페이지 이미지
PDF
ePub

DREDGING.

Beginning at Albany Point, on a straight line through Sodo Lake, the distance to deep water in Willow Pass is about eight miles; over this line the depth at low-water varies from 1 to 5 feet. This is the line that apparently should be adopted for a dredged channel through the lake, for the reason that, except through a very short portion of the lake, (about one-quarter its length,) the old channel of Cypress Bayou has been obliterated by deposit, while that portion that may yet be traced is so narrow and tortuous that it would require widening and straightening by dredging to put it in good condition.

Dredging on the straight line indicated, to obtain a 3-foot channel, would require excavation to a depth of about 2 feet for one mile over Albany Flats, and excavation of about 1 foot, for the same distance, at the foot of Willow Pass; a total excavation of about 35,200 cubic yards for a channel 60 feet wide.

The excavation would run through the remains of a Cypress Swamp, and would consequently be very expensive. Further, it has been shown by the surveys made that Soda Lake is the main settling-basin for the Red River water passing through it.

The deposit in this lake has been so great as to entirely cover the knees of the cypress yet standing in the lake, while in the other lakes these knees are yet uncovered.

The depth of such deposit was not ascertained, nor is it known for how long a time it has been forming, but the covering of cypress-knees indicates a thickness of several feet, and the living trees at points in the lake show it to be a very recent formation. The inference is that since the cause of deposit does and will continue, any dredged channel through the lake must sooner or later be silted up. If, then, dredging be resorted to, the depth should be made considerably greater than actually necessary for the time being, in order to avoid annual dredging to keep the channel open.

I assume, therefore, that the depth of a dredged channel through Soda Lake should be 6 feet at extreme low-water; this would make an average dredging of about 2 feet, sixty feet wide, for a distance of eight miles, equal 563,200 cubic yards of excavation. This, of itself, appears sufficient to condemn dredging, but when there is added to it the dredging that would have to be done in the upper end of Fairy Lake and across the bar at the present mouth of Cypress Bayou, it is clear some other plan.of improvement should be considered.

[ocr errors]

WING-DAMS AT ALBANY POINT.

This was Woodruft's first recommendation. He afterward found that the chutes he proposed to close with dams were so nearly dry at low-water that but little, if any, concentration of low-water discharge would be effected by the dams, and that consequently they would be of little or no use in holding the last stages of a flood in Soda Lake to prolong navi. gation through it.

LOCK AND DAM AT ALBANY POINT.

This was recommended by Lieutenant Woodruff in his final report of survey of Cypress Bayou, and the plan was well calculated to give at all seasons a depth of 6 feet from Albany Point to Jefferson. Having doubts of its low-water effects below Albany Point, I have awaited the results of the further survey ordered before considering the project.

The survey of Mr. Leavitt, besides disclosing difficulties of foundation not anticipated by Lieutenant Woodruff, shows that without the interposition of a second lock and dam between the first and Shreveport, in low-water the bayou and lake below Albany Point, for a distance of nearly 6 miles, would run dry. This is best shown in section on Leavitt's chart herewith, (marked B.)

The first lock would require a lift of 10 feet, the second a lift of 13 feet, at low-water of 1873 at Shreveport, to make 3-foot low-water navi. gation from that point to Jefferson.

The walls and gates of the locks would have to be 26 feet high in order not to be overtopped at a stage of water equal to the high-water of 1866, the highest of which we have record.

In view of the above, the project was rejected, and it was not consid. ered necessary to make detailed plans and estimates for it.

DAM AT ALBANY POINT AND CUT INTO RED RIVER OPPOSITE.

few years.

Woodruft's survey showed the whole fall from Jefferson to Albany Point to be but 3.5 feet at a time when there was 4-foot navigation orer Albany Flats. Of this fall, 2.3 feet was from Jefferson to the head of Fairy Lake. The lake was a level; from the foot of the lake to Albany Point the fall was 1.2 feet.

By placing directly across the water-way at Albany Point a dam having its crest 14 feet below the local high-water mark of 1866, there will be given over Albany Flats a depth of about 6.5 feet, whence, from the above, it is evident there will be given to the foot of Fairy Lake a least depth of 5.3 feet, and from thence to Jefferson a least depth of 3 feet throughout the year.

It is considered that the latter depths will be greater than stated, for the reason that the slopes reported will not probably be very greatly diminished ; also, no allowance is made for the dredging from Jefferson to the head of Fairy Lake, a distance in which the greater fall occurs, for the reason that it is anticipated that the dredged places will fill in a

For perfect safety the figures are based on a level from Albany Point to Jefferson.

