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crown was 16 feet and was covered with a pole grillage having squares of 8 feet. The whole was then covered with stone 1 foot in thickness, top as well as slopes. The sag near the Elmot end was similarly repaired, and the rest of the dam brought up to the 14foot stage by successive courses of brush topped with a heavy layer of stone.

Work was begun November 1, 1894, and completed in January, 1895. The entire repairs cost about $36,000.

SEASON OF 1895

An examination of the dam in the fall of 1895 (Plate LXVI) showed that further settlement had taken place, being most noticeable at the location of the break repaired the previous season and at the sag at the Elmot end. The lowest point of the dam was at about the 10-foot stage. Additional deposits had been made both above and below the dam. Below the dam the deep pool, excavated by the break of the previous years, had shoaled to less than onethird its former depth, and this, as well as several smaller pools, were almost completely land locked, only a small rill, which could be forded, discharging from the larger one. At the Elmot Bar end the depression was about 125 feet long, but not more than 1 foot below the restored elevation of 14 feet. There was no water here, either in front or rear, and the effect of this sag was entirely local, neither the fill above nor that below showing any scour or wash. In view of the favorable conditions prevailing it was not considered necessary to do any work here. On the contrary, it was believed that these depressions, especially the larger one, should be left open, thus forming a convenient and necessary spillway, and thereby preventing the creation of a destructive head in front, besides carrying much unfiltered water through, whose load of silt would tend to accelerate the filling up of the chute below.

SEASON OF 1899

No further work was done in this chute until 1899, and in the meantime changes were taking place. At the old depression near the Elmot end the top was now at about the 12-foot stage. (Plate XVII.) From that point to near the other old depression no appreciable settlement had taken place in the dam. The dam at the other depression had been broken and there was now a gap nearly 100 feet wide and at the deepest place 12 feet deep below low water. From this break to the Tennessee end the dam had settled consid

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erably. The dam was so silted up as to form a solid mass, but where the brush work was exposed to view it was shown to be badly da cayed.

Above and below the middle of the dam and the Elmot end larga deposits had been made. A sand bar, with its crest above the 18-fot stage, had made down and now covered nearly 500 feet of the dam. This fill was continuous to near the head of the chute and joinut Elmot Bar at its head and was largely covered with a growth of willow and cottonwood.

Below the dam, opposite the break and thence to the Tennesete shore, a hole had been scoured out with a maximum depth of 61 feet below low water, This scour extended through the gap

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upstream toward the head of the chute in a deep depression several hundred feet wide, but it was closed at its head by a bar about ? feet above low water. Below the dam this depression gradually de creased in depth, and the filling near the head of Island 30 was su great that there was no discharge through the chute at an 8-font stage.

The Commission this year ordered the closing of all the gaps in this dam and its restoration to its original elevation of 16 feet abore low water. Repairs were begun in October. As no plant could be floated in, the required mats had to be built on the bar. The first work was to mattress in and above the gap, so as to obtain a solid foundation for the closure section and to form a good apron be. hind it. The shape of the required mat was accurately plotted and the mat built accordingly upon ways upon the bar, close to the edge of the pool. In other respects the mat was constructed precisely as a river fascine mat, the construction of which will be described in

ail in the discussion of bank revetment. When the mat was completed, it was swung around 90 degrees, so that the fascines would lie normal to the current, and then dropped into its proper place and sunk. It covers the bottom of the gap, lapped on the old mat below, and above the gap spreads out in a fan shape to cover all the scoured hole. After the mat had been sunk, the construction of the crib designed for closing the gap was begun. In order to avoid the deep water and sloping bottom in the gap, this crib was built curved like an arch springing at either end from the uninjured part of the dam and curving upstream. Its base where it spanned the deepest water had a width of 72 feet, narrowing to 50 feet at either end. Its crown width was 24 feet in the middle, reducing to 16 feet at each end, while its elevation from the base up was 22 feet.

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The length of the whole arch, measured on the middle line, was 148 feet. The crib was built up in tiers, the three bottom ones being 4 feet thick and the two upper ones 5 feet; the whole was built as

one mass. Each tier was stepped back 4 feet on the upstream and Pris 8 feet on the downstream side. A layer of stone was distributed

over the top of each tier before the next one was started, and the top as well as both slopes were covered with a close layer of the same material. The elevation of the rest of the dam was restored by means of brush cribs of varying widths and thicknesses, the width being from 16 to 30 feet, while the maximum thickness was 742 feet, the whole being covered with as much stone as was avail. able. The revetment on the Tennessee shore, just below the dam, having been damaged somewhat by eddy action, two new mats were made to cover the foot of the slope of this bank and to extend some distance up the bank. These were as usual ballasted with stone. All the stone used during the work had to be taken from the old work, as none was stored within hauling distance, and no barges

could be floated in to the chute. Before the work was thoroughly : secured the river began to rise rapidly, and preparations were at

once made and stone was boated up to the dam. This rise of the po river was only temporary, for after coming to a stand for several

days near the 12-foot stage, the river fell again. This rise, however,

did some damage to the dam. The dam was almost impervious, and the by the time the river reached its maximum, the head at the dam was

fully 4 fet, and under the resulting pressure the new work was pushed bodily out of place as much as 8 feet, and at the same time it

settled nearly 3 feet, which allowed water to How over it, and this bent overflow relieved the head. After the water had fallen again, it was

seen that no damage had been done to the work other han the above.

