Imágenes de páginas

or service-bridge, the latter having been lowered at such time after raising the dam as desired, but certainly previous to the formation of ice or the appearance of drift.

Combination of Shutter and A-Frame.-In studying the question of the type of dam to adopt for a movable top it has occurred to the writer that a service-bridge made of A-frames, and the dam itself of Thenard shutters, would be a happy combination. The A-frames would, of course, be first raised, forming a service-bridge from which to raise the shutters, and at the same time they would shut the flow off so that the raising would take place in quiet water. After the shutters are up part or all of the A-frames could be lowered, thus bringing the head of pool against the dam proper. The A-frames, or service-bridge, could remain down until another raising of the dam is necessary, as the shutters will be lowered with the tripping bar. For pool regulation at times when there is a small rise a short length of the service-bridge can be left standing, from which as many shutters as desired can be lowered or raised at will. On the Big Sandy River weirs the wickets are raised by means of a stationary winch on the abutment, from which a wire rope passes out over the service-bridge to a sheave attached to a car, and around this sheave to the wicket. With the A-frame as a service-bridge it would be possible to attach the sheaves directly to the A-frames instead of to a travelling car. With this method, or combination, it would be possible to put on movable tops of great height, as the power required to raise the shutters in quiet water is easily developed; in fact, less power will be required than that now used for throwing the Big Sandy wickets with the tripping bar, which is moved by hand-power gear. The objection to the A-frame dam that drift or ice would interfere would, in this case, lose its significance, as it never would be necessary to lower the frames with drift or ice running. In fact, the chief objection to each of the two types is eliminated when they are used in combination, because the A-frame can be lowered at any time, even before the water reaches normal pool height, and the shutters can be raised without any pressure whatever coming against them during the operation.

In the opinion of the writer the combination suggested would, if properly designed and constructed, solve the problem of a movable top to stationary dams, or of movable dams in the broader sense, with greater success and satisfaction than anything so far tried, or suggested for trial, for either dams or tops.

A movable dam should not be raised against the current; and it should be lowered with the current by methods which will not endanger the lives of those engaged upon it, or injure the dam itself. When down, or when being lowered, it should offer no projections or parts which can cause injury by ice or drift. It should have very few moving parts. It is believed that the Thenard shutter is such a dam, when used in combination with an A-frame service-bridge.

Corps of Engineers

A flashboard on a dam of a type easily operated from the bank possesses undoubted advantages, most of which are brought out in the article. One frequent use of flashboards is for local repairs to the downstream face of the dam during low water. In such cases flashboards, frequently improvised, are erected on or above the crest of the dam to prevent overflow for a short section. It would be an important improvement to this design if its operation could be extended to independent raising and lowering of short sections, or to independent operation by halves.

As regards obstructions to the operation of this dam, I believe there is little to fear from drift and logs. As the writer states, there is little drift at the low stages, and what there is could be easily removed. At the high stages, with the dam up, the smooth surface presented would offer little opportunity for drift to lodge under the heavy pressure of high-water discharge. I am inclined. to fear obstruction more from the deposit of mud in silt-bearing streams, which might fill the entire recess provided for the dam, requiring the expense of frequent cleaning and, under some conditions, being almost impossible.

This dam is almost identical with one designed by Mr. B. F. Thomas, United States Assistant Engineer, and described by him in an article published in the Journal of the Association of Engineering Societies, June, 1896.

Maj. W. W. HARTS
Corps of Engineers

The advantage in using movable dams in canalized rivers, especially where there is considerable range between the high and low water stages, is so obvious that engineers have been engaged for many years in an effort to devise a dam that would satisfactorily combine the good principles of theory with such practical design as would enable it to be successfully used on the various classes of streams met with both in this country and abroad. The great number of inventions proposed at various times attest the interest which has been taken in this class of work. It is unfortunate, however, that so few of these dams have met with success when practically applied, and that even those that are fairly successful under certain conditions should be found to be wholly inapplicable to others.

A prime necessity in all movable dams is simplicity, for when the dam is lowered the river flow passing over it will usually fill the smallest interstices with silt, leaves, and drift which will interfere with its manipulation if the dam is complicated.

The dam proposed by the author for increasing the height of fixed dams apparently differs in no material points from the A-frame dam proposed by Mr. B. F. Thomas, United States Assistant Engineer, some years ago, and adopted on the Upper Ohio at

Dam No. 6. It is said that this dam after its completion was only once lowered, and that before it could be again raised a cofferdam, from which the water could be pumped, was necessary so that the sand and gravel which had collected in the openings and over the frames could be taken out with picks and shovels. It is understood that this dam was never again lowered, and that it now acts as a fixed dam, losing all its advantages as a movable dam. There seems to be no reason why the dam proposed by the author should not be equally subject to a similar injury in any silt-bearing stream.

Furthermore, the use of long chains extending across the stream for raising movable dams has been tried and found to be of doubtful value. An early method of raising the trestles on the movable dam across the Big Sandy, at Louisa, Ky., was proposed at one time, by which a chain was to be used which passed through the heads of the trestles and was wound up on a drum on the abutment. Locking devices to raise and release the trestles, also similar to those indicated in the paper, were suggested by Mr. Thomas, and the writer was sent in 1893 or 1894, while an assistant in the Cincinnati office, to examine and report upon the methods proposed for raising and lowering this dam. The machinery was soon found to be too intricate, and the weight of the trestles and the friction of the chains too great, so that as a practicable method it was later abandoned, and the use of boats for raising and lowering the dam finally adopted. There seems to be no doubt but that the use of chains in cases like the one suggested would necessarily be of very limited application, and the construction of the intricate methods of locking the panels together, and of releasing and attaching them to the moving chain, would not be suitable for locations where all the moving parts could not be easily reached. At such locations other methods might be found better.

