pass remain unchanged from natural causes.

The constructions (the jetties) required to carry the débouché of the canal to the deep water of the pass will cause changes, the general character of which may be predicted.

The area of cross-section of the pass will be diminished by the construction of the jetties proposed by 70,000 square feet.

The area of Breton Island Sound remaining unchanged, the volume of water going through the pass to fill or deplete the sound daily will be, after the jetties are built, the same as if they were not constructed. This will cause increased velocities of the tidal currents and increased scour of the bottom, and of the Breton Island side of the pass, until such time as the present area of cross-section is restored.

The axis of the pass will be shifted to the eastward and its direction slightly changed. The contour of the head of the pass will be considerably changed, and the west end of Breton Island will probably, considering the character of its formation, be abraded unless protected by works of art.

From the character of the bottom in the deeper portions of the pass, it is probable the pass will regain its normal dimensions entirely at the expense of the Breton Island side. Unless the end of the island is protected with such protection as may be necessary, the pass will deepen. The extent of the changes which the jetties will effect can only be defi

nitely ascertained after the construction of the latter. Injurious changes may be prevented by the protection of the west end of Breton Island, as may be found necessary in the course of construction.

While the effect of the proposed jetties on the east side of the pass cannot be accurately predicted, their effect on the west side, on Sable Island, may be predicted with certainty. The angles between the shoreline and the jetties will partially fill with sand and other deposits. This has been the experience of engineers with all jetty constructions on our northern lakes, on our Atlantic seaboard, and in Europe; at the mouths of the Oder, the Vistula, and the Danube, and, recently, at the débouché of the North Sea Canal and of the Suez Canal. In all these cases this filling is considered objectionable, inasmuch as the tidal currents (shore-currents) perpendicular to the jetties are not strong enough to prevent the ultimate growth of the shore-line incident to this filling from extending around the jetty-heads and obliging an extension of the jetties themselves. In the case under consideration, it is possible to secure a very strong current passing the jetty-heads, sufficient to insure us against apprehension of being called upon at any time to extend our constructions.

As there will be no current through the canal, we must expect silting immediately between the heads of the jetties, due to eddies from the currents through the pass. It is apprehended that shoaling of the entrance to the canal from this cause will not be rapid, but that there will be required, to free the entrance from such deposit as may be made, each year, the services of an ordinary dredge-boat for several days, or perhaps weeks.

In the trunk of the canal there will be no silt admitted, the proposed arrangement of sluices for the lift-lock being such as to keep all riverwater required for the lift from passing into the canal below the lock. Slight deposit is anticipated within the lock, the removal of which, quarterly or semi-annually, will be a very simple operation.

There will be deposit of river-silt in the fore-bay of the lock, due to an eddy from the river-current. The amount of this deposit cannot be calculated, and it will require occasional dredging to maintain the depth of this entrance to the canal.

The river-bank at the point selected for the head of the canal, as before stated, is not subject to change, and the depth of the river-bed may be expected to very gradually increase with the advance of the mouth of the river gulfward.

In conclusion, I can see no reason from the above to question the feasibility of the project, as it has heretofore been questioned, on the assumption of engineers, that Breton Island Pass will deteriorate and the canal itself all with silt to such an extent as to require continued and expensive dredging to keep it open.

In regard to the foundations for locks, reference is made to the section of borings (Chart A) and to the specimens sent herewith.

Borings numbered 1, 2, and 3 show, at a suitable depth for the foundation of a lift-lock, a stratum of mud-lump clay, having an indicated thickness of over 20 feet, and its further thickness not ascertained. This I consider well calculated to sustain the lock shown by drawings submitted, if a solid timber platform, here recommended, is employed, or even if a pile and grillage foundation should be adopted.

As no plan of founding will be adopted, even should the construction of the canal be decided upon, except under the advisement of a Board of Engineers, it is not essential that I should here give reasons for favoring the platform.

Boring No. 6, in my opinion, shows that a good foundation may be had for the guard-lock on Sable Island.

Excavation for trunk of canal.-This will be the least difficult part of the work to execute. It will be entirely through an alluvial formation, free, so far as has been ascertained, from rocks, stumps, and buried timber. It can all be done by dredge-boats, provided with suitable carriers to transfer the spoil to the banks.

Canals have been successfully carried through a similar formation by allowing the excavation of a large cross-section with easy slopes, and by giving the embankments a wide base as compared to their heights. The cross-section of this canal and embankments must be determined while excavation is in progress, and be varied to suit the peculiarities of the formation found in the several sections of the work.

For purpose of estimate, a uniform cross-section has been assumed with sides given, the slope found in the river at the head of the canal. The embankments of the canal at some points may require protection of their outer slopes by facines or wicker-work, or by a growth of willows. Throughout their greater part, sodding with Bermuda grass will be sufficient to prevent washing of the slopes. Washing of the sides of the excavation may be avoided by introducing the towage system of navigation now in use on the river Seine, in France, and Elbe, in Germany, which, I think, will be better and more económical than a railroad and towing-locomotives on the embankment.

The jetties.-The location of these is shown on Chart A. The foundation available is shown to be a good one. Similarly-constructed jetties have been employed at the Sulina, mouth of the Danube, proving efficient, substantial, and economical.

There can be no doubt of the ability of even an engineer of ordinary capacity to construct these jetties in the manner indicated by the drawings, so as to withstand the strongest seas to which they may be exposed. The dimensions adopted for the purpose of estimate may require modification in the course of construction, as they are based on calcu lations having factors the value of which could not be accurately determined.

PLANS FOR CONSTRUCTIONS.