As such dam, while answering the purpose of giving good navigation above it to Jefferson, would effectually cut off the navigation below from that above, it would be worse than useless, except a new connection be made with Red River. A cut to Red River from a point above the dam is therefore necessary as a portion of the project.

The difference in level of water-surface between Soda Lake at Albany Point and of Red River (opposite) at Gold Point renders such connec. tion practicable without lowering the surface created by the dam, as shown by the following:

Our surveys indicate the fact that during all seasons of the year the surface of the lake at Albany Point is lower than the surface of the river opposite at Gold Point, viz, at high-water of 1866, when there was between 19 and 20 feet over Albany Flats, the lake-surface was 2.38 feet below that of the river.

In 1872, with 4 feet over the flats, it was 6.3 feet below; and in 1867, with between 2 and 24 feet over the flats, it was 3.8 feet below.

Although no observation was made to determine the difference of level at extreme low-water, viz, when there is but one foot of water over the flats, it is assumed that this difference is probably not less than 2 feet,

inasmuch as the measurements given were all made before the removal of the raft.

Under this assumption the proposed dam and cut should give at all seasons of the year 3 feet navigation over Albany Flats, a sufficient depth for the present, since the river below Shreveport only affords a depth of 20 inches during low-water; and it is not the local trade between Shreveport and Jefferson that is to be served by improvement, but the trade between the Mississippi River and Jefferson.

Besides the improved navigation it is designed to give above Shreveport, this project presents another subject for consideration.

It is claimed that in case the removal of the raft should prove to effect a shortening of navigation through the Soda Lakes, it will also shorten the season below Shreveport.

This is on the supposition that the lakes serve as reservoirs to detain floods of Upper Red River, so that they are longer in passing Shreveport than they would be if these reservoirs did not exist. If this be true, then the reservoirs are useful in lengthening the season of navigation below them.

The plan of a dam and cut at Albany Point appears to present the means not only for preventing any possible deterioration of these lakes as reservoirs, but also a probable means for increasing their efficiency.

In connection with this three plans for a dam have been considered, and these require notice.

1st. A tumbling dam, having its crest 14 feet below the local highwater mark of 1866. This is the one so far assumed for illustration, and the one called for if navigation above it is alone to be considered.

In this case there is but one objection to it, and that may be overcome by a suitable construction. The objection is this: The character of the foundation offered for a tumbling dam is not favorable to permanence of the structure, the soil being of an easily abraded mixture of sand and clay, with strata of sand at intervals. During low-water the bed of the lake at the foot of the dam will be dry, while just above, on Albany Flats, there will be from 3 to 6 feet.

At the commencement of a rise, when the water begins to flow over the dam, the fall over the sereral portions of the length of the dam will range from zero to 15 feet. It will afterward be some time before the lake below fills sufficiently to afford a useful water-cushion to break this fall. At extreme high-water the surface of the upper lake will be from 10 to 14 feet above the crest of the dam, and the lake below will probably be filled to nearly the same level; during the intermediate stages we must expect powerful eddies about the foot of the dam.

These facts suggest difficulties of construction which may certainly be overcome, but at great expense, and by taking every precaution to have workmen and material on hand so as to begin and complete the work in a single low-water season, the length of which may only be safely assumed at 4 months, and those 4 months the most unhealthy of the year.

If we further look to this dam as a means for increasing the efficiency of the Soda Lakes as reservoirs, then it must appear too low to bave any appreciable effect, for it has been shown that during all except the lower stages, water must run from Red River to the lake rather than from the lake to the river, while after the lake has fallen to the crest of the dam, and further discharge must be into the river, the possible fall of three feet, taken in connection with the area of the lake, indicates such a small volume of discharge that it does not need figures to prove it of no value below Shreveport.

20. A dam from Albany Point to the bank of Red River, having its crest 2 feet above the high-water of 1866, on Red River.

As the first plan of dam is the minimum allowable for improved narigation, so this plan appears to offer a maximum for reservoir effect.

It would throw the whole of the lake discharge into Red River through a channel which (following its meanderings) would be 7.6 miles longer between Albany Point and Shreveport than the present route via Twelve-mile Bayou, and 9.1 miles longer than the route via Cross Bayou.

This difference in length would necessarily retard the emptying of the reservoir, but on the other hand we should lose an inportant portion of the present effect of Lower Cross Lake. The latter would then be only a reservoir in proportion as the water was ponded back into it from Shreveport, whereas now it holds a large volume of the flood-waters received through the Sodo Lakes, for a time after the latter hare well run out. This is evidenced by Leavitt's survey, and a section displayed on his chart, where it is shown that the water-surface in the neck between Upper and Lower Cross Lake was 3 feet above the surface in Twelve-mile Bayou, 510 feet distant, and the same above Red River, just above.