The cost of the repairs to the dam in 1899 was about $10,000. No work has since been done in Gold Dust Chute.

SEASON OF 1900

With the next rise of the river, however, a more serious failure took place; this time not at the site of the old gap, but some 400 feet from it toward the Elmot end, where the work had never before given any trouble. This occurred just before the dam became submerged and resulted in a break in the dam, judged to be at the time of submergence of the dam about 125 feet. This break occurred first in the form of a boiling up of water and sand some distance be

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low the dam, and shortly after this the dam itself was seen to settle down and disappear.

The cause of this failure of the dam was no doubt the excessire head forcing the water under and through the foundation of the dam, carrying with it the soft material upon which the dam restai and thus undermining it. The brush and poles were so badly de cayed as to be nothing more than punk, there was no cohesion in the mats, and the whole section quickly disintegrated.

It is believed that the closure of the old gap was an engineering mistake. The sides and bottom of the gap could and should have been protected from further damage, but this should have been all for there was in good condition below the gap a large and efficient apron mattress. The gap in this condition would have formed a convenient spillway and would have prevented the formation of a head too great for the foundation of the dam to stand, and yet the rest of the dam would have acted as a serious impediment to the flow of the current through the chute, and would have caused rapid deposits, probably resulting in the eventual closing of the chute.

During the fall of 1900 a careful survey of Gold Dust Chute was made, and the result is shown on Plates XVI and XVII.

The condition of the dam was found to be as follows: From the Elmot end the dam for half its original length was completely silted up, and for a long distance was buried from sight under a sand bar. which had filled to more than 19 feet above low water. Only in places was the dam still visible, and at these places the sand had piled up, both above and below the dam, to about its crest. The Tennessee end of the dam was still in existence and exposed to view. The new brush placed on it during 1899 had been partly washed away, and the dam had been otherwise somewhat damaged. The

gap in the dam was about 600 feet long with a maximum depth of 18 feet, and both above and below it there were holes of greater depth. Above the dam there was a deep depression that extended nearly to the head of the chute, but below the dam the depression extended only a short distance before it spread out and disappeared. The zero contour (based on the low water of 1879) did not extend completely through the chute, but as the river did not reach this year a very low stage there was throughout the season some discharge through the chute.

On Plate XVII there have been plotted the conditions of the dam and its vicinity in 1899 and 1900. The damage done by the

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breaking of the dam is there shown plainly and no further comment is necessary.

Comparing the survey of 1900 with that of 1893 (Plate LXIV), before the dam was built, we find the following: At the head of Gold Dust Chute in the triangle formed by the upper dikes there had been some scour. From that point to below the dam there had been a large fill on the Elmot side of the chute and a scour near the Tennessee side. In this part of the chute the cross section at low stages had increased, but at medium and high stages there had been a marked increase. From some distance below the dam to the head of Island 30 there had been fills in some places and scours in others, the general condition of this part of the chute being about the same. Elmot Chute had narrowed up very much and was now closed at

the 7-foot stage. The willow growth on the sides of this chute was L encroaching gradually on the chute, and in general it appeared as

though this chute would probably be shortly completely closed. In the chute of Island 30 the general condition was not greatly changed. Taking Gold Dust Chute as a whole, the fills had largely

exceeded the scours, but nevertheless its capacity for carrying 7 water at low and up to nearly medium stages is probably greater

now than in 1893.

Since the break of 1900 no work of any kind has been done in Gold Dust Chute, and its condition seems to have remained about unchanged. This chute is thus still open, in spite of all the works that have been placed in it. The causes of the different failures have all been stated and need not be repeated here.

The total amount expended on the construction of Gold Dust Dam and on its repairs to date has been about $140,000, and this, with the $640,000 expended in 1882-1884 on the dikes at the head of the chute, and the $87,000 in 1889 and 1890 on the dikes in Elmot and Island 30 chutes, makes the total cost of contraction works in Gold Dust Chute $867,000.

EXPERIMENTAL TRIANGULAR FRAME DIKES

The experience with pile dikes and Gold Dust Dam had not been encouraging, and for some time the Commission attempted no further contraction works, but in 1898, upon the suggestion of Col. Amos Stickney, Corps of Engineers, they decided to try experimentally a form of dike that had been used with success on the Missouri River.

In general form this dike consists of a triangular frame work,

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