IMPROVEMENT OF THE YONNE AND THE UPPER SEINE.

The following description is taken from the March number for 1873, of the Annales des ponts et chaussées. This article is so valuable in itself, and so pertinent to our present investigation, that but little of it can be omitted. Unfortunately the illustrative plates do not accompany the text, but are promised in some future number. It is believed, however, that the plates that accompany the other reports sufficiently explain this one. The author is M. Cambuzat, chief engineer des ponts et chaussées.

The great water route connecting Havre, Ronen, and Paris with Lyons and Marseilles, by the Seine and Yonne Rivers, the Burgundy Canal, and the rivers Saône and Rhone, (see map on plate 6,) had, until September, 1871, a very defective section; in fact, a veritable gap, 118 miles long, between Paris and Laroche, where the Burgundy Canal enters the Yonne. In fact, for eight or nine months of the year, from March to November, the descent of loaded boats was only possible, especially on the Youne, twice a week, by the aid of artificial floods or waves from the Upper Yonne, and the draught of water available varied from 24 feet to 3 feet 4 inches, and 3 feet 7 inches at most; so that boats from the Burgundy Canal, drawing from 3 feet 7 inches to 4 feet 7 inches, were obliged to break bulk at Laroche. As to ascending craft, they were generally empty, or only carried a few tons of merchandise. This costly, slow, and altogether insufficient navigation, was accompanied by great fatigue, by danger, and by numerous accidents. Although it was somewhat less difficult on the Seine, navigation was much hindered, and often stopped, between Montereau and Paris. Since the 1st of September, 1871, there has been a continuous navigation between Paris and Laroche, thanks to 17 movable dams constructed on the Yonne, and to 2 cut-offs and 12 movable dams constructed on the Seine. The minimum depth of water in the pools is 5 feet 3 inches, and therefore boats can move up and down with perfect safety, drawing from 4 to 5 feet. At present, while the works are yet incomplete, the greater part of the loads, especially those that come from the Nivernais Canal, do not draw over 4 feet; but when the works of the same kind (that is, 8 movable dams and one cut-off, now being built between Laroche and Auxerre, where the Nivernais Canal enters) are finished, which will be by the end of this year, (1873,) the great advantages will be happily realized which the government had in view, and which were looked for with impatience, but with confidence, by the boating, commercial, industrial, and agricultural interests.

The greatest flood ever known on the Yonne, that of January, 1802, rose 16 feet at Montereau. The low-water discharge between Auxerre and Laroche is 459 cubic feet per second, and the average slope 34 feet per mile; between Laroche and Montereau the discharge is 600 cubic feet per second, and the slope 1 foot 10 inches per mile.

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The first effort to improve the navigation of the Yonne was directed toward checking the flow of the artificial floods by movable dams, in order to prevent their too speedy absorption. On this plan, descending boats and rafts were stopped at each dam, and after all had arrived a new flood carried them to the next one. But the increasing demands of commerce for a continuous navigation caused the adoption of the present sysBetween Laroche and Montereau (the mouth of the Youne) there are now 17 movable dams, of which 6 belonged to the old system of navigation by temporary floods. These had each a navigable pass varying in width from 194 to 230 feet. One of these dams was replaced by a new one, but the others were retained, with some modifications. Each had a lock attached for use during low-water. Eleven new dams were constructed, all of which have locks attached, except those at the heads of cut-offs, whose locks are generally at the lower end of the cut-off. The navigation passes of these new dams are 115 feet in width, and the weir has a minimum length of 164 feet. The sill of the navigation pass is placed at 2 feet below the low-water line. The sill of the weir is placed 20 inches above the same line, and therefore is 3 feet 8 inches higher than the sill of the pass. Two cut-offs across difficult bends

shorten the river a little more than 4 miles. At the head of each cutoff is a guard-lock and at the lower end a lift-lock. At the Courlon cutoff the lift of the lower lock is 12 feet 9 inches. The bottom width of each cut-off is 52 feet.

Wherever dredging was necessary a channel was made 100 feet wide and 5 feet deep at low water.

The works for improving the navigation between Montereau and Laroche, authorized in 1861, were nearly completed in 1868, when a continuation up the Yonne to Auxerre was ordered. These new works were commenced in 1869, and although retarded by the war with Germany, will probably be completed by the end of the present year. These new works consist of eight movable dams, (of which seven have locks,) one cut-off, dredging, bank-protection, and minor works. The dam of La Chaînette, just below Auxerre, has a navigation-pass 138 feet wide, closed by a Poirée needle-dam, and a permanent weir 656 feet long, whose top is even with the surface of the pool. The sole of the navigation-pass is 20 inches below low water. The chamber of the lock attached to the dam is 27 feet wide, and has an available length of 305 feet. This dam was built under the old system of creating temporary floods. The seven new dams have navigation-passes varying in width from 98 to 115 feet, closed by Chanoine wickets, with their soles 2 feet below low water. The weirs, whose soles are 20 inches above low water, are from 82 to 131 feet in length. Six of these weirs have needle-dams on the Poirée system, and one has large Girard shutters 113 feet wide, with a vertical height of 6 feet.