It is questionable if the gain on the one hand would not be balanced by the loss on the other.

There is another and more serious objection to the plan, viz: The river between the dam and Shreveport has not the capacity to carry the volume to be added from the lakes, and to give it the capacity would require radical changes in its bed, involving the destruction of many plantations.

These two considerations condemn the plan of a high dam. 3d. A dam across the foot of Soda Lake, having its crest of the height of No. 2; the crest between the lake and river to be depressed to 9 feet below the bigh-water of 1866.

This is a compromise between the 1st and 2d, and while offering the advantages possessed by both, permits the filling of Cross Lake by direct overflow, and it is thought will not seriously affect the plantations along the river bank.

This is the plan recommended in my communication of May 1, transmitted to the Chief of Engineers in answer to the inquiries of the chairman of sub-committee of House of Representatives on rivers and harbors.

It is the plan I recommend for adoption on two conditions, viz:

1st. That the money required to carry it out be all appropriated before the work is commenced.

2d. That the commerce to be benefited be found to warrant the expenditure. Of this I do not pretend to judge.

The other two plans considered call for but brief mention.

5th. The fall from Red River, through Irishman's Bayou, to the foot of Fairy Lake, January 1, 1872, was 11 feet. It is evident that a dam crossing the head of Sodo Lake, Willow Pass, passing around the foot of Shift-Tail Lake, and connecting with the bank of Irishman's Bayou, might be constructed the same as at Albany Point, and to answer the same purposes.

The length of the dam, as indicated on Woodruff's chart, and its great height, due to the depth of Willow Pass, together with the labor of re-opening Irishman's Bayou, which is filled with raft, shoal, narrow and tortuous, condemns the project, further by cutting off the settling basin afforded by Soda Lake, a great deposit would be induced above the dam, making in a short time, it is anticipated, a second Albany Flats.

6th. Mr. Leavitt suggested a dam across the neck between Upper and Lower Cross Lakes, thence across Twelve-mile Bayou to the bank of Red River, and a cut from the bayou above into the river. This was rejected because of the dimensions of the dam required and the insufficient fall from the river to the bayou, the latter being so slight that it could not have effected the depth over Albany Flats.

The location at Albany Point appears to be a proper mean between the two locations last named.

CONSTRUCTION OF DAV-MATERIAL. The neighboring country affords an abundance of cypress timber, which may be delivered in rafts at the site of the work during highwater. Albany Bluff affords a good clayey soil. Albany and the neighboring bluff's will furnish, it is thought, enough stone (of a quality not suitable for masonry) that will answer for ballast. No other material for construction can be had except from a great distance and at great expense.

The dam must therefore be mainly built of timber.

A pile-dam, judging from the fate of that built across Tone's Bayon, would probably not stand long.

A framed dam would be difficult of construction, costly, and offer no better hope of permanency than one made by piling.

In the Red River raft we found numerous large islands, (comparatively speaking,) formed by an accumulation of timber, cemented together by river-deposit and the roots of willows; these were able to withstand the full force of the river-current during the floods and were difficult of removal by machinery, even when aided by nitro-glycerine.

It is proposed to imitate this natural formation, as is best shown by the drawings herewith, by building a dam with untrimmed cypress-trees, placed butts down-stream, layer over layer, with the interstices filled in with earth from the bluff, and the top and apron ballasted with stone.

On this plan the cost of a tumbling-dam, No. 1, is estimated at $57,661, viz: 4,437 trees, (in place.) at $4.

$17, 7:36 1,576 cubic yards earth, (in dam,) at 50 cents.

788 10,120 cubic yards ballast, (in dam,) at $3.

30, 360 Add 20 per cent. contingencies...

9,777 Total........

57, 661 The cost of dam No. 3 is estimated at $217,314, viz: 24,896 trees, (in place,) at $4.

$99, 584 75,950 cubic yard earth, (in dam,) at 50 cents...

37,975 20,000 cubic yards ballast, (in dam,) at $3.

60,000 Add 10 per cent. contingencies...

19, 755 Total ...

217, 314 Note.-Only 10 per cent. is added in this case because of the magnitude of the work and its offering no more under-water work than No. 1 does.

THE CUT. The place of the cut is indicated on Leavitt's chart. The estimated excavation for it is467,170 cubic yards, which, at 30 cents per cubic yard, will make it cost.. $140, 151 Add 10 per cent. for contingencies..

14,015

Total
Total cost of dam No. 1 and cut
Total cost of dam No.3 and cut

154, 166 211, 827 372, 580

« 이전계속 »