The general and detailed plans for the various constructions pertaining to the canal, and herewith submitted, were drawn up more for the purpose of estimate than with a view to their ultimate adoption without modification. They have been as carefully considered as the time at my command for such purpose would permit.

While some points of the plan are original, the greater number, it will be observed, have been adopted from existing works; for example, the general plan and details for gates, &c. (Due acknowledgment will be found on the drawings.) In all such cases dimensions have been changed to suit the new conditions imposed.

The drawings are intended to be so complete as to render a detailed description unnecessary.

GENERAL DESCRIPTION.

The lift-lock is 400 feet in length between gates, 80 feet wide at top, and has a depth of 27 feet over the sill at mean low tide in the Gulf. The side walls and bottom are of béton, the walls faced with granite.

* North Sea Canal of Holland.

+ Width at bottom, 200 feet; depth, 27 feet; slope, 1 on 2.

These are founded on a solid timber platform, 6 feet in thickness, extending the whole length and breadth of the lock, composed of layers of 12 inch by 12 inch squared timber alternately crossed at right angles and bolted together in the manner adopted for the caissons for piers of the East River suspension bridge, the whole forming a rigid beam calculated to uphold without deflection the weight of the superstructure. This foundation rests upon a stratum of mud-lump clay known to be at least 20 feet in thickness. Seepage is cut off by rows of close piling extended from the lock 60 feet into the bank. The platform is surrounded by a double row of close piling, the heads of the piles being strongly strapped to the platform.

The gates are of iron, circular in plan, and of the kind known as floating gates. Their flotation is calculated, without ballast, for a draught of 28 feet, and for greater depths water-ballast will be introduced or discharged, as may be made necessary by the fluctuations of the tides. The miter sills are wrought-iron trusses. The opening and closing of the gates will be effected by chains and hand-winches.

The sluices for filling and emptying the lock are large cast-iron pipes, laid in the side walls just below the level of extreme low-water in the river. Water is received from the fore-bay to fill the lock, distributed by branch-pipes the whole length of the lock-chamber, then sluiced out through the same pipes into open ditches on either side of the caual. By this method the purest water obtainable from the river is sluiced in on the cushion of salt water already in the lock, and its specific gravity being less than that of the latter, it remains long enough on top to be, with the greater portion of its impurities, sluiced outside of the canal. The sluice gates are arranged with hydraulic lift, and are lowered by their own weight.

The lock is located 400 feet from the river-bank. The approach to it is between two timber wharves, which, besides forming the sides of the entrance, are extended on either side several hundred feet along the river-bank, for the convenience of vessels waiting to pass the lock or awaiting towage to New Orleans. The lower approach is similarly arranged.

All the piles used in this and in other structures are to be carbolized by the Seeley process.

The guard-lock is entirely similar in design and construction to one end of the lift-lock. It is intended to close the canal on occasions of extraordinary storms, which raise the water in the Gulf higher than that in the river.

The details adopted for the jetties are well enough shown by the drawings.

The construction is one of a substantial, though temporary, character, and must ultimately be superseded by one of béton, based on the foundation this will afford.*

ESTIMATES.

The estimates presented have been based on the cost of material and labor for small works undertaken in the neighborhood of the proposed canal.

The magnitude of this work will apparently justify belief that material may be obtained at less cost than that estimated. It is also evident that the material used for coffer-work may be taken up and used in other parts of the work.

* The reason for the adoption of this character of structure is, that it will be economical and can be constructed sooner than permanent jetties.

These considerations induce me to leave out of my estimate amounts required for engineering, superintendence, and contingencies. With this explanation the following estimates are submitted:

For construction of coffer-work for same....

For construction of wharf and excavation at river entrance..

1. Excavation of trunk...

2. Construction of lift-lock.

3. Construction of coffer for same.

4. Construction of jetties....

5. Construction of guard-lock.

6. Construction of coffer for same

SUMMARY.

7. Construction of wharf and river entrance.

Grand total....

$3,966, 673 50 750,900 00 250,938 82 1,949, 497 70 226,885 80

118,068 28

107,839 48

$3,966, 673 50 750,900 00 250,938 82 1,949, 497 70

226,885 80

118,068 28

107,839 48

7,370,803 58

SUMMARY OF CONCLUSIONS.

1st. The construction of the canal is a matter of great importance. 2d. The project is feasible, and its execution presents no great diffi culties to be overcome by the engineer.

3d. The total estimate of cost approximates $7,400,000, in round numbers.

TIME REQUIRED FOR COMPLETION.

If suitable appropriations are made the work can be completed within three years of the date of its commencement.

To do this, an appropriation of $3,000,000 should be made available for the first year, $3,000,000 for the second year, and the balance for the third year.

Should completion be delayed a longer time than three years from date, a new dredge-boat will be required, at an expense of about $250,000, to continue the work of dredging at the mouth of the Mississippi River, maintaining its present effectiveness.

The cost of construction will be increased.
Respectfully submitted.

C. W. HOWELL, Captain of Engineers, U. S. A.

UNITED STATES ENGINEER OFFICE,

New Orleans, La., February

1873.

LIST OF SPECIAL AUTHORITIES CONSULTED.

To ascertain importance of the project.

Statistics from the Bureau of Agriculture, made available through newspaper publi

cations.

Commercial statistics from reports of various commercial bodies.

Report of the Chief of Engineers for 1871, pp. 632-3, &c.

Pamphlet compiled by E. Lorraine, chief engineer James River and Kanawha Canal Company, published by the company in 1869.

Reports of Chief of Engineers, United States Army, since 1867.