Five principal conditions controlled the decision on the position and lift of each dam : 1. The preservation of existing dams.

2. The horizontal plane passing through the tops of the wickets of the navigationpass (whose height fixed the lift of the dam) should be 5 feet above the lower mitersill of the lock next above, and the same distance above the intermediate bars, excepting those which were to be dredged.

3. The height of the natural banks should be at least from 16 to 20 inches above the level of the pool, excepting that certain parts near the dams were to be raised.

4. The space under the keystones of arched bridges, and under the bottom chords of truss-bridges, should be at least 18 feet above the surface of the pools, because the greatest space required for boats is seldom over 16 feet.

5. Finally, a maximum vertical height of from 9 feet 10 inches to 10 feet 2 inches to be given to the wickets of the navigation-pass.

On account of the very small flow in low-water, the surfaces of the pools were always assumed as horizontal.

When the project for movable dams on the Yonne was approved, it was not thought that wickets could be built on the Chanoine system as high as 10 feet, but recently a sluice for navigation has been constructed on the Seine, through the Port á l'Anglais Dam, whose wickets have a vertical height of 12 feet.

1. That, in the first section, Auxerre to Laroche, the 8 dams have lifts varying from 4 to 8 feet, and an average lift of 6 feet. The two locks of the Gurgy cut-off have each a lift of 8 feet. The 9 pools between the mouth of the Nivernais Canal and the La Gravière Dam have lengths varying from four-fifths of a mile to four miles, with an average length of one mile and a half.

2. That the Epineau pool is common to the first and second sections, and has a length of two and one-half miles.

3. That, in the second section, from Laroche to Montereau, the 17 dams have lifts varying from 2 feet 10 inches to 7 feet 5 inches, and a mean lift of 5 feet. The two cut-off locks have lifts of 10 feet 8 inches and 12 feet 9 inches. The 16 pools between the Epineau and the Cannes Dams have lengths varying from seven-eighths of a mile to six and one-third miles, and an average length of three miles.

4. That the distance of two miles between the Cannes Dam and the Montereau bridge is a part of the first pool of the Seine, which ends at the Varennes Dam.

The width of the floor of the navigation-passes of the new dams, with wickets, is from 23 to 33 feet, measured in the direction of the current. The thickness is at least equal to the lift of the dam, and seldom less than 63 feet.

Between Auxerre and Joigny the body of the floor rests directly on solid rock or chalk; the masonry was laid dry in coffer-dams. Between Joigny and Montereau the main body is composed of a bed of beton, poured into an inclosure of piles and plank. On this the floor of the sole, consisting of cut and hammered stone, was placed after the water was removed.

In the masonry floor anchors, iron rods, and cast-iron anchoring-plates are imbedded, in order to bind solidly to the masonry the wooden sill against which the breeches of the wickets rest.

The floor of the weir of the new dams has generally a width of 13 feet and minimum thickness of 64 feet. It is entirely of masonry, or formed by a wooden box filled with beton and covered by a pavement of heavy stone.

The weir lies between a masonry pier 10 feet thick and 20 feet long, which separates it from the pass, and a masonry abutment, which is connected with the bank by two wing-walls.

Below most of the dams there is an apron, formed of heavy riprap of natural or artificial stone. At some of the dams this riprap is held in place by piles driven in quincunx order. The passes of the twenty-two new dams on the Yonne are closed by movable wooden Chanoine thickets, 4 feet wide, with 2-inch intervals. During the season of low-water these intervals are covered by planks to make the dam tighter.

It is well known that each wicket is movable around an axis forming the cap of the horse, which itself turns around its sill, whose journals are held in two boxes fastened in the lower face of the sill of the pass. The wicket, when upright, is inclined at an angle of 15° from the vertical, and laps 3 inches against the upper face of the sill. The top is even with the surface of the pool. The axis of rotation of the wicket is so placed that the height of the breech above the sill is 5% of the total height, and consequently that of the chase is 7. The cap of the horse passes through an eye in the head of a prop, whose foot is supported, when the wicket is up, against a cast iron heurter fastened in the sole of the pass. When the wicket is down the prop is retained in a slide, of which the heurter is the head. When it is desired to lower a wicket, the foot of the prop is tripped by a corresponding projection on the tripping-rod, which is moved horizontally on the sole by means of a wheel and gearing placed in the pier or in a wall of the lock, for each pass is managed by two tripping-rods, each of which acts upon one-half of the wickets, beginning at the middle of the pass.