The fact that a similar dam, when actually constructed, has had such an unfortunate experience would be a good reason, in the minds of many engineers, for not adopting the same principle to the use proposed unless all the causes of past trouble were unquestionably avoided.


Corps of Engineers

Lieutenant Adams points out why this dam was not adopted in the Pittsburg District. If it were certain that one man could operate the dam, I do not think that its high first cost should condemn it. Serious trouble in operation would be almost a certainty, because of the great number of points, trouble at any one of which is possible and might totally block operation pending remedy. Any movable dam whose units fall across the current is sure to be a source of trouble in case of drift and ice during operation. Heavy ice in scraping across a crest sometimes drops a considerable quantity of sand, gravel, and boulders on the cest.

Assistant Engineer

The A-frame movable top to provide increased depths above fixed dams, designed by Lieut. Lewis M. Adams, Corps of Engineers, I believe could be used with success under the conditions described as existing on the Monongahela River at Lock No. 5.

The A-frame and mechanism appear to be exceptionally well designed and simple in operation. The top and recess of the fixed dam are well designed to prevent injury from drift and ice to A-frame when the crest is down. One of the best features of the movable top is that it enables the operators to lower the dam from the lock wall without danger of accident to themselves. The danger attached to lowering the ordinary movable crest with hinged trestles and needles from a maneuvering boat in a rapidly rising river full of drift, especially at night when the drift can not be seen, is considerable.

With no drift running there should be no difficulty in raising the crest of dam, and as the movable crest is used only for procuring additional depths of water at low stages of the river, it should be possible, with careful supervision, to lower the crest on the approach of a rise before sufficient drift appeared to interfere with the lowering of dam. As the author states, the only objection to this type of dam is the possibility of some of the mechanical parts becoming fouled by drift in lowering the crest during a rise. How much danger there is from this source and the practical success of this type of dam can only be determined by trial, but its obvious advantages over the ordinary type of movable crest, under conditions described by Lieutenant Adams in his paper, would seem to justify a trial.

For several reasons, the A-frame dam of Lieutenant Adams could not be adapted to the conditions at Lock No. 11, Kentucky River, where the dam has a 6-foot movable crest. At this lock the elevation of the fixed dam and the lower miter sill of the next lock above are the same, and at low water the draft over the lower sill at Lock No. 12 depends entirely on the extra height of water secured by raising the movable crest. After each rise it becomes necessary to raise the crest on about 3.3 feet of water passing over the fixed dam. At this stage of water, with crest down, the reading on lower sill at Lock No. 12 is slightly less than 6 feet, due to the 3.3 feet of water on dam plus elevation due to surface curve. Only a portion of the crest of the dam can be raised, as the four draining valves are not nearly sufficient to pass the large volume of water passing over the dam at this stage. In other words, the elevation of the pool and the consequent draft over lower sill at Lock No. 12 is regulated largely by raising or lowering a sufficient number of sections of the movable crest. When the movable crest of dam is up and the river starts to rise beyond the capacity of the draining valves, a few sections of the movable crest are lowered, and if this is not sufficient, addi

tional sections are lowered, and again, as soon as the pool commences to drop below the crest, the required number of sections are raised to restore the pool to normal level. The pool could also be regulated by removing alternate needles or pushing back some of the needles at the top, but this is seldom done on account of drift.

Now, at this lock it would be necessary, a large part of the time, to have part of the A-frame dam down, and under these conditions the drift would accumulate around the mechanical parts of the sections partly lowered to such an extent as to obstruct the lowering of the remainder of the dam. The drift on the Kentucky River is exceedingly abundant, and it is not possible to maintain any sort of connection between the part of movable crest up and the part down. The movable crest on Dam No. 11 consists of 26 hinged trestles 6 feet in height, and spaced 8 feet apart, and when up connected by rails supporting the upper end of 2 by 4 inch needles. Originally, these trestles were connected by chains for the purpose of raising the trestles by hand, beginning at the lock wall, without the use of maneuvering boat. During the first winter when these trestles were down, these chains were completely worn out and broken by the constant surging of water in recess made in top of dam for receiving these trestles when lowered. Some of these chains were removed at the abutment end, but the chain connecting the last trestle in the down position was always broken by drift. It is therefore necessary to raise the trestles by grappling into them with grab hooks or other device. This is not an easy thing to do with 4 or 5 feet of water on dam, but a scheme has been devised by the lock men which is quite successful in accomplishing this.

In the foregoing discussion I have attempted to indicate the difficulties of adapting the A-frame crest to the conditions existing at Lock No. 11, often necessitating the maintaining of portions of the crest up to regulate the draft of water over lower sill at Lock No. 12, and incidentally to illustrate the difficulty attending the operation of any fixed dam under such exacting conditions. It is believed that most of the difficulties first encountered in the operation of the movable crest at Lock No. 11 have been overcome. My experience with the operation of a movable crest leads me to believe that they may be desirable adjuncts to provide additional draft at lower water discharge, under similar conditions described by Lieutenant Adams on the Monongahela River at Lock No. 5. The fixed dam is preferable to the movable crest for maintaining normal level where conditions will possibly permit.

The dams on the upper portion of the Kentucky River have an 18foot lift at normal pool stage, with movable crest of dam up. The 6-foot movable crest, of course, decreases the first cost of the lock, less guard above normal pool being necessary. The reaction below dam during high water is not so great. The damage from backwater on low land in creek bottoms is also greatly diminished, which is an item of importance on the Kentucky River.

It would seem advisable where movable crests are resorted to, to establish the height of elevation of the fixed portion of dam at suf

« AnteriorContinuar »