On the other hand, the wickets, when down, are raised by a boat-hook, worked from a boat furnished with windlass and other appliances.

It is quite evident that the trestles, the props, and the tripping-rods are of wrought iron; the slides and the heurters of cast iron.

The weirs of the fifteen new dams between Laroche and Auxerre have been, since their construction, supplied with automatic wickets, with movable counterpoises, on the Chanoine plan. These wooden wickets are 4 feet wide, with 2-inch spaces between. The weir can be made tighter by applying joint-covers over the open spaces between the wickets.

Each weir-wicket is movable, like a pass-wicket, around a horse, which carries a prop; and for each wicket there is a heurter and slide. M. Chanoine had even added tripping-rods, which, however, he did not regard as very necessary. The axis of rotation of a weir-wicket being only 2 inches above the one-third the height of the wicket, it was only necessary that the water in the pool should rise from 4 to 6 inches above the top of the wicket to make it swing; the movable counter-weight placed at the foot of the breech, which kept the wicket up when the pool was at its ordinary level, slipped to the chase when the wicket swung. If the level of the pool fell a certain distance the wicket would swing back, and the counter-weight would fall back to the foot of the breech.

This ingenious system was striking in its simplicity, and was accepted at once after the isolated experiments made at a single dam, while specially pre-occupied in devising means for rapidly passing a flood, without exhausting the pool above or injuring the passage of river-craft; but a great disappointment was experienced when the continuous navigation on the Yonne and Seine, between Paris and Laroche, came to be tested. An official order of May 4, 1868, in approving the provisional regulations for the new method of navigation, authorized the engineers to put in operation the dams built on the Seine and the Yonne, between Paris and Laroche. The official order recommended that this delicate work should be undertaken with all the precautions necessary to prevent injury to navigation. The dams on the Seine were raised between the 18th of May and the 7th of June. The first four dams on the Yonne were closed between the 8th and 10th of June, but the raising of the thirteen others was only finished by the 5th of September, after the closing of the canals. A difficulty immediately arose on account of the co-existence of the artificial floods of the Upper Yonne, whose waves came twice a week, and swung a certain number of weir-wickets, which wickets, although called automatic, would not raise themselves until there was a fall of 3 feet in the pool above, from which circumstance navigation was much hindered. Thanks to the zeal and activity of the engineers, to the careful watch of the conductors, and to the devotion of the lock and dain tenders, it was possible to master the situation during low-water. With boats and different expedients devised by those in charge of the navigation success was obtained in raising the wickets with sufficient rapidity, and the improvement of the new system was finally apparent and conceded by all. But in the month of August the waters discharged from the canals, and those from violent storms, increased by the artificial flood-wave, produced disturbances in the pools that extended to Paris. In consequence of this experience, care was taken to empty several of the upper pools before the arrival of the artificial flood-wave. On the 22d of October a little rise of 8 inches from the Armançon River arrived at Laroche without being announced, and consequently without the precautions prescribed for an artificial flood-wave having been taken below. A complete derangement resulted at all the weirs and in all the pools from Laroche to Paris. Immediately the engineers proposed to establish above each weir, with movable wickets, a foot-bridge, which, with the aid of a windlass and chains, would permit the management of the wickets and the regulation of the level of the pool above; meanwhile all the dams were opened, and the system of artificial flood-waves was continued freely as formerly. A board of three inspector-generals of Ponts et Chausées, charged with an examination, heard those interested at Joigny, at Sens, at Montereau, and at Paris. They adopted the propositions of the engineers, which, as advised by the General Council of Ponts et Chausées, were approved by an official order of December 28, 1869.

In consequence, during the two seasons of 1869 and 1870 foot-bridges for maneuvering were built above each weir with so-called automatic wickets; and but for the unhappy events at the close of 1870, continuous navigation would have been established between Paris and Laroche by the month of September, 1870; which, however, could not be until the 1st of September, 1871, a year later. Each bridge for maneuvering is composed of wrought-iron trestles, like the trestles of Poirée dams, movable around a horizontal axis at right angles to the axis of the weir. Each trestle is opposite the middle of a wicket. These trestles are connected at their caps by two clamp-bars, which fix the width of the bridge. Between these bars is a wooden flooring, which is raised 20 inches above the level of the pool. The two clamp-bars are the rails upon which rolls the truck that carries the hoisting-windlass. Finally, to this windlass reach two chains, one attached to the head of the chase, and the other to the foot of the breech of each wicket. By the help of the windlass, solidly fastened to one or two